investigating cooling and lubrication strategies for...

INVESTIGATING COOLING AND LUBRICATION STRATEGIES FOR SUSTAINABLE MACHINING OF

TITANIUM ALLOYS: IMPACT ON MACHINABILITY AND ENVIRONMENTAL PERFORMANCE

Salman Pervaiz

Licentiate Thesis

School of Industrial Engineering and Management Department of Production Engineering

KTH Royal Institute of Technology

APRIL 2014

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TRITA- IIP-14-02 ISSN 1650-1888 ISBN 978-91-7595-091-4 Akademisk avhandling som med tillstånd av KTH i Stockholm framlägges till offentlig granskning för avläggande av licentiate fredagen den 4 april kl. 10:00 i sal M311, KTH, Brinellvägen 68, Stockholm.

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ABSTRACT The manufacturing sector is one of the most rapidly growing sectors in the industrialized world today. Manufacturing industry is concerned with being more competitive and profitable. Profit margins are directly related to the productivity of the company, and productivity improvements can be achieved by making manufacturing processes more efficient and sustainable. Knowledge of cutting conditions and their impact on machined surface and tool life enable productivity improvement. These days the main emphasis is not only to increase productivity, but also to make processes cleaner and more environmental friendly. This research aims to study machinability of difficult to cut, titanium alloys, in close reference to the application of different cooling/ lubrication strategies and their environmental impact. Total energy consumed (kWh) and carbon dioxide (CO2) emissions produced in machining are common environmental indicators. In this research project environmental implications of the cutting process were calculated in terms of carbon dioxide (CO2) emissions and energy consumption analysis. The experimental work consisted of controlled machining tests with cutting force, surface roughness, power, and flank wear measurements under dry, mist, combination of vegetable oil mist and cooled air (MQL+CA) and flood cutting environments. The current study provides better understanding of the cutting performance of TiAlN coated and uncoated carbide tools. The study also investigated tool failure modes, tool wear mechanisms, surface roughness and energy consumption to improve machinability of Titanium alloys. The study revealed the promising behaviour of minimum quantity lubrication (MQL) under specific ranges of cutting speed for both coated and uncoated tools. Variation in the cutting force showed close link with built up edge (BUE) formation. MQL based systems have huge potential to improve machinability of Titanium alloys and should be investigated further. Keywords: Titanium alloys, Energy consumption, Wear mechanisms

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PREFACE The work reported in this Licentiate thesis was a collaborative project between Production Engineering Department, Royal Institute of Technology and Mechanical Engineering Department, American University of Sharjah. The presented work has been conducted during the years 2011-2013 at the Manufacturing Laboratory, Department of Mechanical Engineering, American University of Sharjah, UAE. The research study was supervised by Professor Cornel Mihai Nicolescu (KTH Royal Institute of Technology) and Dr. Amir Rashid (KTH Royal Institute of Technology) and also co-advised by Dr. Ibrahim Deiab (American University of Sharjah/ University of Guelph). I would like to express my gratitude towards my all three supervisors for their valuable efforts, continuous support and professional mentoring throughout the research work. I would like to acknowledge the financial support of National Research Foundation (NRF) for this project. I would also like to thanks Accu-Svenska AB for supporting the research work by providing MQL booster system. Finally, I would also like to thank my family and friends for their continuous support.

Salman Pervaiz Stockholm, April 2014

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DEDICATION To my beloved wife, children and my parents.

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Table of Contents Copyright Statement I Abstract III Preface IV Dedication V Table of Contents VI Appended Papers X List of Figures XI List of Tables XV List of Abbreviations XVI List of Nomenclature XVII CHAPTER ONE INTRODUCTION

1.1 Evolution of materials used in aerospace engines 1 1.2 Material requirements for aerospace engines and other

applications 3 1.3 Physical properties 5 1.4 Energy consumption and environmental aspects 7

1.4.1 Titanium production 7 1.4.2 Titanium alloys machining 8

1.5 Challenges in the machining of titanium alloys 9 1.6 Research aim and objectives 10 1.7 Organization of thesis 11

CHAPTER TWO LITERATURE REVIEW 2.1 Specific energy and power consumption 13

2.1.1 Energy of chip formation 14 2.1.2 Stress distribution 16 2.1.3 Power consumption in machining operation 18

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2.1.4 Power consumption in turning operation 19 2.2 Machinability of aero-engine alloys 19 2.3 Classification of titanium alloys 20

2.3.1 Machinability of titanium alloys 22 2.4 Cutting tool materials for titanium alloys 25

2.4.1 Tool wear mechanisms and patterns 28 2.5 Surface integrity 33 2.6 Cutting fluid 37 2.7 Environmental friendly cooling strategies 38

2.7.1 Dry cutting 38 2.7.2 Minimum quantity lubrication (MQL) 39 2.7.3 High pressurized cooling (HPC) 40 2.7.4 Cryogenic cooling 41

2.8 Sustainable manufacturing concepts 41 2.8.1 Energy consumption in machining 43 2.8.2 Environmental implication of energy

consumption 47 2.9 Summary of literature review 48 CHAPTER THREE

EXPERIMENTAL METHODS 3.1 Milling experiments 50 3.2 Turning experiments 55

CHAPTER FOUR MACHINABILITY EVALUATION METHODS 4.1 Surface roughness analysis 60 4.2 Cutting force evaluation 62 4.3 Flank wear assessment 63 4.4 Wear mechanism analysis using scanning electron microscopy 64 4.5 Power and energy consumption analysis 65

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4.6 Minimum quantity cooled air lubrication (MQL+CA) system and supporting tool holder 66

CHAPTER FIVE RESULTS AND DISCUSSION 5.1.1 Milling of Aluminum alloy 6061 68 5.1.2 Energy consumption analysis 69 5.1.3 Total machining time 70 5.2.1 Machining of Titanium alloy (Ti6Al4V) 72 5.2.2 Surface roughness analysis 72 5.2.3 Flank wear measurement 74 5.2.4 Wear mechanisms 75 5.3.1 Surface roughness comparison 78 5.3.2 Cutting force comparison 82 5.3.3 Power and energy consumption 84 5.3.4 Tool wear assessment 86 5.3.5 Wear mechanisms in coated and uncoated cutting tools 88

5.4.1 Surface roughness Vs. Energy curves at dry environment 99

5.4.2 Surface roughness Vs. Energy curves at flood environment 101

5.4.3 Observations for similar material removal rate 103 5.4.4 Complimentary results 105

5.4.4.1 Environmental implications of energy 105 consumption 5.4.4.2 Influence of geographical location of 105 CO2 emissions

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5.4.4.3 Estimation of CO2 emissions using energy consumption used in Paper D 106

5.5 Machinability of MQL+CA 5.5.1 Surface roughness analysis 108 5.5.2 Tool wear measurement 108 5.5.3 Cutting temperature analysis 108 CHAPTER SIX

CONCLUSIONS AND FUTURE WORK 6.1 Energy consumption in milling tool path strategies 116 6.2 PVD-TiAlN coated and uncoated carbide tools 116 6.3 Cutting force behaviour 117 6.4 Role of cutting environment 117 6.5 Minimum quantity lubrication and 117 cooled air (MQL+CA) vegetable oil based mist system investigation 6.6 Energy consumption and environmental 118 implications 6.7 Future work 119 REFERENCES 120

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APPENDED PAPERS Paper A: Pervaiz, S., Deiab, I., Rashid, A., and Nicolescu, M., "An

Experimental Analysis of Energy Consumption in Milling Strategies", 2012 IEEE International Conference on Computer Systems and Industrial Informatics – ICCSII’12, Sharjah, UAE, Dec 18-20, 2012.

Paper B: Pervaiz, S., Deiab, I., Darras, B., Rashid, A., and Nicolescu, M., "Performance evaluation of TiAlN- PVD coated inserts for machining Ti-6Al-4V under different cooling strategies”, Advanced Materials Research, Advanced Materials Research Vol. 685 (2013) pp. 68-75.

Also presented in 3rd International Conference on Advanced Materials Research (ICAMR 2013), Dubai, UAE, Jan 19 -20, 2013.

Paper C: Pervaiz, S., Deiab, I., and Darras, B., "Power consumption and tool wear assessment when machining titanium alloys," International Journal of Precision Engineering and Manufacturing, Vol. 14, No. 6, pp. 1-12, 2013.

Paper D: Pervaiz, S., Deiab, I., Rashid, A., Nicolescu, M., and Kishawy,

H., " Energy consumption and surface finish analysis of machining Ti6Al4V”, International Conference on Manufacturing Systems Engineering ICMSE 2013, Venice, Italy, April 14 - 15, 2013.

Paper E: Pervaiz, S., Deiab, I., Rashid, A., Nicolescu, M., and Kishawy, H., “Performance evaluation of different cooling strategies when machining Ti6Al4V,” International Conference on Advanced Manufacturing Engineering and Technologies - NEWTECH 2013, Sweden, October 27-30, 2013.

Words Count: 48,574

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List of Figures Figure 1.1 Materials and operating temperatures in aerospace engines 2 Figure 1.2 Strength vs density materials selection chart 4 Figure 1.3 Strength/ Density ratio versus operating temperature (F°) 6 Figure 1.4 Production technologies from titanium 8 Figure 2.1 Schematic illustration of orthogonal cutting 13 Figure 2.2 Schematic illustration of chip formation by using “Deck of

Cards” approach 14

Figure 2.3 Schematic illustration of forces acting in the cutting zone 15 Figure 2.4 Schematic illustration of flow pattern in chip formation 17 Figure 2.5 Stress distribution on the rake face during cutting 17 Figure 2.6a Phase diagram of Titanium alloys 20 Figure 2.6b Phase diagram Ti-Al alloys 21 Figure 2.7 Sources of heat generation in machining 23 Figure 2.8 Schematic illustration of segmented chip formation 24 Figure 2.9 Machinability ratings of Titanium alloys 24 Figure 2.10 Schematic illustration tool wear patterns 28 Figure 2.11 Abrasive wear observed for different cutting tool materials

when machining titanium alloys 29

Figure 2.12 Adhesive wear observed for different cutting tool materials when machining titanium alloys

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Figure 2.13 Diffusion wear observed for different cutting tool materials when machining titanium alloys

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Figure 2.14 (a) Schematic illustration of the diffusion couple (b) Cross-sectional view of WC/Co carbide tool after exposing to air for 90 min at 800 °C

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Figure 2.15 Chipping/ Flanking observed for different cutting tool materials when machining titanium alloys

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Figure 2.16 (a) Surface tearing (Ti6Al4V), V=100 m/min, f=0.15 mm/ tooth, DoCa = 2.0 mm, DoCr = 8.8 mm [73], (b) White layer in Ti6Al4V machined at V=95 m/ min, f = 0.35 mm/rev, and DoC = 0.10mm

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Figure 2.17 Microhardness behaviour observed in machining FGH 95 (a) Microhardness region (b) Microhardness measurement

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Figure 2.18 Surface roughness behaviour with cutting speed for turning Ti6Al4V (a) ISO-883-MR4 Tool at feed = 0.35 mm/ rev, (b) ISO-890-MR3 Tool at feed = 0.25 mm/ rev

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Figure 2.19 Schematic illustration of MQL 39 Figure 2.20 Through spindle high pressure coolant delivery systems by

Sandvik 40

Figure 2.21 Cumulative growth of federal environmental laws and amendments

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Figure 2.22 Energy consumption breakdown for milling process 44 Figure 2.23 Characterization of power consumption during cutting

processes 45

Figure 2.24 Global CO2 emissions from fossil fuels 47 Figure 3.1 High speed steel (HSS) 02 flute end milling cutter with

code DIN884 51

Figure 3.2 Experimental setup of the experimental setup 53 Figure 4.1 Arithmetical mean of the profile (Ra) as per ISO 4287

standard 60

Figure 4.2 Mitutoyo surface roughness tester SJ 201P 61 Figure 4.3 Cutting force data evaluation system, (a) Kistler 9257b

Dynamometer, and (b) Kistler 5070 charge amplifier 62

Figure 4.4 (a) Mitutoyo tool maker microscope (Model: TM 510), and (b) sample flank wear measurement

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Figure 4.5 (a) Scanning electron microscope (Model: Philips FEI XL 30), Sample SEM images of uncoated carbide tool from paper C, Vc = 30 m/min and f = 0.1 mm/ rev, (b) Dry (c) Mist (d) Flood

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Figure 4.6 (a) PS 3500 power data logger and (b) Sample calculation from paper C for energy consumption calculated for mist condition, vc = 30 m/min, f = 0.1 mm / rev, Depth of cut = 0.8 mm

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Figure 4.7 MQL (vegetable oil based) system with low temperature cool air

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Figure 4.8 (a) Schematic view of Mircona SCLCR 2525 M12-EB tool holder (b) View of actual tool holder used in experimentation

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Figure 5.1 Schematic illustration of milling strategies 69 Figure 5.2 Zigzag, Constant overlap spiral, Parallel spiral and One-

way milling strategies, 4000 rpm 69

Figure 5.3 Zigzag, Constant overlap spiral, Parallel spiral and One-way milling strategies, 2000 rpm

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Figure 5.4 Machining time of milling strategies at 4000 rpm 71 Figure 5.5 Machining time of milling strategies at 2000 rpm 71 Figure 5.6 Total machining length with Zones A, B, C and D 72

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Figure 5.7 Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.1 mm/ rev (b) Vc = 90 m/min, f = 0.1 mm/ rev

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Figure 5.8 Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.2 mm/ rev (b) Vc = 90 m/min, f = 0.2 mm/ rev

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Figure 5.9 Maximum flank wear measurement 74 Figure 5.10a Wear mechanisms in PVD TiAlN coated inserts 77 Figure 5.10a Wear mechanisms in PVD TiAlN coated inserts 78 Figure 5.11b Wear mechanisms in PVD TiAlN coated inserts 79 Figure 5.11b Wear mechanisms in PVD TiAlN coated inserts 80 Figure 5.12 Surface roughness (Ra) measurements 81 Figure 5.13 Statistical analysis of surface roughness (a) Half-normal

plot (b) Residuals vs. Run 83

Figure 5.14 Cutting force at different cutting speeds under dry, mist and flood cooling strategies (a) Uncoated inserts, f = 0.1 mm/ rev (b) Coated inserts, f = 0.1 mm/ rev (c) Uncoated inserts, f = 0.2 mm/ rev, and (d) Coated inserts, f = 0.2 mm/ rev.

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Figure 5.15 Power and Energy consumption in Dry cutting, Uncoated tool, f = 0.1 mm/min

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Figure 5.16 Specific energy consumption by uncoated and coated inserts under dry, mist and flood conditions, (a) Cutting speed of 30 m/ min, (b) Cutting speed of 60 m/ min, and (c) Cutting speed of 90 m/ min

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Figure 5.17 Flank wear on coated and uncoated tool (a) Dry, (b) Mist and (c) Flood

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Figure 5.18 Wear mechanisms in uncoated tool@ 30m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.19 Wear mechanisms in coated tool@ 30m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.24 Energy consumption and surface finish curves for dry cutting at different material removal rates using five feed

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levels f = 0.1 – 0.5 mm/ rev, (a) Vc = 30 m/ min, (b) Vc= 60 m/ min, and (c) Vc= 90 m/ min

Figure 5.25 Energy consumption and surface finish curves for flood cutting at different material removal rates using five feed levels f = 0.1 – 0.5 mm/ rev, (a) Vc = 30 m/ min, (b) Vc= 60 m/ min, and (c) Vc= 90 m/ min

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Figure 5.26 Energy consumption and surface finish at similar material removal rates using different cutting speeds of 30, 60 and 90 m/ min under dry cutting

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Figure 5.27 Energy consumption and surface finish at similar material removal rates using different cutting speeds of 30, 60 and 90 m/ min under flood cutting

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Fig 5.28 Variation in global energy mix 107 Figure 5.29 Equivalent CO2 emissions (g) produced for different

cutting conditions under dry cutting 109

Figure 5.30 Equivalent CO2 emissions (g) produced for different cutting conditions under flood cutting

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Figure 5.31 Surface roughness trends with respect to dry, MQL+CA, and flood cooling strategies, (a) Vc = 90 m/min, (b) Vc = 120 m/min and (c) Vc = 150 m/min

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Figure 5.32 Flank wear for flood, dry and MQL+CA at cutting speed of 90 m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

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Figure 5.35 Sample measurements of cutting temperature under dry environment at cutting speed of 150 m/ min, (a) feed =

0.15 mm/ rev (b) feed = 0.25 mm/ rev

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Figure 5.36 Cutting temperature under dry, MQL+CA and flood environment

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List of Tables Table 1.1 Commercially available high performance alloys 2 Table 1.2 Physical properties of different engineering alloys 5 Table 2.1 Shear stresses and specific horsepower of different

materials 18

Table 2.2 Softening points of tool materials 25 Table 2.3 Advantages and disadvantages of different environmental

friendly cooling strategies employed in machining titanium alloys

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Table 3.1 Nominal chemical composition of Al 6061 51 Table 3.2 Mechanical properties of Al 6061 at room temperature 51 Table 3.3 Specificans of the DIN 844 HSS C08 cutter 52 Table 3.4 Design of experiments 52 Table 3.5 Cutting parameters 53 Table 3.6 Nominal chemical composition of Ti6Al4V 55 Table 3.7 Mechanical properties of Ti-6Al-4V at room temperature 55 Table 3.8 Specificans of the turning cutting insert 56 Table 3.9 Cutting parameters and experimental set up in turing

experiments 57

Table 4.1 Specifications of Mitutoyo surface roughness tester 61 Table 4.2 Specifications of Kistler dynamometer 62 Table 4.3 Specifications of Mitutoyo tool maker microscope 63 Table 4.4 Specifications of power logger 66 Table 4.5 Properties of vegetable oil used in mist (ECULUBRIC

E200L) 66

Table 5.1 Results of ANOVA for surface roughness 80 Table 5.2 Lifecycle estimates of gCO2e/ kWh for electricity

generation procedures 108

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List of Abbreviations ANOVA Analysis of variance BUE Built up edge CCNG Compressed cold nitrogen gas CCNGOM Compressed cold nitrogen gas and oil mist CCS Carbon capture and storage CNC Computer numerical control CVD Chemical vapour deposition COF Coefficient of friction GDP Gross domestic product GHG Greenhouse gas emissions HPC High pressurized cooling HSS High speed steel LCA Life cycle assessment MRR Material removal rate MWF Metal working fluids MQL Minimum quantity lubrication PCD Polycrystalline diamond PCBN Polycrystalline cubic boron nitride PVD Physical vapour deposition SEM Scanning electron microscope TiC Titanium carbide WC MQL+CA

Tungsten carbide Minimum quantity lubrication and cooled air

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List of Nomenclature Symbol Parameter Units

α Rake angle degrees β Friction angle degrees Fc Cutting force N Ft Thrust forces N

Fs Shear force parallel to shear plane N

Fn Normal force perpendicular to shear plane N F Force at tool-chip interface N N Normal force at tool-chip interface N Ø Shear angle degrees i Inclination angle degrees VB Width of flank wear land mm Ra Surface roughness μm Vc Cutting speed m/ min DOC Depth of cut mm f Feed mm/ rev fr Feed rate mm/ min

CHAPTER 1

INTRODUCTION

This chapter describes the background and aim of the research. Organization of the thesis is also elaborated in this chapter

1.1 EVOLUTION OF MATERIALS USED IN AEROSPACE ENGINES

In aerospace industry, titanium and nickel based alloys are preferred over conventional steels and aluminium alloys due to their high strength to weight ratio, fracture toughness, fatigue strength, superior corrosion resistance and ability to operate at higher temperature. The strong point of titanium alloys is that they show higher strength than aluminium alloys and less density than steels, making it suitable for structural applications [1]. Due to these extraordinary properties, titanium and nickel based alloys are used extensively in other demanding sectors like automotive, petrochemical, marine, military, biomedical and nuclear [2, 3].

As titanium and nickel based alloys are well suited to meet the demands of aerospace industry, it is the largest consumer for titanium alloy components. In aerospace industry titanium alloys are used in the construction of air frames, fastening applications, landing gears, jet engine shafts and casings for the front engine fan [4]. An aircraft engine generally consists of three main subassemblies namely compressor, combustor and turbine housed casing. Figure 1.1 shows the cross sectional view of an engine that shows the usage of nickel and titanium alloys in aerospace sector for engine components. Besides aerospace applications, titanium alloys are highly preferred as a composite interface material for advanced aircraft designs [5]. The demand rate of titanium products is increasing rapidly as aerospace industry demands for fuel efficient and light weight aircraft designs. In order to estimate that how extensive titanium alloys are

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being used in an aerospace sector the application areas are presented in Table 1.1.

Figure 1.1: Materials and operating temperatures in aerospace engines [6]

Table 1.1: Commercially available high performance alloys [7, 8, and 9]

Material Alloys Applications

α-alloys/ near α-alloys

T-0.3Mo-0.8Ni Ti-3Al-2.5V Ti-3Al-2.5V-Pd Ti-3Al-2.5V-Ru Ti-5Al-2.5Sn

Chemical processing, desalination, hydrometallurgical extraction Aircraft ducting, tubing, watches, eye glass frames Offshore hydrocarbon production Offshore hydrocarbon production Gas turbine engine parts

α-β alloys Ti-6Al-4V Ti-6Al-7Nb Ti-6Al-6V-2Sn Ti-6Al-2Sn-4Zr-6Mo Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.15Si

Aircraft ducting, Airframe parts, Automotive parts, Consumer products (watches, eye glass frames, etc) Medical implants, surgical instruments Airframe parts Gas turbine engine parts Airframe parts, Space vehicles/ structures

β alloys/ near β- alloys

Ti-10V-2Fe-3Al Ti-3Al-8V-6Cr-4Zr-4Mo Ti-3Al-8V-6Cr-4Zr-4Mo-0.05Pd

Airframe parts, Landing gear parts Geothermal brine energy extraction, navy ship parts, space vehicles Navy ship parts, space vehicles

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1.2 MATERIAL SELECTION REQUIREMENTS FOR AEROSPACE INDUSTRY

In aerospace sector, material selection process is based on high performance and extraordinary thermo-mechanical properties of the engineering materials. There are some driving key factors in the aerospace industry which highly influence the material selection process [10]. These key driving factors and their associated material responses are listed as under; • High engine efficiency, High efficiency in aerospace engines is

generally attained by achieving high pressure ratio in the compressor and high temperature rise in the combustor. High engine efficiency also depends on the 3D aerofoil geometry as well. In financial perspective increasing efficiency results in reduction in the fuel cost.

Material response, In order to achieve high efficiency in the aerospace engine, materials used in the compressor and combustor should be capable of sustaining high operating temperatures. Titanium and nickel based alloys are capable to operate at high temperatures.

• High thrust to weight ratio, Thrust to weight ratio is one of widely

adopted parameters used to evaluate the performance of an aircraft engine. The capability of an engine to generate thrust depends on the operating temperature. If material can operate can operate at high temperature. High thrust to weight combines the requirement of high operating temperature with mass reduction. Generally engineering materials with high strength to weight ratio enhances the performance and reliability of the components.

Material response, Reduction in the structural mass results in an improved aircraft performance by improving the response time, better speed of climb, efficient fuel consumption and survivability. This requirement can be fulfilled by selecting light weight engineering materials with high strength. Titanium and nickel based alloys offer high strength to weight ratio which makes them compatible for aerospace industry. Figure 1.2 shows the plot of all engineering materials on strength (σ) – density (ρ) selection chart. Titanium alloys show higher strength than aluminum alloys and less density than steel, making it suitable for structural applications.

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Fig 1.2: Strength vs density materials selection chart (Adapted from M. F.

Ashby, Materials Selection in Mechanical Design [11]) • Challenging loading conditions, Material selection requirements vary

throughout the aerospace engine. The part of the compressor present at low temperature experiences corrosion, erosion, impact and fatigue as dominant loading conditions. Generally, turbine blades operate at high temperature as a result material experiences creep and corrosion as dominant loads. Whereas at the same time turbine disc operates at low temperature and experiences higher cyclic loads resulting in fatigue load as more critical condition.

Material response, In order to meet the challenges for specific operating conditions, titanium and nickel based superalloys are used

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due to their superior performance characteristics. For high operating temperature derivatives of titanium and nickel based superalloys with tough engineering ceramics are being explored. However to reduce structural mass, derivatives of superalloys, metal composites and ceramics are being investigated.

1.3 PHYSICAL/ MECHANICAL PROPERTIES

Titanium alloys are useful structural material because of their high strength to weight ratio, fracture toughness, fatigue strength, superior corrosion resistance and ability to operate at higher temperatures in the range between 550 – 700 C°. In the Table 1.2 a brief comparison of a titanium alloy (Ti6Al4V) has been presented with conventional steels and aluminium alloy. Table 1.2: Physical properties of different engineering alloys [12]

Properties Ti6Al4V Stainless Steel

Carbon Steel Al 6061

Melting point (K) 1813-1923 1673-1703 1500 855- 925

Density (x103 kg/m3) 4.42 8.02 7.83 2.7 Young’s Modulus (GPa) 113.19 198.94 205.8 68.9

Poisson ratio 0.3-0.33 0.3 0.3 0.33 Fracture toughness (MPa – m1/2) 75 - 50 29

Specific Heat (kJ/(Kg. K)) 0.56 0.5 0.46 0.89

Thermal Conductivity (W/ (m.K))

7.54 16.34 53.5 167

First titanium alloy (Ti6Al4V) was developed in United States in 1954. It is the most commonly used alloy of titanium that is widely utilized in industry due to the better heat resistance, corrosion and erosion resistance, formability, weldability and biocompatibility etc. Consumption of Ti6Al4V is almost 75 – 80% among all different derivatives of titanium alloys.

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Fig. 1.3: Strength/ Density ratio versus operating temperature (F°)

Figure 1.3 represents the plot of strength to weight ratio with respect to the operating temperature range for engineering alloys. It can be observed that titanium alloys offer very impressive strength to weight ratio even at higher temperature making them suitable for the challenging engineering sectors.

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1.4 ENERGY CONSUMPTION AND ENVIRONMENTAL ASPECTS

In the current form, cost involved in the refining, processing and production of titanium is very high as compared to the other metals. It has been observed that cost involved in the phase from melting till refining is five times higher aluminium. If cost is considered for titanium shaped into ingots and finished part, then it has been found ten times higher than the aluminium. The demand rate of titanium products is increasing rapidly as aerospace industry demands for fuel efficient and light weight aircraft designs. In 2009 approximately 21,000 tons of titanium was used in automotive, chemical processing, metallurgical plants, sports, marine, medical and architectural sectors. A careful industrial forecast represents approximately 40 % increase by 2015 [13]. From energy consumption and environmental burden improvement perspectives, in parallel with the optimization of titanium alloys machining, it is important to significantly improve the production process of titanium alloys. 1.4.1 Titanium Production Titanium is manufactured by performing several phases including the extraction, refining, processing and production. In the first phase titanium sponge is extracted from the titanium ore. In the next phase titanium sponge is melted and cast into the ingots shape. Then rolling and forging operations are used to shape ingot into the desired form of plate, billet and rod. At the final stage titanium parts are finished by forging, extrusion, hot and cold forming, machining, and casting processes etc. Figure 1.4 represents the stages and processes involved in the production of titanium. The solid line shows already established technology, whereas dotted line shows that there is potential to improve the process. Titanium parts can be manufactured using titanium in powder form by powder metallurgy based techniques. As shown in the Figure 1.4 that powder can be processed into the final shapes using metal injection molding (MIM), direct powder rolling (DPR) and hot Isostatic pressing (HIP) processes [13].

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Fig. 1.4: Production technologies from titanium (after Chunxiang et al.

[13])

In the conventional titanium production process, melting of titanium sponge is required to cast it into the ingots form. Generally this melting of titanium is performed using electron beam melting (EBM), vacuum arc remelting (VAR) or plasma arc melting (PAM) process. These melting processes consume very large amount of energy and resources. There is a need to develop more energy efficient technologies for the production of titanium alloys. It will reduce the energy consumption and associated environmental burden. 1.4.2 Titanium Machining United Nations world commission on environment and development has defined sustainability as the capability to encounter the need of the present without compromising the capability of future generations to meet their own needs [14]. United States, Department of Commerce has defined sustainable manufacturing as “The creation of manufactured products through economically sound processes that minimize negative environmental impacts while conserving energy and natural resources. Sustainable manufacturing also enhances employee, community, and product safety” [15].

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These days metal cutting sector is under immense pressure to improve environmental performance due to the implementation of strict international environmental regulations. By adopting the sustainable practices in metal cutting sector, environmental performance can be improved significantly under economic conditions. The idea of sustainable manufacturing deals with effective use of material flow, energy, knowledge, health safety, and environmental concerns. In the manufacturing sector, sustainable practices can be employed by minimizing resources (energy, material, and water), improving environmental concerns by reducing the use of toxic and non-biodegradable chemicals, efficiently designing life cycles, and improving working conditions (such as ergonomics and health safety) [16].

The sustainable manufacturing concept aims to reduce the amount of greenhouse gas (GHG) emissions and the ecological footprint. In order to promote sustainable practices in the metal cutting sector it is also important to consider the surface integrity of the machined part to avoid rework. As most of the cutting fluids are environmental hazard in nature. It is also important to limit the usage of cutting fluids. Near dry and minimum quantity lubrication (MQL) techniques are being explored to replace conventional flood cooling method to reduce environmental burden. Machining is the most commonly used operation in the manufacturing sector. In order to reduce greenhouse gas (GHG) emissions and the ecological footprint in machining, processing time and energy consumption also plays an important role. In order to conduct a desired machining operation on a certain machine tool, electrical energy is drawn from an electrical grid system. Electrical energy is generated by using different energy sources such as coal, fossil fuels, and hydraulic, nuclear, solar and wind energy. Each source produces different amounts of greenhouse gas (GHG) emissions, but renewable energy resources (such solar, wind, geothermal and tidal) generate significantly less greenhouse gas (GHG) emissions. Greenhouse gas (GHG) emissions in machining can be reduced by utilizing electricity from renewable sources and by minimizing energy consumption during the machining phase.

1.5 CHALLENGES IN THE MACHINING OF TITANIUM ALLOYS

Despite the increased demand of these alloys in engineering sector, there are difficulties present in the primary and secondary processes [17]. As these alloys maintain high strength and hardness at elevated temperatures, machining is difficult to perform. Properties like low thermal conductivity,

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high strain hardening, high hardness at elevated temperature and high chemical reactivity are responsible of poor machinability rating of high performance alloys. Challenges faced during machining of titanium alloys are mentioned below;

• Titanium has low thermal conductivity of 4 – 16 W m-1 K-1 and high specific heat capacity of 520 J kg-1 K-1. For comparison purpose, thermal conductivity of structural steel is 450 W/ m*K much higher than titanium. Combination of high cutting temperature, high heat capacity and low thermal conductivity of titanium results in poor heat dissipation during the cutting process. As most of the heat stays at the cutting edge because of low thermal conductivity it produces high thermal stresses at the cutting edge resulting in poor tool life.

• Titanium alloys maintain strength at elevated temperatures that makes plastic deformation very difficult during machining phase.

