Study About Minimum Quantity Lubri�cation inBearing Holes Bored with Pcd Tools in AluminumAlloyPaulo Paiva Carvalho  ( paulopaiva01@yahoo.com.br )

Fiat Research Centre: Centro Ricerche FiatGustavo H. N. Fernandes 

Universidade Federal de UberlândiaLucas M. Q. Barbosa 

Universidade Federal de UberlândiaFelipe C. R. de Souza 

Universidade Federal de UberlândiaJosé A. G. de Sousa 

Federal Technological University of ParanáPaulo S. Martins 

Fiat Chrysler AutomobilesElhadji H. T. Ba 

Federal Technological University of ParanáÁlisson R. Machado 

Universidade Federal de Uberlândia

Research Article

Keywords: Aluminum alloys, boring, Minimum Quantity Lubrication, Polycrystalline Diamond tools,sustainable, Green machining

Posted Date: November 2nd, 2021

DOI: https://doi.org/10.21203/rs.3.rs-972806/v1

License: This work is licensed under a Creative Commons Attribution 4.0 International License.  Read Full License

STUDY ABOUT MINIMUM QUANTITY LUBRIFICATION IN

ALUMINUM ALLOYS BEARING HOLES

Paulo P. Carvalho a* ; Gustavo H. N. Fernandes b; Lucas M. Q. Barbosa b; Felipe C. R. de Souza b; José A. G. de Sousa c; Paulo S. Martins a; Elhadji H. T. Ba e; Álisson R. Machado b,f

a Fiat Chrysler Automobiles - FCA, Manufacturing Engineering, Betim - MG, Brazil b Universidade Federal de Uberlândia, ,Mechanical Engineering Faculty, Uberlândia - MG, Brazil cFederal Technological University of Paraná – Mechanical Engineering, Londrina - PR, Brazil d Centro Federal de Educação Tecnológica de Minas Gerais, Belo Horizonte - MG, Brazil e Pontifícia Universidade Católica do Paraná, Mechanical Engineering, Curitiba - PR, Brazil

* Corresponding author. E-mail address: paulopaiva01@yahoo.com.br

Abstract: Cutting Fluid in Abundancy – CFA is a standard cooling technique used in

machining due to the good relationship between quality and productivity. However, due to

sustainability problems, economic, social, and environmental, several research types have

been carried out worldwide about replacing or eliminating these inputs in the cutting process.

In this work, three full factorial Design of Experiment - DoE were used to verify the

performance and feasibility of replacing the Cutting Fluid in Abundance - CFA by Minimum

Quantity Lubrification – MQL during the bearing of holes bored with Polly Cristaline

Diamond – PCD tools in aluminum alloy. The input variables were cutting speed (𝑣𝑐), feed

per knife (𝑓𝑧) and depth of cut (𝑑𝑜𝑐). While output variables were diameter and circularity

deviation. For each machining atmosphere, dry, CFA, and MQL, a full factorial DoE (23)

was performed with test and replicate. A total of 48 runs were performed. The main

conclusion is that MQL showed similar CFA results, as proven by the mean hypothesis test.

Based on the results of this work, it can be affirmed that MQL could replace the CFA

technique.

Keywords: Aluminum alloys; boring; Minimum Quantity Lubrication; Polycrystalline

Diamond tools; sustainable; Green machining.

Statements and Declarat ions The authors have no relevant financial or non-financial interests to disclose.

The authors have no conflicts of interest to declare that are relevant to the content of this article.

All authors certify that they have no affiliations with or involvement in any organization or entity with

any financial interest or non-financial interest in the subject matter or materials discussed in this

manuscript.

The authors have no financial or proprietary interests in any material discussed in this article.

Acknowledgments The authors would like to thanks: the Grupo de Manufatura Sustentável – GMS; the Laboratório de

Ensino e Pesquisa em Usinagem – LEPU at the Federal University of Uberlandia – Brazil, Scanning

Electron Microscopy Laboratory of the School of Chemical Engineering of Federal University of

Uberlandia (FEQUI-UFU) for the SEM analysis, and the Brazilian research agencies CNPq,

FAPEMIG, and Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior - Brasil (CAPES).

2

Declarat ions form

The author(s) is (are) the only responsible for the printed material included in this paper. Furthermore we would like to declare:

a. Consent to participate & Consent for publication:

We herebay, all authors and co-authors below listed, by the meaning of this

statement would like to declare our consentment of participation and our consent

for publication of the article: STUDY ABOUT MINIMUM QUANTITY

LUBRIFICATION IN BEARING HOLES BORED WITH PCD TOOLS IN

ALUMINUM ALLOY.

Author Signature

1 – Paulo Paiva Carvalho (first and corresponding author):

2 – Gustavo Henrique Nazareno Fernandes

3 – Lucas Melo Queiroz Barbosa

4 – Felipe Chagas Rodriguez de Souza

5 – José Aécio Gomes de Sousa

6 – Paulo Sérgio Martins

7 - Elhadji heikh Talibouya Ba

8 – Álisson Rocha Machado (reviewer)

3

1 . Introduct ion

Cutting Fluid in Abundancy – CFA is a standard cooling technique used in machining due

to the good relationship between quality and productivity. CFA plays vital functions in

temperature control, with lubrication and cooling, in addition to removing chips, protecting the

machine-tool-set against oxidation [1]–[3]. Said et al. (2019) [4] estimates that in 2016 alone,

the global consumption of non-biodegradable mineral-based CFs was ~ 13,726 million tons,

with an increasing forecast of increase. According to Benedicto et al. (2017) [5], in 2022,

lubricants' global demand can reach 43.9 million tons.

