Effects of Coolants on Improving Machining Parameters while Mach-inability Titanium Alloy (Ti-6Al-4V): A Review

Moaz H. Ali1, a, R. Balasubramanian1, Bashir Mohamed1 and Basim A. Khidhir2, b

1COE, University Tenaga Nasional, Kuala Lumpur, Malaysia

2FK, University Industry Selangor, Bestari Jaya, Malaysia

amuezhm@hotmail.com, bBak-time@hotmail.com

Keywords: component; Coolants, Improving machining, Titanium alloy.

Abstract. In this review, present day, the best versatile material Titanium alloy (Ti-6Al-4V) plays

the most vital role in the manufacturing of aero engines as a base material. It has been observed that

machining this type of alloys is the complicated tasks with optimum machining results. This

research investigates the consequences of machining parameters by making use three types of

technique: firstly, nitrogen gas technique; secondly, high pressure coolant (HPC); thirdly, cryogenic

cooling technique as the coolant’s techniques compared to the normal coolant. Utilizing liquid

nitrogen and gas nitrogen as a coolant will decrease friction, heat at the cutting zone, surface

roughness, the amount of power consumed in a metal cutting process in addition significantly

increase the tool life, dimensional accuracy and so improve the productivity.

Introduction

Machining is the most widespread metal shaping process in mechanical manufacturing industry.

All over the world, machining operations such as turning, milling, drilling, grinding and shaping to

consume a huge amount of money and time while machining of titanium alloys (Ti–6Al–4V).

Because it is having a high strength to weight rate, high toughness, high rigidity, low elasticity

models and high chemical reactivity at elevated temperatures.

Sharma [1] studied researchers have focused their attention on improvements in conventional

turning, so that tool life can be enhanced or tool failure can be avoided. By tool failure is meant

damage so large that the tool has no ability to remove material from a work piece. This tool damage

cannot be avoided, but we can find out ways to minimize it by analyzing its causes [2]. Stress and

the temperature at the tool surface influence the damage at the cutting tool [3]. Tool damage is very

sensitive to changes in the cutting conditions (cutting speed, feed rate and depth of cut) and

presence of cutting fluid. While tool damage cannot be avoided, it can often be reduced if its failure

mechanism and the factors controlling it are properly understood [4]. Tool damage is classified as:

adhesion, thermal damage (plastic deformation, thermal diffusion and chemical reaction),

mechanical damage (abrasion, chipping, fracture and fatigue). Out of these damages, thermal

damage increases drastically with increasing temperature. Adhesion wear is also a temperature

dependent phenomenon. The temperatures at which thermal damage and adhesion damage occur to

vary with a tool and work material combination. Thus heat generation in turning leads to a

reduction in tool life and as a result reduced surface finish [2]. The diffusion type thermal damage

occurs at high cutting temperature if cutting tool and work material elements diffuse mutually into

each other’s structure. The plastic deformation type of thermal damage is observed when a cutting

tool at high cutting temperature cannot withstand the compressive stress on its cutting edge [5, 6].

In metal turning operation, material is subjected to extremely high strain, and the elastic

deformation forms a very small proportion of the total deformation; therefore, it may be assumed

that all the energy is converted into heat [7]. Ying-lin [8] found that for very high temperature in the

machining zone can damage the finished surface of titanium alloys easily. Another impairment

caused by the high temperature is the melting of titanium alloy chips and adhesion to the tool and

on the machined surface.

Applied Mechanics and Materials Vols. 110-116 (2012) pp 1657-1666Online available since 2011/Oct/24 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.110-116.1657

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Where as Palanisamy [9] explain that the cutting fluids are used to remove heat generated at the

work-piece and tool interface during the machining process. These fluids act as both a coolant to

reduce tool temperature and also as a lubricant. Poor coolant delivery and low pressure can lead to a

number of adverse effects while machining titanium and its alloys. These effects include premature

tool failure due to abrasion and thermal shock resulting in severe chipping of the tool [10].

However, cutting fluids, which are supplied at high pressure (above 70 bars) and accurately directed

at the cutting zone can counter all these problems. The coolant pressure of machining centres has

progressively risen from initial pressures of around 4 bars too much higher pressures in the range of

70 bars and above [11]. A number of investigations [12–18] have shown that applying high-

pressure coolant (HPC) technology not only increases production efficiency, by increasing the

cutting speed but also improves chip removal, resulting in increased tool life while machining

titanium alloys [19]. It has also been shown that HPC results in better surface integrity and lower

compressive residual stress, which improves the properties of the machined work-piece [15].