• Titanium alloys have high yield point and low plasticity which gives it good elastic properties. During machining phase cutting tool bounces back like a spring because of high elasticity of titanium resulting in chatter [2].

• Less contact area on rake face between tool and workpiece material causes high magnitude of stresses at cutting edge [2].

• Formation of segmented chips form pulsating load at cutting edge [34,35, 39].

• Due to high chemical reactivity of titanium alloys chips tend to weld at tool tip and cutting edge which results in catastrophic tool failure and severe edge chipping [2].

1.6 RESEARCH AIMS AND OBJECTIVES

Motivation of the research work has been described as under;

These days metal cutting sector is under immense pressure to improve environmental performance due to the implementation of strict international environmental regulations. In the metal cutting sector, utilization of cutting fluids is being questioned due to their negative impact on the environment. Majority of the commonly used cutting fluids are toxic and non-biodegradable in nature. The main drawback of using these cutting fluids is the waste disposal after being used. At the same time energy consumed in

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the machining process has strong connection with the amount of greenhouse gas (GHG) emissions.

The aims of this research are listed as under;

• The main objective of this research is to study the feasibility aspects of shifting from conventional (dry and flood) to the environmental friendly cooling strategies.

• Cutting fluid application has direct influence on the machining performance of the cutting process. The process of moving from one cooling strategy to other was examined by analysing the machining performance (tool life assessment, wear mechanisms, and machined surface quality) and the energy requirements (power and energy).

• The current study investigates energy consumption and its associated environmental implications by considering carbon dioxide CO2 emissions.

1.7 ORGANIZATION OF THE THESIS

The thesis is based on five research papers enclosed in six chapters. Paper A is based on the preliminary experimentation to investigate the behaviour of energy consumption under milling operation using Al-6061 alloy as workpiece material. In paper B, machinability of Titanium alloy (Ti-6Al-4V) is investigated using TiAlN-PVD coated carbides. The study used limited cutting parameters and surface roughness and flank tool wear were used as machinability evaluation criteria. Paper C, further extends the machinability evaluation of Ti6Al4V using uncoated and TiAlN coated carbide tools. The study utilized comprehensive cutting parameters and different cooling strategies during investigation. The machinability was evaluated using surface roughness, tool wear, cutting force and energy consumption criteria. The study also discussed the detected wear mechanisms in detail. In paper D, energy consumption and surface roughness plots are created against material removal rate (MRR). These curves will be helpful in order to optimize the energy consumption and surface roughness at desired material removal rate (MRR). In Paper E, machining performance of vegetable oil based MQL system was investigated experimentally.

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The thesis consists of six chapters:

• Chapter one provides a brief introduction about the topic and describes the aims and objectives of the study.

• Chapter two provides literature review on the machinability of

titanium alloys and sustainability concepts in machining.

• Chapter three describes the methodology and experimental setup adopted to perform the conducted research.

• Chapter four demonstrate the machinability evaluation criteria used

this research.

• Chapter five summarises the important findings and results obtained from all four appended papers.

• Chapter six concludes the research work and recommends

proposals for future work.

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CHAPTER 2

LITERATURE REVIEW

This chapter presents a brief review of the available literature.

2.1 SPECIFIC ENERGY AND POWER CONSUMPTION

In this section, a brief review of the theoretical concepts of metal cutting and specific energy and power consumption is introduced. In the material removal processes, material is removed using shearing operation in the form of small chips. Orthogonal machining process is the basic fundamental model used to understand the machining operation. Figure 2.1 represents the schematic illustration of orthogonal cutting. In orthogonal cutting orientation, the cutting edge position is perpendicular to the direction of cutting speed.

Figure 2.1: Schematic illustration of orthogonal cutting [18]

Metal cutting process is highly complex nonlinear and combined thermo-mechanical operations. The major complications are because of high strain

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and high strain rate in the primary shear zone and high friction at tool- chip interface along secondary shear zone. Chip formation is also very complex in nature because it involves huge plastic deformation [19]. Piispanen [20] illustrated the chip formation process at low and high cutting speed with respect to the shear angle approach. The chip formation process was explained with the help of deck of cards placed at shear angle (Ø) as shown in Figure 2.3. Each parallelogram shaped card represents a chip segment. As the cutting tool moves with certain velocity, each card slides on the neighbouring card representing chip flow action. This approach assumes friction at tool face during the cutting process as elastic instead of plastic. The approach also assumes no built-up-edge (BUE) and chip curl.

Figure 2.2: Schematic illustration of chip formation by using “Deck of

Cards” approach [18, 20]

2.1.1 Energy of Chip Formation

During the machining process, total energy consumed per unit time (power) can be computed by taking the product of primary cutting force component (Fc) and the cutting velocity (Vc). As cutting process is very complex in nature and energy consumed in cutting depends on many cutting parameters. In order to normalize the energy consumption it is generally divided by the material removal rate (MRR). Material removal rate can be computed by multiplying the area being cut with the velocity perpendicular to that area. Area being cut can be calculated by taking the product of uncut chip thickness (t) and width of the sample being cut (w) as shown in Figure 2.3. Thus, the energy per unit time, or specific energy, u, can be calculated as shown in Eq. 2.1:

u = 𝐹𝑐𝑉

𝑡 𝑤 𝑉𝑐 = 𝐹𝑐

𝑡 𝑤 (2.1) [21]

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Figure 2.3: Schematic illustration of forces acting in the cutting zone [21]

Specific energy consumed during a machining process can be divided into four different portions. These four different portions will be discussed in detail.

• Shear energy per unit volume (us), In order to estimate this component of energy consumption, energy involved in the shearing of material is divided by material removal rate as shown in Eq. 2.2. Where Fs and Vs are shear force and shear velocity respectively.

us = 𝐹𝑠 𝑉𝑠𝑡 𝑤 𝑉𝑐

(2.2) [21]

• Friction energy per unit volume (uf), it is the energy consumed for sliding action of chip on the rake face. It is computed by taking into consideration the sliding velocity of chip (V chip) over the rake face. Eq. 2.3 shows the formula for friction energy calculation.

uf = 𝐹 𝑉𝑐ℎ𝑖𝑝𝑡 𝑤 𝑉𝑐

(2.3) [21]

• Kinetic (momentum) energy per unit volume (um), it is the energy required to accelerate the chip. Generally it is neglected as it very less as compared to over portions of energy. But in case of high speed machining, it is important to take into account this energy as well. Momentum force is represented as Fm. Eq. 2.4 shows the formula for kinetic energy calculation.

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um = 𝐹𝑚 𝑉𝑠𝑡 𝑤 𝑉𝑐

(2.4) [21]

Fm can be computed as Fm = ρV2twγ. Where ρ is density and γ is shear strain.

• Surface energy per unit volume (ua), it is the energy utilized to create a new uncut surface during machining operation. This component can be computed by using surface energy of the material being cut (T) as shown in Eq. 2.5.

ua = 𝑇. 2 𝑉𝑤𝑡 𝑤 𝑉𝑐

= 2𝑇𝑡

(2.5) [21]

2.1.2 Stress Distribution In case of metal cutting operation, high normal and shear stresses are formed in the primary and secondary shear zones. These normal and shear stresses are formed due to presence of high plastic deformation in the primary shear zone and friction in the secondary shear zone. For the primary shear zone, it is assumed that both normal and shear stresses are distributed uniformly over the shear plane. Area at shear plane (As) can be calculated by taking cutting area (t.w) and shear angle (Ø) into account.

As = 𝑡 . 𝑤𝑆𝑖𝑛Ø

(2.6) [21] Taking this shear area into consideration normal (σ) and shear (τ) stresses can be calculated by the formulas mentioned below;

τs = 𝐹𝑠𝐴𝑠

(2.7a) [21] & σs = 𝐹𝑛𝐴𝑠

(2.7b) [21] There are several theories about the stress distribution on the rake face. The classical approach assumes that stress distribution obeys coulombs sliding law and is uniformly distributed. In this case coefficient of sliding friction (µ) is the ratio between friction (F) and normal (N) forces.

µ = 𝐹𝑁

(2.8) [21] However, a large number of experimental evidences are there to negate that stresses are uniformly distributed over the rake face. These experiments

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were conducted using scanning electron microscope, photoelastic tool and quick stop arrangements. These experimental studies revealed that there are different chip flow patterns available in the chip formation. The flow pattern near the tool tip was seized as shown in Figure 2.4. However, on the rest of the tool face the flow patterns shows sliding contact. Due to the seizure at the tool tip, material close to the tool surface is stationary which facilitates relative shearing. Generally seizure is called as sticking region.

Figure 2.4: Schematic illustration of flow pattern in chip formation [21] The actual normal and shear stress distributions were represented by several researchers. The non-linear behaviour of stress distribution by developed by Zorev [22] as shown in Figure 2.5.

Figure 2.5: Stress distribution on the rake face during cutting [22]

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2.1.3 Power Consumption in Machining Operations

Generally machine tools are rated in terms of power. Specific power (Ps) can be computed by dividing the input power (Fc.Vc) with material removal rate (MRR) 1. Specific power provides a measure of difficulty involved while machine a certain material. Total power in cutting process can be computed by the product of specific power with material removal rate. Specific horse power for different materials has been presented in the Table 2.1.

P = Ps · MRR (2.9) [21] Table 2.1: Shear stresses and specific horsepower of different materials [21]

Material Shear stress (psi)

Specific horsepower

(hp/in3./min)

Hardness (HB)

Magnesium

28000

0.17

…

1100 aluminium alloy 16700 … … 6061-T4aluminium alloy 35722 0.35 … 2024-T4aluminuim alloy 50000 0.46 … Copper 44850 0.78 … 60-40 Brass 47000 … … 65-35 Brass 50000 … … 70-30 Brass 56940 0.59 … AISI 1020Steel 61500 0.58 150-175 AISI 1112 Steel 63500 0.5 150-175 Type 304 Stainless steel 105000 1.1 – 1.9 … Titanium 173500 1.9 …

Here it is also important to incorporate the efficiency (ɳ) of machine tool into the consideration. Due to the presence of friction and wear in between different parts of the machine tool there are different losses present. The gross power (Pg) can be represented as under;

1 Nomenclature used in equations was adopted from ASM International: Machining handbook, Volume 16.

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Pg = 𝑃ɳ

(2.10) [21]

2.1.4 Power Consumption in Turning Operation

In order to estimate the power consumption in a turning operation material removal rate (MRR) should be defined with respect to the turning operation for basic power consumption equation (P = Ps · MRR). Cutting speeds are generally selected from literature or metal cutting handbooks for specific workpiece materials. The cutting speed (Vc) is then used to compute spindle rpm (N) utilizing the workpiece diameter (D) as shown in the equation mentioned below;

Vc = πDN (2.11) [21] Similarly the amount of travel for a cutting tool is known as feed and generally represented for one revolution of the workpiece. Feed is recommended by the manufacturer with respect to the tool and workpiece materials.

2.2 MACHINABILITY OF AEROSPACE MATERIALS Machinability is the ease with which a material can be machined [21]. Assessment of machinability can be based on several parameters like tool life/ tool wear, cutting conditions, workpiece/ tool material, power and thrust forces generated during machining, chip formation, cutting temperature, etc. For practical considerations, machinability is judged by using tool life/ tool wear, cutting forces and surface roughness based criteria [23]. However, a material with superior machinability rating in one criterion can show unacceptable rating when observed from other criterion. Aero engine components are generally manufactured by titanium and nickel based alloys. These materials offer high strength to weight ratio, high chemical wear resistance and high hot hardness value. However they offer extreme difficulty while machining. Due to the level of difficulty involved in the machining of these materials, they are termed as difficult-to-cut materials. Difficulties in the machinability of aerospace alloys can be traced backed to the below listed causes;

• Low heat dissipation by the chips and workpiece material produces high thermal stresses at the cutting edge. When machining aerospace alloys, cutting temperature is approximately twice higher than the value observed in machining of conventional steel.

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• Another important consideration is that aerospace alloys specifically titanium alloys are extremely chemically reactive which facilitates adhesion mechanism and built-up-edge formation. Due to this property titanium tends to weld at the cutting edge resulting in poor tool life.

• High cutting pressure loads are formed at the cutting edge of the tool as reduced contact surface area is involved in the cutting process.

• Segmented chip formation during the cutting of aerospace alloys results in cyclic cutting forces. This pulsating nature of cutting load results in vibrations and self-induced chatter.

• Titanium alloys have low modulus of elasticity and high yield stress value. This elastic behaviour supports the spring back action during the cutting process and excites chatter.

2.3 CLASSIFICATION OF TITANIUM ALLOYS Titanium alloys are normally categorized into four main types which are α-alloys, near α-alloys, α-β alloys and β alloys [24]. At ambient temperature titanium contains closed pack hexagonal microstructure that is known as α-phase. The alloy elements that increase transformation temperature are known as α- stabilizers. These elements are aluminum, oxygen, nitrogen and carbon. Figure 2.6a is schematic illustration of two phase diagrams with an α- and a β stabilizing element respectively.

Fig 2.6a: Phase diagram of Titanium alloys [25]

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In general α-alloys are hard and exhibit high hardening affinity and creep resistance. Increase in oxygen and nitrogen content results in higher strength. However decrease in oxygen, aluminum and nitrogen results in better ductility and fracture toughness. These alloys are utilized in cryogenic applications. Near α-alloys act more like α-alloys but they contain small quantities of β-phase as well. Typical examples of near α-alloys are Ti8Al1Mo1V and Ti6Al5Zr0.5Mo0.25Si. Titanium α and near α-alloys are used in manufacturing of steam turbine blades and autoclaves. In α-β alloys both stabilizers are present in high proportions. In industry they are used for high strength applications with temperature range 350 – 400 Co. Most popular alloys in this category are Ti6Al4V and Ti4Al2Sn4Mo0.5Si. Alloying elements such as molybdenum, silicon and vanadium are known as β – stabilizers. The β alloys are denser in nature and offer high strength at low operating temperatures. Aerospace industry is exploring these alloys for structural applications. Table 1.1, in previous chapter 1, shows commercially available titanium based alloys being widely used in industry.

Fig 2.6b: The Ti - Al phase diagram [127]

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The most intensively investigated titanium phase diagram is the Ti-Al system. Besides α and β phases of traditional titanium alloys, there are several intermetallic phases present such as α 2-Ti3Al, γ-TiAl, TiAl2 and TiAl3. Out of these titanium aluminides, α 2-Ti3Al and γ-TiAl are widely used in high temperature applications. However, TiAl2 and TiAl3 are very brittle in nature [127].

2.3.1 Machinability of Titanium alloys Titanium alloys are normally categorized into four main types which are α-alloys, near α-alloys, α-β alloys and β alloys [24]. Titanium and its alloys are difficult to cut materials, especially β - alloys are famous for their complexities during machining. Poor machinability of titanium alloys has been reported due to their inherent properties such as high chemical reactivity and low thermal conductivity result in poor machinability. Presence of high temperature in cutting zone results in poor tool life and accelerated abrasion, adhesion and diffusion wear mechanisms. Trent and Wright [26] revealed that 99% of the work is converted into heat that caused high temperature in cutting tool and workpiece surface. Temperature on tool face changes with change in cutting speed and exposure time [27]. Longer time duration and higher cutting speed results in higher cutting temperature in the cutting zone [28]. Abele and Frohlich [30] reports that titanium has low thermal conductivity of 4 – 16 W m-1 K-1 and high specific heat capacity of 520 J kg-1 K-1. Combination of high cutting temperature, high heat capacity and low thermal conductivity of titanium results in poor heat dissipation during the cutting process. As most of the heat stays at the cutting edge because of low thermal conductivity it produces high thermal stresses at the cutting edge. Combination of high thermal stresses and chemical affinity of titanium alloys facilitate tool failure through diffusion and adhesion wear mechanisms. It has been reported [31] that approximately 80% of the heat generated during machining of Ti6Al4V is transferred to the cutting tool due to low thermal conductivity of workpiece and fast flowing chip removal cannot take heat away from the cutting zone.

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Figure 2.7: Sources of heat generation in machining [29]

Aerospace alloys, especially titanium alloys have very high chemical reactivity [25]. In general all tool materials react with titanium alloys at elevated temperatures. Due to high chemical reactivity of titanium alloys chips tend to weld at tool tip and cutting edge which results in catastrophic tool failure and severe edge chipping. High tendency of built up edge (BUE) formation is present due to high chemical reactivity of these alloys. Aerospace alloys maintain strength at elevated temperatures that makes plastic deformation very difficult during machining phase. Adequate plastic deformation is required to facilitate the chip formation mechanism. For any specific material, selection of cutting speed decides if the chip will be continuous, discontinuous or segmented. For all conventional materials chip morphology changes from continuous to discontinuous as cutting speed and feed rates increases. It has been reported that for titanium alloys segmented chips are formed at all levels of cutting speed. In segmented chip formation, material deforms plastically ahead of the tool. Fig. 2.8 shows schematic illustration of segmented chip formation. Fracture occurs in the form of shear band when certain strain level is reached in the cutting process. The chips formed under these conditions are segment like in shape [32]. The phenomenon of cyclic chip formation generates variable forces during machining phase [33]. It is reported in literature [34, 35] that cyclic nature of cutting forces are generated due to serrated chip formation and low Young modulus which results in excessive chatter on cutting tool.

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Figure 2.8: Schematic illustration of segmented chip formation [34]

The American Iron and Steel Institute (AISI) machined many alloys and compared the normal cutting speed, tool life and surface finish to the one attained when machining B1112. Materials with rating above 1.00 were easier to machine than B1112. Likewise, materials with ratings less than 1.00 were difficult to machine. For example, Inconel is an alloy that is very difficult to machine and it has a rating of 0.09.

Figure 2.9: Machinability ratings of Titanium alloys [36]

If the criterion of machinability is cutting force and energy consumption, α titanium alloys have comparatively less tensile strengths and generate comparatively lower cutting forces in contrast to that produced during machining of α−β alloys, β and near β alloys. Hence machinability of α−β alloys, β and near β alloys is lower than α titanium alloys.

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2.4 CUTTING TOOL MATERIALS FOR TITANIUM ALLOYS Low machinability rating of titanium alloys is responsible of creating high thermal stresses and pulsating loads at the cutting edge. Combination of high stress level at cutting edge and high chemical reactivity of titanium starts tool wear at accelerated rate. Abele and Frohlich [30] revealed that high cutting temperature was obtained at higher cutting speeds. Titanium has low thermal conductivity that results in poor heat dissipation from cutting zone. In order to perform machining at high cutting speeds cutting tool material show exhibit excellent hot hardness to with stand elevated temperatures during cutting phase. Table 2.2 shows the softening point1 temperature for all known cutting tool materials. Table 2.2: Softening points of tool materials [2]

Tool Materials

High Speed Steel

Cemented Carbides (WC/Co)

Aluminum oxide

(Al2O3)

Cubic boron nitride (CBN)

Diamond Based Tools

Softening Point

Temperature (C°)

600 1100 1400 1500 1500

Cemented carbide, Komduri and Reed [37] suggested that titanium alloys are difficult to machine above cutting speed of 60 m/ min using cemented carbide tools because of high temperature and chemical affinity. The study observed segmented chip formation and higher resulting stresses at apex of tool in machining titanium alloys. Combination of higher stresses and chemical reactivity of titanium results in rapid adhesion of workpiece and erosion of tool material. Zhang et al. [28] conducted an experimental study to evaluate diffusion wear in high speed machining of Ti6Al4V using micro-grain straight cemented carbides. Tool -chip interface was analysed in this study. The EDX results revealed that tool particles (WC and Co) were diffused in Ti6Al4V chips. Diffusion of Cobalt into tool results in pulling out of WC from tool that helps in crater wear mechanism. Jiang and Shivpuri [39] developed a wear model based on diffusion rate to address crater wear in WC/ Co tool. They validate the numeral model with published experimental data. Jawaid et al. [40] conducted experiment on Ti

1 The softening point is the temperature at which a material softens beyond some arbitrary softness. It shows the thermal softening tendency of cutting tool materials.

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– 6% Al 2% Sn 4% Zr 6% Mo using two grades of fine grain size straight1 tungsten carbide tools. Experimentation was performed using three levels of cutting speed (60, 75 and 100 m/ min) and two levels of feed (0.25 and 0.35 mm/ rev). The study revealed that fine grain size carbide grade has longer tool life. Chipping was the main reason of tool failure at flank face. Coated cemented carbides, Coatings are performed to improve machining performance of carbide tools. Hardness, thermal and chemical properties of coating increases resistance to abrasion, adhesion, diffusion and oxidation wear mechanisms. Jawaid et al. [41] performed face milling experiments on Ti6Al4V using PVD – TiN and CVD - TiCN/ Al2O3 coated inserts. The study pointed out that CVD tools outperformed PVD tools. Wear starts on flank and rake face by coating delamination and then extends into attrition and diffusion wear. Wang and Ezugwu [42] executed machining tests on Ti6Al4V using TiN and TiN/ TiCN/ TiN - PVD coated tools. The results revealed that flank wear, chipping and flaking on rake face and nose were the major failure modes in PVD coated inserts. The study pointed out that TiN single coated inserts out performed TiN/ TiCN/ TiN multilayer coated tools at higher feed rate. Rahim and Sasahara [43] conducted drilling test on Ti6Al4V using TiAlN coated drills under palm oil (PO) and synthetic ester (SE) based minimum quantity lubrication (MQL) system. The study revealed that MQL (PO) produced less cutting forces pointed at lower friction coefficient. Ceramic tools, these tools attracted researchers for machining applications especially for high speed cutting conditions. Ceramic tools are widely used in industry because of their high hot hardness2. It is reported in literature [44] that mixed ceramics tools can be used for machining nickel base alloys at cutting speeds ten times higher than carbide tools. Choudury et al. [44] reported that alumina whisker tools can machine aero-engine alloys up to cutting speed of 750 m/ min and feed rate of 0.375 mm/ rev. Jianxin et al. [45] performed turning tests on Inconel 718 using Al2O3/ TiB2 /SiCw tools with different proportions of TiB2 particles and whisker. The experiments were conducted at cutting speed range of 50 – 180 m/min. Dominant wear mechanism found on flank face was abrasion and on rake face were adhesion and diffusion. The study revealed that ceramic tools performed with stable wear rate for cutting speed less than 80 m/ min. Lo Casto et al.

1 Straight Tungsten carbide (WC), the hard phase, together with cobalt (Co), the binder phase, forms the basic Cemented Carbide structure from which other types of Cemented Carbide have been developed 2 Hot hardness means ability of material to retain it bulk hardness and geometry at elevated temperatures.

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[46] conducted turning test on nickel based alloys using cemented carbide (WC/ TiC /Co), zirconia toughened alumina (ZTA), silicon nitride, alumina based and alumina whisker tools. The SEM micrographs revealed erosion of tool and chemical wear as dominant wear mechanisms in ceramic tools. Narutaki et al. [47] executed high speed machining experiments on Inconel 718 using whisker reinforced alumina, silicon nitride and TiC mixed alumina. SiC whisker tool showed less notch wear under 300 m/ min cutting speed. TiC mixed alumina showed less comparatively less wear at cutting speeds above 400 m /min. Cubic boron nitride (CBN) tools, these are produced by hexagonal structured crystals of boron nitride in presence of 1400° C temperature and 6000 MPa pressure [7]. Although CBN tools are the second hardest cutting tool material, but due to high cost their utilization is only limited to finishing operations. Zoya and Krishnamurthy [48] evaluated performance of CBN tools for machining titanium alloys. Experimentation was performed using cutting speed range of 150 – 350 m/ min and feed rate of 0.5 mm / rev. The study recommended cutting speed range of 185 – 220 m/ min for machining titanium alloys. The study revealed chipping and notch wear as main failure modes. Diffusion was the mainly found on rake face of the tool. Critical temperature1 for CBN tools was identified to be 700° C Ezugwu et al. [50] executed experiments on Ti-6Al-4V using CBN tools under three different coolant flow rates. Experimentation was conducted using conventional, 11M Pa, and 20.3 M Pa pressurized coolant flow. It was observed that 11 M Pa and 20.3 M Pa strategies enhanced 68% and 150% of tool life respectively. The study revealed that increasing CBN content results in severe chipping and notch wear at cutting edge. Diamond based tools, In general diamond coated tools and poly crystalline diamond tools are used for machining of titanium alloys. Many researchers have investigated machining performance of PCD and diamond coated tools with respect to the conventional carbides, ceramics and CBN tools. Velasquez et al. [51] performed high speed turning experiments on Ti6Al4V using carbide tool with polycrystalline diamond (PCD) tip. Cutting speed used in experiments range from 20 – 660 m/ min. The study exposed white layer formation characteristics with respect to cutting speed. Oosthuizen et al. [52] conducted milling experiments on Ti6Al4V using PCD coated tools. Experimentation was performed using 100 – 500 m/ min cutting speed and 0.025 – 0.050 mm/ z feed rate. It was observed that PCD

1 Critical temperature of the cutting tool corresponds to the value after which tool wear occurs at higher rate [128].

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tools outperformed carbide tools at higher cutting speeds. A slower wear progression was observed in PCD tools at cutting speed of 200 m/ min. Adhesion wear mechanism was found in PCD tool when machining Ti6Al4V. 2.4.1 Tool wear mechanisms and patterns In the machining process, due to the mechanical contact between the cutting tool and workpiece material, gradual wear is observed at the cutting edge of the tool. Tool wear is defined as change in shape of the cutting tool during the cutting process [53]. Wear patterns formed on the cutting tool during machining result from different types of wear mechanisms. Literature has revealed that tool wear is a complex function of workpiece and tool materials, tool geometry, cutting conditions and cutting environment. To understand the tool wear patterns it is essential to investigate the tool wear mechanisms [54]. The type and rate of tool wear depend on the workpiece and tool materials, cutting conditions, cutting environment and dynamic characteristics of machine tool used during cutting. The rate and type of tool wear is deciding factor towards tool life. Tool life of the cutting tool is measured by measuring the flank and crater wear using ISO 3685:1993 standard [57].

Figure 2.10: Schematic illustration tool wear patterns [56]

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Abrasive wear, When the two surfaces are in sliding contact the hard particles of harder material rub against the less hard surface [53]. Similarly in machining hard particles such as carbides and nitrides rub against the cutting tool material during cutting process. The process of abrasion is dependent upon the relative hardness of abrading particles and abrading materials. Abrasion wear rate increases rapidly with increase in temperature at cutting zone. It is due to the fact that increase in temperature lowers material hardness level [57]. Below in Figure 2.11, there are some reported cases of abrasion wear.

Turning, Ti6Al4V (Uncoated Carbides) V=150,200,250 m/min, Ezugzu et al. [50]

Turning, Ti alloy, PCD tool, Corduan et al. [58]

Figure 2.11: Abrasive wear observed for different cutting tool materials when machining titanium alloys

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Adhesive wear, In adhesive wear mechanism, two metals are forced together at high temperature and pressure that result in welding of two materials. This mechanism is also observed in the machining [53, 60]. The particles of workpiece metal tend to weld at the cutting edge of the cutting tool during machining process. This deposition of workpiece material is termed as built-up-edge (BUE). Built up edges (BUE) forms and then break at regular intervals during cutting. Every time built-up-edge (BUE) breaks it also peels off some tool material resulting in adhesive tool wear. Below in Figure 2.12, there are some reported cases of adhesive wear.

Milling, (TA15) alloy, PCBN tool, V=250-350m/min, Honghua et al. [62]

Milling, (TA15) alloy, PCD , V=250-350m/min, Honghua et al. [62]

Figure 2.12: Adhesive wear observed for different cutting tool materials when machining titanium alloys

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Diffusion wear, during the cutting operation, workpiece and tool materials are in contact. It has also been found in literature that atomic particles of workpiece material diffuse to the cutting tool material that weakens the tool [26]. Diffusion wear is highly dependent on the cutting temperature and chemical affinity between tool and workpiece materials. Normally diffusion wear appears on the rake face of the cutting tool. Diffusion wear mechanism is a dominant tool wear mechanism for higher cutting speed conditions [63]. Crater wear is shown below in Figure 2.13. Crater wear can be formed due to the mechanical rubbing action of the hard particles on the rake face or complex atomic diffusion between tool and chip materials. Crater wear generally shows that tool material is diffused into the chip material.

Milling, Ti6Al4V

TiCN/ Al2O3 CVD Coated tool Jawaid et al. [41]

Figure 2.13: Diffusion wear on the rake face of cutting tool materials when

machining titanium alloys

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Chemical wear, during the cutting process high cutting temperature is generated that facilitates chemical reactions like oxidation and creation of other chemical compounds [57]. This oxide layer is too brittle in nature and breaking off this oxide layer sometimes results in chipping of tool material [26]. Jianxin et al. [64] studied the diffusion and oxidation chemical wear by developing a diffusion couple, just like an interface between Ti6Al4V and WC/Co tool as shown in Figure 2.14. The study showed oxidation layer formed by exposing the WC/Co carbide tool in air for 90 min at 800 °C.

Figure 2.14: Schematic illustration of the diffusion couple [64]

Chipping and Flaking, when small amount of material peels off from cutting edge, it is called as chipping [53]. Chipping can lead to gross fracture at later stages as well. It is unpredictable in nature but most commonly found in the tools with low fracture toughness. Chipping is formed due to mechanical and thermal shocks during machining [65]. Chipping is found in uncoated tool. Flaking is similar in concept to the chipping, but it means large amount of material will be peeled off from tool surface. Flaking is generally observed in coated tool materials. Figure 2.15 shows different cases of chipping and flaking observed in different cutting tools when machining titanium alloys.

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Chipping Case, Milling, Ti-6242S (Uncoated tool)

V= 150m/min, Ginting and Nouari. [5]

Flaking Case, Milling, Ti6Al4V TiN PVD Coated tool, V= 55, 65, 80

and 100 m/min, Jawaid et al. [41]

Chipping Case, Turning, Ti6Al4V

Wurtzite boron nitride (wBN) tool at 80 min, V =75m/min, Bhaumik et al. [38]

Table 2.15: Chipping and Flanking observed for different cutting tool materials when machining titanium alloys

2.5 SURFACE INTEGRITY Surface integrity is based on a group of properties of a machined surface that influence the performance of this surface during post processing and service life [66]. The properties directly linked with surface integrity are surface roughness, texture, microhardness, residual stresses, microstructure, and surface cracking etc. These properties affect the service life by governing wear and frictional behaviour of contacting bodies, controlling lubrication efficiency, and growth of surface crack etc [67]. Surface condition is very critical if part has to be used under fatigue loading [68].

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Carbide cracking/ Tearing, most of the coating materials and workpiece materials consist of small carbide particles. It has been observed that at lower values of depth of cut in micro-machining, 50μm approximately, carbide particles were smeared against the workpiece surface. The reported size of carbide particle is 20 μm approximately [66]. Tearing can be seen as shown in Figure 2.16a. White layer formation, One of the most commonly observed microstructural defects in titanium and nickel alloys is called white layer formation. It is a condition where upper most surface of machined workpiece contains high hardness as compared to the bulk material inside [69]. This upper hard layer appears white under microscope due to which it is named as white layer [70]. White layer usually consists of very fine grains. White layer is very hard and brittle in nature that can support cracks to grow easily in to the material and it has direct influence on fatigue life of the product [71, 72]. White layer observed in the machining of Ti6Al4V is presented in Figure 2.16b.