However, due to sustainability problems, economic, social, and environmental, several

research types have been carried out worldwide about replacing or eliminating these inputs in

the cutting process. For example, according to Tai et al. (2014) [6], the costs for maintenance

and operation of CFA in a Powertrain system can reach 17% of the part manufacturing cost.

Similar values were estimated by King et al. (2001) [7], 7 to 17%. Sharma et al. (2016) state

that this value is between 16 to 20% [8], and for Demirbas & Kobya (2017) [9], it can reach

30% in some particular cases.

Regarding CFAs' social and environmental problems, Shashidhara & Jayaram (2010) [10]

estimate that ~ 80% of machining operators leave are associated with CFA poisoning. This is

because it contains several additives, many of them toxic, such as fungicides and bactericides,

which cause severe damage to human health, such as allergic or respiratory problems,

degenerative diseases, and even cancer [11]. Besides, they cause environmental damage by

contaminating soils, sewers, rivers, and water sources [12], [13].

From the sustainable point of view, Dry Machining - DM, seems to be the most

advantageous method. However, according to Dhar et al. (2006) [14], it is impossible to

perform DM in Aluminum-Silicon alloys' drilling operations due to their high ductility.

Without proper cooling and lubrication, the chip adheres to the tool and can cause it to break

prematurely. Regarding Dry Machining, many authors have performed comparative

performance studies with CFA and MQL. In most cases, MQL results were better in cutting

power, tool wear, surface integrity, temperature, etc. [4], [15]–[17].

MQL has been widely studied as an excellent alternative concerning CFA since it:

(i) combines both cooling and lubricating properties; (ii) reduces initial investment with

machinery, pumping system, filtration, big tanks, CFA cooling machines; (iii) reduces energy,

CFA, and water consumption; (iv) increases chip recycling rate; (v) causes lower

environmental, social and economic impacts; (vi) acts cooling manly the tool, keeping the

4

benefit of high temperatures in the piece that causes material softening, lower shear resistance

and hardness, facilitating the cutting process; (vii) better penetration in tool/piece interface

because of high pressure, and since CFA tends to evaporate before accessing cutting zone;

(viii) lower metallic particles generations; (ix) equal or even better machining performance [4],

[6], [15], [17], [18].

In this work, it was proposed to investigate the use of MQL in the boring operation of

Aluminum-Silicon alloy (AA384) compared to dry and CFA atmosphere. It was used thin-

walled AA384 parts of a motor vehicle transmission housing. This bearing holes investigation

was carried out using the same parameters adopted in an automotive production line. The

diameter and circularity deviation of the hole was chosen as output variables.

1 .1 . Boring operat ion

Wang et al. (2013) [19] and Bhattacharyya et al. (2006) [20] define boring as precision

machining. Its main objective is to widen holes diameter to achieve narrow final tolerances,

such as diameter, deviation of circularity, cylindricality, roughness, and position. It is a

complex operation that requires perfect alignment between pieces/tools. A misalignment

between the actual and the theoretical hole center causes: an unbalance of the cutting forces

due to higher removal of material from the side of the hole about the opposite side; tool

deflection and consequently difficulties in obtaining the finished hole characteristics; variation

in depth of cut at each cutting edge of the tool at each rotation; deflection and alteration of the

tool beat, which also contributes to increased cutting force unbalance [20][21].

Jun et al. (2006) [21] studied the effects of tool beating (misalignment) and cutting force

on machined holes' using a boring bar with four sharp edges. They introduced a maximum

misalignment of 0.1 mm and a beat whose maximum displacement of the tool, measured before

and during the cutting, was 0.2 and 0.1 mm, respectively. It caused variations of 0.08 mm in

diameter in relation to the nominal value and 0.1 mm amplitude in the circularity deviation.

Figure 1 (a) (b) (c) shows the schematic drawing of tool displacement and Figure 1 (e) (f)

shows its effect on boring.

It is noticed, Figure 12 (d), that the tools` displacement decreases during the cutting due

to the restriction imposed by the material but does not reach zero. In addition, this restriction

changes the cutting force due to the depth of cut momentary variation in each cutting edge

responsible for irregular hole machining. Finally, the tool displacement is transferred to the

hole according to vibration frequency, generating quantitative errors such as circularity and

diameter deviations, poor roughness, etc.

5

In finishing holes in Aluminum alloy parts, the boring operation is essential for obtaining

tolerances, usually with H7 adjustment. For example, Bezerra et al. (2001) [22] analyzed the

influence of cutting parameters on the qualitative characteristics of holes with 12 mm in

diameter, using a wider of 11.997 mm of a medium diameter and 140 mm in length. According

to the authors, smaller variations in diameter and circularity deviation occurred at low cutting

speeds. However, these results contradict the literature. At low cutting speeds, there is a higher

probability of forming a Buildup-Edge - BuE, which would lead to more significant variations

in diameter, which would not happen at high cutting speeds.

1 .2 . Minimum Quant i ty Lubri f icat ion - MQL

Minimum Quantity Lubrification (MQL) system works through a jet of a line pressurized

air (5 ~ 15 𝑏𝑎𝑟), which interacts with oil from a reservoir, fragmenting it, and forming a mixture

of pressurized air and micro drops of oil, called a jet. This mixture is then injected into the

Figure 1: Tool displacement and effects. (a) Lateral view; (b) Top view; (c) Schematic representation of the removed chips section area; (d) Tool displacement; (f) Forces before,

during and after boring

Source: Adapted from [20][21]

6

cutting zone by a nozzle, with an approximate diameter of ∅ = 1 𝑚𝑚, assisting the cooling

and lubricating process [23].