Sharma [1] the coolant effect reduces the temperature in cutting zone and the lubrication action

decreases cutting forces. Thus the friction coefficient between the tool and chip becomes lower in

comparison to dry machining [20]. Minimizing the friction between the cutting edge of the tool and

work piece, corrosion control, chip ejection and washing are the functions of the cutting fluid in

machining [21].

Researchers have focused their attention on issues related to environmental problems, fluid

disposal, toxicity, filterability, misting, staining, surface cleanly-ness and indirect cost involved

while using coolant/lubricants and have tried to find out ways to improve the current techniques of

cooling during turning in order to reduce the temperature at the cutting zone and to

eliminate/minimize these problems. The lubricating action is more important to low cutting speed,

whereas the cooling effect is more important for higher cutting speed due to large increase in heat

generated by the chip removal process [5].

Then Yildiz [22] said the purpose of the application of the cutting fluids in metal cutting was

stated as reducing cutting temperature by cooling and friction between the tool, chip and work-piece

by lubrication [23]. Chip’s formation and curl, which affects the size of the crater wear and the

strength of the cutting tool edge, is also affected when coolant is carried out during machining.

Generally, a reduction in temperature results in a decrease in wear rate and an increase in tool life.

However, a reduction in the temperature of the work-piece can increase its shear stress, so that the

cutting force may be increased, and this can lead to a decrease in tool life [24].

Ali Khan [25] observed by [26] when machining Titanium alloys (Ti–6Al–4V) with high-

pressure coolant supplies and the recorded surface roughness values were found to be below the

tool rejection criteria.

[27] Introduced a new technique of application of cryogenic coolant during machining of Ti–

6Al–4V. They designed a special micro-nozzle for application of liquid nitrogen. Liquid nitrogen

was introduced through the gap between the chip breaker and the rake surface. The new technique

provides an effective way of cooling the tool using very low flow rate of nitrogen. Tool life was

reported to enhance five times to that using emulsion cooling. An innovative and economical

dispensing method of applying liquid nitrogen and nitrogen gas were introduced by [28] In this

study, liquid nitrogen and nitrogen gas cooling application methods and their effects on production

in machining operations have been reviewed in detail. Most of the liquid nitrogen and nitrogen gas

cooling applications in machining studies have been examined in turning operations even though

there were its applications in other machining operations such as drilling, milling and grinding.

1658 Mechanical and Aerospace Engineering

Fig.1- Techniques for reduction in heat generation during turning [1].

Nitrogen Gas Technique

For instance, Ying-lin [8] able to obtain that for when there was no nitrogen gas, the shape of

chips was continuous and undue. It was obvious that the chip was composed of several serrated

small chips. This characteristic was more obvious when observing the chip from its back side

“Fig.2”. The reason why this configuration was formed during high speed machining of Ti-6Al-4V

can be explained as follows. Chips that absorbed the majority of machining heat and could not be

removed away immediately from the machining zone would be softened and burnt by the high

temperature when the edge cut into the material. The temperature softening effect in the primary

shear zone was especially stronger than the other part and resulted in the plastic instability, leading

to catastrophic shear failure along the shear surface. So, after every constant length along the

cutting edge trajectory, a serrated chip would be formed. A longer chip composed of several

serrated parts was generated when an edge cut out the work-piece material. The fact that the chip

length was almost the same as the length of cutting layer showed that the chip did not break from an

edge cutting-into to cutting-out work-piece material. The dark blue color of the chip showed it was

burnt in the machining process. The continuous and burnt chips adhered to cutting edge and

machined surface would result in the poor integrity of machined surface, and increased cutting force

at the same time. This conclusion agreed well with the analysis of cutting force. “Fig.2 (b)” shows

the shape of chips obtained when nitrogen gas arrived at the machining zone. It was every obvious

that the color, length and shape of the chips were changed. The length was about 1/3−1/4 that with

nitrogen gas; the color was even gray; and the shape was not serrated.

Applied Mechanics and Materials Vols. 110-116 1659

Fig. 2- Configuration of chips under high speed machining of Ti-6Al-4V: (a) Without nitrogen gas;

(b) With nitrogen gas [8].

All these differences showed that, in an edge cutting process, the cutting layer was broke into

3−4 parts, and the chips did not burn but was removed away from machining zone because the

existence and high speed flowing of nitrogen gas. So, the chips did not adhere to the cutting tool,

and the abrasion of tool could be improved greatly. The surface integrity could be polished

correspondingly. The majority of machining heat was absorbed by chips. With nitrogen gas, not

only chip burning was prevented but also machining heat was removed by the flowing nitrogen gas.

So, when the surface was just finished, the temperature increment was very low (nearly the room

temperature), which could prevent the increase of hardness on the top layer of the finished surface.

In general, elements that affect chip shape include work-piece material properties, machining

parameters and tool geometry [29]. But based on above analysis, nitrogen gas is another factor that

affects chip shape of titanium alloys.