(a)

(b)

Figure 2.16: (a) Surface tearing (Ti6Al4V), V=100 m/min, f=0.15 mm/ tooth, DoCa = 2.0 mm, DoCr = 8.8 mm [73], (b) White layer in Ti6Al4V machined at V=95 m/ min, f = 0.35 mm/rev, and DoC = 0.10mm [74] Work hardening layer formation, Materials with high work hardening behaviour form a work hardening layer in response to the machining operation. Titanium alloys have high tendency to form this work hardening layer under machining phase [75]. Presence of this extremely hard layer on top of bulk material makes further processing very difficult. Pawade et al. [73] studied influence of machining parameters and cutting edge geometry on surface integrity of Inconel 718. In the study machining affected zones

35

were observed and degree of work hardening was measured for cutting experiments. The study revealed high values of microhardness in the vicinity of 200 μm below the machined surface. The study revealed that degree of work hardening increased with depth of cut and cutting edge geometry. The study showed that chamfered plus honed cutting edge geometry produced highest degree of work hardening. Figure 2.17 shows the behaviour of hardness under machined surface.

(a)

(b)

Figure 2.17: Microhardness behaviour observed in machining FGH 95 (a) Microhardness region (b) Microhardness measurement [69]

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Surface roughness, it is most widely treated as main indicator to estimate surface integrity. Titanium and nickel alloys are prone to produce high cutting temperature in cutting zone due to which high tool wears are reported in several studies [5, 76]. Factors like cutting speed, feed rate and depth of cut are found to be effective parameters influencing surface finish [40, 53]. Che-Haron et al. [77] investigated effect of cutting speed in machining of Ti6Al4V through turning experiments. The study was performed using straight tungsten carbide tools. It revealed that at lower speed of 45 m/min surface roughness increases in the start and then at the end of tool life surface tends to become smoother. At higher speeds of 60, 75 and 100 m/ min surface roughness increased rapidly towards the end of tool life as shown in Figure 2.18.

(a)

(b)

Figure 2.18: Surface roughness behaviour with cutting speed for turning Ti6Al4V (a) ISO-883-MR4 Tool at feed = 0.35 mm/ rev, (b) ISO-890-MR3 Tool at feed = 0.25 mm/ rev [77]

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2.6 CUTTING FLUIDS The accurate usage of lubricants/ coolants during machining processes significantly prolongs cutting tool life. Lubricants/ coolants are used to lubricate the cutting zone and dissipate the heat efficiently from the cutting zone. The effect of lubrication and cooling results in lower cutting forces during machining. In order to dissipate heat rapidly water based fluids are more efficient than the oil based, but oil based lubricants provides better lubrication. As available in literature [78], a weak solution of rust inhibitor and/or water-soluble oil (5 to 10%) is the most commonly used fluid for high-speed titanium machining processes. However for slow speed machining of titanium alloys chlorinated or sulphurized oils are recommended for better lubrication. Chlorine containing fluids, if chlorine based cutting fluids are used in the machining of alloys that may be subject to stress corrosion cracking, careful post-machining and cleaning operations must be performed afterwards. In order to investigate the applicability of chlorinated cutting fluids, US Air Force Materials Laboratory [78] concluded some interesting findings. The findings are mentioned below [78];

“Sulfurized and chlorinated soluble-oil emulsions used in low-stress grinding and end milling/end cutting did not degrade the high-cycle fatigue strength of annealed Ti-6Al-4V (34 HRC) at 25 °C (75 °F) and 315 °C (600 °F) relative to results from a neutral soluble-oil emulsion. Sulfurized and chlorinated soluble-oil emulsions used in abusive grinding did not degrade the 25 °C (75 °F) high-cycle fatigue strength of Ti-6Al-4V relative to results from a neutral soluble-oil emulsion. Sulfurized and chlorinated oils and soluble-oil emulsions as crack tip environments did not accelerate 25 °C (75 °F) fatigue crack propagation rates in Ti-6Al-4V at 1 cpm and 1800 cpm relative to results in laboratory air environment. A 100 h exposure under stress to sulfurized and chlorinated soluble oil emulsions did not affect 25 °C (75 °F) bend test results from low-stress ground and end milled end cut Ti-6Al-4V relative to results from a neutral soluble oil emulsion”.

Simon et al. [129] performed experiments and utilized auger analysis to study the influence of cutting fluids containing chlorine. It was found that films were formed on the surface with thickness equal to or less than 150 nm with chlorine content of 3% at the most. Similar behaviour with 100-150 nm thick film and 1.5% content was observed when machining

38

titanium with demineralized water. It was concluded that prohibition of machining titanium with chlorinated cutting fluids cannot be continued. 2.7 ENVIRONMENTAL FRIENDLY COOLING STRATEGIES

Metal working fluids (MWF) are essential in the machining of titanium alloys to increase tool life, improve surface finish and chip removal from the cutting zone. Metal working fluids acts as lubricant or coolant during machining. These days metal working fluids (MWF) are being questioned extensively for their economics and environmental related issues. These lubricants and coolants impose danger to environment due to their toxicity and non-biodegradability. In order to make machining process sustainable in nature, toxicity has to be reduced whereas biodegradability has to be enhanced. This section provides a brief over view of different cooling strategies utilized to improve the machinability of titanium alloys.

2.7.1 Dry cutting

Dry cutting of titanium alloys is consider as the ideal desired approach but most of the literature strongly recommends that generous amount of coolant should be used while cutting titanium alloys. Cutting fluids offer an advantage of clearing chips easily, however in dry cutting dust and difficult chip removal is experienced. Dry cutting also results in higher friction and higher cutting temperature that initiates higher and rapid wear rates.

Ginting and Nouari [5] examined the machinability of Titanium alloy Ti6242S under dry condition using uncoated carbide inserts. The study analysed surface roughness, cutting temperature, flank wear and chip formation to evaluate machinability. Adhesion was found responsible for flank wear, whereas abrasion and diffusion produced crater wear. Nabhani [49] investigated the machinability of Ti 48 titanium alloy using PCD (SYNDITE)1, PCBN (AMBORITE)2 and CVD-TiN /TiCN /TiC multi layered carbide insert under dry conditions. The study showed that PCD tool outperformed other tools. Better performance of PCD (SYNDITE) was attributed to the reason that carbon substrate of the tool reacts with titanium to form TiC layer. This layer provides protection again abrasion and diffusion [49, 130]. 1 Syndite is a composite material that combines diamond with the toughness of tungsten carbide. 2 PCBN tool with low CBN content is called an AMBORITE

http://en.wikipedia.org/wiki/Composite_material

http://en.wikipedia.org/wiki/Diamond

http://en.wikipedia.org/wiki/Tungsten_carbide

http://en.wikipedia.org/wiki/Tungsten_carbide

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2.7.2 Minimum Quantity Lubrication (MQL)

Due to the negative influence of lubricants on environment, many researchers have focused their work to utilize small amount of cutting fluid during machining. These procedures are termed under minimum quantity lubrication (MQL) methods. Rahim and Sasahara [43] conducted an experimental comparative study using palm oil based and synthetic ester based MQL systems. The study was performed to investigate the effectiveness of palm oil as lubricant in MQL system. The study revealed that palm oil based MQL arrangement out performed synthetic ester based MQL system. Zeilmann and Weingaertner [79] performed drilling experiments on Ti6Al4V using uncoated and coated drills (TiALN, CrCN and TiCN) under MQL environment. The study measured cutting temperature during drilling operation to evaluate the performance of MQL technique. The study revealed that internal MQL arrangement performed better than external MQL arrangement.

Figure 2.19: Schematic illustration of MQL [80]

Wang et al. [81] executed orthogonal turning experiments on Ti6Al4V using dry, flood and MQL cutting environments. The study was conducted under continuous and interrupted cutting cases. The study pointed out that MQL performed better than flood cooling at higher cutting speeds due to better lubrication capacity. The study also revealed that MQL was more effective in interrupted cutting scenario. Cia et al. [82] performed end milling experiments to investigate the controlling parameters for MQL system. The study used oil flow rates of 2 ml/h – 14 ml/h for optimized value. The study revealed that diffusion wear was present for low oil supply rates 2ml/h – 10ml/h, however at 14ml/h relatively less diffusion wear was found. Yasir et al. [83] utilized physical vapour deposition (PVD) coated cemented carbide tools to machine Ti6Al4V using MQL system. The study

40

utilized coolant flow rates of 50 – 100 ml/h at three cutting speed levels of 120, 135 and 150 m/ min. Improved tool life was observed at 135 m/min with high flow rates. Mist 1 was found more effective for worn tool. A possible explanation of getting relatively better performance at 135 m/ min speed can be attributed to the better lubrication capacity at higher flow rate of 100 ml/h.

2.7.3 High pressurized cooling (HPC)

Klocke et al. [84] performed machining experiments on Titanium alloys to investigate the effect of high pressurized coolant supply. The study analysed cutting tool temperature, tool wear, chip formation and cutting forces. The study pressurized the cutting fluid up to 300 bars (55l/min) and compared the effects with conventional flood cooling. The study revealed that 25% cutting tool temperature reduction and 50% tool wear improvement, in best case, were achieved using high pressure coolant. Nandy et al. [85] performed machining experiments on Ti6Al4V using uncoated K20 cemented carbide inserts using conventional wet, high pressure neat oil and high pressure water soluble oil. Machining tests were conducted using cutting speeds of 90, 100 and 111 m/min and supply pressures of 70, 100 and 140 bar. The studies revealed 250% improved tool life when compared to conventional wet cutting. Sandvik coromant has also revealed sample testing studies to show better tool life and higher material removal rates (MRR) for Ti6Al4V and Inconel 718. The coolant delivery arrangement is shown in Figure 2.20.

Figure 2.20: High pressure coolant delivery systems by Sandvik Coromant

[86] 1 Small cutting oil droplets suspended in air is called as mist.

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2.7.4 Cryogenic cooling

When cutting operation is conducted at very low temperature, generally lower than – 153 °C, it is termed as cryogenic machining [87]. Su et al. [88] performed end milling machining experiments on titanium alloys to evaluate the performance of different cooling strategies by analysing oil mist, compressed cold nitrogen gas (CCNG) at 0, and−10 ◦C, and compressed cold nitrogen gas and oil mist (CCNGOM) as the cooling strategies. The study revealed that compressed cold nitrogen gas and oil mist (CCNGOM) cooling strategy outperformed other strategies by resulting longer tool life. A possible hypothesis to explain this better performance can be attributed to the arrangement of combining oil mist with liquid nitrogen setup. The combination provides lubricant and coolant both at same time. Yildiz et al. [89] reviewed the application methods of cryogenic coolants. The study revealed that cryogenic coolants effectively controlled the cutting temperature at cutting zone, and provided good tool life with reasonable surface finish. Sun et al. [90] evaluated the machining performance of titanium alloys by utilizing cryogenic compressed air. The study showed great potential of cryogenic compressed air cooling strategy as it reduced tool wear significantly. Bermingham et al. [91] performed machining experiments using cryogenic cooling technique. Cutting speed and material removal rate were kept constant during the study, however feed rate and depth of cut were varied to analyse cutting force. The study revealed that less heat was generated for low feed rate and high depth of cut. Table 2.3 shows a brief comparison of different cooling strategies used for the machining of titanium alloys. 2.8 SUSTAINABLILITY CONCEPTS IN MACHINING Currently the manufacturing sector is under enormous burden to improve environmental and ecological performance due to the development of strict international environmental protocols. By implementing the sustainable practices in the metal cutting sector, environmental performance and economics can be improved considerably.

Sustainability concepts can be incorporated in manufacturing sector by implementing the following practices [92 - 94];

• Reducing amount of input resources like energy, material and water.

• Improving environmental quality of resources, reducing the use of toxic and non-biodegradable chemicals.

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• Efficient designing of life cycles. • Adopting eco-friendly manufacturing technologies. • Improving ergonomic, health and safety requirements, equity and

fairness, and employee personal development. Table 2.3: Advantages and disadvantages of different environmental friendly cooling strategies employed in the machining of titanium alloys under continuous cutting processes

Dry

Minimum Quantity

Lubrication (MQL)

High Pressurized Cooling (HPC) Cryogenic Cooling

No lubricant/

coolant required, no

need for coolant disposal

Poor

temperature control in the cutting zone

Poor chip

removal and dust formation

Poor tool life

Poor surface finish

Very small amount

(ml/min) of lubricant/ coolant is

involved

For machining difficult to cut materials not

effective due to low cooling capacity

[130] Better chip removal as compared to dry

Extended tool life at higher cutting

speeds as compared to dry but highly

dependent on arrangement of

nozzle Improved surface finish but highly

dependent on arrangement of

nozzle

Very high quantity

of lubricant/ coolant is required

Superior temperature control

in cutting zone

Superior chip removal

Improved tool life [84]

Improved surface finish but highly

dependent on arrangement of coolant delivery

system and pressure involved

Generally lubricants/

coolants are compressed air or

liquid nitrogen

Superior temperature control in cutting zone

[87]

Better chip removal as compared to dry

Improved tool life [87]

Inconsistency in surface integrity as it

is highly dependent on the pairing of tool and workpiece materials

[130]

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Figure 2.21: Cumulative growth of federal environmental laws and

amendments [95] Figure 2.21 provides an overview of increasing federal environmental laws and their amendments. The sustainable manufacturing intends to lessen the amount of greenhouse gas (GHG) emissions and the associated carbon footprints. Machining operations are most commonly performed operations in manufacturing industry. To lower greenhouse gas (GHG) emissions and carbon footprint during machining process processing time and energy consumption play a vital role. 2.8.1 ENERGY CONSUMPTION IN MACHINING Machining operations are normally conducted on manual or computer numerical controlled (CNC) machine tools. These machine tools used electrical grid as an input energy. Greenhouse gas (GHG) emissions are directly linked with the energy sources a country utilized to generate electricity. If a country is using clean energy resources (solar, wind, geothermal and tidal etc) then the associated greenhouse gas (GHG) emissions will be lower than the electricity generated using conventional energy sources (fossil fuels, coal and hydal). The energy utilized by machine tool to perform certain machining task is directly proportional to the production of greenhouse gas (GHG) emissions. By optimizing the

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electricity consumption in machine tool greenhouse gas (GHG) emissions can also be reduced. Several researchers have focused their work to establish a link between machining process and the environmental issues associated with machining operations. Munoz and Sheng [96] developed an analytical model to demonstrate the environmental issues during machining phase. The study was based on the energy consumption, tool wear, cutting mechanics and lubrication. The study concluded that energy consumption is strongly linked with geometric features, material type and lubrication technique. Kordonowy [97] conducted a very detailed study on energy consumption by machine tools. The work considered six different machines (injection molding, manual and automatic milling and automatic lathes) during the study. The study identified the portions of energy, consumed during different operations of the machine tool. Figure 2.22 shows the breakdown of energy consumption during machine operation on a milling machine.

Figure 2.22: Energy consumption breakdown for milling process [97]

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Dahmus and Gutowski [98] performed a study to investigate the environmental aspects of machining. The study revealed that energy consumed in cutting process is comparatively lower than the total energy consumed during machining cycle. Drake et al. [99] developed a framework to characterize energy utilization of the machine tool. The framework was composed of six steps based procedure. The framework suggested that major portion of energy consumption was utilized by the machine tool controllers. The spindle system utilized 35% of the total energy consumption. Schlosser et al. [100] also developed a model for energy consumption during the manufacturing operation. Figure 2.23 shows the characterization of power consumption during machining operation. The model was verified for drilling process. The model was found in good agreement with the experimental results.

Figure 2.23: Characterization of power consumption during cutting processes [100]

Avram and Xirouchakis [101] proposed a method to estimate energy consumption during machining phase. The method provided geometries

46

suitable for 2.5 D1 milling. The method was verified experimentally and found in good agreement. He et al. [102] established an energy model for machine tools based on a task oriented approach. The study separated the energy consumption into two categories namely constant and variable energies. The study was focused on development of a model to estimate variable energy during cutting process. The study also optimized energy consumption by machine parameter selection approach. Hu et al. [103] created a model to estimate energy in machining using an on-line energy monitoring system. The study divided power into constant and variable energies. The values of constant energies were saved and used by the system, however variable power involved in cutting was calculated using energy balance formulations. The study provided a way to calculate energy consumption with using any power/ energy measuring device. Rajemi et al. [104] generated a model to estimate total energy required in performing a machining task. The model was optimized for minimum energy consumption in reference with economic tool life criterion. The work provided good understanding to apprehend the relation between economic and environmental concerns. Mativenga and Rajemi [105] reviewed formerly proposed methodology to select optimum cutting parameters. The study provided a link between energy intensity and energy related cost involvement. Mori et al. [106] studies the behaviour of power consumption in machining centers using different sets of energy cutting parameters. The study provided useful data about the behaviour of energy consumed by spindle motor and servo motors. The study suggested that to optimize energy consumption of the process, synchronisation of spindle acceleration and feed system is critical. Neugebauer et al. [107] executed machining experiments based on turing and drilling operations. The study was designed to analyse energy efficiency in machine tools. The study incorporated tool selection and cutting parameters as well. The study exposed that tool selection and feed rate play significant role towards energy efficiency of machine tool. Balogun and Mativenga [108] also established a mathematical model to predict direct energy consumption of the machine tool. The new model covered the deficiencies of previously created models. The study efficiency created a link between the energy consumption in machine modules, spindles, auxiliary units and motion states during machining. 1 2.5D machining is when all the machining is in the same plane and that plane coincides with one of the planes of the milling machine [131]

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2.8.2 ENVIRONMENTAL IMPLICATIONS OF

ENERGY CONSUMPTION Environmental implications, associated with a machining process, are directly linked with the energy mix of the region. Geographical location is of primary importance when evaluating the environmental implications of a manufacturing process. If the region is producing electricity using coal and fossil fuels, then even a small amount of energy consumption will contribute more towards greenhouse gas (GHG) emissions. By selecting a region where energy mix has more clean resources (solar, wind, tidal and geothermal etc) results in lower emissions.

Figure 2.24: Global CO2 emissions from fossil fuels [110]

Kong et al. [111] executed milling experiments using five different milling tool path strategies. The energy consumption was measured in the study and then represented in the form of greenhouse gas (GHG) emissions. To investigate the environmental implications of the process energy mix of California and New York were utilized. Gutowski [82] proposed a framework in which carbon (CO2) emissions were divided into four aspects. The aspects were population, GDP per population, energy per GDP and carbon energy per. The study showed direct link of carbon emissions with

48

population increase. There will be increase in demand of manufacturing products with increasing population. To meet this increase in demand manufacturing sector has to increase production rate. 2.9 SUMMARY OF LITERATURE REVIEW Findings of the literature review are presented in this section. Furthermore areas which require more investigation are pointed out. In order to avoid the harmful effects of cutting fluids on environment and worker’s health, ideally dry cutting is the best solution. But due to excessive heat generation in dry cutting, it limits the cutting speed and results in low tool life and surface finish. The studies show good potential of cryogenic LN2 cooling method, but machinability findings are not consistent for all tool and workpiece materials. The main cause of inconsistency is related to the change in tool and workpiece properties. It means that different pairs of tool and workpiece materials should be investigated. MQL technique appears to be a possible solution as it reduces friction but it has low cooling capacity due to the absence of coolant. They are difficult to be used for difficult to cut materials because of the presence of high cutting temperatures. However more machinability investigations are required for an arrangement where it is possible to combine the cooling method in the MQL system. It has also been observed that MQL systems are explored experimentally and there is a lack of theoretical knowledge behind it. Similarly it is very rare in literature to find a numerical model for an MQL system. Uncoated carbide tools are referred as potential materials to machine titanium alloys at lower cutting speeds of 30 – 40 m/ min. It has been reported that K type uncoated tools performed comparatively better than other uncoated tools. As the cutting speed increases temperature in cutting zone also increases. Higher cutting temperature causes thermal softening of tool and diffusion starts at very rapid rate. At higher cutting speeds crater wear forms rapidly and increase in cutting speed shifts crater wear closer to the cutting edge. Tool failure in carbide material is due to heat generation in the primary deformation zone. However, high shear stresses are reported in the secondary deformation zone. The combination of high temperature in primary deformation zone and high stresses at secondary deformation zone causes very short tool life at higher cutting speed. For machining nickel alloys adhesion facilitates built up edge formation and attrition results in notching. Several studies showed that coated carbide tools outclassed

49

their uncoated equivalents when machining high performance alloys. As majority of heat is rejected into the cutting tool material, it is recommended that both cutting tool and coating materials have thermal conductivity lower than workpiece being machined. Lower thermal conductivity of tool material improves tool life. Ceramic tools show poor machinability when machining titanium alloys because they are prone to react with titanium alloys. Cubic boron nitride (CBN) tools are tested at higher cutting speeds of 300 – 400 m/ min by many researchers. The results point out 185 – 220 m /min to be a recommended cutting speed range for CBN/ PCBN tools for machining titanium alloys. It has been observed for machining nickel alloys that tool life was sensitive to the cutting speed. Decrease in cutting speed can increase tool life up to 250%. It has also been observed for CBN tools that percentage of CBN content in cutting tool can improve tool life. Best performance of CBN tool was observed when CBN content was in between 40 – 60%. Poly crystalline diamond (PCD) tools proved to be the best choice when machining titanium alloys due to their low solubility to titanium. However they show poor machinability when machining nickel alloys as diamond reacts with nickel alloys easily. Although diamond tools show good wear resistance and high hot hardness. Diamond tools are highly reactive when temperature exceeds 700 C°. The problem in diamond tools is high tooling cost involved in poly crystalline diamond (PCD) tools that disturbs machining economics. Dynamic characteristics of machine tool like rigidity and stability also affects machinability. Several studies focusing uncoated and coated carbides have presented ambiguous results about their machining performance. One possible reason of these results is that machine tool condition and dynamic character of machine tool has not been reported in most of the available literature.

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CHAPTER 3

EXPERIMENTAL METHODS

This chapter presents the experimental methods used in the research work.

This chapter covers the experimental methods and techniques utilized during this research work. The chapter also provides detailed information about the workpiece and tool materials used, design of experiments and actual test setup. The current research project deals with a combination of milling and turning experiments. In paper A, milling experiments were performed using Aluminum alloy (Al 6061). This preliminary research work was conducted using cheaper material (Al 6061) just as a reference to understand the basics of machinability and energy consumption related concepts. In papers B, C and D, turning experiments were performed using Titanium alloy (Ti6Al4V). In this portion of the work machinability and environmental implications of the machining process were investigated. In paper E, machinability of Ti6Al4V was investigated using vegetable based MQL system in combination with low temperature (0 – 6 °C) air.

3.1 MILLING EXPERIMENTS a) Workpiece Material Aluminum alloy (Al 6061)

Aluminum alloy, Al 6061 was used as a work piece material for milling experiments. Here the main aim of the study was to understand the trends of energy consumption with cutting parameters. Aluminium is different than titanium so machinability aspects were not compared with each other. Experimentation was conducted under the flood condition on a CNC machining center. The nominal chemical composition of the workpiece material is mentioned in Table 3.1.

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Table 3.1: Nominal chemical composition of Al 6061

Element Wt. % Element Wt. %

Si Fe Cu Mn Mg

0.40 – 0.8 0.7 0.15 – 0.40 0.15 0.8 – 1.2

Cr Zi Ti Al

0.04 – 0.35 0.25 0.15 Remaining

Table 3.2 represents the mechanical properties of the Aluminum alloy 6061 used during the cutting experiments. Table 3.2: Mechanical properties of Al 6061 at room temperature

Properties Values Properties Values

Tensile strength Yield strength Elongation

310 MPa 276 MPa 12-17%

Poison ratio Modulus of elasticity Hardness (HRB)

0.33 68.9 GPa

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b) High Speed Steel (HSS) End Milling Cutter

End milling cutters were used to machine Aluminum alloy (Al 6061) under milling experiments. The HSS end milling 02 flutes cutters have the code of DIN 844. Figure 3.1 and Table 3.3show the cutter and basic features of the cutter.

Figure 3.1: High speed steel (HSS) 02 flute end milling cutter with code

DIN884

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Table 3.3: Specificans of the DIN 844 HSS C08 cutter [83]

Mill Dia. (mm)

Shank Dia. (mm)

Length of cut (mm)

Overall length (mm)

8.0 10.0 19.0 69.0 c) Design of experiments

Experiments were designed to investigate the power and energy consumption during four different milling tool path strategies. Experiments were designed by using Design Expert software package. Full factorial 1 design was used for designing experiments to investigate the influence of cutting parameters on energy consumption in detail. Table 3.4 shows all 24 runs for milling experiments.

Table 3.4: Design of experiments

Test No. Milling Strategy Spindle Speed (rpm) Feed (mm/ min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Zigzag Zigzag Zigzag Zigzag Zigzag Zigzag Constant OverlapSpiral Constant Overlap Spiral Constant Overlap Spiral Constant Overlap Spiral Constant OverlapSpiral Constant OverlapSpiral Parallel Spiral Parallel Spiral Parallel Spiral Parallel Spiral Parallel Spiral Parallel Spiral One-way One-way One-way One-way One-way One-way

4000 4000 4000 2000 2000 2000 4000 4000 4000 2000 2000 2000 4000 4000 4000 2000 2000 2000 4000 4000 4000 2000 2000 2000

200 300 400 200 300 400 200 300 400 200 300 400 200 300 400 200 300 400 200 300 400 200 300 400

1 A design in which every setting of every factor appears with every setting of every other factor is a full factorial design

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PS3500 power data logger was used to monitor the power consumed for each machining test. A software package named power sight manager was used to perform and validate calculations for energy consumption. d) Test Setup End milling experiments were conducted under the conventional flood conditions on a CNC vertical machining center. The cutting parameters used are shown in Table 3.5. Figure 3.2 shows the schematic illustration of the experimental setup. Table 3.5: Cutting parameters

Pocketing of Al 6061 blocks

Dimension of pocket Depth of cut No of passes Lubrication Technique Spindle Speed (rpm) Feed (mm/ min) Milling Strategies

100 x 100 x 6 mm 3 mm (full immersion) 2 Flood Two levels ( 2000 - 4000) Three levels (200 – 300 - 400) Zigzag, constant overlap spiral, parallel spiral and one way spiral

Figure 3.2: Experimental setup of the experimental setup

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e) Results It has been observed that one-way milling strategy consumed more energy in almost all cases of pocket milling. However, zigzag milling strategy consumed the least energy among all milling strategies. Constant overlap spiral and parallel spiral strategies were found in between zigzag and one-way milling strategy. The study revealed that one-way milling strategy consumed more energy because a large number of repetitive movements were present in it.

Increase in feed rate resulted in less energy consumption. This is due to the fact that higher feed rate increases material removal rate. Higher material removal rate consumes less energy when machining similar volume of material. It establishes that optimum selection of material removal rate in a machining phase can result in efficient energy consumption. Material removal rate can influence environmental burden of machining phase.

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3.2 TURNING EXPERIMENTS a) Titanium alloy (Ti6Al4V) for turning experiments

The workpiece material used in the turning test was α – β titanium alloy Ti-6Al-4V. Stock of Ti6Al4V material was available in the form of cylindrical rod. The chemical composition (wt. %) and mechanical properties of Ti6Al4V are mentioned in Table 3.6 and 3.7 respectively. Table 3.6: Nominal chemical composition of Ti6Al4V

Element Wt. % Element Wt. %

H N C Fe

0.005 0.01 0.05 0.09

V Al Ti

4.40 6.15

Balance

Table 4 represents the mechanical properties of the Titanium alloy Ti6Al4V used during the cutting experiments. Table 3.7: Mechanical properties of Ti-6Al-4V at room temperature

Properties Values Properties Values


993 MPa 830 MPa 14%

Poison ratio Modulus of elasticity Hardness (HRC)

0.342 114 GPa

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b) Turning Cutting Inserts (Coated and Uncoated carbides)

The experimentation on turning setup was conducted using two types of cutting tools, coated and uncoated carbide inserts. Table 3.8 shows the general specifications of turning inserts. These cutting inserts were selected on recommendation of tool supplier (Sandvik).

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Table 3.8: Specificans of the turning cutting insert

Turning Cutting Insert

1. Coated Carbide - TCMT 16 T3 04-MM 1105

- The substrate consists of a fine grained WC with 6% Co for high hot hardness and good resistance against plastic deformation. - The new thin PVD TiAlN coating with excellent adhesion, also on sharp edges, guarantees toughness, even flank wear and outstanding performance in heat resistant super alloys.

2. Uncoated Carbide - TCMT 16 T3 04-KM H13A

- Combines good abrasive wear resistance and toughness for medium to rough turning of heat resistance steel and titanium alloys.

c) Design of experiments Turning experiments were conducted under dry, conventional flood and mist conditions on a CNC turning center. Out of the turning experiments, four papers B, C, D and E are produced. All experiments were conducted using full factorial model. d) Experimental test setup Turning experiments were conducted under the dry, mist and flood cooling conditions on a CNC Turning Center. Mitutoyo Roughness Tester SJ 201P has been utilized for the measurement of surface roughness of generated surface. To minimize experimental error each surface roughness measurement was repeated three times and only the average values are reported in this study. A Mitutoyo tool maker microscope was used to evaluate flank wear at the cutting edge. Scanning Electron Microscope (SEM), Philips FEI XL30, was utilized to investigate the wear mechanisms at flank face. Figure 1 shows the schematic illustration of experimental setup. Kistler Multi Channel Dynamometer was utilized for measuring the cutting forces generated during drilling operations. PS3500 power data logger has been used to capture the power utilized during each cutting test.