There is no literary consensus about the oil flow rate in MQL. Values were found between

0.01 to 2 000 l/h, meanwhile, in CFA it was estimated to be around 1 200 l/h, reaching up to

24 000 l/h [6], [15], [23]–[27]. For instance, Obikawa et al. (2009) [28], using the same

machinery and machining conditions, found a flow rate of approximately 30 l/h of emulsifiable

cutting fluid ( 8 to 10% v.v.), while for MQL, it was 10 to 100 ml/h.

Figure 2 shows an example of an external MQL system with an ejector nozzle adapted

for turning operation used by Marques et al. (2017) [29]. The most common oils used in MQL

are vegetable, esters, or synthetic fluids due to more excellent physical-chemical stability at

high pressures, greater fluidity, and more remarkable anti-wear ability [24].

According to Sharma et al. (2016) [8], the MQL can be divided regarding the application

way into two categories, internal (single or double channel) or external (conventional or

ejector), as shown in Figure 3. In the case of internal application, Figure 3 ( 1 ) (2 ) , i t is

done inside the tool with channels designed for this purpose. Boswell et al. (2017) [15] explain

that the internal channels can be simple, where oil and air are mixed along with the tool and

before being injected, or dual channels in which oil and air are atomized just before the cutting

zone.

Figure 2: Layout of an external MQL system with ejector nozzle used in turning

Source: adapted from [29]

7

In the external MQL with a conventional nozzle, Figure 3 ( 3 ) ( 4 ) , the oil/air mixture is

made first in an atomizer and later delivered to the nozzle. Finally, in the ejector nozzle, the

mixture is made just before being injected. Of the four forms, the one with the best results is

the internal atomization by double channel. This technique is mostly favorable for processes

where access to the cutting zone is practically impossible, such as drilling, threading, or

widening operations.

According to Tai et al. (2014) [6], the Ford Motor Company launched its first production

line using MQL in 2005 at the United States factory. Due to the successful application for

prismatic components, such as cast-iron block, heads, crankshaft, motor block, and Aluminum

alloy transmission housings, MQL machining has become standard for acquiring new

production lines, being included in the BOP list1 (Bill Of Process).

Two atmospheres, dry and MQL, were studied by Khettabi et al. (2017) [16] during high-

speed milling of three types of Aluminum Alloys, 2024, 6061, and 7075. Input variables were,

dry and MQL (7, 14, 21 and 28 mm3/s), cutting speed (300 ~1 500 m/min) with constant feed

of 0.055 (mm/rev) and depth of cut (1.0 mm). The output variables analyzed were cutting

1 BOP (Bill Of Process) is a descriptive of the process that contains the best practices and worldwide standards of performance, quality and productivity, being, therefore, used as standard of acquisition of processes.

Figure 3: Ways of MQL: (1) Internal single channel; (2) double channel; (3) external ejector nozzle; (4) conventional nozzle.

Source: Adapted from [6], [15], [43]

8

forces, particle emission, and surface finish. MQL performed better in terms of forces and

surface finish but worse for particle emission.

Jayal et al. (2007) found more significant cylindricity error deviations to the nominal

diameter of 12 mm when applying dry machining during the drilling of cast Aluminum alloy

in comparison to MQL. The deviations found were approximately 0.01500 mm for the MQL

technique and a 0.01575 mm deviation for DM.

Thus, there is an excellent appeal for the use of MQL. However, even with superior

performance concerning dry machining and, in many cases, also to CFA, the effects of

overheating can influence the quality of products, especially those manufactured in aluminum

alloy, which limits or even prevents the use of the MQL on a large scale in industrial processes.

Furthermore, CFA's cooling effect is much higher than MQL due to its thermal properties, such

as specific heat (CP). The air, water, and oil CP values are, respectively: 1.04 kJ/kgK, 4.18

kJ/kgK, and 1.92 kJ/kgK [30]. For this reason, the success of using the MQL in substitution to

CFA depends on the correct choice of machining factors.

1 .3 . Aluminum al loys

Aluminum alloys are of significant and increasing importance materials, as it has: (i)

exceptional strength-to-weight ratio (ii) high corrosion resistance; (iii) good mechanical

properties (iv) excellent thermal and electrical conductivity; (v) good machinability; (vi) low

melting point; (vii) abundantly available in the Earth crust [31], [32]. These properties make

them widely used in virtually all kinds of industries and purposes, including automotive,

aerospace, naval, electronic, medical, electromechanics, civil, electrical, among others [31].

For example, the total amount of aluminum parts in a European car in 2012 was estimated at

around 140 kg [33]; today, this value is nearly 180 kg, expected to reach 250 kg in 2025 [31]

example of the applicability of this light metal [32].

Aluminum-Silicon alloy is of great relevance, mainly due to the Al-Si eutectic presence's

high fluidity. According to Yan et al. (2013) [34], the Al-Si2 alloy is an essential Al-Si series

used in engine manufacturing blocks, heads, pistons, etc. According to these authors, silicon

acts as a vital reinforcement by forming lamellae distributed in the ductile matrix. In this way,

the physical and mechanical properties of the Al-Si alloy are influenced by the morphology,

volume fraction, and specific mechanical and physical properties of eutectic silicon. Other

elements added to the Al-Si system produce important molten alloys: AI-Si-Cu, AI-Si-Mg, AI-

Si-Cu-Mg, AI-Si-Cu-Fe. The addition of copper contributes to strength, and silicon improves

moldability and increases the final product's abrasion resistance [35].

