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European Journal of Scientific ResearchISSN 1450-216X Vol.22 No.2 (2008), pp.153-162

© EuroJournals Publishing, Inc. 2008

http://www.eurojournals.com/ejsr.htm

High Speed Milling of Ti-6Al-4V Using Coated Carbide Tools

Nagi Elmagrabi

Department of Mechanical and Material

Malaysia National University, Bangi

E-mail: [email protected]

Che Hassan C.H



Jaharah A.G



F.M. Shuaeib

Mechanical Eng. Dept, Garyounis University

Benghazi, Libya

Abstract

The new challenge in machining is to use high cutting speed in order to increase the

productivity. This is the main reason for rapid tool wear. For titanium and its alloy this

problem is more severe due to its low thermal conductivity (about 6.6Wm_1K_1 for Ti–6Al–4V). This poor machinability has limited cutting speed to less than 60 m/min in

industrial practice (Komanduri & Von-Turkovich 1981; Chandler 1989; Che Haron, et al.

2001). Furthermore, titanium alloys are generally difficult to machine at cutting speeds of over 30m/min with high-speed steel (HSS) tools, and over 60m/min with cemented

tungsten carbide (WC) tools which result in very low productivity. In this work, dry slot

milling tests were carried out on Titanium Alloys (Ti–6Al–4V) with uncoated and coatedcarbide cutting tools. The experimental tests were performed at various cutting speeds of

50, 80 and 105 m/min, with depth of cuts of 1, 1.5 and 2 mm and feed rates of 0.1, 0.15 and

2 mm/tooth respectively. Tool life and the quality of the surface finish were the factors thatdetermine the performance of the cutting tool. Surface finish was studied based on the

surface roughness and the microhardness underneath the machined surface. Microstructureof the sub-machined surface was observed in order to investigate the metallurgicalalteration. It was found that the PVD coated carbide tool has a better tool life; with a

maximum tool life of 11.5 minutes. Surface roughness is more sensitive to the feed rate and

the depth of cut.

Keywords: coated carbide, dry machining, Tool life, Machining, Titanium alloy.

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High Speed Milling of Ti-6Al-4V Using Coated Carbide Tools 154

1. IntroductionTitanium and titanium alloys possess several excellent properties including excellent corrosion

resistance, very high strength-to-weight ratio, light weight and the ability to maintain their properties atextremely high temperatures. There are many applications in which titanium and its alloys are used.

The straight tungsten carbide (WC-Co) grade still remains the first choice in face milling (Ezugwu and

Machado 1988) and turning (Hartung and Kramer 1982)of titanium alloys. However, the machining of

titanium alloys is a major production problem, because of the frequent lower cutting speed. In turning

operations, the cutting speeds are limited to about 45 m /min when using straight-grade cementedcarbide (WC-Co) ( Jawaid et al. 1999), while in face milling operation, when coated carbide tools(PVD and CVD) were used, the best cutting conditions with respect to the highest tool life of 30 min

was achieved at cutting speed of 55m/min (Dearnly and Grearson 1986). In some cases, machining

without using cutting fluids (dry machining) can be successfully implemented in the industrialmachining applications. Cutting fluids are used in metal cutting process as a lubricant and a coolant.

Their absence in machining means a high friction and a high cutting temperature at the tool -

workpiece interface. This condition drastically affects both the tool wear and the tool life. Therefore,dry machining represents a great challenge to manufacturing engineers because of the high

temperatures generated especially when machining aerospace materials such as titanium and nickel

based super alloys. The aim of the present work is to investigate the performance of the coated and

uncoated carbide tools in green (dry) slot milling of Ti64.

2. Experimental procedureThe slot milling tests were carried out on a Sabre 750 Cincinnati CNC vertical machining center which

was controlled by an Achramatic 850 sx- controller. The machining tests were conducted under the

conditions shown in Table 1. The work piece material selected is Titaium-Ti-6Al-4V. This type of

material belongs to the group of alpha-beta alloy used largely for commercial purposes. The dimensionof the Ti-6Al-4V test pieces is 100x100x160 mm. These test pieces were pre-machined 2 mm thick of each surface before the experiment in order to remove the residual stress and aging at the outer layer.