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Table 3.9: Cutting parameters and experimental set up in turning experiments

For Paper B Cutting Insert Depth of cut (mm) Cutting Speed (m/min) Feed (mm/ rev) Machining Length (mm) Machining Environment

TCMT 16 T3 04-MM 1105 (TiAlN Coated) 0.8 mm 30 – 90 (Two levels) 0.1 – 0.2 (Two levels) 120 Dry , Mist and Flood (Three levels)

For Paper C Cutting Insert Depth of cut (mm) Cutting Speed (m/min) Feed (mm/ rev) Machining Length (mm) Machining Environment

TCMT 16 T3 04-MM 1105 (TiAlN Coated) TCMT 16 T3 04-KM H13A (Uncoated Carbides) 0.8 mm 30 - 60- 90 (Three levels) 0.1 – 0.2 (Two levels) 120 Dry , Mist and Flood (Three levels)

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For Paper D

Cutting Insert Depth of cut (mm) Cutting Speed (m/min) Feed (mm/ rev) Machining Length (mm) Machining Environment

TCMT 16 T3 04-KM H13A (Uncoated Carbides) 0.8 mm 30 - 60- 90 (Three levels) 0.1 - 0.2 – 0.3 – 0.4- 0.5 (Five Levels) 125 Dry - Flood (Two levels)

For Paper E Cutting Insert Cutting Speed Feed rate Depth of cut Cooling strategy

CCMT 12 04 04 MM 1105 (Coated Carbides) 90 – 120 – 150 m/ min 0.15 – 0.2 – 0.25 mm/ rev Constant 0.8 mm Dry, Flood and Low temperature air (Sub-zero, 0 to - 4 C°) + Vegetable oil based mist (Internal) MQL+CA, 4 ml/ min

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e) Results TiAlN coated inserts have better thermal stability then uncoated inserts, due to formation of a dense and adhesive protective Al2O3 layer at higher temperatures which protects the tool from diffusion wear mechanism. This feature of TiAlN coated insert makes it performs better at higher cutting speeds. At low cutting speeds coated inserts did not perform well because coated inserts are brittle in nature and exhibit high friction coefficient. It was observed in the study that increase in material removal rate reduces energy consumption significantly. It is due to the fact that machining time plays dominant role towards consumption of energy. It was also observed that surface roughness and energy consumption decreased by increasing cutting speed and material removal rate. Reduction in energy consumption with increase in feed rate is logical because high feed rate results in faster machining and less processing time. Cutting speed of a machining process is directly linked with cutting force. Higher cutting speed generates low cutting forces which results in less energy consumption. However limitation of using higher cutting speed is that it generates high amount of heat during cutting process. High cutting temperature results in poor tool life and accelerated tool wear mechanisms. It was observed that MQL+CA cooling technique performed better than dry in almost all cases and in some conditions out performed flood environment as well. In general high temperature is present in the cutting zone during the machining of Ti6Al4V. Due to high cutting temperature, oil in MQL strategy evaporates easily without providing proper lubrication. Mixing of MQL (vegetable oil based) with cool air provides better result at cool air try to reduce temperature facilitating MQL to lubricate properly. This clearly shows potential of MQL+CA strategy as a possible replacement of flood cooling.

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CHAPTER 4

MACHINABILITY EVALUATION METHODS

This chapter presents the methods used in the analysis of the experimental data. The main parts of the analysis were energy consumption and investigation of the tool wear mechanisms in the cutting tools used in the turning experiments.

4.1 SURFACE ROUGHNESS ANALYSIS The study utilized Mitutoyo surface roughness tester SJ 201P to measure the measure the arithmetical mean deviation of the profile (Ra) 1. The study utilized the measurement procedure as suggested by the standard ISO 4287. The arithmetical mean (Ra) of the profile represents the deviations “Z” for reference length l as shown in Figure 4.1. Figure 4.2shows the tester used for experimentation. Table 4.1 shows the general specifications of the surface tester used during experimentation.

Ra = 1𝑙 ∫ |𝑍(𝑥)| 𝑑𝑥𝑙0 (4.1)

Figure 4.1: Arithmetical mean of the profile (Ra) as per ISO 4287 standard

[112]

1 Arithmetical mean of the profile (Ra) as per ISO 4287 standard

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Figure 4.2: Mitutoyo surface roughness tester SJ 201P

Table 4.1: Specifications of Mitutoyo surface roughness tester [113]

Model Mitutoyo SJ 201P

Drive Unit Drive speed Evaluation length Mass Detector Provided Detecting method Material of stylus Radius of skid curvature Stylus tip radius Measuring force Display Units Roughness standard Sampling length Display range Ra

0.25 mm/s, 0.5mm/s and 0.8 mm/s 12.5 mm 190g Differential Inductance Diamond 40 mm 5μm 4mN JIS, DIN, ISO, ANSI 0.25mm, 0.8 mm and 2.5 mm 0.01 – 100 μm

For all of the cutting experiments roughness values were measured three times and only average values are reported to reduce experimental errors.

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4.2 CUTTING FORCE EVALUATION Cutting force data was measured and collected during the machining experiments. The cutting force data was collected in the Z (Fz), X (Fx) and Y (Fy) directions using Kistler (Model: 9257b) dynamometer. In order to obtain signal multichannel (04) charge amplifier (Model: 5070) was used. The output signals from charge amplifier were presented on a personal computer using Dynoware software package.

(a) (b)

Figure 4.3: Cutting force data evaluation system, (a) Kistler 9257b Dynamometer, and (b) Kistler 5070 charge amplifier

Table 4.2: Specifications of Kistler dynamometer

Model 9257b (Dynamometer) 5070 (Charge amplifier)

Calibration Range (kN)

Fx direction Fy direction Fz direction

Sensitivity (pC/N)

Fx direction Fy direction Fz direction

0-5 (kN) 0-5 (kN) 0-5 (kN) -7.916 -7.902 -3.707

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4.3 FLANK WEAR ASSESSMENT Tool wear at the flank face of the cutting tool is termed as flank wear. The amount of tool wear was measured in the machining experiments using standard ISO 3685:1993 [56]. The maximum values of width of the flank wear land were measured during experiments. To investigate this flank wear, Mitutoyo tool maker microscope (Model: 510) was used in the presented research work. Figure 4.4 shows the Mitutoyo tool maker microscope.

(a) (b)

Figure 4.4: (a) Mitutoyo tool maker microscope (Model: TM 510), and (b) sample flank wear measurement

Table 4.3: Specifications of Mitutoyo tool maker microscope Model Mitutoyo TM 510 XY range Effective area of table Max. workpiece height Max. workpiece weight Total magnification Transmitted illumination Reflected illumination

100x50mm 150x92mm 107mm 5kg 30X Light source: Tungsten bulb (24V, 2W) Green filter, Light intensity adjustable Light source: Tungsten bulb (24V, 2W) Light intensity adjustable

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4.4 WEAR MECHANISM ANALYSIS USING SCANNING ELECTRON MICROSCOPY After each turning experiment the cutting inserts were collected and cutting edges were marked with different colors, and placed in plastic bags that were marked with parameters like cutting speed, test number and cutting tool material. Figure 4.5 shows the Philips FEI XL30 scanning electron microscope used to study the tool wear mechanisms. The micrographs of cutting inserts were obtained under hi-vacuum scanning modes. For the present study micrographs were taken using spot size of 3 and beam acceleration voltage as 30 kV.

(a)

(b)

Figure 4.5: (a) Scanning electron microscope (Model: Philips FEI XL 30,

(b) Sample SEM images of uncoated carbide tool under Dry, Vc = 30 m/min and f = 0.1 mm/ rev

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4.5 POWER AND ENERGY CONSUMPTION ANALYSIS In order to investigate the energy consumed during each cutting experiment, PS3500 power data logger has been used to capture the power utilized during the cutting test. Figure 4.6a and b show power data logger used in experiments and a sample calculation performed to compute energy consumption using power data.

(a)

(b)

Figure 4.6: (a) PS 3500 power data logger and (b) Sample calculation from paper C for energy consumption calculated for mist condition, vc = 30

m/min, f = 0.1 mm / rev, Depth of cut = 0.8 mm

http://www.testequipmentdepot.com/powersight/images/ps3500.gif

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Table 4.4: Specifications of power logger Model PS 3500 Operating Range Measurement Rate Frequency Measurement

0 - 50 degrees C, Relative humidity to 70% (non-condensing) Analyses two cycles per second of each voltage and current input at 16 μs; uses 130 samples per cycle @ 60 Hz. All measurements updated once per second Range: DC, 45 - 66 Hz, 360 - 440 Hz fundamental, Display on meter: (PS3500 ONLY), Accuracy: ±0.5%

4.6 MINIMUM QUANTITY COOLED AIR LUBRICATION (MQL+CA) SYSTEM AND SUPPORTING TOOL HOLDER A cooling strategy in which a mixture of low temperature air with internal vegetable oil based mist (MQL+CA) was employed during machining experiments. Vegetable oil in MQL was operation at the flow rate of 4.6 ml/ min. The vegetable oil (ECULUBRIC E200L) was provided by ACCU-Svenska AB. The flow rate of mist was controlled by regulating the low temperature air and oil supply. The information about the vegetable oil is shown in Table 4.5. Figure 4.6 shows MQL+CA system under operation at CNC turning center.

Table 4.5: Properties of vegetable oil used in mist (ECULUBRIC E200L) [114]

Properties Description

Chemical description Health hazard Flash point Ignition point Density (at 20 C0) Viscosity (at 20 C0) Partition coefficient

A fraction of natural triglycerides, easily biodegradable substances Not hazard to human health 325 C0 365 C0 0.92 g/ cm3 70cP < 3%

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Figure 4.7: MQL (vegetable oil based) system with low temperature cool air

To support the internal cooling from the MQL+CA system, special tool holder was also required. Special tool holder (Mircona SCLC R 2525 M12-EB) with two internal passages was utilized with this MQL+CA system. Figure 4.7a shows the schematic illustration of the internal cooling passages in tool holder. Figure 4.7b shows the tool holder used in the experimentation.

Figure 4.8: (a) Schematic view of Mircona SCLCR 2525 M12-EB tool holder (b) View of actual tool holder used in experimentation

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CHAPTER 5

RESULTS AND DISCUSSION

This chapter discusses the results obtained from the experimental work. Most of the results came from appended published papers. In addition to these appended papers, additional work is also reported in this chapter.

5.1.1 Milling of Aluminum alloy 6061 Aluminum alloy 6061 is one of the most commonly used alloys of 6000 series. It is widely used in the manufacturing of aircraft wings and fuselages, automotive components like spacers and food cans etc. Milling operation is most commonly performed on Al 6061. In the current study, four milling strategies namely zigzag, constant overlap spiral, parallel spiral and one-way have been analyzed with respect to the power and energy consumption during the machining phase. Power and energy consumption by the machining process is one of the key parameters related to sustainability calculation and environmental burden analysis. This work aims to provide a better understanding the concept of energy consumption during the milling process by comparing different strategies to help select the optimum milling strategy with respect to the power and energy consumption. The study also shows the influence of cutting parameters such as feed rate and cutting speed on energy consumption.

a) Zigzag b) Constant Overlap

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c) Parallel Spiral d) One Way

Figure 5.1: Schematic illustration of milling strategies

To evaluate each strategy properly two levels of spindle speed (2000 – 4000 rpm) and three levels of feed (100, 200 and 300 mm/ min) were used. 5.1.2 Energy Consumption Analysis The block of Al 6061 was machined to create pockets using end milling operation. More information about the setup can be found in paper A. Energy consumed during each pocketing operation has been calculated using the power signal..

Figure 5.2: Zigzag, Constant overlap spiral, Parallel spiral and One-

way milling strategies, 4000 rpm, Depth of cut 3mm (full immersion)

Figure 5.2 shows that at spindle speed of 4000 rpm, one-way milling strategy consumed more energy to produce pocket of similar dimension.

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Constant overlap spiral milling strategy ranked second at lower and medium feed. However at higher feed constant overlap spiral strategy consumed almost equal energy to parallel spiral strategy. Figure 5.3 shows that at lower feed all of the milling strategies consumed more or less the same energy. However one-way milling strategy again consumed more energy than others at medium and higher feed levels.

Figure 5.3: Zigzag, Constant overlap spiral, Parallel spiral and One-

way milling strategies, 2000 rpm, Depth of cut 3mm (full immersion) 5.1.3 Total Machining Time Figures 5.4 and 5.5 represent the total machining time required to machine a pocket by each strategy with respect to the energy consumption. This can be observed easily that one way milling strategy consumed more time to machine the desired pocket. More machining time points out at the fact that one way strategy adopted the longest path to machine. The lowest machining time was obtained for zigzag strategy at high feed rate of 400 mm/ min for high cutting speed of 4000 rpm. This can also be seen that all of the milling strategies consumed more or less the same amount of energy at low speed and low feed rate.

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Figure 5.4: Machining time of milling strategies at 4000 rpm

Figure 5.5: Machining time of milling strategies at 2000 rpm

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5.2.1 Machining of Titanium alloy (Ti-6Al-4V) High performance alloys, such as titanium and nickel alloys are used in aerospace, automotive, defence, dental and orthopaedic sectors because of their ability to work at higher operating temperature and high strength to weight ratio. These high performance alloys are used in manufacturing of gas turbines, space crafts, rocket engines, nuclear reactors, submarines, petrochemicals and glass industries. These alloys are termed as “Difficult-to-cut materials” because of their low machinability rating. The quality of surfaces produced using specific material removal process is characterized by measuring surface integrity. High value of surface roughness is critical for fatigue life of any engineering component. Ti-6Al-4V was machined using single coated TiAlN- PVD coated carbide inserts. Experimentation was conducted using constant depth of cut under two levels of cutting speeds and feed rates. The performance of TiAlN coated tool was investigated under dry, mist and flood cutting environments. Surface roughness trends were observed and wear mechanisms at early stage machining were observed using scanning electron microscope. Comparatively fair results were observed at higher cutting speeds. 5.2.2 Surface roughness analysis In order to investigate the surface roughness behaviour with respect to machining length, total length of workpiece was divided into four zones A, B, C and D as shown in Figure 5.6.

Figure 5.6: Total machining length with Zones A, B, C and D

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(a) (b)

Figure 5.7: Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.1 mm/ rev (b) Vc = 90 m/min, f = 0.1 mm/ rev

Figures 5.7 (a) and (b) show the roughness finish obtained for TiAlN PVD coated tools under 30 m/ min of cutting speed and 0.1 - 0.2 mm/ rev of feed rates. Figures 5.7 (a) and (b) represents the low and high level of cutting speeds at low feed. At higher cutting speed of 90 m/ min the surface roughness increases and then decreases uniformly. This shows that sharp edge of tool produces high surface roughness but after some time surface finish improves.

(a) (b)

Figure 5.8: Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.2 mm/ rev (b) Vc = 90 m/min, f = 0.2 mm/ rev

Figures 5.8a and 5.8b show that feed rate is directly proportional to surface roughness (Ra). Surface roughness values at feed of 0.2 mm/ rev were higher than the values obtained for 0.1 mm/ rev. Figure 5.8b shows comparatively better results at high cutting speed and feed rate. An important observation was that surface roughness was lowest for mist

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environment when working at low feed rate. Dry environment provided lower surface roughness at higher feed rate. To illustrate this behaviour state of the art for the titanium alloys machining was checked. But in literature a study [115] was found for steel where similar trends were observed. Seah et al. [115] also observed that flood cooling enhanced wear rate for machining steels. They concluded that crater wear was shifted near the cutting edge by the application of flood coolant.

5.2.3 Flank Wear Measurement Figure 5.9 represents flank wear at different cutting conditions and environments. The highest value of flank wear was measured at high cutting speed of 90 m/ min and high feed of 0.2 mm/ rev under dry cutting condition. It indicates the presence of high cutting temperature. High cutting temperature has influence on tool wear, surface integrity, tool life, chip formation mechanism and thermal deformation of the tool [116]. William and Tabor [117] examined role of cutting fluids in metal cutting. They discussed friction mechanism at tool chip interface. The study proposed a formulation of interconnecting capillaries at the interface.

Figure 5.9: Maximum flank wear measurement

The combination of high temperature and chemical affinity of titanium alloys initiates wear at very high rate. Wear mechanisms such as adhesion, abrasion and diffusion depends on the tool and workpiece materials and the cutting temperature. Titanium alloys have low thermal conductivity as a

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result temperature remains at the cutting edge and the region nearby. To lower cutting temperature at cutting edge different cooling strategies can play a vital role. Figure 5.9 showed that mist and flood cooling performed equally good job at higher cutting speeds. It also reveals the potential of mist as a cooling strategy because of the pressure for adopting environmental friendly machining techniques. 5.2.4 Wear Mechanisms Due to the complex interaction of tool, workpiece and chip removal high temperature is produced in the cutting zone. At this high temperature different wear mechanisms initiates can start at very faster rate.

Vc = 30 m/min, f = 0.1 mm / rev

DRY

MIST

FLOOD

Figure 5.10a: Wear mechanisms in PVD TiAlN coated inserts

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Vc = 30 m/min, f = 0.2 mm / rev

DRY

MIST

FLOOD

Figure 5.10b: Wear mechanisms in PVD TiAlN coated inserts Mainly these wear mechanisms are adhesion, abrasion, diffusion, coating delamination, notch and micro-chipping etc. These wear mechanism were present on both faces of the tool. On rake face it is present in the form of crater wear and on flank face it is known as flank wear. Figure 5.10 represents the SEM micrograph of tool wear at cutting speeds of 30 and 90 m/min under dry, mist and flood conditions. Micrograph for 30m/ min and feed 0.1 mm/ rev showed coating delamination, adhesive wear and abrasive wear as major wear mechanisms for all cutting environments. However with increase in feed rate adhesive and abrasive wears were magnified as shown in the micrograph for 30 m/ min and feed 0.2 mm/ rev. The wear mechanisms for Ti6Al4V were found in accordance with the literature [89-90]. The micrographs in Figure 5.10 make it clear that at low cutting speed dry environment was worst. Dry cutting conditions provided friendly environment for the initiation of adhesive tool wear as workpiece material found attached with tool. However, mist and flood environments performed equally well at low cutting speed.

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Vc = 90 m/min, f = 0.1 mm / rev

DRY

MIST

FLOOD

Figure 5.11a: Wear mechanisms in PVD TiAlN coated inserts

Vc = 90 m/min, f = 0.2 mm / rev

DRY

MIST

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FLOOD

Figure 5.11b: Wear mechanisms in PVD TiAlN coated inserts

5.3.1 Surface roughness comparison The paper B provides an idea of machinability of Ti6Al4V using PVD-TiAlN coated carbide inserts under different dry, mist and flood cooling strategies. Paper B utilized limited levels of cutting conditions; however paper C extends this investigation further. In paper C, machinability of Ti6Al4V is investigated using PVD-TiAlN coated and uncoated carbides under dry, mist and flood conditions. Paper C provides comparison of both cutting tools by discussing surface roughness, cutting forces, flank wear and associated wear mechanisms and energy consumption. Figures 5.12a, b, c and d compare the roughness values for coated and uncoated tools at feed 0.1 mm/ rev and 0.2 mm/ rev under dry, mist and flood environments. At low cutting speed of 30 m/ min under dry conditions observed roughness (Ra) values for both coated and uncoated inserts were approximately same. However at cutting speed of 30 m/ min uncoated insert gave better surface quality for both mist and flood conditions. At cutting speed of 60 m/ min for dry condition both inserts performed in a similar manner. But at 60 m/ min coated inserts provided more roughness than the uncoated inserts for both mist and flood environments. At 60 m/ min the lowest value of surface roughness was obtained using uncoated insert under mist condition. Similar trends but with better surface finish were observed at 90 m/min for both coated and uncoated inserts. Previous research, [28- 118], points out that at higher

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cutting speed, cutting force decreases and cutting temperature increases. Uncoated carbides cannot withstand high temperature and results in rapid adhesion and diffusion wear. TiAlN coating resists heat to extend tool life by lowering coefficient of friction.

(a) (b)

(c) (d)

Figure 5.12: Surface roughness (Ra) measurements

For dry condition coated tool performed better than uncoated tool for all cutting speeds of 30, 60 and 90m/ min. For mist and flood environments coated tool gave high roughness values at low cutting speed i.e. 30 m/ min. Coated inserts gave good result at cutting speeds of 60 and 90 m/ min. Uncoated inserts performed better than coated inserts at low cutting speed for both mist and flood condition. It can be seen that coated inserts provides comparatively fair results at higher cutting speeds this might be because of the wear and heat resistant nature of TiAlN coating. However uncoated inserts were found superior for low cutting speeds because of relatively low cutting temperature at low cutting speed. Better surface finish was obtained

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under dry condition for both coated and uncoated inserts. Similar trends were observed by Seah et al. [118] when machining steel alloys. Their study revealed that coolant shifted crater wear towards the cutting edge. The surface roughness data was also analysed by using analysis of variance (ANOVA) method. Table 4 shows that except feed rate none of the factors contributed considerably towards the surface roughness. The model F value of 140.06 implies that the model is significant. In the present study A, B, C, D, AB, AC, AD, CD, ABC, ABD and ABCD were significant model terms. Table 5.1 Results of ANOVA for surface roughness

Cutting Parameters

Sum of Squares df Mean

Square F - Test Contribution (%)

Model 149.61 35 4.27 140.06 98.55

A-Cutting Speed 1.12 2 0.56 18.35 0.74 B-Feed 142.61 1 142.61 4672.83 93.94

C-Coolant 0.40 2 0.20 6.47 0.26 D-Cutting

Tool Material 0.55 1 0.55 18.13 0.36 AB 0.35 2 0.18 5.78 0.23 AC 0.62 4 0.16 5.11 0.41 AD 1.06 2 0.53 17.38 0.70 BC 0.18 2 0.09 3.00 0.12 BD 0.02 1 0.02 0.75 0.02 CD 0.21 2 0.10 3.43 0.14

ABC 0.31 4 0.08 2.50 0.20 ABD 0.86 2 0.43 14.07 0.57 ACD 0.11 4 0.03 0.89 0.07 BCD 0.07 2 0.03 1.12 0.05

ABCD 0.39 4 0.10 3.20 0.26

Error 2.20 72 0.03 -- 1.45 Total 151.81 107 -- -- 100.00

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(a)

(b)

Figure 5.13: Statistical analysis of surface roughness (a) Half-normal plot (b) Residuals vs. Run

Half-normal probability plot is a powerful graphical tool that points out at important factors and their interactions [119]. As shown in Figure 5.13 (a), a list of effects and their interactions were reported based on their

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magnitudes. Insignificant factors appear on or close to the near zero line. It was observed that factor B (feed rate) is the most important factor for the presented study. Residual vs. run plot is a special scatter plot which shows different drifts in data. In residual vs. run plot each residual is plotted against experimental run order. 5.3.2 Cutting force comparison In Figures 5.14, it was observed as a general trend that cutting forces decrease with increasing cutting speed up to a certain range, and after passing that range cutting force again starts increasing. Figures 5.14 (a) and (b) represent that at low feed of 0.1 mm/ rev cutting force reduced slightly for both uncoated and coated tools when cutting speed was increased from 30 m/ min to 60 m/ min. However cutting force increased again when cutting speed was raised from 60 m/ min to 90 m/min. This variation in cutting force is attributed with built up edge (BUE) phenomenon and cutting temperature behaviour. Previous studies [28-39] revealed that temperature in cutting zone is directly related to the cutting speed. Higher cutting velocities generate elevated temperatures in cutting zone that enhances thermal softening of workpiece material. Built up edge (BUE) formation is based on cutting conditions and combination of workpiece and cutting tool material. Reduction in cutting forces points out that built up edge increases effective rake angle resulting in lower cutting forces. Fang and Wu [120] also observed reduction in cutting forces with increase in cutting speed for machining Ti6Al4V and Inconel 718. Other studies [121] also revealed that cutting force decreases with increase in cutting speed. The present study also revealed that cutting force was lower, when cutting speed was close to the range of 60 m/ min. Komanduri and Reed [37] also found that uncoated carbides exhibit excessive tool wear above 60 m/ min. The cutting force increased at higher cutting speed of 90 m/min because at higher cutting speed there is no built up edge (BUE) formation. Figures 5.14 (a) and (b) shows cutting force for coated and uncoated inserts under dry, mist and flood conditions. At low feed of 0.1 mm/ rev and cutting speed of 30 m/ min forces produced in dry conditions were lower than forces obtained in mist and flood conditions. Similar cutting force behaviour was observed for flood cooling at feed of 0.2 mm/ rev. The finding related to negative impact of flood cooling was against the common belief of coolant application. A possible explanation of this finding could be that by introducing the coolants in cutting zone reduces the temperature

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of cutting tool as well as workpiece material. This cooling effect of workpiece material reduces thermal softening tendency. As a result higher cutting forces are generated in order to cut material. Seah et al. [115] also found similar behaviour of flood cooling when machining AISI 1045 and AISI 4340 steel grades using uncoated tungsten carbides. His study revealed that flood cooling favors crater wear to grow near the cutting tip that makes it much weaker. Beno et al. [98] also found similar results of increasing cutting force under MQL for machining Wasaloy.

(a) (b)

(c) (d)

Figure 5.14: Cutting force at different cutting speeds under dry, mist and flood cooling strategies (a) Uncoated inserts, f = 0.1 mm/ rev (b)

Coated inserts, f = 0.1 mm/ rev (c) Uncoated inserts, f = 0.2 mm/ rev, and (d) Coated inserts, f = 0.2 mm/ rev.

Figures 5.14 (a) and (b) shows that coated inserts generated less cutting force at higher cutting speeds. The lowest cutting force was observed with coated insert under dry condition at 60 m/ min cutting speed. Highest cutting force was observed with coated insert under mist condition at 30 m/

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min cutting speed. It concludes that TiAlN coated inserts were better for higher cutting speed selection. Higher cutting speed generates high cutting temperature in the cutting zone that allows diffusion wear mechanism to start rapidly. TiAlN coated tools performed better at higher cutting speeds due to the formation of highly dense Al2O3 protective layer. This protective layer prevents diffusion wear mechanism at higher cutting speeds. Figures 5.14 (c) and (d) depict the generated cutting forces at constant feed of 0.2 mm/ rev for three different levels of cutting speed. It states that uncoated inserts generated more cutting force at all levels of cutting speeds. However, performance was equally good for uncoated at 90 m/ min cutting speed under flood environment when compared with coated inserts. At higher feed level of 0.2 mm/ rev the lowest cutting force was observed with coated insert under dry condition at 60 m/ min cutting speed. The general trend of cutting force was that it decreased with increasing cutting speed in both uncoated and coated inserts, whereas it increased with further increase in cutting speed. 5.3.3 Power and Energy Consumption Energy consumption was compared for both inserts to examine which tool is more energy efficient. Figure 5.15 displays a sample calculation of power and energy consumed during a turning experiment for specific cutting conditions under dry condition.

Figure 5.15: Power and Energy consumption in Dry cutting, Uncoated tool,

f = 0.1 mm/min

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Figure 5.15 also shows that power demand increases with increase in cutting speed but machine time reduces due to variation in workpiece diameter. For the current study machining length and cutting speeds were kept constant due to which variation in machine speed was obtained by workpiece diameter.

(a) (b)

(c)

Figure 5.16: Specific energy consumption by uncoated and coated inserts under dry, mist and flood conditions, (a) Cutting speed of 30 m/ min, (b)

Cutting speed of 60 m/ min, and (c) Cutting speed of 90 m/ min

For appropriate energy comparison in machining tests specific energy consumption was calculated with respect to material removal rate (mm3/ min) as shown in Figure 5.16. Machining with low feed rate takes more

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time because of slower tool movement, however similar length can be machined in less time using higher feed rates. As less time was involved in machining at high feed rate that result in reduced energy consumption. Lower cutting speeds resulted in high energy consumption when constant machining length was used. It was observed that this reduction in energy consumption was attributed to machining time. The machining time was reduced due to variation of workpiece diameter in order to adjust machine speed. At cutting speed of 30 m/ min, coated inserts consumed more power than the uncoated inserts.

At cutting speed of 30 m/ min and feed of 0.1 mm/ rev it was observed that mist and flood consumed more power. Higher power consumption for mist and flood is due to higher cutting forces and power drawn by hydraulic pump. At cutting speed of 60 m/ min dry environment consumed minimum energy for both feed levels. At higher cutting speed of 90 m/ min mist environment showed potential of better heat dissipation from the cutting zone at both feed levels. Flood environment gave higher energy consumption in most of the cases which shows that high cutting forces were produced under flood conditions. Comparatively less energy consumption was observed for coated inserts at cutting speeds of 60 m / min and 90 m/ min. However uncoated inserts showed comparatively better performance at low cutting speed of 30 m/ min.

The general trends obtained for energy consumption can be summarized as follows: higher energy consumption was obtained at low cutting speed due to variation in diameter when machining length was kept constant. It is because machining time reduced when workpiece diameter changed. The coated carbides under mist condition provided comparatively less energy consumption because of efficient control of friction in cutting zone. TiAlN coating is also helpful in reducing coefficient of friction (COF). TiAlN coated tools showed comparatively less energy consumption in all cases, especially at higher cutting speeds. Higher feed rate results in less energy consumption.

5.3.4 Tool Wear Assessment

As shown in Figure 5.17, higher flank wear rate was observed for higher feed rate under all cutting environments. Tool coatings are useful in reducing the coefficient of friction between chip and tool. By reducing the friction coefficient heat generation during metal cutting operation can also be reduced. This phenomenon can increase the tool performance and tool life. Higher flank wear rate in uncoated insert at high cutting speed and feed points out at high amount of heat generation for these cutting conditions. Improved heat transfer can be obtained using mist and flood environments.

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(a)

(b)

(c)

Figure 5.17: Flank wear on coated and uncoated tool (a) Dry, (b) Mist and (c) Flood

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The coated insert provided the highest flank wear at high cutting speed of 90 m/ min and feed of 0.2 mm/ rev. This point out at the fact that titanium alloys have low thermal conductivity as a result the heat generated during the machining operation stays in the region of cutting. As the cutting environment was dry there were less chances of heat dissipation. Due to the excessive heat generation and friction phenomenon coating delamination and abrasion were responsible for large wear zone at flank face of the coated insert Experimentation showed good results for the same cutting condition under mist and flood environments. The experiments revealed that improved heat dissipation can reduce the coating delamination which can result in low flank wear. Figure 5.17 (b) represents the plotted results for flank wear of coated and uncoated inserts under mist conditions. Under mist condition results obtained for flank wear were better than the dry conditions. Another observation was related to the coated inserts that they performed better than uncoated tools at higher cutting speed of 60 and 90 m/ min. However uncoated tools were fairly good for low cutting speed of 30 m/ min. Figure 5.17 (c) shows flank wear of coated and uncoated inserts under flood conditions. Flank wear results at low cutting speed 30 m/ min for flood environment were better than mist especially for coated inserts. Coated inserts comparatively performed better than uncoated tools at higher cutting speed of 90 m/ min. 5.3.5 Wear Mechanisms in coated and uncoated tools

a) Wear mechanisms at Vc = 30 m/ min In agreement with the literature it has been observed that adhesion, diffusion and abrasion were the main wear mechanisms for Ti-6Al-4V alloy. Scanning electron microscope (SEM) has been utilized to study the underlying wear mechanisms present in turning of titanium alloys. Figure 5.18a shows the SEM micrographs of wear at flank face of uncoated insert under cutting speed of 30 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. Adhesion and abrasion were found as major wear mechanisms in uncoated carbide tools. It was also observed that wear rate was much higher for dry environment. The combination of high temperature and chemical reactivity creates a conducive environment for the adhesion and diffusion to start at rapid rate. Figure 5.18b shows that

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diffusion was also present in coated inserts under dry conditions. But for mist and flood environment diffusion was not observed clearly. As titanium alloys have very high chemical reactivity that results in an extreme wear at flank face. The particles of workpiece material were found attached at the nose and flank face of the cutting tool. The main wear mechanism responsible for this behaviour is adhesion. Adhesive forces between the tool and workpiece material result in adherence of small particles at cutting edge. These small welded particles have potential to form built up edge (BUE). Built up edge (BUE) was observed in the shown micrographs. SEM micrographs of coated and uncoated inserts revealed that abrasive wear mechanism. The combination of high temperature and stresses at the tool face starts the abrasion wear mechanism. Chipping of tool material was observed as shown in the Figures 5.18 and 5.19 micrographs. As the chipped material moves in between the tool and machined surface it results in the abrasive wear. The study also pointed out the fact that flank wear rate was higher at high feed. Depending on the tool and workpiece material, both abrasive and diffusion wear mechanism are more rapid at higher cutting speeds.