9

Alloys with high silicon content usually are used in the casting of more complex products

in permanent matrices. A1-Si-Cu/Mg alloys with more than 3 – 4 % copper or magnesium are

heat treatable. According to Asghar, Requena, and Kubel (2010) [36], adding copper and

magnesium in Al-Si alloys improves its mechanical resistance at room temperature due to its

formation of precipitates of Mg2Si and Al2Cu. However, this effect of precipitation hardening

reduces when these alloys are exposed to temperatures above 150 ºC. Alloys with high silicon

content (> 12% Si) have a less thermal expansion, giving an advantage in high temperatures.

When the silicon content exceeds about 12-13%, the crystalline structure has primary silicon

crystals dispersed in the eutectic structure. When these crystals are evenly distributed, the

product tends to have excellent wear resistance [35].

According to Couto et al. (2010, apud Parreira, 2014) [37], [38], the alloys of the Al-Si

system, in general, have, in addition to high fluidity, low shrinkage in castings, high resistance

to corrosion, good weldability, easy brazing and low coefficient of thermal expansion.

Furthermore, for Requena et al. (2009) [39], eutectic silicon substantially improves the

Aluminum alloy properties due to the transfer of charge from the ductile Aluminum alloy

matrix to the hard silicon particles.

Kishawy et al. (2005) state that the silicon percentage also influences the alloy's

machinability. Since that, its particles have a greater hardness than any other microstructure

phase. In this way, abrasion becomes the tool's primary wear mechanism as the average

temperature in the cutting zone does not exceed the alloy's melting point (on average 650 ºC).

Even close to this temperature, cutting tools would not be influenced by thermally activated

wear.

Niknan, Zedan, and Songmene (2014) [40] affirm that the machining of aluminum alloys,

although classified as easy machining about tool wear and cutting power, becomes more

difficult due to the tool wear abrasion caused by particles of hard silicon. Bezerra et al. (2001)

[22] also comment that the more excellent resistance to abrasion conferred by Si contributes to

increased tool wear and, consequently, to the machining process's cost. Yan et al. (2013) [34]

state that Al-Si alloys are more susceptible to the formation of a Built-up Edge (BuE) both on

the rake and flank face, mainly with materials that have a chemical affinity with the aluminum

alloy. Coz et al. (2012) [41] studied the influence of cutting geometry on dry milling of

aerospace Aluminum alloy AA7075 - AlZnMgCu1.5 and found adhesion as the primary wear

mechanism due to higher cutting temperatures.

Regarding the tool’s material, PCD (Polycrystalline Diamond) has been recognized for its

high performance due to the possibility of using High-Speed Machining - HSM without

10

affecting the tool’s life. Furthermore, the properties of PCD depend on the size of the diamond

grain, which provides more excellent resistance to abrasion since the hardness of the silicon

particles is lower than that of the diamond, this being the critical point to justify the high

performance of PCD boring tools [34].

2 . Mater ia ls & Methods

2 .1 . Design of Exper iments (DoE) and procedures

Three full factorial Design of Experiment - DoE with three variables in two levels (±) (3𝑥 (23)) were used to verify the performance and feasibility of replacing the CFA with MQL.

The input variables were: cutting speed (𝑣𝑐), feed per knife (𝑓𝑧), and depth of cut (𝑑𝑝). For

each atmosphere, DM, CFA, and MQL, a DoE (23) was performed with a test and replicate.

A total of 48 runs were performed, as shown in Tables 3 and 4. The parameter values were

based on the data used in the automotive industry production line.

A variance analysis (ANOVA) and a hypothesis tests were used to plan and analyze results

using Minitab software version 17. A significance level(α) of 0.05 was defined in the bilateral

t-test for comparing diameter and circularity deviation. A hypothesis test was proposed for

MQL, CFA, and DM as following:

𝐻0: 𝜇𝑀𝑄𝐿 = 𝜇𝐶𝐹𝐴 𝑜𝑟 𝜇𝐷𝑀 for the case, MQL diameter and circularity deviation is

statistically equal to CFA or DM;

𝐻1: 𝜇𝑀𝑄𝐿 ≠ 𝜇𝐶𝐹𝐴 𝑜𝑟 𝜇𝐷𝑀 for the case, MQL diameter and circularity deviation is

statistically different from CFA or DM. The following assumptions were stated:

(i) The data represent random samples from normal populations;

(ii) 𝑦𝑡𝑖 = �� + (��𝑡 − ��) + (𝑦𝑡𝑖 − ��𝑡); where i=1, 2,…,𝑛𝑡;

(iii) 𝜀𝑡𝑖 = 𝑦𝑡𝑖 − ��𝑡𝑖 and 𝜀𝑡𝑖 are independent; identical and randomly distributed with

𝑁(0, 𝜎2). Thus, values higher than the significance level indicate the impossibility of rejection of

the null hypothesis (H0), i.e., indicate that the results of both samples under study are part of

the same population, and, therefore, there is no difference between the results. For values

greater than 0.05, it is possible to affirm a difference between the tests' means.