Table 1: Slot milling test conditions

Cutting conditions:

Axial depth of cut (mm) : 1 to 2

Radial depth of cut (mm): 8 constant

Cutting speed (m/min): 50 to 105

Feed rate F (mm/tooth): 0.1 to 0.2

Cutting Fluid: Dry Cutting

Cutter Geometry:

Diameter: 12 mm

Insert: No. of Insert one

ISO grade :(S20-S30)–XOMX090308TR ME06, F40 ( PVD Coated Carbide –Ti-N)

3. Workpiece materialAmong the titanium alloy used in aerospace industries, the titanium alloy from alpha-beta group Ti-

6Al-4V is the most widely used. The composition (wt %) and the mechanical properties of the materialselected for the study are shown in Tables 2 and 3 respectively.

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155 Nagi Elmagrabi, Che Hassan C.H, Jaharah A.G and F.M. Shuaeib

Table 2: Composition (wt%) of Ti-6Al-4V

Content O H N C Fe V Al Ti

Wt% - 0.005 0.01 0.05 0.09 4.40 6.15 Balance

Table 3: Mechanical Properties of Ti-6Al-4V at room temperature

Tensile strength (MPa) Yield strength (MPa) Elongation Modulus of elasticity (GPa) Hardness (HRC)

993 830 14 114 36

4. Cutting tool materialsPVD coated cemented carbide is used in this experiment. This insert is mounted on a 12 mm diameterslot milling cutter. Only one tooth is used at each experiment. The insert diameter is 8 mm. The notable

data for the insert and the coating material are shown in Tables 4, 5 and 6 respectively.

Table 4: Cutting tool composition

Content WC Co

wt % 87 13

Table 5: Mechanical properties of cutting tool

Particle size Hardness 25ºC Density Modulus of elasticity Coefficient of thermal expansion

0.8 μm 1470 Hv10 14.5 g/cm3 580 GPa 5.5 x 10-6 /K

Table 6: Geometry of cutting tool

Cutting rake angle Side clearance angle Helix angle

24º 11º 15º

5. Research methodIn this work, the effect of cutting condition parameters on the titanium machinability characteristics in

terms of tool life is investigated using the design of experiment statistical approach. Table 7 shows thedesign matrix factors considered and their high and low levels. These levels are estimated based on

literature values and several trial experiments. However, it should be mentioned here that the result of

the analysis are presented for the coated carbide tool only. The omission of other results has beendecided due to space limitations and also based on the general trend of the superior performance of the

coated carbide tool over the uncoated one. This can be observed by comparing the tool life values of

each design point of both the coated and the uncoated carbide tools of Table (A1) of Appendix A. Forexample, the maximum tool life of the coated carbide tool is 10.85 minutes while for the uncoated

carbide tool it is 6.125 minutes. This observed superior performance is noticed not only on the

maximum value but also it is extended on most of the design points (cutting conditions).

Table 7: the ranges of the used machining cutting condition parameters

Factors Cutting speed m/sec Feed rate mm Depth of cut mm

Low level 50 0.1 1

High level 105 0.2 2

After design matrix creation the designed experiments were carried out. The machining was

stopped at each cutting path of 0.102m and the insert was periodically removed from the tool holder.

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The flank wear on the tool and surface finish of the work material was measured accordingly. The

work piece surface roughness was measured in accordance with cutting tool deterioration criterion,which was measured in terms of flank wear using a tool marker microscope. The experiment was

stopped when VB1 reached 0.4 mm or the VBmax reached 0.7 mm; anyone occurs first. The arithmetic

surface roughness value (R a ) was adopted and measured on the machined surface parallel to feed

motion with a portable surface roughness tester (Mpi Mahr perthometer model). The R a values of the

machined surface were obtained by averaging the surface roughness values at three locations on thecentre path of workpiece width. Figure 1 shows experiment number (1) for coated carbide tool and the

measured response (The surface roughness).

Figure 1: Surface roughness measurement

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Figure 2: Coated Carbide (PVD) cutting condition Vc=105m/min. Feed =0.2mm/toot,Depth= 1.5mm

6. Results and discussion6.1 Tool life

The result of design matrix with the tool life is shown in Table (A1) of Appendix A. This result shows

the strong relationship between the cutting speed and tool life as shown in design points 8 and 12 with

tool life 0.553 and 3.393 minutes respectively. At any condition of cutting process, the cutting speeddominates the tool life. This table also can tell us that the high tool life was obtained at lower values of

feed rates, which are clear from the design points 1 and 15 with tool life of 6.5 and 10.859 minutes

respectively. On the other hand, the tool life does not seem to be much affected by the depth of cut ascompared with feed rate and cutting speed. This is clear from the design points 1 and 14, which give

the same tool life of 6.5 minutes. However, the depth of cut affects the tool life at higher feed as shown

in the design points 12 and 15 with tool life 3.393 and 10.85 minutes respectively for the same cuttingspeed value. The chemical reactivity of titanium alloy results in an excessive tool wear in the

machining of this material. Further, it is observed in Figure 2 that for cutting speed of 105 m/min the

tool deteriorates quickly, and the workpiece material could be observed melted at the cutting edge. The

chip was observed melted at the cutting edge of the tool as shown in Figure 3 promoting the diffusion

and the weakness of the tool. The adhesion wear is due to the high temperature and pressure duringcutting which cause welding to occur. Evidence of adhesion of workpiece material at the cutting edge

is given in Figure 4.