Uncoated Tool at Vc = 30 / min and feed = 0.1 mm/ rev

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Figure 5.18: Wear mechanisms in uncoated tool, (a) Dry (b) Mist (c) Flood

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Coated Tool at Vc = 30 / min and feed = 0.1 mm/ rev

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Figure 5.19: Wear mechanisms in coated tool, (a) Dry (b) Mist (c) Flood

b) Wear mechanisms at Vc = 60 m/ min Figure 5.20 shows the SEM micrographs of wear at flank face of uncoated insert under cutting speed of 60 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. Figure 5.20 shows high amplitude of abrasive and adhesive wears. It indicates that uncoated carbides did not perform well at cutting speed 60 m/ min. Figure 5.20b showed promising behaviour of mist cooling strategy. Figure 5.21 shows the SEM micrographs of wear at flank face of coated inserts under cutting speed of 60 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. Figures 5.19 and 5.21 show that coated inserts did not perform well for the low and medium cutting speeds of 30m/ min and 60 m / min respectively. The study exposed an observation that coated inserts at high feed of 0.2 mm/ rev represents less wear. This phenomenon of coating delamination should occur before adhesion at low cutting speeds for the coated inserts. Another observation was the dominant adhesion phenomenon. This adhesion phenomenon was observed for cutting speed of 60 m/ min and feed of 0.1 mm/ rev where the particles of work piece material were strongly welded with the tool material. As

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proposed by the literature [122] adhesion phenomenon comes after the delamination of coating

Uncoated Tool @ Vc = 60 / min and feed = 0.1 mm/ rev

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Coated Tool @ Vc = 60 / min and feed = 0.1 mm/ rev

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c) Wear mechanisms at Vc = 90 m/ min

Figures 5.22 and 5.23 show the SEM micrographs of wear at flank face of uncoated and coated inserts under cutting speed of 90 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. In Figure 5.22, the abrasive wear was very evident in uncoated carbides. Figure 5.22 b shows

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improved results under mist condition. It was observed that land affected by abrasion was magnified at high cutting speed and feed rate. The coated inserts performed better at high cutting speed of 90 m/ min as shown in Figure 5.23. This suggests that as titanium alloys have low thermal conductivity so during machining most of the heat stays at the cutting edge. Low cutting speed and low feed means that cutting edge stays longer in the region of high temperature. The combination of high temperature and friction results in rapid coating delamination at low cutting speeds which gives high wear at the flank face. Temperature can be controlled by using mist and flood environments. The experiments show that mist environment gave better result than the dry environment.

Uncoated Tool at Vc = 90 / min and feed = 0.1 mm/ rev

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Coated Tool at Vc = 90 / min and feed = 0.1 mm/ rev

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5.4.1 Roughness vs. Energy Curves at Dry Environment Paper D presents an experimental study to examine behaviour of energy consumption and surface finish under different material removal rates. Energy consumption data was also interpreted in the form of equivalent CO2 emissions with reference to the energy mix of United Arab Emirates. Graphical representations of energy consumption and surface finish were generated for better understanding and visualization. These plots can be a useful tool for environmental sustainability assessment. Figure 5.24 shows plots for energy consumption and surface finish for different material removal rates calculated at constant speeds of 30, 60 and 90 m/ min with five different feed levels ranging from 0.1 – 0.5 mm/ rev. Figure 5.24 (a) represents that energy consumption decreased with increase in material removal rate. Trends line was fitted using second order polynomial equations.

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(a)

(b)

(c)

Figure 5.24: Energy consumption and surface finish curves for dry cutting at different material removal rates using five feed levels f = 0.1 – 0.5 mm/ rev, (a) Vc

= 30 m/ min, (b) Vc= 60 m/ min, and (c) Vc= 90 m/ min

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Figure 5.24 shows that surface roughness increased with increase in material removal rate. Increase is surface roughness was observed due to increase in the feed rate. However energy consumption decreased with increasing material removal rate. Energy consumption decreases due to increase in feed rate. High feed rate means faster machining with less processing time. The intersection point shows the best optimized value of surface roughness with respect to the energy consumption. Similar trends for energy consumption and surface roughness were observed in Figures 5.24 (b) and (c). With increase in cutting speed energy consumption decreased whereas minor difference in surface roughness was observed when compared to the cutting speed of 30 m/min. Point of intersection between both curves was lowered with increase in cutting speed. It points out that increase in cutting speed lowers both energy consumption and surface roughness but literature criticize high cutting speeds with high amount of heat generated. The major limitation of using high cutting speed is high amount of heat generation that directly affects cutting tool life. As titanium alloys show poor heat dissipation due to their low thermal conductivity, presence of high amount of heat in cutting zone results in severe and rapid tool wear. 5.4.2 Roughness vs. Energy Curves at Flood Environment Figure 5.25 shows plots for energy consumption and surface finish for different material removal rates calculated at constant speeds of 30, 60 and 90 m/ min with five different feed levels ranging from 0.1 – 0.5 mm/ rev. Figure 5.25 (a) shows that energy consumption decreased with increase in material removal rate. Optimal point at the intersection of both curves was slightly shifted towards higher material removal rate when compared with dry cutting. Similar trends for energy consumption and surface roughness were observed in Figures 5.25 (b) and (c). With increase in cutting speed energy consumption decreased whereas minor difference in surface roughness was observed when compared to the cutting speed of 30 m/min. Point of intersection between both curves was lowered with increase in cutting speed. It points out that increase in cutting speed lowers both energy consumption and surface roughness but literature criticize high cutting speeds with high amount of heat generated.

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(a)

(b)

(c)

Figure 5.25: Energy consumption and surface finish curves for flood cutting at different material removal rates using five feed levels f = 0.1 – 0.5 mm/ rev, (a) Vc

= 30 m/ min, (b) Vc= 60 m/ min, and (c) Vc= 90 m/ min

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5.4.3 Observations for similar material removal rate (MRR) Figures 5.26 and 5.27 show that material removal rate of 80 mm3/ sec was maintained using two different cutting speeds of 30 and 60 m/ min. In the first case cutting speed of 30 m/ min was used with feed of 0.2 mm/ rev to attain 80 mm3/ sec. However for second reading cutting speed of 60 m/min was used with feed of 0.1 mm/ rev to reach 80 mm3/ sec. It was observed that for material removal rate of 80 mm3/ sec less energy consumption and better surface roughness was obtained for cutting speed of 60 m/ min. Similar behaviour was observed for material removal rates of 160, 120 and 240 m/ min. This means that to minimize energy consumption and achieve good surface finish higher removal rates should be utilized by increasing the cutting speed. But cutting speed is directly linked with cutting temperature in the cutting zone that can affect tool life and associated wear mechanism significantly.

Figure 5.26 Energy consumption and surface finish at similar material removal rates

using different cutting speeds of 30, 60 and 90 m/ min under dry cutting

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Figure 5.27 Energy consumption and surface finish at similar material removal rates

using different cutting speeds of 30, 60 and 90 m/ min under flood cutting

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5.4.4 Complimentary Results (Extension From Paper D) 5.4.4.1 Environmental implications of energy consumption A significant amount of greenhouse gases (GHG) is released in atmosphere due to the processes metal cutting sector. Carbon dioxide (CO2) emission is one of the most critical GHG emissions. To protect the environment strict legislations are being developed and implemented by the global community. Manufacturing sector is also under immense pressure to avoid all environmental hazardous practices. Energy consumption during manufacturing operations is one of the key parameters that play an important role towards environmental burden. By optimizing energy requirements for a given machining operation greenhouse gases can be reduced. 5.4.4.2 Influence of geographical location on CO2 emissions Electricity generation in a specific region depends upon the type of energy resources of that region. Carbon dioxide (CO2) emissions generating from the electricity consumption of a machine tool highly depends on the geographical site. Figure 5.27 denotes global energy mix to display the variation in energy mix of different locations. It can be observed in Figure 5.28 that main sources of energy in Middle East are based on fossil fuels (oil and gas).

Fig 5.28: Variation in global energy mix [123]

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5.4.4.3 Estimation of CO2 emissions using energy consumption used in Paper D Energy consumption measured in paper D, were interpreted in terms of CO2 emissions. Table 5.2 shows equivalent CO2 emissions per Kilo-Watt hour produced by using different energy resources. These equivalent CO2 emissions are calculated by considering complete life cycle assessment (LCA) of each energy resource. According to the energy mix of United Arab Emirates, electricity generation is mainly produced by the combustion of fossil fuels. This presented study utilized 778 gCO2e/ kWh in order to estimate equivalent CO2 emissions. Table 5.2: Lifecycle estimates of gCO2e/ kWh for electricity generation procedures [124 – 125]

Technology Capacity/ Configuration/ Fuel

Estimates gCO2e/ kWh

Wind 2.5 MW, offshore 9 Hydroelectric 3.1 MW, reservoir 10 Wind 1.5 MW, onshore 10 Biogas Anaerobic digestion 11 Hydroelectric 300kW, run – river 13 Solar thermal 80 MW, parabolic trough 13 Solar PV Polycrystalline silicone 32

Biogass Short rotation forestry steam turbine

35

Natural gas Various combined cycle turbines

443

Fuel cell Hydrogen from gas reforming 664 Diesel Generators , turbine types 778 Heavy Oil Generators , turbine types 778 Coal Generators with scrubbing 960 Coal Generators without scrubbing 1000

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Figure 5.29: Equivalent CO2 emissions (g) produced for different cutting

conditions under dry cutting

Figure 5.29 revealed that under dry cutting highest CO2 emission was produced at lowest cutting speed and feed level. Increase in cutting speed results in decrease of CO2 emissions. Similarly high feed level results in reduced CO2 emissions.

Figure 5.30: Equivalent CO2 emissions (g) produced for different cutting

conditions under flood cutting

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5.5 MACHINABILITY OF MQL+CA

This paper E presents performance evaluation of different cooling strategies when machining Ti6Al4V. The study was conducted using dry, conventional flood and a mixture of low temperature air with vegetable oil based mist cooling strategies. Each cooling strategy was examined in reference with tool life, cutting temperature and surface roughness. The study explored a combination of sub-zero temperature air and vegetable oil based mist as possible environmentally benign alternative to conventional cooling methods. 5.5.1 Surface roughness analysis

Surface roughness was measured for all of the machining tests. Surface roughness defines the integrity of surface generated after machining. Surface roughness is more critical for the components manufactured from titanium alloys, because these alloys are termed as difficult to cut materials.

(a) (b)

(c)

Figure 5.31: Surface roughness trends with respect to dry, MQL+CA, and flood cooling strategies, (a) Vc = 90 m/min, (b) Vc = 120 m/min and (c) Vc =

150 m/min

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Figure 5.31a represents the surface roughness obtained for different cooling strategies at the cutting speed of 90m/ min. The dry cutting condition provided higher surface roughness at feed levels of 0.15 and 0.20 mm/ rev. At higher feed of 0.25 mm/ rev flood environment provided higher roughness. The MQL+CA based cooling strategy performed better than dry conditions at 0.15 and 0.20 mm/ rev and outclassed other strategies at higher feed of 0.25 mm/ rev. With increase of feed level surface roughness increases for all cooling strategies. Figure 5.31b represents surface roughness trends for different cooling strategies at cutting speed of 60 m/ min. It has been observed that MQL+CA cooling strategy outperformed other cooling strategies at all three levels of feed. It was also observed that cutting speed of 60 m/ min flood environment provided the highest surface roughness at all feed levels. Seah et al. [115] also performed machining tests on steel specimen using flood cooling techniques. The study revealed that wear rate was higher for flood environment due to the shifting of crater wear near the cutting edge. This can be a possible reason for higher surface roughness in flood environment. Figure 5.31c represents the plots for surface roughness for all cooling strategies at cutting speed of 150 m/ min. Figure 5.31c also showed higher surface roughness under dry cooling strategy at all feed levels. MQL+CA strategy provided comparatively better surface roughness at all feed levels. As a general trend MQL+CA performed comparatively better at higher cutting speeds. Generally in MQL systems cooling component is absent. As a result unsatisfactory performance is obtained when machining difficult to cut materials with MQL system. Our hypothesis is that by combining sub-zero temperature air, cooling component is added to the MQL system. Improved performance can be attributed to the combination of lubrication and cooling at same time. 5.5.2 Tool wear measurement During machining operation, cutting tool experiences loss of tool material and deformation. With the passage of time this wear increases at different locations of the cutting edge. Under the normal cutting parameters flank wear grows on the flank face and crater wear grows on the rake face. Flank tool wear is of great importance as it directly influences dimensional accuracy and surface integrity of the generated surface. Figure 5.32 shows the tool life observed at cutting speed of 90 m/ min for three levels of feed (0.15-0.20-0.25 mm/ rev). It was observed that MQL+CA lubrication technique out-performed dry and flood cooling at low feed of 0.15 mm/ rev as shown in Figure 5.32a. However at higher feed levels MQL+CA resulted in low tool life like dry environment as shown in

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(a)

(b)

Figure 5.32: Flank wear for flood, dry and MQL+CA at cutting speed of 90 m/

min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev Figures 5.32b and 5.32c. Figures 5.31b and 5.31c also shows that flood cooling gave comparatively better tool life. The general trend observed in Figure 5.32 shows that increase in the feed level results in less tool life. A

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possible explanation of this phenomenon is the low fracture toughness of cermet tools as found in agreement with literature [126]. Rapid crack formation and propagation was the reason of failure at higher feed levels. The reason of poor performance of MQL+CA at high feed rates can be the insufficient time provided to lubrication. MQL+CA cooling technique provided encouraging result at low feed rate only. At higher feed level of 0.25 mm/ rev even flood environment showed poor tool life. Figure 5.33 shows the tool life observed at cutting speed of 120 m/ min for three levels of feed (0.15-0.20-0.25 mm/ rev). It has been observed that MQL+CA technique out-performed dry and flood environment at low feed level of 0.15 mm/ rev as shown in Figure 5.33a. At feed level of 0.2 mm/ rev MQL+CA tool life was found little better than dry environment. Unexpectedly at higher feed level of 0.25 mm/ rev flood environment performed worst among other cooling techniques. Seah et al. [115] has also observed the negative effect of cutting fluids when machining steels. His work reveals that cutting fluid can enhance crater wear rate at the rake face. High crater wear rate weakens the cutting edge by excessive chipping. A possible explanation of low tool life under flood environment can be attributed by the presence of rapid crater wear rate and chipping at cutting edge. Performance of MQL+CA cooling technique was reasonable. When MQL system is used alone without cool air, the oil film evaporates rapidly because of the presence of high temperature in the cutting zone. The concept of cool air was used to reduce the cutting temperature in cutting zone. The main cause of reasonable performance of MQL+CA technique is that it takes the benefit of both cool air and MQL which makes it compatible for machining Titanium alloys.

(a)

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(b)

(c)

Figure 5.33: Flank wear for flood, dry and MQL+CA at cutting speed of 120 m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

Figure 5.34 shows the tool life observed at cutting speed of 150 m/ min for three levels of feed (0.15-0.20-0.25 mm/ rev). At low feed level of 0.15 mm/ rev, MQL+CA cooling technique performed as good as flood cooling environment. At feed levels of 0.20 mm/ rev all of the cutting environments performed almost in a similar way resulting very short tool life. A general trend was observed that higher feed rate and cutting speed resulted in shorter tool life.

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(a)

(b)

(c)

Figure 5.34: Flank wear for flood, dry and MQL+CA at cutting speed of 150 m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

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5.5.3 Cutting Temperature Analysis

Cutting temperature is an important and decisive factor towards machinability evaluation. It is a good measure to evaluate the effectiveness of a cooling strategy. Figure 5.35 shows sample calculation of cutting temperature using infrared camera.

(a)

(b)

Figure 5.35: Sample measurements of cutting temperature under dry environment at cutting speed of 150 m/ min, (a) feed = 0.15 mm/ rev (b) feed = 0.25 mm/ rev

Figure 5.36a, b and c represent plots for average values of cutting temperatures recorded under dry, MQL+CA and flood environment. It has been observed that at all levels of cutting speeds MQL+CA strategy has reduced cutting temperature. Flood cooling was found the most efficient way of heat dissipation. It was observed that MQL+CA strategy decreased average temperature by 26.6 % than the temperature obtained in dry environment at cutting speed of 90 m/ min. Similarly 17.9 % and 17.5%

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reduction was observed in MQL+CA strategy for cutting speeds of 120 and 150 m/ min.

(a)

(b)

(c)

Figure 5.36: Cutting temperature under dry, MQL+CA and flood environment, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25

mm/ rev

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CHAPTER 6

CONCLUSIONS AND FUTURE WORK

This chapter presents the conclusions drawn from the research work. In addition to the conclusions, plan for future work is also reported in this chapter.

6.1 Energy consumption in milling tool path strategies It was observed that increase in feed rate resulted in less energy consumption. This is due to the fact that higher feed rate increases material removal rate. Higher material removal rate consumes less energy when machining similar volume of material. It establishes that optimum selection of material removal rate in a machining phase can result in efficient energy consumption. Material removal rate can influence environmental burden of machining phase. It has been observed that one-way milling strategy consumed more energy in almost all cases of pocket milling. However, zigzag milling strategy consumed the least energy among all milling strategies. Constant overlap spiral and parallel spiral strategies were found in between zigzag and one-way milling strategy. The study revealed that one-way milling strategy consumed more energy because a large number of repetitive movements were present in it. It appears that the energy consumed for material removal was almost the same for all tool paths, but the movements and functions of machine tool in idle state make energy consumption different. 6.2 PVD-TiAlN coated and uncoated carbide tools It has been observed that for machining of Ti6Al4V alloys PVD-TiALN coated and uncoated performs under different cutting speed ranges. In general the uncoated inserts performed better at low cutting speed of 30 m/ min, whereas PVD-TiAlN coated inserts were superior at cutting speeds of 60 m/ min and 90 m/ min. Adhesion and abrasion wear mechanism were predominantly observed in the SEM micrographs of flank wear for uncoated inserts. High wear rate at cutting speed above 60 m/min indicates presence of elevated cutting temperature. High cutting temperature

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increases the rate of adhesion and diffusion wear mechanisms. Coating delamination was first observed for coated inserts. After coating delamination process adhesion, abrasion and diffusion mechanisms were accountable for the flank wear of the coated inserts. 6.3 Cutting force behaviour It was observed in the study that cutting force decreased for both tools with increase in cutting speed and after passing 60 m/ min it again starts increasing. This variation is due to the built up edge formation. Built up edge increases effective rake angle that results in lower cutting forces. At higher cutting speeds there is no built up edge formation that results in higher cutting forces. Higher cutting speed produces high cutting temperature that reduces cutting force due to thermal softening of workpiece material. Uncoated inserts generated more cutting forces at low cutting speed, pointing out the fact that there will be more tool wear and less tool life. However coated tools generated less force at higher cutting speeds. 6.4 Role of cutting environment Environmental condition plays an important role towards flank wear propagation. Dry environment showed rapid increase in flank wear with increasing cutting speed. However mist and flood environments showed equally good results for flank wear. Flood environment showed better result at low feed and cutting speed than mist environment. This points out at the potential of Minimum Quantity Lubrication (MQL) technique for higher cutting speeds to reduce environmental burden. The study showed that by using MQL and flood cooling techniques magnitude of adhesion wear mechanism can be minimized. 6.5 Minimum quantity lubrication and cooled air (MQL+CA) vegetable oil based mist system examination It was observed that MQL+CA cooling technique performed better than dry in almost all cases and in some conditions out performed flood environment as well. In general high temperature is present in the cutting zone during the machining of Ti6Al4V. Due to high cutting temperature, oil in MQL strategy evaporates easily without providing proper lubrication. Mixing of MQL (vegetable oil based) with cool air provides better result at cool air try to reduce temperature facilitating MQL to lubricate properly. This clearly

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shows potential of MQL+CA strategy as a possible replacement of flood cooling. Surface roughness analysis shows that MQL+CA out-performed dry cutting in almost all cases. However, MQL+CA provided better finish than the flood environment at higher cutting speed level of 150 m/ min. It was found that MQL+CA strategy decreased average cutting temperature by 26.6%, 17.9% and 17.5% than the temperature obtained in dry environment at cutting speed levels of 90, 120 and 150 m/ min respectively. 6.6 Energy consumption and environmental implications Power and energy consumption data showed that low feed and cutting speed gave more energy consumption. Dry environment showed less energy consumption as compared to mist and flood environments. Difference in energy consumed for dry, mist and flood environment was due to the cutting force behaviour and increase in power requirement due to coolant pumping. However, increasing feed results in less energy consumption because it reduces the machining cycle time. Although increase in feed results in higher surface roughness. To optimize a machining operation there should be a compromise between required surface roughness and reduction in energy consumption. In the presented work graphical plots of energy consumption and surface roughness were constructed under different material removal rates (MRR). Graphical plots of energy consumption and surface roughness intersect each other at certain location pointing out at the optimized value. These curves are useful tool to predict the amount of energy required for achieving desired surface roughness at specific material removal rate for certain machine tool. The study also revealed that highest CO2 emission was produced at lowest cutting speed and feed level. Increase in cutting speed results in decrease of CO2 emissions. Similarly high feed level results in reduced CO2 emissions. But, increase in both cutting speed and feed results in poor machining performance. It is well known that high feed results in poor surface finish and high cutting speed generates high temperature at cutting interface. Greenhouse gas (GHG) emissions are region specific in nature as they depend on the energy mix of the region. If the energy mix is based on cleaner energy sources (solar, wind, tidal, geothermal etc.) then even high value of energy consumption results in low emissions. In order to reduce environmental impact of certain industrial task, nominated site should be assessed critically with reference to the energy mix of the region as a part of the feasibility study. Priority should be given to the sites where more clean energy resources are present in the energy mix.

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6.7 Future work The literature has revealed that machining of Titanium alloy (Ti6Al4V) is difficult due to poor heat dissipation in the cutting zone. By utilizing an efficient cooling strategy heat can be rejected to improve machinability and tool life. In the future work cooling techniques will be analyzed further in detail to explore the machinability of Ti6Al4V. We plan to conduct experiments using a special hardware designed to combine MQL based system with sub-zero temperature air at different oil flow rates. The enhanced cooling technique will be environmental friendly technique as MQL system will utilize vegetable oil and low temperature air. The effectiveness of this technique will be evaluated in reference with conventional cooling strategies. At the same time efforts will be directed to run finite element based machining simulations to validate the experimental work. The data obtained from machining simulations will be used to develop a computational fluid dynamic (CFD) model for MQL+CA system.

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APPENDED PUBLICATIONS

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Paper A: AN EXPERIMENTAL ANALYSIS OF ENERGY CONSUMPTION IN MILLING STRATEGIES Authors: Salman Pervaiz, Ibrahim Deiab, Amir Rashid and Cornel Mihai Nicolescu Presented at IEEE International Conference on Computer Systems and Industrial Informatics – ICCSII’12, Sharjah, UAE, Dec 18-20, 2012 Available at: DOI: 10.1109/ICCSII.2012.6454527

PAPER A

http://dx.doi.org/10.1109/ICCSII.2012.6454527

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AN EXPERIMENTAL ANALYSIS OF ENERGY CONSUMPTION IN MILLING STRATEGIES

S. Pervaiz, I. Deiab, A. Rashid and M. Nicolescu Department of Mechanical Engineering, American University of Sharjah, Sharjah, United Arab Emirates Department of Production Engineering, KTH Royal Institute of Technology, Stockholm, Sweden Abstract:

Pocket milling operation is one of the widely used milling operations. CAM packages offer different tool path strategies to execute a machining operation. In the presented work zigzag, constant overlap spiral, parallel spiral and one-way tool path strategies were compared in terms of power and energy consumption for pocket milling of Al 6061 aluminum alloy. All pocketing operations were conducted using 8 mm diameter High Speed Steel (HSS) end milling cutters. Energy utilization was analysed for all tool path strategies. This work aims to develop better understanding towards sustainability concept in core machining phase.

Keywords: Computer aided manufacturing, tool path strategies, Al-6061, pocket milling

INTRODUCTION Pocket milling operation is one of the most frequently used operations carried out on a CNC machining center. Milling operations using 2.5D, 3D and 5D tool paths are being used to manufacture complex features on mechanical components. As mentioned by Held [1] almost 80% of the mechanical components produced by milling can be produced by pocket milling operation. Nowadays different CAM software packages are available in market to simplify the implementation of tool path strategies. Many researchers have focused on the issues related to the tool path strategies. Tlusty et al. [2] revealed the differences in radial engagement of the milling cutter for corner and straight path. The study was conducted for end milling operation. Zhang et al. [3] investigated four tool path strategies with respect to machining accuracy, surface quality and form accuracy. The study revealed that parallel tool path strategy with 2 axes driving mode can

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produced more accurate form surfaces with good surface finish. Chen et al. [4] developed a computer aided planning system for the optimization of tool path for shoe mold. The proposed system resulted in reduction of processing time and increase in efficiency. Korosec et al. [5] conducted a study related to the organization and assessment of free form surface model complexity using self-organized kohonen neural network (SOKN). The results displayed improved mean surface roughness profile (Ra) for reorganized surface. Toh [6] studied the effects of tool path strategies in close reference of different orientations. The study was conducted on offset, raster and single direction raster strategies. The study suggested that in vertical upward milling tool life was best for the work piece orientation of 15°. Monreal et al. [7] studied the influence of tool path strategies on cycle time under high speed conditions. The study proposed a mechanistic approach for the evaluation of cycle time. Experimentation was conducted using zig-zag tool path strategy. The study showed that estimated values of average feed rate were in good agreement with machine tool performance under high speed milling. Ramos et al. [8] evaluated texture, surface roughness and dimensional deviations for different finishing milling strategies. The study was based on radial, raster and 3D offset milling strategies under constant cutting conditions. The study revealed that 3D offset strategy performed better than others. Gologlu et al. [9] utilized Taguchi method for the evaluation of surface roughness in pocket milling. The study was conducted for different tool path strategies under different cutting parameters. The study revealed that feed rate was most crucial parameter for one direction and spiral tool path strategies. Whereas depth of cut was most influential parameter for back and forth tool path strategy. Lopez de Lacalle et al. [10] studied tool path strategies with reference to the dimensional errors. The study was based on the average tool deflection force. The proposed method ology reduced dimensional errors from 30μm to 4μm for three axes and less than 15μm for five axes. Avram et al. [11] proposed a method for the evaluation of energy consumption required by machine tool system for milling of the part. The study utilized geometries suitable for 2.5D milling. The numerical outcomes were also validated experimentally. Shao et al. [12] conducted an experimental study based on face milling operation. The study was focused on cutting conditions and average flank wear. The study used cutting power model to predict tool wear. The simulated results were found in good agreement with experimental results. Kong et al. [13] conducted a study on tool path evaluation to relate the outcomes with environmental sustainability. The study revealed that selection of tool path strategy plays

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an important role towards the amount of energy consumption and greenhouse gas (GHG) emissions. In this paper four milling strategies namely zigzag, constant overlap spiral, parallel spiral and one-way have been analyzed with respect to the power and energy consumption during the machining phase. Power and energy consumption by the machining process is one of the key parameters related to sustainability calculation and environmental burden analysis. This work aims to provide a better understanding the concept of energy consumption during the milling process by comparing different strategies to help select the optimum milling strategy with respect to the power and energy consumption. The study also shows the influence of cutting parameters such as feed rate and cutting speed on energy consumption.

EXPERIMENTAL SETUP Pocketing experiments were conducted under the flood condition on a CNC machining center. Aluminum alloy, Al 6061 was used as a work piece material. The nominal chemical composition of the workpiece material is mentioned in Table I. High Speed Steel (HSS) made end milling cutters of diameter 8 mm with 2 flutes were utilized for this study. Stock of Al 6061 material was available in the form of sheet with dimensions of 3000 x 150 x 10 mm. The pocket dimensions were 100 x 100 x 6 mm in all cases. Pockets were created by using two passes of depth 3 mm each. Figure 1 shows the schematic representation of experimental setup.

Figure 1. Schematic illustration of experimental setup

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Table 1 Nominal chemical composition of Al 6061 (wt. %)

Element % Element %

Si Fe Cu Mn Mg

0.40 – 0.8 0.7

0.15 – 0.40 0.15

0.8 – 1.2

Cr Zi Ti Al

0.04 – 0.35 0.25 0.15

Remaining

The presented study involved 24 sets of experiments using zigzag, constant overlap spiral, parallel spiral and one-way milling strategies.

Table II Summary of Cutting Conditions

Pocketing of Al 6061 blocks

Machine Tool Dimension of pocket (mm) No of passes Lubrication Technique Spindle Speed (rpm) Feed (mm/ min)

CNC Vertical Machining Center 100 x 100 x 6 mm 2 Flood Two levels ( 2000 - 4000) Three levels (100 – 200 - 300)

Milling Strategies

- Zigzag - Constant Overlap

Spiral - Parallel Spiral - One way Spiral

To evaluate each strategy properly two levels of speed and three levels of feed were used. Table II shows the summary of experimental cutting conditions used for the presented study. To reduce the experimental error each measurement was repeated four times and only the average values were reported in the presented work. PS3500 power data logger was used to monitor the power consumed for each machining test. A software

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package named power sight manager was used to perform and validate calculations for energy consumption.

Zigzag b) Constant Overlap

c) Parallel Spiral d) One Way

Figure 2. Schematic illustration of milling strategies

Table III Design of Experiments Test No. Milling Strategy Spindle Speed

(rpm) Feed (mm/

min)

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Test No. Milling Strategy Spindle Speed (rpm)

Feed (mm/ min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Zigzag Zigzag Zigzag Zigzag Zigzag Zigzag Constant OverlapSpiral Constant Overlap Spiral Constant Overlap Spiral Constant Overlap Spiral Constant OverlapSpiral Constant OverlapSpiral Parallel Spiral Parallel Spiral Parallel Spiral Parallel Spiral Parallel Spiral Parallel Spiral One-way One-way One-way One-way One-way One-way

4000 4000 4000 2000 2000 2000 4000 4000 4000 2000 2000 2000 4000 4000 4000 2000 2000 2000 4000 4000 4000 2000 2000 2000

200 300 400 200 300 400 200 300 400 200 300 400 200 300 400 200 300 400 200 300 400 200 300 400

Table III represents the design of experiments used for the presented experimental study. Figure 2 represents the schematic illustration of four milling strategies used in this study.

RESULTS AND DISCUSSION

Energy Consumption Analysis Energy consumed during each pocketing operation has been calculated using the power signal. A software package, Power Sight Manager, was employed for the calculation and monitoring on energy consumption. Figure 3 represents a sample calculation performed for energy consumption (kWh). As shown in the Figure 3 calculations were performed for power consumed during both passes of pocketing operation. Constant overlap spiral strategy for speed 4000 rpm and feed 400 mm/ min was utilized in this sample calculation. The energy consumption in this case was 0.18 kWh. The total machining time for machining the pocket was 10 minutes and 5 seconds.