11

Table 3: Values of each level for the factors VC, fZ e ap

Factor Level

-1 1 𝒗𝒄(m/min) 842 1263 fZ (mm/rev) 0.15 0.30 dp (mm) 0.40 0.65

Table 4: Complete full factorial planning (23) with a replicate

Standard

Order

Execution

order

VC (m/min)

fZ

(mm/rev) ap (mm)

Atmosphere (A)

3 1 842 0.30 0.40

MQL, DM or

CFA

14 2 1263 0.15 0.65 2 3 842 0.15 0.65 6 4 1263 0.15 0.65 9 5 842 0.15 0.40

13 6 1263 0.15 0.40 5 7 1263 0.15 0.40

16 8 1263 0.30 0.65 11 9 842 0.30 0.40 8 10 1263 0.30 0.65

12 11 842 0.30 0.65 15 12 1263 0.30 0.40 10 13 842 0.15 0.65 7 14 1263 0.30 0.40 4 15 842 0.30 0.65 1 16 842 0.15 0.40

2 .2 . Measuring procedures

The circularity and diameter deviation was measured at 16.5 mm from the reference plane,

Figure 4, using eight equally distributed touches. A template containing the angular division

markings was positioned over the hole to ensure equity, reference the cylindrical and angular

reference holes. The least-squares technique was adopted to measure the circularity deviation

because it is the most recommended form by the equipment manufacturers because of the

controlled characteristic. Also, the holes' diameter was measured, with a nominal value of

67+0.03 mm, measured in the same places of the circularity deviation and with the same number

of touches.

Figure 4: Diameter and circularity measurement distance from the reference plane. Dimensions in mm.

12

The hole in question is the same finished in the industry with a 67+0.03(H7) mm diameter

and a circularity deviation of 0.015 mm. Figure 5 (a) shows the production part's drawing and

identifying the hole chosen for analysis. The specimens were generated from the cut of the top

of the piece, as shown in Figure 5 (b).

Figure 5: a) transmission housing design with identification of bearing hole chosen for

analysis, b) Specimens extracted from production parts

Table 5 shows the measurements made for the diameter and circularity deviation of the

hole under study. The cutting fluid application was made through three nozzles positioned at

approximately 120º for cooling with CFA and with two nozzles positioned at approximately

180º for machining with MQL. After assembly, the tool beat was measured using a comparator

clock with 0.001mm resolution. The beat found was 0.015 mm.

Table 5: Results of diameter and circularity deviation obtained in the machining of thinning of the hole

ap (mm) 0,40 0,65

Ø C Ø C

Average (mm) 66.204 0.041 65.696 0.032

Standard deviation (mm) 0.032 0.013 0.016 0.014

Maximum (mm) 66.277 0.067 65.720 0.067

Minimum (mm) 66.181 0.013 65.667 0.013

Range (mm) 0.096 0.054 0.053 0.054

Ø: diameter and C: circularity deviation, both measured in millimetres (mm)

2 .3 . Mater ia l s & Machinery

The experiments were carried out using materials shown in Table 6. Table 7 is shown the

composition of aluminium-silicon alloy used. Figure 6 shows the PCD cutting tool used in the

experiments.

13

Table 6: Materials used in the experiment

Piece 48 fused Al-Si alloy evidence bodies AA384.0, with the composition shown in Table 6

Tools 1 PCD boring bar with a diameter of 67.10 mm

Cutting

fluids

Emulsion concentration of 8%, used as a coolant in machining with cutting fluid in abundance.

INTEGRAL OIL PLANTOCUT 40 SB, manufacture FUCHS, kinematic viscosity: 40 mm2/s at 40ºC, used as cutting fluid for MQL jet generation

Table 7: Composition of aluminium-silicon alloy used

Elements (%) Impurities (% maximum) Al

Si Cu Fe Zn Mn Ni Mg Ti Pb Sn Other

11.0 to 12.5 1.75 to 2.5 0.7 to 1.1 1.5 0.5 0.3 0.3 0.2 0.15 0.1 0.1 Balance

Figure 6: The cutting tool used in experiments, a) drawing of the boring bar, b) geometry of

the PCD plate.

The experiments were carried out using the following equipment available at the

Machining and Automation Laboratory of the Department of Mechanical Engineering of

Federal University of Minas Gerais – UFMG, according to Table 8.

Table 8: Equipment used for machining, measuring, and statistical tests

Machining

equipment

Romi Discovery 560 machining center, Siemens 810D numerical control, with a 7.5 kW power and maximum rotation of 7500 rpm.

MQL device, setup for 30 ml/h oil flow.

Wear analysis Scanning Electron Microscope (SEM) built-in EDS function (X-ray dispersive energy spectroscopy)

Diameter and

circularity

measurements

TESA coordinate measuring machine (MMC), MICRO HITE 3D model with REFLEX SCAN software, resolution of 0.001 mm, (calibrated in April 2017)

Statistical analysis Computer with Minitab software® version 17

14

3 . Resul ts & Discuss ion

3 .1 . Diameter Analys is

Figure 7 (a) (b) (c) shows the values of the pairs tested for atmosphere MQL, CFA, and

DM, respectively. Figure 7 (a) shows good repeatability between the test and replica values,

except for pair 1 (parts 1 and 9), whose difference was 0.011 mm. In the CFA test, Figure 7

(b), using the lowest cutting parameters, 𝑣𝑐 and 𝑑𝑝. Diameter values obtained for pair 1 (parts

1 and 9) were smaller than the tool's diameter. Similarly, Yan et al. (2013) [34], where the

hole’ diameter obtained in one of the tests was smaller than the tool's diameter with a

combination of low cutting speed, depth of cut, and high feed. MQL, Figure 7 (a), also

presented diameter values smaller than the tool's diameter in pair 4 (parts 5 and 16). Also, these

values correspond to the combination of the lowest cutting parameters, 𝑣𝑐 (842 m/min), fZ (0,15

mm/rev) e ap (0,4 mm.).