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Figure 3: Coated Carbide (PVD) cutting condition V=50m/min. Feed =0.1mm/tooth Depth= 1.5mm

Figure 4: Result of EDAX analysis on the chip showing evidence of adhesion of work material( in Figure 3)

6.2 Surface roughness

Figure 5 shows the typical surface roughness values recorded when machining titanium Ti64 with PVD

coated carbide insert under dry cutting conditions. In order to compare the degree of surface variation

of the surface roughness, two cutting speeds of 50 and 105 m/min at feed rates of 0.1 and 0.15mm/tooth, at a depth of cut 1.5mm were chosen to signify the low and high cutting conditions when

slot milling Ti-6Al-4V. As shown in Figure 5 the surface roughness values at the start of cut were

slightly high due to the sharp edge of the tools when cutting at all cutting speeds investigated. Results

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also indicate that the surface roughness values decreased as the cutting speed increased. The

improvement on surface roughness was obtained at the lowest feed rate of 0.1mm/tooth. Thisexperiment has proved that higher feed rate results in higher value of surface roughness. However, the

roughness values recorded were unstable during the intermediate cutting process.

Figure 5: Surface roughness vs. cutting time when slot milling Ti64 with PVD Tools at depth of cut= 1.5mm

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0 5 10 15

Time (min)

R o u g h n e s s ( m i c r o n )

Roughness of

Ti64

v=50,V=0.1

Roughness of

Ti64

V=50,f=0.2

Roughness of

Ti64 V=105,

V=0.2

Roughness of

Ti64

V=105,F=0.1

6.3 Surface hardness

Figure 6 shows the micro hardness value underneath the machine surface at various cutting conditions.In order to compare the degree of hardness deviation of the machined surface, two cutting speeds of 50

and 105 m/min at feed rates of 0.1and 0.15 mm/tooth were chosen to signify the low and high cutting

conditions when slot milling Ti-6Al-4V. Results show that microhardness readings of the top surfacelayer (up to 0.02 mm) are always higher than the average hardness of the base material. This is

probably due to the work-hardening effect of the work material. This applies even at the lowest cutting

condition (cutting speed = 50 m/min & feed rate=0.1mm/tooth). The temperature below the machined

surface is retained due to the low thermal conductivity of titanium alloy. The hardness values at 0.1mm increase significantly under all cutting conditions. This indicates that hardening has occurred 0.01

below the machined surface. The hardness of the subsurface at 0.04mm is always lower than the base

material. This slight decrease in hardness was probably due to over aging of titanium as a result of veryhigh temperature generated on the surface of the workpiece. Between 0.07 to 0.12 mm beneath the

surface, the hardness readings increased to values near the average hardness of the work material.

Maximum hardness readings at different cutting conditions were recorded at 0.015 to 0.02 mm. Thehighest hardness recorded was 420HV when machining at cutting speed of 105m/min and feed rate of

0.2mm after the coated carbide tool has failed. This highest hardness reading recorded at 0.015 mm

beneath the machined surface shows that the microstructure was heavily deformed (Figure 6a).

However, when machining with worn tools, the hardness approached the base material only 0.20 mmbeneath the machined surface.

Figure 6: Microhardness value beneath the machined surface at depth cut =1.5mm

Microhadness of Ti-64, d=1.5mm, dry

0

50

100

150

200

250

300

350

400

450

0 0.1 0.2 0.3 0.4 0.5

Depth Beneath the Surface,mm

M i c r o h a r d n e s s , H V

v=105, f=0.2 t=33.18

sec

Ave, hardness(HV)

v=105,f=0.1 t=265.74

sec

v=50,f=0.2 t=203.58

sec

v=50 ,f=0.1, t=651sec

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Figure 6A: Deformed microstructure beneath the machined surface at depth cut =1.5mm

6.4 Metallurgical alteration

Figure 7 shows the microstructure of the machined surface with PVD tool produced during dry

machining of Ti64. This figure also illustrates the subsurface microstructural deformation caused by

machining consisted of deformed grain boundaries in the direction of cutting and elongation of grains.