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Figure 3. Power and energy consumption for 1st and 2nd pass, Constant

Overlap Spiral, 4000 rpm, 400mm/min

The following plotted figures are presented to show the trends of energy consumption obtained for used cutting conditions.

Figure 4. Energy consumption (KWH) of Zigzag milling strategy for

4000 and 2000 rpm

Figure 4 represents energy consumption for zigzag milling strategy. Six experiments were performed using two levels of cutting speed and three levels of feed. Figure 4 shows that for zigzag strategy higher cutting speed consumed more energy than lower cutting speed. Whereas increase in feed resulted in reduction in energy consumption. The lowest case of energy

1st Pass

2nd Pass

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consumption was observed for operation conducted at low speed and high feed values.

Figure 5. Energy consumption (KWH) of Constant overlap spiral

milling strategy for 4000 and 2000 rpm

Figures 5, 6 and 7 represent energy consumption for constant overlap spiral, parallel spiral and one-way milling strategies. It has been observed that energy consumption for all of these milling strategies was more at higher speed and less at higher feed rates. Figure 8 represents energy consumption of different milling strategies at high speed of 4000 rpm.

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Figure 6. Energy consumption (KWH) of Parallel spiral milling strategy for 4000 and 2000 rpm

Figure 7. Energy consumption (KWH) of One-way milling strategy for

4000 and 2000 rpm

Figure 8. Zigzag, Constant overlap spiral, Parallel spiral and One-way

milling strategies, 4000 rpm

It was observed that one-way milling strategy consumed more energy to produce pocket of similar dimension. Constant overlap spiral milling strategy ranked second at lower and medium feed. However at higher feed

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constant overlap spiral strategy consumed almost equal energy to parallel spiral strategy.

Figure 9. Zigzag, Constant overlap spiral, Parallel spiral and One-way

milling strategies, 2000 rpm

Figure 9 represents energy consumption of different milling strategies at lower speed of 2000 rpm. It was observed that at lower feed all of the milling strategies consumed more or less the same energy. However one-way milling strategy again consumed more energy than others at medium and higher feed levels.

Total Machining Time Figure 10 and 11 represent the total machining time required to machine a pocket by each strategy with respect to the energy consumption. This can be observed easily that one way milling strategy consumed more time to machine the desired pocket. More machining time points out at the fact that one way strategy adopted the longest path to machine. Figure 10 represents that highest machining time was consumed by one way milling strategy at low feed rate for high cutting speed. The lowest machining time was obtained for zigzag strategy at high feed rate for high cutting speed. This can also be seen that all of the milling strategies consumed more or less the same amount of energy at low speed and low feed rate.

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Figure 10. Machining time of milling strategies at 4000 rpm

Figure 11. Machining time of milling strategies at 2000 rpm

CONCLUSION Pocketing operation is one of the most widely performed milling operations in mold / die making industry. Computer aided manufacturing packages offer different tool path strategies to perform pocketing operation. Work piece manufactured using these different tool path strategies behave different from each other. This presented work compares four different tool

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path strategies with respect to the energy consumption and machining time during the machining phase.

The conclusions drawn from this study are mentioned below. An observation was related to the feed level of the work piece. Increase in feed rate resulted in less energy consumption. This is due to the fact that higher feed rate increases material removal rate. Higher material removal rate consumes less energy when machining similar volume of material. It establishes that optimum selection of material removal rate in a machining phase can result in efficient energy consumption. Material removal rate can influence environmental burden of machining phase.

It has been observed that one-way milling strategy consumed more energy in almost all cases of pocket milling. However, zigzag milling strategy consumed the least energy among all milling strategies. Constant overlap spiral and parallel spiral strategies were found in between zigzag and one-way milling strategy. The study revealed that one-way milling strategy consumed more energy because a large number of repetitive movements were present in it. It appears that the energy consumed for material removal was almost the same for all tool paths, but the movements and functions of machine tool in idle state make energy consumption different.

ACKNOWLEDGMENT Authors acknowledge the financial support from Emirates foundation, UAE.

REFERENCES M. Held, “On the Computational Geometry of Pocket Machining,” Springer

Berlin, 1991. J. Tlusty, S. Smith, and C. Zamudia, “New NC routines for quality in

milling,” Annual CIRP, 39 (1), pp 517–521, 1990. X. F. Zhang, J. Xie, H. F. Xie, and L. H. Li, “Experimental investigation on

various tool path strategies influencing surface quality and form accuracy of CNC milled complex freeform surface,” Int J Adv Manuf Technol, 59: pp 647–654, 2012.

H. Chen, H. T. Yau, and C. C. Lin, “Computer-aided process planning for NC tool path generation of complex shoe molds,” Int J Adv Manuf Technol, 58: pp 607–619, 2012.

M. Korosec, and J. Kopac, “Improved surface roughness as a result of free-form surface machining using self-organized neural network,” Journal of Materials Processing Technology, 204: pp 94–102, 2008.

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C. K. Toh, “ A study of the effects of cutter path strategies and orientations in milling,” Journal of Materials Processing Technology, 152: pp 346–356, 2004.

M. Monreal, and C. A. Rodriguez, “ Influence of tool path strategy on the cycle time of high - speed milling,” Computer – Aided Design, 35: pp 395 – 401, 2003.

A. M. Ramos, C. Relvas and J. A. Simoes, “The influence of finishing milling strategies on texture, roughness and dimensional deviations on the machining of complex surfaces,” Journal of Materials Processing Technology, 136: pp 209–216, 2003.

C. Gologlu and N. Sakarya, “The effects of cutter path strategies on surface roughness of pocket milling of 1.2738 steel based on Taguchi method,” Journal of Materials Processing Technology, 206: pp 7–15, 2008.

L. N. Lopez de Lacalle, A. Lamikiz, J. A. Sanchez and M. A. Salgado, “Tool path selection based on the minimum deflection cutting forces in the programming of complex surfaces milling,” International Journal of Machine Tools & Manufacture, , 47: pp 388 – 400, 2007.

O. L. Avram and P. Xirouchakis, “Evaluating the use phase energy requirement of machine tool system,” Journal of Cleaner production, 19: pp 699 – 711, 2011.

H. Shao, H. L. Wang, and X. M. Zao, “A cutting power model for tool wear monitoring in milling,” International Journal of Machine Tools & Manufacture, , 44: pp 1503 – 1509, 2004.

D. Kong, S. Choi, Y. Yasui, S. Pavanaskar, D. Dornfeld, and P. Wright, “Software – based tool path evaluation for environmental sustainability,” Journal of Manufacturing Systems , 30: pp 241 – 247, 2011.

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Paper B: PERFORMANCE EVALUATION OF TIALN- PVD COATED INSERTS FOR MACHINING TI-6AL-4V UNDER DIFFERENT COOLING STRATEGIES

Authors: Salman Pervaiz, Ibrahim Deiab, Basil Darras, Amir Rashid and Cornel Mihai Nicolescu

Published in: Advanced Materials Research, Advanced Materials Research Vol. 685 (2013) pp. 68-75.

Presented in: 3rd International Conference on Advanced Materials Research (ICAMR 2013), Dubai, UAE, Jan 19 -20, 2013. DOI: 10.4028/www.scientific.net/AMR.685.68

PAPER B

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PERFORMANCE EVALUATION OF TIALN- PVD COATED INSERTS FOR MACHINING TI-6AL-4V UNDER DIFFERENT COOLING STRATEGIES S. Pervaiz, I. Deiab, B. Darras, A. Rashid and M. Nicolescu Department of Mechanical Engineering, American University of Sharjah, Sharjah, United Arab Emirates Department of Production Engineering, KTH Royal Institute of Technology, Stockholm, Sweden

Keywords: Wear mechanisms, Cooling strategies, Titanium alloys. Abstract. This paper presents an experimental study of machining Ti-6Al-4V under turning operation. All machining tests were conducted under dry, mist and flood cooling approaches by using a single layer TiAlN coated carbide cutting inserts. All cutting experiments were conducted using high and low levels of cutting speeds and feed rates. The study compared surface finish of machined surface and flank wear at cutting edge under dry, mist and flood cooling approaches. Scanning electron microscopy was utilized to investigate the flank wear at cutting edge under various cooling approaches and cutting conditions. Investigation revealed that TiAlN coated carbides performed comparatively better at higher cutting speed.

Introduction High performance alloys, such as titanium and nickel alloys are used in aerospace, automotive, defense, dental and orthopedic sectors because of their ability to work at higher operating temperature and high strength to weight ratio. These high performance alloys are used in manufacturing of gas turbines, space crafts, rocket engines, nuclear reactors, submarines, petrochemicals and glass industries [1, 2]. These alloys are termed as “Difficult-to-cut materials” because of their low machinability rating. The quality of surfaces produced using specific material removal process is

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characterized by measuring surface roughness and surface integrity. High value of surface roughness is critical for fatigue life of any engineering component [3]. Many researchers have conducted studies towards the machinability of high performance alloys. Nabhani [4] performed quick stop machining tests to examine the tool wear rate using PCBN (AMBORITE) and PCD (SYNDITE). The study revealed that PCD (SYNDITE) out classed PCBN (AMBORITE). Ginting et al. 5] conducted an experimental study on Ti-6242S titanium alloy for end milling process using uncoated cemented carbide tools under dry condition. Results revealed that good surface finish was obtained under high cutting speed for feed level 0.15 mm/tooth. Experimental results were validated using numerical simulation. Zoya et al. [6] conducted turning experiments on α-β titanium alloys using Cubic Boron Nitride (CBN) cutting tools. The study recommends cutting speed range of 180-220 m / min for good surface finish. Temperature of 700 C° was recommended as a tool failure criteria. Ozel et al. [7] executed experimental study for machining Ti6Al4V to examine force and tool wear rate using uncoated, single layer TiAlN coated, and multi-layer TiAlN + cBN coated carbide inserts. This study also proposed a finite element simulation code to predict chip formation, forces, temperatures and tool wear. Simulation shows good agreement with actual orthogonal cutting measurements. Venugopal et al. [8] performed an experimental investigation to analyse tool life of microcrystalline uncoated carbide inserts under dry, wet and cryogenic conditions for machining of titanium alloy. Study revealed that cryogenic cooling results in reduced crater and flank tool wear. Venugopal et al. [9] in another study analysed the growth of tool wear in turning of titanium alloy under cryogenic machining. Scanning electron microscope results show deposition of titanium chip material on the inserts. Su et al. [10] conducted an experimental investigation on different cooling strategies and respective tool wear rate in high speed end milling of titanium alloys. Study utilized dry, flood coolant, nitrogen-oil mist, compressed cold nitrogen gas (CCNG) at 0, and−10 ◦C, and compressed cold nitrogen gas and oil mist (CCNGOM) as the cooling strategies. Study showed that compressed cold nitrogen gas and oil mist (CCNGOM) cooling strategy provides longer tool life. Yildiz et al. [11] presented a review on the application methods of cryogenic coolant and its impact on cutting tool and workpiece material properties. The review states that cryogenic coolant is useful in controlling the cutting temperature at cutting zone, and gives extended tool life with good surface finish. Sun et al. [12] investigated the machining performance of titanium alloy using cryogenic compressed air. The study revealed reduction in wear rate and built up edge (BUE)

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formation. Bermingham et al. [13] studied machining of titanium alloy using cryogenic cooling. They used constant cutting speed and material removal rate (MRR) with variable feed rate and depth of cut. The study revealed decrease in cutting force with the application of cryogenic coolant. Micrographs of chips illustrated less deformation and heat generation under low feed and high depth of cut as compared to high feed and low depth of cut. In this paper, Ti-6Al-4V was machined using single coated TiAlN- PVD coated carbide inserts. Experimentation was conducted using constant depth of cut under two levels of cutting speeds and feed rates. The performance of TiAlN coated tool was investigated under dry, mist and flood cutting environments. Surface roughness trends were observed and wear mechanisms at early stage machining were observed using scanning electron microscope. Comparatively fair results were observed at higher cutting speeds.

Experimental Setup Machining experiments were conducted under the dry, mist and flood cooling conditions on a CNC Turning Center. Mitutoyo Roughness Tester SJ 201P has been utilized for the measurement of surface roughness of generated surface. To minimize experimental error each surface roughness measurement was repeated three times and only the average values are reported in this study. A tool maker microscope was used to evaluate flank wear at the cutting edge. Scanning Electron Microscope (SEM), Philips FEI XL30, was utilized to investigate the wear mechanisms at flank face. Figure 1 shows the schematic illustration of experimental setup.

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Fig. 1 Schematic illustration of experimental setup

In order to investigate the surface roughness behavior with respect to machining length, total length of workpiece was divided into four zones A, B, C and D as shown in Figure 2. Table 2 shows the cutting parameters utilized in this presented study.

Fig. 2 Total machining length with Zones A, B, C and D

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Table 1 Machining Parameters

Machining Parameters

Cutting Insert

TCMT 16 T3 04-MM 1105 Triangular shape Insert thickness = 0.1563” Nose radius = 0.0157” Coating : TiAlN PVD Rake : Positive

Depth of cut (mm) Cutting Speed (m/min) Feed (mm/ rev) Machining Length (mm)

0.8 mm 30 - 90 0.1 – 0.2 120

Machining Environment Dry , Mist and Flood

Results and Discussion

Surface Roughness Analysis Surface finish was measured for all of the cutting tests. The total machining length of 120 mm was further divided into four zones to illustrate the trend of surface roughness. TiAlN –PVD coated inserts were used at high and low levels of cutting speeds and feed rates. Figure 3 (a) and (b) shows the roughness finish obtained for TiAlN PVD coated tools under 30 m/ min of cutting speed and 0.1 - 0.2 mm/ rev of feed rates. Figure 3a represents the low and high level of cutting speeds at low feed. At higher cutting speed of 90 m/ min the surface roughness increases and then decreases uniformly. This shows that sharp edge of tool produces high surface roughness but after some time surface finish improves.

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(a)

(b)

Fig. 3 Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.1 mm/ rev (b) Vc = 90 m/min, f = 0.1 mm/ rev Figure 4 (a) and (b) shows the roughness finish obtained for TiAlN PVD coated tools under 90 m/ min of cutting speed and 0.1 - 0.2 mm/ rev of feed rates. Figure 4a and 4b show that feed rate is directly proportional to surface roughness (Ra). Surface roughness values at feed of 0.2 mm/ rev were higher than the values obtained for 0.1 mm/ rev. Figure 4b shows comparatively better results at high cutting speed and feed rate. An important finding was that surface roughness was lowest for mist environment when working at low feed rate. Dry environment provided lower surface roughness at higher feed rate. Seah et al. [20] also observed that flood cooling enhanced wear rate for machining steels. They concluded that crater wear was shifted near the cutting edge by the application of flood coolant.

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(a)

(b)

Fig. 4 Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.2 mm/ rev (b) Vc = 90 m/min, f = 0.2 mm/ rev Flank Wear Measurement Maximum value of flank wear at cutting edges was measured by using tool maker microscope. Flank wear is affected by cutting speed and feed rate. However, literature [14] suggests that wear rate is more sensitive to cutting speed. Figure 5 represents flank wear at different cutting conditions and environments. The highest value of flank wear was measured at high cutting speed of 90 m/ min and high feed of 0.2 mm/ rev under dry cutting condition. It indicates the presence of high cutting temperature. High cutting temperature has influence on tool wear, surface integrity, tool life, chip formation mechanism and thermal deformation of the tool [15]. William and Tabor [16] examined role of cutting fluids in metal cutting. They discussed friction mechanism at tool chip interface. The study proposed a formulation of interconnecting capillaries at the interface.

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Fig. 5 Maximum flank wear measurement

The combination of high temperature and chemical affinity of titanium alloys initiates wear at very high rate. Wear mechanisms such as adhesion, abrasion and diffusion depends on the tool and workpiece materials and the cutting temperature. Titanium alloys have low thermal conductivity as a result temperature remains at the cutting edge and the region nearby. To lower cutting temperature at cutting edge different cooling strategies can play a vital role. Figure 5 showed that mist and flood cooling performed equally good job at higher cutting speeds. It also reveals the potential of mist as a cooling strategy because of the pressure for adopting environmental friendly machining techniques.

Early Stage Wear Mechanisms At Low Cutting Speed

As the literature suggest [14, 18] that due to the complex interaction of tool, workpiece and chip removal high temperature is produced in the cutting zone. At this high temperature different wear mechanisms initiates can start at very faster rate. Mainly these wear mechanisms are adhesion, abrasion, diffusion, coating delamination, notch and microchipping etc. These wear mechanism were present on both faces of the tool. On rake face it is present in the form of crater wear and on flank face it is known as flank wear. Scanning electron microscopy was utilized in order to investigate the tool wear mechanism at early stage of machining Ti6Al4V. Figure 6 represents the SEM micrograph of tool wear at low cutting speed of 30 m/min under dry, mist and flood conditions. Micrograph for 30m/ min and feed 0.1 mm/

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rev showed coating delamination, adhesive wear and abrasive wear as major wear mechanisms for all cutting environments. However with increase in feed rate adhesive and abrasive wears were magnified as shown in the micrograph for 30 m/ min and feed 0.2 mm/ rev. The wear mechanisms for Ti6Al4V were found in accordance with the literature [18-19].

a) Dry, vc = 30 m/min, f = 0.1 mm / rev b) Mist, Vc = 30 m/min, f = 0.1 mm / rev

c) Flood, vc = 30 m/min, f = 0.1 mm / rev

d) Dry, vc = 30 m/min, f = 0.2 mm / rev e) Mist, vc = 30 m/min, f = 0.2 mm / rev

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f) Flood, vc = 30 m/min, f = 0.2 mm / rev

Fig.6 SEM images of wear at coated carbide tool under dry conditions The micrographs in Figure 6 make it clear that at low cutting speed dry environment was worst. Dry cutting conditions provided friendly environment for the initiation of adhesive tool wear as workpiece material found attached with tool. However , mist and flood environments perfomed equally good at low cutting speed.

Early Stage Wear Mechanisms At High Cutting Speed Figure 7 represents the SEM micrograph of tool wear at high cutting speed of 90 m/min under dry, mist and flood conditions. At high cutting speed coating delamination, adhesion and abrasion were observed in all cutting environments. Micrographs for higher cutting speed of 90 m/ min shows worst case in dry cutting condition at high feed of 0.2 mm/ rev. In dry cutting condition coating delamination, adhesion and abrasion mechansims were clearly observed. This points out at high value of cutting temperature generated in the cutting zone. The absence of cutting flood was the main reason for high value of cutting temperature. The best case was observed for flood cooling at high cutting speed of 90 m/min and low feed of 0.1 mm/ rev. The micrograph for flood cooling at 90 m/min and 0.1 mm/ rev shows the cutting edge in good shape without any adhesion. However, abrasion and thermal cracks were found at flank face. Mist environment shows promising results at higher cutting speed. Micro-chipping and abrasion were mainly observed at the cutting edge under mist conditions.

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a) Dry, vc = 90 m/min, f = 0.1 mm / rev b) Mist, Vc = 90 m/min, f = 0.1 mm / rev

c) Flood, vc = 90 m/min, f = 0.1 mm / rev d) Dry, vc =30 m/min, f = 0.2 mm / rev

e) Mist, vc = 30 m/min, f = 0.2 mm / rev f) Flood, vc = 30 m/min, f = 0.2 mm / rev

Fig. 7 SEM images of wear at coated carbide tool under dry conditions

Conclusions The consulsion drawn from this presented study of machining titanium alloy Ti6Al4V using TiAlN- PVD coated inserts are as follows;

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1. Surface roughness trends were observed in total accordance with

literature. Surface roughness values obtained during experiments were high at higher feed rate of 0.2 mm/ rev for both levels of cutting speeds. Although surface roughness was more sensitive to feed rate.

2. It was observed that TiAlN-PVD coated tools performed better at higher cutting speed of 90 m/ min and low feed of 0.1 mm/ rev under flood cooling. The worst case was observed at cutting speed of 90 m/min and 0.2 mm/ rev feed under dry conditions. This points out at the key role of cutting fluids for TiAlN-PVD coated tools .

3. The dominant wear mechanisms were coating delamination, adhesion and abrasion. Coating delamination was observed in the initial phase of machining. Adhesion and abrasion were observed as a major source of wear propagation at flank face.

4. Adhesion was more evident under dry cutting environment. Whereas less magnitude of adhesion was observed under mist and flood cooling techniques.

5. Mist environment shows promising results towards tool wear. Under the concept of green manufacturing, mist cooling technique showed potential of replacing flood cooling technique.

AKNOWLEDGEMENTS

Authors acknowledge the financial support from Emirates foundation, UAE, National Research Foundation and American university of Sharjah Research office. References [1] Ezugwu, E.O., 2005, Key improvements in the machining of

difficult-to-cut aerospace superalloys, International Journal of Machine Tools & Manufacture.45; 1353–1367.

[2] Ezugwu, E. O., Wanga Z. M., Machadop, A. R., 1998 ,The machinabilityof nickel – based alloys: a review, Journal of Material Processing Technology, 86 (1-3); 1 – 16.

[3] G. Weiping, X. Honglu, L. Jun and, Y. Zhufeng, “Effects of drilling process on fatigue life of open holes,” Tsinghua Science and Technology, vol. 14, pp. 54-57, 2009

[4] Nabhani, F., 2001, Machining of aerospace titanium alloys,

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Robotics and Computer Integrated Manufacturing, 77; 99-106. [5] Ginting, A., Nouari, M., 2006, Experimental and numerical studies

on the performance of alloyed carbide tool in dry milling of aerospace material, International Journal of Machine Tools & Manufacture.46; 758–768.

[6] Zoya, Z.A., Krishnamurthy, R., 2000, The performance of CBN tools in the machining of titanium alloys, Journal of Materials Processing Technology. 100; 80–86.

[7] Ozel, T., Sima, M., Srivastava, A.K., Kaftanoglu, B., 2010, Investigations on the effects of multi-layered coated inserts in machining Ti–6Al–4V alloy with experiments and finite element simulations, CIRP Annals - Manufacturing Technology. 59: 77-82.

[8] Venugopal, K.A., Paul, S., Chattopadhyay, A.B., 2007, Tool wear in cryogenic turning of Ti-6Al-4V alloy, Cryogenics, 47; 12-18.

[9] Venugopal, K.A., Paul, S., Chattopadhyay, A.B., 2007, Growth of tool wear in turning of Ti-6Al-4V alloy under cryogenic cooling, Wear , 262; 1071-1078.

[10] Su, Y., He, N., Li, L., Li, X.L., 2006, An experimental investigation of effects of cooling/lubrication conditions on tool wear in high-speed end milling of Ti-6Al-4V, Wear , 261; 760-766.

[11] Yildiz, Y., Nalbant, M., 2008, A review of cryogenic cooling in machining processes, International Journal of Machine Tools & Manufacture, 48 ; 947–964.

[12] Sun, S., Brandt, M., Dargusch, M.S., 2010, Machining Ti–6Al–4V alloy with cryogenic compressed air cooling, International Journal of Machine Tools & Manufacture, 50; 933–942.

[13] Bermingham,M.J., Kirsch, J., Sun, S., Palanisamy, S., Dargusch, M.S., 2011, New observations on tool life, cutting forces and chip morphology in cryogenic machining Ti-6Al-4V, International Journal of Machine Tools & Manufacture, 51; 500–511.

[14] Shaw, M. C., Metal Cutting Principles, 2nd edition, Oxford University Press, 2005.

[15] Abukhshim, N. A., Mativenga, P.T., and Sheikh, M. A., , Heat generation and temperature prediction in metal cutting: A review and implications for high speed machining, International Journal of Machine Tools & Manufacture, 46; 782–800.

[16] Willaiams, J. A., and Tabor, D., 1977 ,The role of lubricants in machining, Wear, 43; 275 – 292.

[17] Trigger, K. J. and Chao, B.T., “Mechanism of crater wear of cemented carbide tools,” Transactions of ASME, Vol. 78, No. 5, 1956.

[18] Jawaid, A., Che-Haron, C.H. and Abdullah, A., “Tool wear

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characteristics in turning of Titanium Alloy Ti - 6246,” Journal of Materials Processing Technology, Vol. 92-93, pp. 329-334, 1999.

[19] Corduan, N., Himbart, T., Poulachon, G., Dessoly, M., Lambertin, M., Vigneau, J. and Payoux, B., “ Wear Mechanisms of New Tool Materials for Ti-6AI-4V High Performance Machining,” CIRP Annals - Manufacturing Technology, Vol. 52, Issue 1, pp. 73-76, 2003

[20] Seah, K. H. W., Li, X., and Lee, K. S., “The effect of applying coolant on tool wear in metal machining,” Journal of Material Processing and Technology, Vol 48, pp 495 -501, 1995.

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Paper C: POWER CONSUMPTION AND TOOL WEAR ASSESSMENT WHEN MACHINING TITANIUM ALLOYS

Authors: Salman Pervaiz, Ibrahim Deiab, Basil Darras

Published in: International Journal of Precision Engineering and Manufacturing, Vol. 14, No. 6, pp. 925-936, 2013. DOI: 10.1007/s12541-013-0122-y

PAPER C

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POWER CONSUMPTION AND TOOL WEAR ASSESSMENT WHEN MACHINING TITANIUM ALLOYS S. Pervaiz 1, 2, I. Deiab 1, B. Darras 1, 1 Department of Mechanical Engineering, American University of Sharjah, Sharjah, United Arab Emirates 2 Department of Production Engineering, KTH Royal Institute of Technology, Stockholm, Sweden

Keywords: Machinability, Wear mechanisms, Energy consumption, Titanium. Abstract. Titanium alloys are of interest for aerospace industries due to their high strength to weight ratio, outstanding corrosion and erosion properties and ability to operate at higher temperature. They are classified as difficult to cut materials because of their low thermal conductivity, high chemical reactivity and high strength at elevated temperature. The machinability rating of titanium alloys is low compared to other materials from many aspects. This study focuses on studying the cutting tool wear and power consumption when machining Titanium alloys under different cutting conditions. Design of experiments were used to develop a test matrix that cover the range of cutting conditions recommended for machining titanium alloys. Cutting forces, power consumption, tool wear and surface roughness were measured and analyzed. Tool wear mechanisms were also studied using scanning electron microscopy.

1. Introduction

Titanium alloys are being used extensively for the manufacturing of air-crafts, aero-engines, biomedical equipment and chemical processing units. Titanium alloys exhibit very good strength at high temperature and low density. Titanium alloys offer good corrosion resistance making them suitable for marine industry [1]. However, titanium alloys show poor machinability rating. Main causes of poor machinability are low thermal

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conductivity, high strength at elevated temperature, high chemical reactivity and low young’s modulus [2].

Concept of environmental conscious machining has been spread all over the world. Minimizing the power consumption in the machining phase of a product can save cost and reduce the global warming potential associated with machining. More energy usage in machining phase means more CO2 equivalent emissions in environment. Several researchers have worked in the area of machining to minimize power consumption. Gutowski et al. [3] performed an environmental examination of a machining process. The study revealed that out of total energy very less amount of energy is required for cutting. Munoz et al. [4] developed an analytical approach to demonstrate the environmental impacts of the machining operations. The study was based on power consumption, cutting mechanics and coolant flow. This study exposed that power consumption utilized by a machining process depends upon complexity involved in geometry, material and coolant selection. Kordonowy [5] performed energy calculations for various machine tools. This work was based on injection molding, manual milling, automated milling and automated lathe machine. The study presented a complete examination of power consumption utilized in different phases of machine. Drake et al. [6] suggested a framework to describe power consumption in machine tools. The framework suggested a six steps process to characterize energy consumption. The study concluded that most of the energy consumption was used in machine controller. Study shows that 35% of the total energy was used by spindle. Diaz et al. [7] conducted an analysis of machine tool to develop more efficient energy consumption strategy. The study developed a method of using specific energy as a function of process rate. The proposed method provides accurate energy consumption without actually measuring power demand. In another study Diaz et al. [8] presented design and operation strategies to reduce energy consumption. The study was conducted using kinetic energy recovery system (KERS), process parameter selection strategy and web-based energy estimation tool. The study exposed that KERS has potential of saving 25% of energy. Kara et al. [9] developed an empirical model to establish the relationship between energy consumption and process parameters. This model was verified on different milling and turning machines. The proposed model predicts energy consumption with an accuracy of 90%.

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Jawaid et al. [10] conducted an experimental investigation to evaluate the machinability of Ti-6246 alloy under dry condition. Experimentation was conducted at constant depth of cut with four different levels of cutting speed using uncoated cemented carbide tools. The study revealed that finer grain size tools performed better than the tools with coarser grain size. Abrasion wear mechanism was observed at the flank face of the cutting insert. Corduan et al. [11] examined the wear mechanisms of PCD (polycrystalline diamond), CBN (carbon boron nitride) and TiB2 coated carbides for the machining of titanium-based alloys. The study pointed out that the PCD tools performed best at cutting speed of 150 m/ min. CBN tools were suitable for finishing cutting conditions. Whereas, TiB2 coated inserts worked well under 100 m/min cutting speed. It has been observed that adhesion and diffusion were the dominant wear mechanisms. Elmagrabi et al. [12] conducted an experimental study to investigate the performance of the coated and uncoated carbide tools under dry conditions. The experimentation was focused on slot milling of Ti 6Al 4V. The study revealed that PVD coated carbide tool has high life. Surface roughness was more dependent on feed rate and depth of cut. This paper presents an experimental study and analysis to evaluate the performance of coated and uncoated carbide inserts under different machining environments. In literature there is very limited research work available with respect to the power consumption in machining Titanium alloys. In the presented study; performance of cutting inserts based on the tool wear were examined carefully with respect to the energy consumption during each machining test. This contribution of energy consumption can be a useful source of information for sustainability computations.

2. Experimental Setup Machining tests were carried out on a CNC Turning Center. Titanium alloy Ti 6Al 4V was selected as a workpiece material. The nominal chemical composition of the workpiece material is given in Table 1. Experiments were conducted utilizing two types of cutting inserts. The nominal specifications for these inserts are given in Table 2. Experiments were performed under the conditions shown in Table 3 by utilizing a constant depth of cut and length to machine. Dimensions of raw material work piece are 300 mm in length and 95 mm in diameter.

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Table 1 Nominal chemical composition of Ti 6Al 4V (wt. %)

Element % Element %

C Al Fe V Ti

< 0.08 5.5 – 6.75

< 0.4 3.5 – 4.5 Balance

H N O

< 0.05 < 0.01

< 0.2

Table 2 Cutting Inserts properties

Cutting Inserts

1. Coated Carbide - TCMT 16 T3 04-MM 1105 • The substrate consists of a hard fine grained WC with 6% Co

for high hot hardness and good resistance against plastic deformation.

• The new thin PVD TiAlN coating with excellent adhesion, also on sharp edges, guarantees toughness, even flank wear and outstanding performance in heat resistant super alloys.

2. Uncoated Carbide - TCMT 16 T3 04-KM H13A • Combines good abrasive wear resistance and toughness for m

edium to rough turning of heat resistance steel and titanium alloys.