It is also noticed a similar behavior of diameter reduction with the increase of the cutting

parameters between CFA and DM, pairs 4 and 5, which correspond, in general, to the

combinations of the highest cutting parameters considered, mainly 𝑣𝑐 (1 263 m/min) and fZ

(0,30 mm/rev). The data show that the CFA values are lower than those achieved by dry

machining, which proves the influence of refrigeration on the machining process.

Table 9 shows the P-Value values relative to variance analysis (ANOVA) for each

atmosphere. Significant P-Value values lower than the adopted level, 𝛼 = 0.05 , were

highlighted in Table 9 to facilitate visualization.

15

Figure 7: Values of the diameter test pairs in machining, a) MQL, b) CFA, c) DM

Table 9: P values for diameter

Source DF MQL CFA DM

Model 7 0.029 0.000 0.004 Linear 3 0.105 0.000 0.002 VC 1 0.024 0.024 0.014

fZ 1 0.541 0.000 0.001

dp 1 0.541 0.015 0.656 2nd order interactions 3 0.016 0.138 0.022 𝒗𝒄 𝒙 𝒇𝒛 1 0.046 0.298 0.312 𝒗𝒄 𝒙 𝒅𝒑 1 0.116 0.972 0.218 𝒇𝒛 𝒙 𝒅𝒑 1 0.012 0.038 0.006

3rd order interactions 1 0.172 0.891 0.382 𝒗𝒄 𝒙 𝒇𝒛 𝒙 𝒅𝒑 1 0.172 0.891 0.382

Error 8 Total 15

In CFA, all three factors, 𝑣𝑐 , 𝑓𝑧 and 𝑑𝑝 were significant, as well as the interaction between 𝑓𝑧 𝑥 𝑑𝑝. This trend was not repeated for MQL, whose variations in the significance of the

16

factors demonstrate the difference in machining behavior concerning CFA. In MQL, the factors

with the most significant influence were: 𝑣𝑐 and the interactions between 𝑣𝑐 𝑥 𝑓𝑧 and 𝑓𝑧 𝑥 𝑑𝑝.

Dry machining showed significance for 𝑣𝑐 𝑥 𝑓𝑧 and 2nd order interaction between 𝑓𝑧 𝑥 𝑑𝑝.

Figure 8 (a) (b) (c) shows the factors that presented statistical significance for machining

with MQL, CFA, and DM, respectively.

When using MQL, Figure 8 (a), the diameter tends to increase about 0.006 mm with an

increase in 𝑣𝑐 . In addition to the second-order interactions, there is an increase in the diameter

of 0.15 mm with an increase from 𝑣𝑐 and 𝑓𝑧 . The diameter reduction is related to the

combination of larger 𝑓𝑧 an 𝑑𝑝. In MQL and CFA techniques, it is noted that the cutting speed

exerts the same influence, i.e., the diameter increase with the cutting speed, on average of 0.005

mm for both techniques. However, these results are contrary to those found by Yan et al. (2013)

[39] when machining Aluminum-Silicon alloy with a boring bar with a diameter of 33 mm

and 6 cutting edges. These authors found a tendency to reduce the diameter with the increase

of the cutting speed in all the feed ranges studied, although the diameters were always larger

than the tool's diameter, except in one of the tests.

Bezerra et al. (2001) [21] found using a 12 mm diameter and 8 cutting edges, the same

ratio of diameter increases with the increase in cutting speed (Figure 8 (a) (b)) in the machining

of Aluminum-Silicon alloy boring for both speed and feed. The increase in cutting speed causes

an increase in both the cutting temperature and the tool beat increase, generating larger

diameters. As found in the literature, MQL produces higher cutting temperatures than CFA. It

was expected to find in this work larger diameters for MQL. However, the results were similar,

although not all cutting parameters were significantly equal for both atmospheres.

Regarding the feed rate, the results found by Yan et al. (2013) [39] showed an alternation in

the tendency to increase or decrease depending on the rotation (cutting speed) adopted.

However, comparing results with the same rotation range (4000 and 6000 rpm), the increase in

the feed caused an increase in diameter, contrary to that found in the present study.

Similar results were found by Bezerra et al. (2001) [22] for feed, i.e., higher feed tends to

reduce the diameter. It happens because high feed reduces the contact time with the part, which

reduces the effects of heating caused by the cutting speed, besides contributing to the increase

of the tool's contact area, which would influence the cut's stability. It can be verified by

analyzing the combination with fZ and 𝑑𝑝 in both the MQL and CFA. The use of the highest

feed with the highest depth of cut resulted in smaller diameters.

17

Figure 8: Linear significant parameters and second-order interactions on the diameter in the machining technique, a) MQL, b) CFA, c) DM

The final effect of similarity in the behavior between CFA and DM is the significant feed

contribution reducing the diameter, compared with the lower influence of the increase in

diameter due to the cut speed and depth of cut. It can be confirmed by the P-Value presented

in Table 9 for DM, whose contribution percentage is 72.01% for the feed, against 6.66% of 𝑣𝑐 ,

8.26% of 𝑑𝑝 and 5.23% of 𝑓𝑧 𝑥 𝑑𝑝, as calculated by Equation (Eq.) 1.