These types of defect are typically reported during the machining of titanium alloys ( Haron 2001;Ezugwu and Wang 1997 & Yang and Liu 1999). At this stage, the tools were worn out. More

significant alteration was observed at cutting speed of 77.5 m/min. This explains that there is more

work hardening occurred at this cutting condition. Higher density of grain formed beneath the top layercontributes to higher value of hardness. This explains why higher hardness is obtained at the sub-

surface. Softening at the top layer takes place due to the over aging ( Che Haron & Jawaid 2005).

Figure 7: Microstructure of machined surface after 1.449 min of cu77.5m/min , feed=0.2mm/tooth and depth

=2mm

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7. ConclusionFrom this experiment we can conclude that:

• At any cutting condition the cutting speed and feed rate are the dominant factors in

controlling tool life.

• Effect of feed rate on surface roughness is more significant compared to other cuttingparameters.

• The top layer of the machined surface experiences work hardening process, which is higherthan the average hardness of the workpiece material.

• Flank wear, adhesion and thermal crack at high cutting speed are the dominant failure modewhen machining Ti64 with PVD carbide tool.

AcknowledgementThe author wishes to thank Mr Rosli Ahmad and Mr Faizul bin Sharron for their invaluable assistance

with many practical aspects of the work.

References[1] Jawaid, A. Che Haron, C.H. Abdullah, A. (1999). "Tool Wear Characteristics in Turning of Titanium Alloy Ti-6246." Journal Material Processing and Technology: 329-334.

[2] Chandler, H. W. (1989). "'Machining of Reactive Metals'ASM Handbook-Machining,."10: 844-

857.[3] Che Haron, C.H. Ginting, A.& Goh.J.H. (2001). "Wear of coated and uncoated carbides in

turning tool steel." Journal of Materials Processing and Technology 116:49–54.

[4] Che Haron, C.H. (2001). "Tool life and surface integrity in turning titanium alloy." Journal of

Materials Processing Technology: 231-237.[5] Che Haron,C.H. and Jawaid, A. (2005). "The effect of machining on surface integrity of

titanium alloy Ti-6% Al-4% V." Journal of Materials Processing Technology: 188-192.

[6]

Dearnly, P. A. and Grearson, A. N. (1986). "Evaluation of principle wear mechanisms of cemented carbide & ceramics used for machining titanium alloy IMI 318." Mater.Sci.Tech.: 47-

48.

[7] Ezugwu, E. O. and Machado, A. R. (1988). "Face milling of aerospace materials." proceeding

of the first International Conferenceon on the Behaviour of Materials in Machining 3.1-3.11.

[8] Ezugwu, E. O. and Wan, Z. M. (1997). "Titanium alloys and their machinability- a review."

Journal of Materials Processing Technology 68: 262-274.

[9] Hartung.P.D and Kramer, B. M. (1982). "Tool Wear in Machining Titanium” Ann.CIRP 31 1:75-80.

[10] Komanduri, R. and Von-Turkovich B. F. (1981). "New Observations on the Mechanism of

Chip Formation When Machining Titanium Alloys." Wear 69: 179-188.

[11]

Yang,X and Li, C.R. (1999). "Machining titanium and its allots." Machining Sci.Technol., 3(1):107-139.

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Appendix ATable (A1): Design matrix for titanium machining with coated and uncoated carbide tools

Machining factors Response (Tool life) ( min)Design

pointcutting speed

(m/min)feed rate (mm) depth of cut (mm)

Coated carbide

(PVD)Uncoated carbide

1 77.5 0.1 1 6.5 5.500

2 105 0.1 1.5 4.429 3.650

3 77.5 0.15 1.5 2.672 3.9604 77.5 0.15 1.5 2.338 2.310

5 50 0.15 1 7.238 5.740

6 77.5 0.2 1 3.237 3.735

7 105 0.15 2 0.492 1.098

8 105 0.2 1.5 0.553 0.820

9 50 0.15 2 6.7226 4.208

10 105 0.15 1 2.696 2.451

11 77.5 0.15 1.5 3.34 4.290

12 50 0.2 1.5 3.393 4.597

13 77.5 0.2 2 1.449 3.289

14 77.5 0.1 2 6.5 5.000

15 50 0.1 1.5 10.85 6.126

16 77.5 0.15 1.5 2.672 3.96017 77.5 0.15 1.5 3.674 1.980

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