Mitutoyo Roughness Tester SJ 201P was used for the measurement of surface roughness. To minimize the experimental error each measurement of surface roughness was repeated four times and only the average values were reported. Tool flank wear was measured with a toolmaker microscope. Scanning electron microscopy was utilized to study the major wear mechanisms. Kistler Multi Channel Dynamometer was utilized for measuring the cutting forces generated during drilling operations. PS3500 power data logger has been used to capture the power utilized during each cutting test. Figure 1 shows schematic representation of the experimental setup.

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Fig. 1 Experimental Setup

Table 3 Cutting Conditions


Depth of cut (mm) Cutting Speed (m/min) Feed (mm/ rev) Length to machine (mm)

0.8 mm Constant Three levels ( 30 – 60 - 90) Two levels (0.1 – 0.2) 120

Machining Environment - Dry - Mist - Flood

3. Results and Discussion

3.1 Surface Roughness Analysis Surface roughness values were recorded for all of the turning tests performed by using both coated and uncoated inserts. Surface roughness has been plotted for both cases as shown in following figures below.

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Figures 2 and 3 compare the roughness values for coated and uncoated tools at feed 0.1 mm/ rev under dry, mist and flood environments.

Fig. 2 Surface roughness, f = 0.1 mm/ rev, Coated tools under dry, mist and

flood environments.

Fig. 3 Surface roughness, f = 0.1 mm/ rev, Uncoated tools under dry, mist

and flood environments.

At low cutting speed of 30 m/ min under dry conditions observed roughness (Ra) values for both coated and uncoated inserts were approximately same. However at cutting speed of 30 m/ min uncoated insert gave better surface quality for both mist and flood conditions. At cutting speed of 60 m/ min for dry condition both inserts performed in a similar manner. But at 60 m/

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min coated inserts provided more roughness than the uncoated inserts for both mist and flood environments. At 60 m/ min the lowest value of surface roughness was obtained using uncoated insert under mist condition. Similar trends but with better surface finish were observed at 90 m/min for both coated and uncoated inserts. previous research, [18, 19, 23] ,points out that at higher cutting speed, cutting force decreases and cutting temperature increases. Uncoated carbides cannot withstand high temperature and results in rapid adhesion and diffusion wear. TiAlN coating resists heat to extend tool life by lowering coefficient of friction.

Fig. 4 Surface roughness, f = 0.2 mm/ rev, Coated tools under dry, mist and

flood environments.

Figure 4 and 5 represents the comparison of roughness (Ra) values for coated and uncoated tool at higher feed of 0.2 mm/ rev for dry, mist and flood environments. Similar trend was noted before [21-22]. For dry condition coated tool performed better than uncoated tool for all cutting speeds of 30, 60 and 90m/ min. For mist and flood environments coated tool gave high roughness values at low cutting speed i.e. 30 m/ min. Coated inserts gave good result at cutting speeds of 60 and 90 m/ min. Uncoated inserts performed better than coated inserts at low cutting speed for both mist and flood condition. It can be seen that coated inserts provides comparatively fair results at higher cutting speeds this might be because of the wear and heat resistant nature of TiAlN coating. However uncoated inserts were found superior for low cutting speeds because of relatively low cutting temperature at low cutting speed. Better surface finish was obtained under dry condition for both coated and uncoated inserts. Similar trends were onserved by Seah et

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al. [25] when machining steel alloys. Their study revealed that coolant shifted crater wear towards the cutting edge.

Fig. 5 Surface roughness, f = 0.2 mm/ rev, Uncoated tools under dry, mist and flood environments.

3.1.1 Statistical Analysis of Surface Roughness

The surface roughness data was also analysed by using analysis of variance (ANOVA) method. ANOVA technique is very useful statistical method for efficient decision making. ANOVA divides total variation into responsible sources. The study utilized Design Expert 8 to analyse surface roughness data statistically. ANOVA was implemented to determine the percentage contribution of process parameters on surface roughness. Outcome of ANOVA results in F value that makes results significant from each other. Higher F value shows that variation in that parameter causes significant change in the response parameter [1, 2]. Table 4 shows that except feed rate none of the factors contributed considerably towards the surface roughness. The model F value of 140.06 implies that the model is significant. In the present study A, B, C, D, AB, AC, AD, CD, ABC, ABD and ABCD were significant model terms.

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Table 4 Results of ANOVA for surface roughness

Cutting Parameters

Sum of Squares df Mean

Square F - Test Contribution (%)

Model 149.61 35 4.27 140.06 98.55 A-Cutting Spe

ed 1.12 2 0.56 18.35 0.74 B-Feed 142.61 1 142.61 4672.83 93.94

C-Coolant 0.40 2 0.20 6.47 0.26 D-Cutting

Tool Material 0.55 1 0.55 18.13 0.36 AB 0.35 2 0.18 5.78 0.23 AC 0.62 4 0.16 5.11 0.41 AD 1.06 2 0.53 17.38 0.70 BC 0.18 2 0.09 3.00 0.12 BD 0.02 1 0.02 0.75 0.02 CD 0.21 2 0.10 3.43 0.14

ABC 0.31 4 0.08 2.50 0.20 ABD 0.86 2 0.43 14.07 0.57 ACD 0.11 4 0.03 0.89 0.07 BCD 0.07 2 0.03 1.12 0.05

ABCD 0.39 4 0.10 3.20 0.26

Error 2.20 72 0.03 -- 1.45 Total 151.81 107 -- -- 100.00

Figure 6 shows different statistical graphs obtained from surface roughness data. Half-normal probability plot is a powerful graphical tool that points out at important factors and their interactions [Benski, 1989]. As shown in Figure 6 (a), a list of effects and their interactions were reported based on their magnitudes. Insignificant factors appear on or close to the near zero line. It was observed that factor B (feed rate) is the most important factor for the presented study. The normal probability plot graphically represents normal distribution of data set. Figure 6 (b) represents normal distribution of surface roughness data set. A straight line shows data followed normal distribution approximately. Residual vs. run plot is a special scatter plot which shows different drifts in data. In residual vs. run plot each residual is plotted against experimental run order. Figure 6 (c) shows residual values of

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surface roughness data against experimental runs. Residual data was found randomly distributed over experimental runs.

Fig. 6 Statistical analysis of surface roughness (a) Half-normal plot (b)

Normal plot of residuals (c) Residuals vs. run

3.2. Cutting Force Analysis The cutting forces significantly control cutting temperature, tool wear, tool life, surface integrity and distortions in workpiece, fixtures and cutting tool due to instabilities in machining dynamics. In figure 7, It was observed as a general trend that cutting forces decrease with increasing cutting speed up to a certain range, and after passing that range cutting force again starts increasing. Figures 7 (a) and (b) represent that at low feed of 0.1 mm/ rev cutting force reduced slightly for both uncoated and coated tools when cutting speed was increased from 30 m/ min to 60 m/ min. However cutting force increased again when cutting speed was raised from 60 m/ min to 90 m/min. This variation in cutting force is attributed with built up edge (BUE) phenomenon and cutting temperature behavior. Previous studies [19-20]

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revealed that temperature in cutting zone is directly related to the cutting speed. Higher cutting velocities generate elevated temperatures in cutting zone that enhances thermal softening of workpiece material. Built up edge (BUE) formation is based on cutting conditions and combination of workpiece and cutting tool material. Reduction in cutting forces points out that built up edge increases effective rake angle resulting in lower cutting forces. Fang and Wu [26] also observed reduction in cutting forces with increase in cutting speed for machining Ti6Al4V and Inconel 718. Other studies [18, 23] also revealed that cutting force decreases with increase in cutting speed. The present study also revealed that cutting force was lower, when cutting speed was close to the range of 60 m/ min. Komanduri and Reed [17] also found that uncoated carbides exhibit excessive tool wear above 60 m/ min. The cutting force increased at higher cutting speed of 90 m/min because at higher cutting speed there is no built up edge (BUE) formation. The cutting force measurement in figures 7 (a) and (b) also revealed that uncoated inserts have generated less cutting forces than the coated inserts at low cutting speed of 30 m/ min and feed of 0.1 mm/ rev. This higher magnitude of cutting force points out at potential of high tool wear in coated inserts at low cutting speeds. However less cutting forces were observed for coated inserts at cutting speeds of 60 – 90 m/ min. This shows that coated inserts performed comparatively better at higher cutting speeds. Adhesion and abrasion mechanisms were found at lower cutting speeds. Higher cutting speeds and elevated cutting temperature favors diffusion to be a dominant tool wear mechanism. Diffusion wear limits the performance of uncoated carbides at higher cutting speeds. Slightly higher cutting force was observed in uncoated tool at cutting speed of 90 m/min indicating unstable wear rate. Similar behavior was observed in case of coated inserts. Higher cutting speeds can produce elevated temperature at cutting zone but TiAlN coating is heat and wear resistant in nature. Groover [24] also commented on the relationship of cutting force with shear area in the cutting zone. Low cutting forces for coated carbide tools point at less shear area in cutting zone. Less shear area means high value of shear plane angle that is favorable for easy machining because of less power consumption and temperature in cutting zone. Figure 7 (a), (b), (c) and (d) states that increase in feed results in higher cutting forces for both uncoated and coated carbides. Figure 7 (a) and (b) shows cutting force for coated and uncoated inserts under dry, mist and flood conditions. At low feed of 0.1 mm/ rev and cutting speed of 30 m/ min forces produced in dry conditions were lower than forces obtained in mist and flood conditions. Similar cutting force

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behavior was observed for flood cooling at feed of 0.2 mm/ rev. The finding related to negative impact of flood cooling was against the common belief of coolant application. A possible explanation of this finding could be that by introducing the coolants in cutting zone reduces the temperature of cutting tool as well as workpiece material. This cooling effect of workpiece material reduces thermal softening tendency. As a result higher cutting forces are generated in order to cut material. Seah et al. [25] also found similar behavior of flood cooling when machining AISI 1045 and AISI 4340 steel grades using uncoated tungsten carbides. His study revealed that flood cooling favors crater wear to grow near the cutting tip that makes it much weaker. Thomas et al. [28] also found similar results of increasing cutting force under MQL for machining Wasaloy.

Fig. 7 Cutting force at different cutting speeds under dry, mist and flood

cooling strategies (a) Uncoated inserts, f = 0.1 mm/ rev (b) Coated inserts, f = 0.1 mm/ rev (c) Uncoated inserts, f = 0.2 mm/ rev, and (d) Coated

inserts, f = 0.2 mm/ rev.

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Figure 7 (a) and (b) shows that coated inserts generated less cutting force at higher cutting speeds. The lowest cutting force was observed with coated insert under dry condition at 60 m/ min cutting speed. Highest cutting force was observed with coated insert under mist condition at 30 m/ min cutting speed. It concludes that TiAlN coated inserts were better for higher cutting speed selection. Higher cutting speed generates high cutting temperature in the cutting zone that allows diffusion wear mechanism to start rapidly. TiAlN coated tools performed better at higher cutting speeds due to the formation of highly dense Al2O3 protective layer. This protective layer prevents diffusion wear mechanism at higher cutting speeds. Figure 7 (c) and (d) depicts the generated cutting forces at constant feed of 0.2 mm/ rev for three different levels of cutting speed. It states that uncoated inserts generated more cutting force at all levels of cutting speeds. However, performance was equally good for uncoated at 90 m/ min cutting speed under flood environment when compared with coated inserts. At higher feed level of 0.2 mm/ rev the lowest cutting force was observed with coated insert under dry condition at 60 m/ min cutting speed. The general trend of cutting force was that it decreased with increasing cutting speed in both uncoated and coated inserts, whereas it increased with further increase in cutting speed. 3.3. Power and Energy Consumption Power and energy consumption has also been observed for both coated and uncoated inserts. Energy consumption was compared for both inserts to examine which tool is more energy efficient. Figure 8 displays a sample calculation of power and energy consumed during a turning experiment for specific cutting conditions under dry condition. Based on the sample calculation shown in figure 8 energy consumption has been computed for all of the cutting tests. Figure 8 also shows that power demand increases with increase in cutting speed but machine time reduces due to variation in workpiece diameter. For the current study, the cutting speeds were kept constant by varying spindle speed, RPM, to compensate for the reduction in workpiece diameter. The change in RPM changed the machining time for different cutting speeds. In present study machining length was kept constant for cutting experiments. It can be observed that change in spindle speed influences machining time.

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Fig. 8 Power and Energy consumption in Dry cutting (a) Uncoated tool, f =

0.1 mm/min (b) Coated tool , f = 0.1 mm/min (c) Uncoated tool, f = 0.2 mm/min (d) Coated tool, f = 0.2 mm/min

For appropriate energy comparison in machining tests specific energy consumption was calculated with respect to material removal rate (mm3/ min). Figures 9, 10 and 11 show that more energy was consumed at low feed of 0.1 mm/ rev. Machining with low feed rate takes more time because of slower tool movement, however similar length can be machined in less time using higher feed rates. As less time was involved in machining at high feed rate that result in reduced energy consumption. Lower cutting speeds resulted in high energy consumption when constant machining length was used. It was observed that this reduction in energy consumption was attributed to machining time. The machining time was reduced due to

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variation of workpiece diameter in order to adjust spindle speed, RPM. At cutting speed of 30 m/ min, coated inserts consumed more power than the uncoated inserts.

Fig. 9 Specific energy consumption by uncoated and coated inserts at

cutting speed of 30 m/ min under dry, mist and flood conditions At cutting speed of 30 m/ min and feed of 0.1 mm/ rev it was observed that mist and flood consumed more power. Higher power consumption for mist and flood is due to higher cutting forces and power drawn by hydraulic pump. At cutting speed of 60 m/ min dry environment consumed minimum energy for both feed levels. At higher cutting speed of 90 m/ min mist environment showed potential of better heat dissipation from the cutting zone at both feed levels. Flood environment gave higher energy consumption in most of the cases which shows that high cutting forces were produced under flood conditions. Comparatively less energy consumption was observed for coated inserts at cutting speeds of 60 m / min and 90 m/ min. This is linked with cutting forces produced by coated inserts. However uncoated inserts showed comparatively better performance at low cutting speed of 30 m/ min.

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cutting speed of 60 m/ min under dry, mist and flood conditions The general trends obtained for energy consumption can be summarized as follows: higher energy consumption was obtained at low cutting speed due to variation in diameter when machining length was kept constant. It is because machining time reduced when workpiece diameter changed. The coated carbides under mist condition provided comparatively less energy consumption because of efficient control of friction in cutting zone. TiAlN coating is also helpful in reducing coefficient of friction. TiAlN coated tools showed comparatively less energy consumption in all cases, especially at higher cutting speeds. Higher feed rate results in less energy consumption.

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cutting speed of 90 m/ min under dry, mist and flood conditions 3.4. Tool Wear Assessment The interaction between tool, workpiece and chip results in different types of tool wear. The literature classifies these types as adhesive wear, abrasive wear, delamination wear, diffusion wear, microchipping, fatigue, notch, gross wear and plastic deformation [13, 14]. These wear mechanisms are dominant on the rake face as a crater wear and flank wear on the flank face. Under normal machining conditions flank wear predominate crater wear and defines the failure criteria for cutting tools. Maximum values of flank wear were measured using a tool maker microscope. The values obtained for uncoated and coated inserts under dry condition are plotted in figure 12. It can be observed that flank wear rate is very rapid for coated carbides. Higher flank wear rate was observed for higher feed rate under all cutting environments. Tool coatings are useful in reducing the coefficient of friction between chip and tool. By reducing, the friction coefficient heat generation during metal cutting operation can also be reduced. This phenomenon can increase the tool performance and tool life. Higher flank wear rate in uncoated insert at high cutting speed and feed

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points out at high amount of heat generation for these cutting conditions. Improved heat transfer can be obtained using mist and flood environments.

Fig. 12 Flank wear, Coated and Uncoated tool under Dry environment

The coated insert provided the highest flank wear at high cutting speed of 90 m/ min and feed of 0.2 mm/ rev. This points out the fact that titanium alloys have low thermal conductivity as a result the heat generated during the machining operation stays in the region of cutting. As the cutting environment was dry there were less chances of heat dissipation. Due to the excessive heat generation and friction phenomenon coating delamination and abrasion were responsible for large wear zone at flank face of the coated insert Experimentation showed good results for the same cutting condition under mist and flood environments. The experiments revealed that improved heat dissipation could reduce the coating delamination, which can result in low flank wear. Figure 13 represents the plotted results for flank wear of coated and uncoated inserts under mist conditions. Under mist condition results obtained for flank wear were better than the dry conditions. Another observation was related to the coated inserts that they performed better than uncoated tools at higher cutting speed of 60 and 90 m/ min. However, uncoated tools were good for low cutting speed of 30 m/ min.

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Fig. 13 Flank wear, Coated and Uncoated tool under Mist environment

The micrographs shown in figures 15 - 20 point out at the underlying wear mechanisms for both uncoated and coated tools. The particles of workpiece material were found attached at the nose and flank face of the cutting tool. These small welded particles have potential to form built up edge (BUE).

Fig. 14 Flank wear, Coated and Uncoated tool under Flood environment

Figure 14 shows flank wear of coated and uncoated inserts under flood conditions. Flank wear results at low cutting speed 30 m/ min for flood environment were better than mist especially for coated inserts. Coated inserts comparatively performed better than uncoated tools at higher cutting speed of 90 m/ min. Scanning electron microscopy presented in the later part of the paper is in agreement with the values obtained in figure 14. The

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particles of workpiece material were found attached at the nose and flank face of the cutting tool for both coated and uncoated tools. Similar behavior of workpiece material was reported for flood environment.

Fig. 15 SEM images of wear at uncoated carbide tool at Vc = 30 m/min

and f = 0.1 mm/ rev, (a) Dry (b) Mist (c) Flood

3.4.1 Wear mechanisms in uncoated and coated carbides at cutting speed of 30 m/ min

It has been observed that adhesion, diffusion and abrasion were the main wear mechanisms for Ti-6Al-4V alloy, as was reported earlier [10-11]. Scanning electron microscope (SEM) has been utilized to study the underlying wear mechanisms present in turning of titanium alloys. Figure 15 shows the SEM micrographs of wear at flank face of uncoated insert under cutting speed of 30 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. Adhesion and abrasion were found as major wear mechanisms in uncoated carbide tools. It was also observed that wear rate was much higher for dry environment. The combination of high temperature and chemical reactivity creates a friendly environment for the adhesion and diffusion to start at rapid rate. Figure 17 shows that diffusion was also present in coated inserts under dry conditions. But for mist and flood environment diffusion was not observed clearly. As titanium, alloys have very high chemical reactivity that results in an extreme wear at flank face. The particles of workpiece material were found attached at the nose

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and flank face of the cutting tool. The main wear mechanism responsible for this behavior is adhesion. Adhesive forces between the tool and workpiece material result in adherence of small particles at cutting edge. These small welded particles have potential to form built up edge (BUE). Built up edge (BUE) was observed in the shown micrographs. SEM micrographs of coated and uncoated inserts revealed that abrasive wear mechanism. The combination of high temperature and stresses at the tool face starts the abrasion wear mechanism. Chipping of tool material was observed as shown in the figure 15 and 16 micrographs. As the chipped material moves in between the tool and machined surface, it results in the abrasive wear. The study also pointed out the fact that flank wear rate was higher at high feed. Depending on the tool and workpiece material, both abrasive and diffusion wear mechanism are more rapid at higher cutting speeds.

Fig. 16 SEM images of wear at Coated carbide tool at Vc = 30 m/min and f

= 0.1 mm/ rev, (a) Dry (b) Mist (c) Flood

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Fig. 17 SEM images of wear at Uncoated carbide tool at Vc = 60 m/min


3.4.2 Wear mechanisms in uncoated and coated inserts at cutting speed of 60 m/ min Figure 17 shows the SEM micrographs of wear at flank face of uncoated insert under cutting speed of 60 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. Figure 17 shows high amplitude of abrasive and adhesive wears. It indicates that uncoated carbides did not perform well at cutting speed 60 m/ min. Figure 17b showed promising behavior of mist cooling strategy. Figure 18 shows the SEM micrographs of wear at flank face of coated inserts under cutting speed of 60 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. Figure 16 and 18 shows that coated inserts did not perform well for the low and medium cutting speeds of 30m/ min and 60 m / min respectively. The study exposed an observation that coated inserts at high feed of 0.2 mm/ rev represents less wear. Coating delamination phenomenon was also observed. This phenomenon of coating delamination should occur before adhesion at low cutting speeds for the coated inserts. Another observation was the dominant adhesion phenomenon. This adhesion phenomenon was observed for cutting speed of 60 m/ min and feed of 0.1 mm/ rev where the particles of work piece material were strongly welded with the tool material. As proposed by the

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literature [15] adhesion phenomenon comes after the delamination of coating.

Fig.18 SEM images of wear at Coated carbide tool at Vc = 60 m/min and f


Fig. 19 SEM images of wear at Uncoated carbide tool at Vc = 90 m/min


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Fig. 20 SEM images of wear at Coated carbide tool at Vc = 90 m/min and f


3.4.3 Wear mechanisms in uncoated and coated inserts at cutting speed of 90 m/ min Figure 19 and 20 shows the SEM micrographs of wear at flank face of uncoated and coated inserts under cutting speed of 90 m/min and feed of 0.1 mm/ rev using dry, mist and flood environments. In figure 19, the abrasive wear was very evident in uncoated carbides. Figure 19b shows improved results under mist condition. It was observed that land affected by abrasion was magnified at high cutting speed and feed rate. The coated inserts performed better at high cutting speed of 90 m/ min. This suggests that as titanium alloys have low thermal conductivity so during machining most of the heat stays at the cutting edge. Low cutting speed and low feed means that cutting edge stays longer in the region of high temperature. The combination of high temperature and friction results in rapid coating delamination at low cutting speeds which gives high wear at the flank face. Whereas for high cutting speed and high feed cutting edge travels rapidly out of the region of high temperature which gives less wear. Temperature can be controlled by using mist and flood environments. The experiments show that mist environment gave better result than the dry environment.

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4. Conclusions The conclusions drawn from the turning of titanium alloy Ti – 6Al- 4V using coated PVD TiAlN and uncoated carbide inserts are as follows:

1. In general the uncoated inserts performed better at low cutting

speeds whereas PVD TiAlN coated inserts were superior at cutting speeds of 60 m/ min and 90 m/ min as was indicated by other researchers [16] and [18-23].

2. It was observed that cutting force decreased for both types of inserts used with increase in cutting speed and after passing 60 m/ min it again starts increasing. This variation is due to the built up edge formation. Built up edge increases effective rake angle that results in lower cutting forces. At higher cutting speeds there is no built up edge formation which resulted in higher cutting forces. Higher cutting speed produces high cutting temperature that reduces cutting force due to thermal softening of workpiece material. Uncoated inserts generated more cutting forces at low cutting speed, pointing out the fact that there will be more tool wear and less tool life. However coated tools generated less force at higher cutting speeds.

3. TiAlN coated inserts have better thermal stability then uncoated

inserts, due to formation of a dense and adhesive protective Al2O3 layer at higher temperatures which protects the tool from diffusion wear mechanism. This feature of TiAlN coated insert makes it performs better at higher cutting speeds. At low cutting speeds coated inserts did not perform well because coated inserts are brittle in nature and exhibit high friction coefficient.

4. Power and energy consumption data showed that low feed and

cutting speed gave more energy consumption. Dry environment showed less energy consumption as compared to mist and flood environments. Difference in energy consumed for dry, mist and flood environment was due to the cutting force behavior and increase in power requirement due to coolant pumping. However, increasing feed results in less energy consumption because it reduces the machining cycle time. Although increase in feed results in higher surface roughness. To optimize a machining operation there should be a compromise between required surface roughness and reduction in energy consumption.

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5. Environmental condition plays an important role towards flank

wear propagation. Dry environment showed rapid increase in flank wear with increasing cutting speed. However mist and flood environments showed equally good results for flank wear. Flood environment showed better result at low feed and cutting speed than mist environment. This points out at the potential of Minimum Quantity Lubrication (MQL) technique for higher cutting speeds to reduce environmental burden.

6. Adhesion and abrasion wear mechanism were predominantly

observed in the SEM micrographs of flank wear for uncoated inserts. High wear rate at cutting speed above 60 m/min indicates presence of elevated cutting temperature. High cutting temperature increases the rate of adhesion and diffusion wear mechanisms. Coating delamination was first observed for coated inserts. After coating delamination process adhesion, abrasion and diffusion mechanisms were accountable for the flank wear of the coated inserts.

ACKNOWLEDGEMENT

The Authors acknowledge the financial support of Emirates foundation;

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Paper D: ENERGY CONSUMPTION AND SURFACE FINISH ANALYSIS OF MACHINING Ti6Al4V

Authors: Salman Pervaiz, Ibrahim Deiab, Amir Rashid, Cornel Mihai Nicolescu and Hossam Kishawy

Published in: World Academy of Science, Engineering and Technology 76, 2013. Presented in: International Conference on Manufacturing Systems Engineering, ICMSE 2013, Venice, Italy, April 14 - 15, 2013. Available at: www.waset.org/journals/waset/v76/v76-22.pdf

PAPER D

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ENERGY CONSUMPTION AND SURFACE FINISH ANALYSIS OF MACHINING Ti6Al4V S. Pervaiz 1, 2, I. Deiab 1, A. Rashid 2, M. Nicolescu 2 and H. Kishawy 3 1 Department of Mechanical Engineering, American University of Sharjah, Sharjah, United Arab Emirates 2 Department of Production Engineering, KTH Royal Institute of Technology, Stockholm, Sweden 3 Faculty of Engineering and Applied Sciences, University of Ontario Institute of Technology, Oshawa, Ontario, CANADA

Keywords: Energy Consumption, CO2 Emission, Ti6Al4V. Abstract. Greenhouse gases (GHG) emissions impose major threat to global warming potential (GWP). Unfortunately manufacturing sector is one of the major sources that contribute towards the rapid increase in greenhouse gases (GHG) emissions. In manufacturing sector electric power consumption is the major driver that influences CO2 emission. Titanium alloys are widely utilized in aerospace, automotive and petrochemical sectors because of their high strength to weight ratio and corrosion resistance. Titanium alloys are termed as difficult to cut materials because of their poor machinability rating. The present study analyzes energy consumption during cutting with reference to material removal rate (MRR). Surface roughness was also measured in order to optimize energy consumption.

I. INTRODUCTION A significant amount of greenhouse gases (GHG) is released in atmosphere due to the metal cutting sector. To protect the environment strict legislations are being developed and implemented by the global community. Manufacturing sector is also under immense pressure to avoid all environmental hazardous practices. Energy consumption during manufacturing operations is one of the key parameters that play an

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important role towards environmental burden. By optimizing energy requirements for a given machining operation greenhouse gases can be reduced. Many researchers have focused their work to optimize energy consumption with respect to the cutting conditions. Interaction between minimum cost and minimum energy consumption for machining operations revealed that minimum energy criterion resulted in less cost, energy consumption and carbon foot print [1]. Reference [2] explored utilization of polynomial networks to develop models for multistage turning. The study investigated possibilities of maximizing production and minimizing production cost. An analytical model was developed to determine the environmental burden of core machining phase [3]. The research utilized energy utilization, cutting mechanics and lubricant flow rate for developing machining model. This study revealed that energy consumed by a machining process is a function of product geometry, workpiece material and cutting environment. In general electrical energy is consumed in a machine tool to perform machining task. Reference [4] revealed detailed analysis of energy consumption used to perform different tasks during machining. The experimentation was conducted using injection molding, manual/ automatic milling and automated lathe machines. Reference [5] describes a methodology of calculating environmental burden of a machining operation. The study also provided formulation to calculate equivalent CO2 emissions using electrical energy consumption. Reference [6] proposed an online energy monitoring method for machine tool. It was revealed that energy efficiency can be increased by reducing idle time through efficient managerial skills or by optimizing cutting parameters through technical means.

A framework consists of six steps process to characterize energy consumption was recommended to illustrate power and energy consumption [7]. The research work revealed that a high portion of the energy consumption was utilized in machine controller and idle movements. It was revealed that spindle utilized 35% of total energy. Reference [8] recommends design and process based approaches to minimize energy utilization. The research analysed different model based on kinetic energy recovery system (KERS), process parameter selection strategy and web-based energy estimation tool. It was observed that KERS can save energy up to 25%. An empirical expression was formulated to

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explain the interaction between energy utilization and cutting conditions [9]. Experimental validation of model was performed using different milling and turning machine tools. Reference [10] represents a model for prediction of energy foot print of machined components. The work was conducted using turning experiments. The study also discussed boundaries and interaction of machining economics and environmental impact of reduction in energy consumption. Different machining strategies were investigated to analyse energy consumption of a machine tool [11]. Different components of a machine tool were treated as variables. All numerical results were verified experimentally. The study was useful to evaluate different part programs with respect to their energy consumption. Reference [12] shows machining performance of six different cutting fluids. The study was conducted using four vegetable based and two semi-synthetic/ mineral based cutting fluids. Experimentation was designed using Taguchi (L18) mixed level parameter design. The study revealed that sunflower and canola based cutting fluids performed better than other available cutting fluids. This paper presents an experimental study to examine behavior of energy consumption and surface finish under different material removal rates. Energy consumption data was also interpreted in the form of equivalent CO2 emissions with reference to the energy mix of United Arab Emirates. Graphical representations of energy consumption and surface finish were generated for better understanding and visualization. These plots can be a useful tool for environmental sustainability assessment.

II. EXPERIMENTAL SETUP Machining experiments were conducted on a CNC turning center under dry cutting environment. Mitutoyo Roughness Tester SJ 201P was utilized for the measurement of surface finish. Each surface roughness reading was repeated four times in order to minimize experimental error and then average values were reported in the study. Power logger was employed to monitor power and energy consumption. Power sight manager was used as data acquisition software. Fig. 1 shows the schematic representation of experimental setup.

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Fig. 1 Schematic representation of experimental setup

For any metal cutting operation cutting tool material, workpiece material, cutting conditions (depth of cut, cutting speed and feed rate) and cutting environment plays an important role. Previous studies [12]-[14] showed that for any machine tool energy consumption is manly dependent on material removal rate of process. The present study used Titanium alloy Ti 6Al 4V as a workpiece material. Titanium alloys are nominated as difficult to cut materials due to their low thermal conductivity and high heat capacity. Cutting environment plays significant role towards the machinability of titanium alloys. To analyse and understand the core mechanisms dry and flood cutting environments were used for this study. The composition of Ti6Al4V is provided in Table 1. Experimentation was performed using uncoated carbide cutting inserts. The specifications for inserts are reported in Table 1.

The study was conducted using three different levels of cutting speeds and five levels of feed. Dry and flood cutting environment was utilized during the study. However depth of cut and machining length were kept constant.

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TABLE I CUTTING CONDITIONS


Workpiece material

Ti6Al4V C: < 0.08%, Al: 5.5 – 6.75%, Fe:<0.4%, V:3.5-4.5%, H: 0.05%, N:0.01%, O<0.2%

Insert type Uncoated carbide TCMT 16 T3 04-KM H13A

Depth of cut Cutting Speed Feed Machining length Machining Environment

0.8 mm 30 – 60 – 90 (m/min) 0.1 – 0.2– 0.3 – 0.4 – 0.5 (mm/ rev) 125 mm Dry - Flood

III. RESULTS AND DISCUSSION Power consumed during each machining test was recorded and analyzed using power sight manager software. After filtering the power signal energy consumption (KWh) was calculated. Fig. 2 shows a sample calculation for power and energy consumed during turning of Ti6Al4V. A sample plot for energy consumption is shown in Fig. 2 Energy consumed during the process was approximately 0.036 kWh.