𝑐𝑜𝑛𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 (%) = 𝑆𝑢𝑚 𝑜𝑓 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙 𝑠𝑞𝑢𝑎𝑟𝑒𝑠𝑆𝑢𝑚 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 𝑠𝑞𝑢𝑎𝑟𝑒𝑠 (1)

DM showed significant factors like CFA, except for cutting speed, which presented

inverse behavior. This inversion in the diameter behavior with the increase in cutting speed

18

may be related to the reduction of torque and thrust due to the increase in cutting temperature,

which reduces cutting and feed forces because of reducing the material's mechanical strength.

Thus, with lower acting forces, the lower the effect of the vibration generated in the cut

and, consequently, the greater the cut's stability. This effect was observed in DM because at

low cutting speeds and advances, the diameter increases about 0.023 mm of the tool diameter,

reducing this increase to about 0.013 mm of the tool diameter, especially when considering the

combined effect of 𝑓𝑧 𝑥 𝑑𝑝 . In this case, the smallest diameter values were found when

combined with the highest value of 𝑑𝑝.

The increase in 𝑓𝑧 causes an increase in chip thickness, which makes cutting more

effective. A similar result was also obtained by Wang et al. (2013) in the tests carried out on

the aluminium-silicon alloy boring. The authors concluded that silicon particles present in the

aluminium alloy matrix would be more easily cut than plucked due to reduced mechanical

resistance caused by increased temperature.

3 .2 . Roundness Analys is

Figure 9 (a) (b) (c) shows the cylindricality results of the test pairs for machining with

MQL, CFA, and DM, respectively. For machining with MQL, a more significant difference

was observed between the pair test values about the other techniques used, with an average of

0.007 mm. Pair 1 presented one of the highest values of circularity deviation associated with

the diameter behavior, pair 1, which presented a value below the tool diameter, as shown in

section 3.1.

Better repeatability between values was observed for CFA, but most values were above

the tolerance specified for the circularity deviation (0.015mm). For DM, it is noted that the

lowest mean value, 0.016 mm, and the highest average value found of 0.074 mm,

corresponding to the lowest values of VC (842 m/min) and dp, (0.4) mm.

Table 10 shows the values of variance analysis, ANOVA, for the studied techniques,

whose P-Value values are lower than the established significance level, 𝛼 = 0.05, denote the

statistical significance of the cut-off parameter.

CFA significant parameters were the same as for the diameter, i.e., 𝑣𝑐 , 𝑓𝑧 , 𝑑𝑝 and

combinations 𝑣𝑐 𝑥 𝑓𝑧 and 𝑓𝑧 𝑥 𝑑𝑝. For MQL, the significant factors were 𝑣𝑐 , 𝑑𝑝 and interactions 𝑣𝑐 𝑥 𝑓𝑧 and 𝑓𝑧 𝑥 𝑑𝑝. For dry machining only 𝑓𝑧 was significant, and the interactions 𝑣𝑐 𝑥 𝑑𝑝and 𝑓𝑧 𝑥 𝑑𝑝.

19

Figure 9: Test pair values for circularity in machining, a) MQL, b) CFA, c) DM

Table 10: ANOVA with P-Value values for circularity deviation

Source DF MQL CFA DM

Model 7 0,007 0,000 0,000 Linear 3 0,004 0,000 0,000 VC 1 0,004 0,003 0,695

fZ 1 1,000 0,000 0,000

ap 1 0,004 0,000 0,400 2nd order interactions 3 0,021 0,000 0,000 VC * fZ 1 0,027 0,002 0,073 VC * ap 1 0,933 0,095 0,002

fZ * ap 1 0,013 0,000 0,000

3rd order interactions 1 0,705 0,061 0,217 VC * fZ * ap 1 0,705 0,061 0,217 Error 8 Total 15

Analyzing the MQL, comparing the diameter values of Figure 8 (a) with Figure 10 (a),

an inversion in the trend was observed because the circularity deviation reduces with the

increase in the cutting speed, as occurred with the depth of cut. This behavior may be linked to

20

aluminium alloy's addition on the cutting edge and the occurrence of a BuE, especially at low

speeds, which would cause an increase in the deviation of the circularity of the hole. As BuE

tends to reduce with the increase of 𝑣𝑐 , the circularity deviation tends to reduce.

The reduction of the circularity deviation with the increase of the depth of cut using MQL

is related to the increase of chip thickness. The analysis of the interaction between 𝑓𝑧 and dp

can confirm it because the lowest circularity deviation values were obtained with these

parameters' highest values. Similar results were found by De Chiffre et al. (2009) [42] in five

different cutting conditions, using MQL, with external application. It was tested a combining

of two levels of cutting depth (p = 0.1 and 0.05 mm) with two feed levels (𝑓𝑧 = 0.3 and 0.2

mm/rev). Among other results pointed out by the authors, the use of low values of dp such as:

0.05 mm are not recommended due to the reduction of chip thickness increasing the probability

of chip breaks, depending on the radius of the tool's hadl.

CFA results showed the opposite trends concerning MQL for cutting speed. However, at

the lowest cutting speed, the value of circularity deviation is similar, i.e., both machining starts

with average values between 0.020 and 0.026 mm, and CFA the values tend to increase with

the cutting speed. In this range, there was an increase in the circularity deviation with the

increase in cutting speed.

For the DM, the increase of the circularity deviation with the increase of 𝑓𝑧 of about 0.02

mm, a value higher than the tolerance itself. This result is opposite to that found for the

diameter, which tends to reduce with increased 𝑓𝑧 . This reversal in trends is exceptionally

harmful to the process because it hinders the adjustment of parameters since both deviations of

circularity and diameter define the product's qualitative characteristics.

3 .3 . Performance Analys is o f Atmosphere (MQL, CFA, and

DM)

Figure 12 (a) (b) (c) shows the optimized values for the three machining techniques,

MQL, Fluid, and Dry, respectively. Considering that machining performance should be

considered for both characteristics, the values were adjusted simultaneously to minimize the

deviation of circularity and the target value for the diameter of 67.015mm, using the "optimized

response" function of the Minitab® version 17.

21

Figure 10: Significant parameters for linear and second-order circularity, a) MQL, b) CFA, c) DM

By comparative analysis of the three graphs' values, it is noted that MQL performed better

than CFA and DM. For the adjusted values, it would be possible to use the maximum cutting

parameters for machining with MQL, which would result in a shorter machining time, 0.5

seconds. For fluid machining, optimized cutting parameters would result in a machining time

of 0.75 seconds, depending on the use of the lowest value for 𝑣𝑐 .

Regarding DM, the optimized results were above the tolerance established for the

circularity deviation and at the upper limit for the diameter, whose values were 0.0163 mm for

the circularity deviation and 67.0297 mm for the diameter. In the tested pairs 4 and 5, the

circularity deviation values were between 0.015 and 0.021 mm, reducing the optimized

calculated values.

22

Figure 12: Cutting parameters optimized for diameter and circularity for machining, a)

MQL, b) CFA, c) DM. Two hypothesis tests were performed, which evaluated each technique's data to compare

machining techniques' performance and the possibility of substitution, especially between the

machining with MQL and Fluid. Table 11 shows the mean, standard deviation, maximum, and

minimum calculated for diameter and circularity deviation in each machining technique.

Table 11: Diameter data and circularity deviation for calculation of the mean hypothesis test

Diameter Circularity deviation

MQL CFA DM MQL CFA DM

Average 67.014 67.019 67.031 0.021 0.022 0.043 Standard Deviation 0.007 0.009 0.007 0.007 0.007 0.018 Maximum 67.022 67.031 67.043 0.031 0.035 0.082 Minimum 66.996 67.002 67.020 0.010 0.012 0.015

23

Table 12 shows the calculation of the mean hypothesis tests for the MQL versus Fluid and

MQL versus Dry techniques for diameter and circularity deviation. The P-Values greater than

the level of significance adopted, α = 0.05, indicates the impossibility of rejection of the null

hypothesis, H0, i.e., as established in the DoE. Thus, it is concluded that the values found for

both diameters and circularity deviation with MQL and CFA are part of the same population

and, therefore, are interchangeable, which means that both techniques are compatible and MQL

can replace CFA.

Table 12: P-Value results for the test of mean hypotheses for diameter and circularity deviation

The P-Values less than 0.05 indicate the rejection of the null hypothesis and, consequently,

the alternative hypothesis, H1. It means that the values found for the MQL and Dry techniques

do not belong to the same population and are therefore not interchangeable. It means that DM

could not replace MQL.

4 . Conclusions

In this work, three full factorial Design of Experiment - DoE with three variables in two

levels (±) (3𝑥 (23) were used to verify the performance and feasibility of replacing the CFA

by MQL. The input variables were: cutting speed (𝑣𝑐), feed per knife (𝑓𝑧), and depth of cut (𝑑𝑝). The output variables were diameter and circularity deviation. For each atmosphere,

dry, CFA, and MQL, a DoE (23) was performed with a test and replicate. A total of 48 runs

were performed. The main conclusions were:

• The cutting speed (𝑣𝑐), was the significant standard parameter between machining

techniques for both diameter and circularity deviation, except for the circularity

deviation in dry machining;

• Diameter tends to increase with increasing cutting speed for machining with MQL and

Fluid and reduce for dry machining;

Feature Analysis Hypothesis tried P-Value

Diameter

MQL x CFA H0: µMQL = µFluido

H1: µMQL ≠ µFluido 0,073

MQL x Seco H0: µMQL = µSeco

H1: µMQL ≠ µSeco 0,000

Circularity deviation

MQL x CFA H0: µMQL = µFluido

H1: µMQL ≠ µFluido 0,429

MQL x Seco H0: µMQL = µSeco

H1: µMQL ≠ µSeco 0,001

24

• The circularity deviation tends to reduce with increasing cutting speed for machining

with MQL and increase with increasing cutting speed for dry machining;

• The feed rate (𝑓𝑧), was the parameter that exerted the most significant influence to the

cutting speed, regardless of the trend, both for the diameter and for the circularity

deviation;

• The diameter tends to reduce, increasing the feed per knife when using CFA and DM.

An inverse situation occurred with the circularity deviation because the increase in

the feed per knife caused an increase in the circularity deviation;

• The depth of cut was the most significant variable for the circularity deviation in MQL

and DM. There was a tendency to decrease with the increase of depth of cut, possibly

due to the more excellent stability generated in the cut due to the increase in chip

thickness and consequent reduction of tool vibration;

• The depth of cut was the least significant variable but presented great second-order

interaction with 𝑣𝑐 and 𝑓𝑧;

• MQL showed similar results to CFA, as proven by the mean hypothesis test. The

optimized cutting parameters for the deviation of circularity and diameter were within

the specified tolerances, resulting in a shorter cutting time, 0.5s for MQL and 0.75s

for CFA. Consequently, MQL was more productive than CFA, considering the

conditions tested;

• Finally, based on this work's results, it can be affirmed that MQL could replace the

CFA technique.

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