Fig. 2 Power and energy consumption, Material removal rate = 240 mm3/ sec, Cutting speed = 60 m/min, Feed = 0.3 mm / rev, Depth of cut = 0.8

mm, Dry environment

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A. Dry Environment To set the reference base line, experimentation was first performed under dry cutting environment. Fig. 3 shows plots for energy consumption and surface finish for different material removal rates calculated at constant speed of 30 m/ min and different feed levels. Fig. 3 represents that energy consumption decreased with increase in material removal rate. Trends line was fitted using second order polynomial equations.

Fig. 3 Energy consumption and surface finish at different material removal

rates, Vc= 30 m/ min, f = 0.1 – 0.5 mm/ rev However as found in literature [15]–[16], surface roughness increased with increase in material removal rate. Increase is surface roughness was observed due to increase in the feed rate. The intersection point shows the best optimized value of surface roughness with respect to the energy consumption. Fig. 4 shows behavior of energy consumption at all feed levels using different cutting speeds. It is observed that energy consumption is more sensitive to feed rate then cutting speed. However increase in both feed rate and cutting speed results in lower energy consumption. Fig. 5 represents plots for energy consumption and surface finish for different material removal rates calculated at constant speed of 60 m/ min and different feed levels.

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Fig. 4 Energy consumption


rates, Vc= 60 m/ min, f = 0.1 – 0.5 mm/ rev Similar trends for energy consumption and surface roughness were observed. With increase in cutting speed energy consumption decreased whereas minor difference in surface roughness was observed when compared to the cutting speed of 30 m/min. Point of intersection between both curves was lowered with increase in cutting speed. It points out that increase in cutting speed lowers both energy consumption and surface

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roughness but literature criticize high cutting speeds with high amount of heat generated.

Fig. 6 represents plots for energy consumption and surface finish for different material removal rates calculated at constant speed of 90 m/ min and different feed levels. At higher cutting speed best surface finish was obtained at expense of less energy consumed was known from the literature. The major limitation of using high cutting speed is high amount of heat generation that directly affects cutting tool life. As titanium alloys show poor heat dissipation due to their low thermal conductivity, presence of high amount of heat in cutting zone results in severe and rapid tool wear.


rates, Vc= 90 m/ min, f = 0.1 – 0.5 mm/ rev

Fig. 7 shows that material removal rate of 80 mm3/ sec was maintained using two different cutting speeds of 30 and 60 m/ min. In the first case cutting speed of 30 m/ min was used with feed of 0.2 mm/ rev to attain 80 mm3/ sec. However for second reading cutting speed of 60 m/min was used with feed of 0.1 mm/ rev to reach 80 mm3/ sec. It was observed that for material removal rate of 80 mm3/ sec less energy consumption and better surface roughness was obtained for cutting speed of 60 m/ min. Similar behavior was observed for material removal rates of 160, 120 and 240 m/ min. This means that to minimize energy consumption and achieve good surface finish higher removal rates should be utilized by increasing the cutting speed. But cutting speed is directly linked with cutting temperature in the cutting zone that can affect tool life and associated wear mechanism

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significantly [17].

Fig. 7 Energy consumption and surface finish at similar material removal

rates using different cutting speeds of 30, 60 and 90 m/ min

B. Flood Environment In addition to dry cutting conditions the study was repeated for similar cutting conditions under emulsion based flood cooling environment.

Fig. 8 shows plots for energy consumption and surface finish for different material removal rates calculated at constant speed of 30 m/ min and different feed levels. Fig. 8 shows that energy consumption decreased with increase in material removal rate. Optimal point at the intersection of both curves was slightly shifted towards higher material removal rate when compared with dry cutting.

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Fig. 9 represents plots for energy consumption and surface finish for different material removal rates calculated at constant speed of 60 m/ min and different feed levels. Both curves and their intersection followed the similar trend as in Fig. 8.

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Fig. 10 represents plots for energy consumption and surface finish for different material removal rates calculated at constant speed of 90 m/ min and different feed levels. Similarly like the previous Fig. 9 optimal point was shifted further downward in Fig. 10.



Fig. 11 Energy consumption and surface finish at similar material removal

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rates using different cutting speeds of 30, 60 and 90 m/ min Fig. 11 shows the similar behavior as explained previously in Fig. 7 for dry cutting environment. Higher material removal rates maintained by using higher cutting speeds resulted in better surface finish and less energy consumptions.

IV. CONCLUSION The conclusions drawn from the dry and wet machining of titanium alloy Ti – 6Al- 4V by using uncoated carbide inserts are as follows:

• It was observed in the study that increase in material removal rate

reduces energy consumption significantly. It is due to the fact that machining time plays dominant role towards consumption of energy.

• Increase in material removal rate results in higher cutting load at the contact area in cutting tool and workpiece. However this increase in cutting load does not significantly increases energy consumed during cutting.

• It was observed that energy consumption for cutting process is highly sensitive to feed rate as compared to the cutting speed.

• It was also observed that surface roughness and energy consumption decreased by increasing cutting speed and material removal rate. Reduction in energy consumption with increase in feed rate is logical because high feed rate results in faster machining and less processing time. It is found in agreement with literature [18] – [20] that cutting speed of a machining process is directly linked with cutting force. Higher cutting speed generates low cutting forces which results in less energy consumption. However limitation of using higher cutting speed is that it generates high amount of heat during cutting process. High cutting temperature results in poor tool life and accelerated tool wear mechanisms.

• Graphical plots of energy consumption and surface roughness intersect each other at certain location pointing out at the optimized value. These curves can be utilized to predict the amount of energy required for achieving desired surface roughness at specific material removal rate.

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• It was observed that optimized value at intersection point of two curves shifted below by an increase in material removal rate.

.

ACKNOWLEDGMENT The Authors acknowledge the financial support of National

Research Foundation (NRF) UIRCA2012-21838.

REFERENCES

[1] P. T. Mativenga, and M. F. Rajemi, “Calculation of optimum cutting parameters based on minimum energy footprint,” CIRP Annals – Manufacturing Technology, vol 60, pp. 149 – 152, 2011.

[2] B. Y. Lee, and Y. S. Tarngb, “Cutting-parameter selection for maximizing production rate or minimizing production cost in multistage turning operations,” Journal of Materials Processing Technology, vol 105, pp. 61-66, 2000.

[3] A. A. Munoz, and P. Sheng, “ An analytical approach for determining the environmental impact of machining processes,” Journal of Materials Processing Technology, vol. 53, pp. 736-758, 1995.

[4] D. N. Kordonowy, “A power assessment of machining tools,” Bachelor of Science Thesis in Mechanical Engineering, Massachusetts Institute of Technology, Massachusetts, 2002.

[5] H. Narita, N. Desmira, and H. Fujimoto, “Environmental burden analysis for machining operation using LCA method,” The 41st CIRP Conference on Manufacturing Systems, 2008, 65 – 68.

[6] S. Hu, F. Liu, Y. He, and T. Hu, “An on-line approach for energy efficiency monitoring of machine tools,” Journal of cleaner production, vol. 27, pp. 133 – 140, 2012.

[7] R. Drake, M. B. Yildirim, J. Twomey, L. Whitman, J. Ahmad, and P. Lodhia, “Data collection framework on energy consumption in manufacturing,” Proceedings from Institute of Industrial Engineers Research Conference, Orlando, Florida, 2006.

[8] N. Diaz, S. Choi, M. Helu, Y. Chen, S. Jayanathan, Y. Yasui, D. Kong, S. Pavanaskar, D. Dornfeld, “ Machine tool design and operation stragtegies for green manufacturing,” Proceedings of the 4th CIRP International Conference on High Performance Cutting, 2011.

[9] S. Kara, W. Li, “Unit process energy consumption models for material removal processes,” CIRP Annals – Manufacturing Technology, vol. 60, pp. 37 – 40, 2011.

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[10] M. F. Rajemi, P. T. Mativenga, A. Aramcharoe, “Sustainable machining: selection of optimum turning conditions based on minimum energy considerations,” Journal of Cleaner Production, vol 18 (10 - 11), pp. 1059-1065.

[11] O. I. Avram and P. Xirouchakis, “Evaluating the use phase energy requirements of a machine tool system,” Journal of Cleaner Production vol 19 (6-7), pp. 699-711, 2011.

[12] M. H. Cetin, B. Ozcelik, E. Kuram, E. Demirbas, “Evaluation of vegetable based cutting fluids with extreme pressure and cutting parameters in turning of AISI 304L by Taguchi method,” Journal of Cleaner Production, vol 19 (17-18), pp. 2049-2056, 2011.

[13] W. Li, and S. Kara, “An empirical model for predicting energy consumption of manufacturing processes: a case of turning process,” Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, vol.225 (9), pp.1636-1646, 2011.

[14] T. Gutowski, J. Dahmus, A. Thiriez, “Electrical energy requirements for manufacturing processes,” Proceedings of 13th CIRP International Conference on LCE, Leuven, 2006.

[15] T.H.C. Childs, K. Sekiya, R. Tezuka, Y. Yamane, D. Dornfeld, D.E. Lee, S. Min, P.K. Wright, “Surface finishes from turning and facing with round nosed tools,” Annals of CIRP, vol-57, pp.89-92, 2008.

[16] O.B. Abouelatta, and J. Madi, “Surface roughness prediction based on cutting parameters and tool vibration in turning operations,” Journal of Materials Processing Technology, 118, pp.269-277, 2001.

[17] P. A. Viktor, and S. Shvets, “The Assessment of plastic deformation in metal cutting,” Journal of Materials Processing Technology, vol 146 – 02, pp.193-202, 2004.

[18] D. G. Flom, “High-Speed Machining,”' in: Bruggeman and Weiss (eds.) Innovations in Materials Processing, Plenum Press, 1983, pp. 417-439.

[19] E. F. Smart, and E. M. Trent, “Temperature distributions in tools used for cutting iron, titanium and nickel,” Int. J. Prod. Res., vol 13, 265-290, 1975.

[20] T. D. Marusich, “Effects of friction and cutting speed on cutting force,” Proceedings of ASME Congress, November 11-16, New York, 2001.

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Paper E: PERFORMANCE EVALUATION OF DIFFERENT COOLING STRATEGIES WHEN MACHINING Ti6Al4V

Authors: Salman Pervaiz, Ibrahim Deiab, Amir Rashid, Cornel Mihai Nicolescu and Hossam Kishawy

Presented in: Proceedings of the International Conference on Advanced Manufacturing Engineering and Technologies - NEWTECH 2013, Sweden, October 27-30, 2013. Available at: http://kth.diva-portal.org/smash/get/diva2:660817/FULLTEXT08.pdf

PAPER E

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Performance Evaluation of Different Cooling Strategies when Machining Ti6Al4V Salman Pervaiz1, 2, Ibrahim Deiab2, Amir Rashid1, Mihai Nicolescu1

and Hossam Kishawy3

1 Department of Production Engineering, KTH Royal Institute of Technology, Stockholm, Sweden 2 Department of Mechanical Engineering, American University of Sharjah, Sharjah, UAE 3 Faculty of Engineering and Applied Sciences, University of Ontario Institute of Technology, Oshawa, Ontario, CANADA Email of corresponding author: [email protected] ABSTRACT

Titanium alloys have replaced heavier, weaker and less serviceable engineering materials from demanding industries like aerospace, automotive, biomedical, petrochemical and power generation. High strength-to-weight ratio, high operating temperature and excellent corrosion resistance makes them suitable for challenging tasks. However their low thermal conductivity, high chemical reactivity and high strength at elevated temperature account for their poor machinability rating and short tool life. To overcome issues with poor heat dissipation generous amount of coolant is required in machining titanium alloys. Utilization of these coolants is being questioned because of their environmental and health issues. The paper presents performance evaluation of different cooling strategies when machining Ti6Al4V. The study was conducted using dry, conventional flood and a mixture of low temperature air with vegetable oil based mist cooling strategies. Each cooling strategy was examined in reference with tool life, cutting temperature and surface roughness. The study explored a combination of sub-zero temperature air and vegetable oil based mist as possible environmentally benign alternative to conventional cooling methods.

KEYWORDS: Titanium alloys, Cooling strategies, Vegetable oil mist

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1. INTRODUCTION Titanium alloys offer wide range of applications in aerospace, automotive, marine, petrochemical and biomedical sectors. Their high strength to weight ratio, extraordinary corrosion resistance and ability to operate at elevated temperatures makes them suitable for demanding engineering industries. Machinability of an engineering material is assessed by measuring tool life, material removal rate, cutting forces, power consumption, and surface roughness [1]. Titanium alloys exhibit poor machinability rating due their low thermal conductivity, high chemical reactivity at elevated temperatures and ability to maintain high hardness at elevated temperatures [2].

Metal working fluids are employed in the machining operation to enhance tool life, improve surface finish and chip removal from the cutting zone. These days metal working fluids are being questioned extensively for their economics and environmental related issues. These lubricants and coolants impose danger to environment due to their toxicity and non-biodegradability. In order to make machining process sustainable in nature, toxicity has to be reduced whereas biodegradability has to be enhanced. Near dry machining and minimum quantity lubrication (MQL) techniques are employed to encourage sustainability in machining. These techniques utilises very small amount of lubricant to reduce friction in the cutting zone. Rahim and Sasahara [3] conducted an experimental comparative study using palm oil based and synthetic ester based MQL systems. The study was performed to investigate the effectiveness of palm oil as lubricant in MQL system. The study revealed that palm oil based MQL arrangement out performed synthetic ester based MQL system. Zeilmann and Weingaertner [4] performed drilling experiments on Ti6Al4V using uncoated and coated drills (TiALN, CrCN and TiCN) under MQL environment. The study measured cutting temperature during drilling operation to evaluate the performance of MQL technique. The study revealed that internal MQL arrangement performed better than external MQL arrangement.

Wang et al. [5] executed orthogonal turning experiments on Ti6Al4V using dry, flood and MQL cutting environments. The study was conducted under continuous and interrupted cutting cases. The study pointed out that MQL performed better than flood cooling at higher cutting speeds due to better lubrication capacity. The study also revealed that MQL was more effective in interrupted cutting scenario. Cia et al. [6] performed end milling experiments to investigate the controlling parameters for MQL system. The

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study used oil flow rates of 2 ml/h – 14 ml/h for optimized value. The study revealed that diffusion wear rate was present for low oil supply rates 2ml/h – 10ml/h, however at 14ml/h no diffusion wear rate was found. Klocke et al. [7] performed machining experiments on Titanium alloys to investigate the effect of high pressurized coolant supply. The study analysed cutting tool temperature, tool wear, chip formation and cutting forces. The study pressurized the lubricant up to 300 bars (55l/min) and compared the effects with conventional flood cooling. The study revealed that 25% cutting tool temperature reduction and 50% tool wear improvement, in best case, were achieved using high pressure coolant.

Yasir et al. [8] utilized physical vapour deposition (PVD) coated cemented carbide tools to machine Ti6Al4V using MQL system. The study utilized coolant flow rates of 50 – 100 ml/h at three cutting speed levels of 120, 135 and 150 m/ min. The study revealed that mist outperformed others at the cutting speed of 135 m/min. Improved tool life was observed at 135 m/min with high flow rates. Mist was found more effective for worn tool. Su et al. [9] performed end milling machining experiments on titanium alloys to evaluate the performance of different cooling strategies by analysing the rate of tool wear. The study used dry, conventional flood, nitrogen-oil mist, compressed cold nitrogen gas (CCNG) at 0, and−10 ◦C, and compressed cold nitrogen gas and oil mist (CCNGOM) as the cooling strategies. The study revealed that compressed cold nitrogen gas and oil mist (CCNGOM) cooling strategy outperformed other strategies by resulting longer tool life. Yildiz et al. [10] reviewed the application methods of cryogenic coolants. The study revealed that cryogenic coolants effectively controlled the cutting temperature at cutting zone, and provided good tool life with reasonable surface finish. Sun et al. [11] evaluated the machining performance of titanium alloys by utilizing cryogenic compressed air. The study showed great potential of cryogenic compressed air cooling strategy as it reduced tool wear significantly. Bermingham et al. [12] performed machining experiments using cryogenic cooling technique. Cutting speed and material removal rate were kept constant during the study, however feed rate and depth of cut were varied to analyse cutting force. The study revealed that less heat was generated for low feed rate and high depth of cut.

In this presented study turning experiments were performed on Ti6Al4V using coated carbide tools. All of the experiments were performed using constant depth of cut under three levels of cutting speeds, feed rate and cutting environments. The study aimed to evaluate the performance of each cooling strategy by analysing surface finish, cutting temperature and tool wear.

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2. EXPERIMENTAL SETUP

2.1. Workpiece Material The workpiece material used in the turning test was α – β titanium alloy Ti-6Al-4V. Stock of Ti6Al4V material was available under ASTM B381standard specifications, in the form of cylindrical rod. The chemical composition (wt. %) and mechanical properties of Ti6Al4V are mentioned in Table 1 and Table 2 respectively.

Table 1. Nominal chemical composition of Ti6Al4V

Element Wt. % Element Wt. % H N C Fe

0.005 0.01 0.05 0.09

V Al Ti

4.40 6.15

Balance

Table 2. Mechanical properties of Ti-6Al-4V at room temperature Properties Values Properties Values


993 MPa 830 MPa 14

Poison ratio Modulus of elasticity Hardness (HRC)

0.342 114 GPa

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2.2. Cutting Tool Material Physical vapour deposition (PVD) coated cermet turning inserts were utilized in the experimentation work for the presented study. The specified cutting insert came with two cutting edges. Each cutting edge was used for single experimental run. Data for the cutting insert has been shown in Table 3.

Table 3. Cutting tool specifications

Sandvik Cutting Insert ISO Code: CCMT 12 04 04 MM 1105

Material Rake Relief angle Inscribed circle Insert thickness

Cermet inserts Positive 7 Degrees 1/2" 0.1875"

Nose radius Coating Cutting direction Mounting hole dia.

0.0157" PVD Neutral 0.203" Screw clamp

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2.3. Cutting Environment

The study utilized three different types of cooling strategies in order to investigate the machinability of Ti6Al4V. These cooling strategies are named as dry, conventional emulsion based flood and mixture of low temperature air with internal vegetable oil based mist (MQL+CA). Vegetable oil in MQL was operation at the flow rate of 4.6 ml/ min. The vegetable oil (ECULUBRIC E200L) was provided by ACCU-Svenska AB. The flow rate of mist was controlled by regulating the low temperature air and oil supply. The information about the vegetable oil is shown in Table 4.

Table 4. Properties of vegetable oil used in mist (ECULUBRIC E200L) [13]

Properties Description

Chemical description Health hazard Flash point Ignition point Density (at 20 C0) Viscosity (at 20 C0) Partition coefficient

A fraction of natural triglycerides, easily biodegradable substances Not hazard to human health 325 C0 365 C0 0.92 g/ cm3 70cP < 3%

2.4. Machining Tests and Cutting Conditions

All of the turning experiments were conducted on a CNC tuning center. A Mitutoyo roughness tester SJ 201P was used for the measurement of surface finish of the generated surface. In order to add repetition in the study each machining experiments was repeated for two times. Infrared camera, UFPA – T170, was utilized to measure the temperature in cutting zone under different cooling strategies. The study was conducted using three different levels of cutting speed and feed rate as shown in Table 5. Fig. 1 shows schematic illustration of experimental set up.

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Fig.1. Schematic illustration of experimental setup Table 5. Cutting conditions

Machining Parameters Levels

Cutting Speed Feed rate Depth of cut Cooling strategy

90 – 120 – 150 m/ min 0.15 – 0.2 – 0.25 mm/ rev Constant 0.8 mm Dry, Flood (2 l/ min) and Low temperature air (Sub-zero, 0 to - 4 C°) + Vegetable oil based mist (Internal) MQL+CA, 4 ml/ min

3. RESULTS AND DISCUSSION

3.1 Surface Roughness Analysis

Surface roughness was measured for all of the machining tests. Surface roughness defines the integrity of surface generated after machining. Surface roughness is more critical for the components manufactured from titanium alloys, because these alloys are termed as difficult to cut materials. Elevated temperatures in cutting zone, high chemical reactivity and high hardness at elevated temperatures are the main causes for low machinability rating of titanium alloys.

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(a) (b)

(c)

Fig.2. Surface roughness trends with respect to dry, MQL+CA, and flood cooling strategies, (a) Vc = 90 m/min, (b) Vc = 120

m/min and (c) Vc = 150 m/min

Fig. 2(a) represents the surface roughness obtained for different cooling strategies at the cutting speed of 90m/ min. The dry cutting condition provided higher surface roughness at feed levels of 0.15 and 0.20 mm/ rev. At higher feed of 0.25 mm/ rev flood environment provided higher roughness. The MQL+CA based cooling strategy performed better than dry conditions at 0.15 and 0.20 mm/ rev and outclassed other strategies at higher feed of 0.25 mm/ rev. With increase of feed level surface roughness increases for all cooling strategies. Fig. 2(b) represents surface roughness trends for different cooling strategies at cutting speed of 60 m/ min. It has been observed that MQL+CA cooling strategy outperformed other cooling strategies at all three levels of feed. It was also observed that cutting speed of 60 m/ min flood environment provided the highest surface roughness at

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all feed levels. Seah et al. [14] also performed machining tests on steel specimen using flood cooling techniques. The study revealed that wear rate was higher for flood environment due to the shifting of crater wear near the cutting edge. This can be a possible reason for higher surface roughness in flood environment.

Fig. 2 (c) represents the plots for surface roughness for all cooling strategies at cutting speed of 150 m/ min. Fig. 2(c) also showed higher surface roughness under dry cooling strategy at all feed levels. MQL+CA strategy provided comparatively better surface roughness at all feed levels. As a general trend MQL+CA performed comparatively better at higher cutting speeds.

3.2 Tool Wear Measurement

During machining operation, cutting tool experiences loss of tool material and deformation. With the passage of time this wear increases at different locations of the cutting tool. Under the normal cutting parameters flank wear grows on the flank face and crater wear grows on the rake face. Flank tool wear is of great importance as it directly influences dimensional accuracy, topographic information and surface integrity of the generated surface. Flank tool wear evaluation is the most commonly used tool life criteria used in the metal cutting sector. In accordance with the standard ISO 3685: 1993 (E) [15], average flank wear of 0.3 mm was used in all cutting tests as tool life criteria.

Fig. 3 shows the tool life observed at cutting speed of 90 m/ min for three levels of feed (0.15-0.20-0.25 mm/ rev). It was observed that MQL+CA lubrication technique out-performed dry and flood cooling at low feed of 0.15 mm/ rev as shown in Fig. 3a. However at higher feed levels MQL+CA resulted in low tool life like dry environment as shown in Figs. 3b and 3c. Figs. 3b and 3c also shows that flood cooling gave comparatively better tool life. The general trend observed in Fig. 3 shows that increase in the feed level results in less tool life. A possible explanation of this phenomenon is the low fracture toughness of cermet tools as found in agreement with literature [16]. Rapid crack formation and propagation was the reason of failure at higher feed levels. The reason of poor performance of MQL+CA at high feed rates can be the insufficient time provided to lubrication. MQL+CA cooling technique provided encouraging result at low feed rate only. At higher feed level of 0.25 mm/ rev even flood environment showed poor tool life.

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Fig. 4 shows the tool life observed at cutting speed of 120 m/ min for three levels of feed (0.15-0.20-0.25 mm/ rev). It has been observed that MQL+CA technique out-performed dry and flood environment at low feed level of 0.15 mm/ rev as shown in Fig. 4a. At feed level of 0.2 mm/ rev MQL+CA tool life was found little better than dry environment. Unexpectedly at higher feed level of 0.25 mm/ rev flood environment performed worst among other cooling techniques. Seah et al. [14] has also observed the negative effect of cutting fluids when machining steels. His work reveals that cutting fluid can enhance crater wear rate at the rake face. High crater wear rate weakens the cutting edge by excessive chipping. A possible explanation of low tool life under flood environment can be attributed by the presence of rapid crater wear rate and chipping at cutting edge. Performance of MQL+CA cooling technique was reasonable.

Fig.3. Flank wear for flood, dry and MQL+CA at cutting speed of 90

m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev When MQL system is used alone without cool air, the oil film evaporates rapidly because of the presence of high temperature in the cutting zone. The

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concept of cool air was used to reduce the cutting temperature in cutting zone. The main cause of reasonable performance of MQL+CA technique is that it takes the benefit of both cool air and MQL which makes it compatible for machining Titanium alloys. Fig. 5 shows the tool life observed at cutting speed of 150 m/ min for three levels of feed (0.15-0.20-0.25 mm/ rev). At low feed level of 0.15 mm/ rev, MQL+CA cooling technique performed as good as flood cooling environment. At feed levels of 0.20 mm/ rev all of the cutting environments performed almost in a similar way resulting very short tool life. A general trend was observed that higher feed rate and cutting speed resulted in shorter tool life.


m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

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m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

3.3 Cutting Temperature Analysis

Cutting temperature is an important and decisive factor towards machinability evaluation. It is a good measure to evaluate the effectiveness of a cooling strategy. Fig. 6 shows sample calculation of cutting temperature using infrared camera.

(a) (b)

Fig.6. Sample measurements of cutting temperature under dry environment at cutting speed of 150 m/ min, (a) feed = 0.15 mm/ rev (b) feed = 0.25 mm/ rev

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Fig.7a,7b and 7c represent plots for avergae values of cutting temperatures recorded under dry, MQL+CA and flood environment. It has been observed that at all levels of cutting speeds MQL+CA strategy has reduced cutting temperature. Flood cooling was found the most efficient way of heat dissipation. It was observed that MQL+CA strategy decreased average temperature by 26.6 % than the temperature obtained in dry environment at cutting speed of 90 m/ min. Similarly 17.9 % and 17.5% reduction was observed in MQL+CA strategy for cutting speeds of 120 and 150 m/ min.

Fig.7. Cutting temperature under dry, MQL+CA and flood environment

4. CONCLUSIONS The conclusions drawn from the machining of Titanium alloy

Ti6Al4V using coated cermet inserts are as follows;

1. It was observed that coated cermet inserts poorly performed when machining Titanium alloy Ti6Al4V.

2. It was observed that MQL+CA cooling technique performed better than dry in almost all cases and in some conditions out performed flood

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environment as well. In general high temperature is present in the cutting zone during the machining of Ti6Al4V. Due to high cutting temperature, oil in MQL strategy evaporates easily without providing proper lubrication. Mixing of MQL (vegetable oil based) with cool air provides better result at cool air try to reduce temperature facilitating MQL to lubricate properly. This clearly shows potential of MQL+CA strategy as a possible replacement of flood cooling.

3. Surface roughness analysis shows that MQL+CA out-performed dry cutting in almost all cases. However, MQL+CA provided better finish than the flood environment at higher cutting speed level of 150 m/ min.

4. It has been found that MQL+CA strategy decreased average cutting temperature by 26.6%, 17.9% and 17.5% than the temperature obtained in dry environment at cutting speed levels of 90, 120 and 150 m/ min respectively.

Acknowledgement

The Authors acknowledge the financial support of Emirates foundation; National Research Foundation (NRF) and Natural Sciences and Engineering Research Council (NSERC). The authors would like to thank Accu-Svenska AB for supporting the research work by providing MQL booster system. References [1] E. M. Trent and P. K. Wright, “Metal Cutting,” 3rd Edition, Butterworth

Heinemann, Boston, 2000. [2] A. R. Machado and J. Wallbank, “Machining of Titanium and Its

Alloys: A Review,” Proceedings of the Institution of Mechanical Engineers Part B: Management and Engi-neering Manufacture, vol. 204, no. 11, pp. 53-60, 2005.

[3] E.A. Rahim and H. Sasahara, “A study of the effect of palm oil as MQL lubricant on high speed drilling of titanium alloys,” Tribology International 44, pp. 309–317, 2011.

[4] R. P. Zeilmann, W. L. Weingaertner, “ Analysis of temperature during drilling of Ti6Al4V with minimal quantity of lubrication,” Journal of Material Processing Technology, Vol. 179, pp. 124–7, 2006.

[5] Z. G. Wang, M. Rahman, Y. S. Wong, K. S. Neo, J. Sun, C. H. Tan, H.

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Onozuka, “Study on orthogonal turning of titanium alloys with different coolant supply strategies,” International Journal of Advanced Manufacturing Technology, Vol. 42, pp.621–632, 2009.

[6] X. J. Cai, Z. Q. Liu, M. Chen and Q. L. An, “An experimental investigation on effects of minimum quantity lubrication oil supply rate in high-speed,” Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, Vol. 226(11), pp. 1784–1792, 2012.

[7] F. Klocke, H. Sangermann, A. Kramer, and D. Lung, “Influence of a High-Pressure Lubricoolant Supply on Thermo-Mechanical Tool Load and Tool Wear Behaviour in the Turning of Aerospace Materials, ” Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, Vol. 225, 52, 2011.

[8] M.S.A. Yasir, C.H. Che Hassan, A.G. Jaharah, H.E. Nagi, B. Yanuar, A.I. Ghusri, “ Machinability of Ti6Al4V under dry and near dry condition using carbide tools,” The Open Industrial and Manufacturing Engineering Journal, Vol. 2, pp.1-9, 2009.

[9] Y. Su, N. He, L. Li, X. L. Li, “An experimental investigation of effects of cooling/lubrication conditions on tool wear in high-speed end milling of Ti-6Al-4V,” Wear , Vol. 261, pp. 760-766, 2006.

[10] Y. Yildiz, M. Nalbant, “A review of cryogenic cooling in machining processes,” International Journal of Machine Tools & Manufacture, Vol. 48, pp. 947–964, 2008.

[11] S. Sun, M. Brandt, M.S. Dargusch, “Machining Ti–6Al–4V alloy with cryogenic compressed air cooling,” International Journal of Machine Tools & Manufacture, Vol. 50, pp. 933–942, 2010.

[12] M.J. Bermingham,J. Kirsch, S. Sun, S. Palanisamy, M. S. Dargusch, “New observations on tool life, cutting forces and chip morphology in cryogenic machining Ti-6Al-4V,” International Journal of Machine Tools & Manufacture, Vol. 51, pp. 500–511, 2011.

[13] ECOLUBRIC E200L , Material Saftey Data Sheet, Retrived on 25 June 2013 from the source http://www.accu-svenska.se/wp-content/uploads/2012/04/ECULUBRIC-E200L-CAS-8002-13-9-eng.pdf

[14] K.H.W. Seah, X. Li, and K.S. Lee, “The effect of applying coolant on tool wear in metal machining,” Journal of Material Processing and Technology, Vol 48, pp. 495 -501, 1995.

[15] ISO 3685, Tool life testing with single-point turning tools, ISO

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Speed Machining of Austenitic Stainless Steel,” Journal of Applied Sciences, Vol. 6, pp.779-784, 2006.

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