Prussure Technique

Besides that, Palanisamy [9] could get the different types of chips were generated during the

turning operation, and these chips have been classified according to ISO 3685 for tool life testing

[30]. When machining with standard coolant pressure (6 bar), short lustrous tubular chips (with an

average length of 30mm as shown in Fig. 3a) were generated when the insert was new and long

tubular chips (average length of 210mm) dull bluish in colour were produced towards the end of the

tool’s life. The application of HPC at 90 bars has resulted in very loose arc-shaped chips (with an

average length of 5mm as shown in Fig. 3b) at the beginning and then short washer-type helical

chips (average length of 40mm) after 10 minimums of cutting.

Serrated chip

Adhered burnt

chips

1660 Mechanical and Aerospace Engineering

Fig. 3- Chips obtained after1min cutting time (first cut) from the application of (a) standard

pressure coolant and (b) HPC [9].

Cryogenic Coolant Technique

However, Ali Khan [25] found the results show that nitrogen liquid and nitrogen gas cooling

enhances tool life. Tool life with a cutting speed of 100m/min (depth of cut and feed rate are 0.5mm

and 0.1mm/rev, respectively) using conventional coolant is 13.45 min, where for the same cutting

conditions the life is 57.45 min in nitrogen liquid and nitrogen gas cooling. It shows that the

application of nitrogen liquid and nitrogen gas coolant has increased the tool life by 4.27 times.

Again, at a higher cutting speed of 250m/min tool lives with conventional coolant and nitrogen

liquid and nitrogen gas cooling are 22.5 and 4.62 min, respectively with the same feed rate and

depth of cut. This shows an increase of tool life by 4.87 times. It can be observed that tool life at a

higher depth of cut is substantially low compared to that at a low depth of cut. For a cutting speed

of 200 m/min, tool lives for nitrogen liquid and nitrogen gas cooling and conventional cooling are

20.7 and 4.2 min at a depth of cut of 1mm, whereas the same are 25.92 and 5.4 min at a depth of cut

of 0.5mm.

That, he [25] found from his experimental as shown in “Fig.5” the SEM image of the flank wear

after a machining time of 2 min with conventional coolant at a cutting speed, feed and depth of cut

of 250 m/min, 0.5mm/rev and 1mm, respectively. The image shows extensive wear of the flank as

well as the face and this is due to heavy cutting conditions. The flank surface suffered from attrition

and abrasion wear. A crater wear is seen on the tool rake face that is caused by adhesion and

diffusion due to high temperature. But “Fig.6” shows that a little wear took place on the flank and

face of the tool while using cryogenic coolant after a machining time of 2min under the same

cutting conditions. The cutting edge suffers minor micro-chipping for high cutting speed. “Fig.7”

shows the flank wear of the tool after a machining time of 3min with conventional coolant while

“Fig.8” shows the same for cryogenic coolant under the same machining conditions (v = 200

m/min, f = 0.1mm/rev and t = 0.5 mm). “Fig.8” shows a little flank wear in cryogenic cooling. Less

abrasion and attrition wear is observed at the flank surface as compared to “Fig.6” (v = 250 m/min)

because of lower cutting speed (v = 200 m/min). “Fig.7” shows an extensive flank wear during

machining with conventional coolant. Flank wear is caused by abrasion. Micro-cracks are also

observed on the cutting edge. “Fig.9” illustrates the flank wear of the tool after 20 min of machining

Short washer-type

helical chips

Short

lustrous

tubular

chips

Applied Mechanics and Materials Vols. 110-116 1661

with conventional coolant while “Fig.10” shows the same for cryogenic cooling under the same

cutting conditions (v = 100 m/min, f = 0.05mm/rev and t = 0.5 mm). “Fig.10” shows a very little

flank wear after a machining time of 20min with cryogenic coolant.

Fig. 4(a)-Tool life at different cutting speeds (f = 0.1mm/rev) [25].

Fig. 4(b)-Percentage increase in tool life at different cutting speeds [25].

Fig. 5- Flank wear after 2min of machining with conventional coolant (v = 250 m/min, f =

0.1mm/rev and t = 1 mm) [25].

Fig. 6- Flank wear after 2min of machining with cryogenic coolant (v = 250 m/min, f =

0.1mm/rev and t = 1 mm) [25].

1662 Mechanical and Aerospace Engineering

Fig. 7- Flank wear after 3min of machining with conventional coolant (v = 200 m/min, f =

0.1mm/rev and t = 0.5 mm) [25].

Fig. 8- Flank wear after 3min of machining with cryogenic coolant (v = 200 m/min, f =

0.1mm/rev and t = 0.5 mm) [25]

.

Fig. 9- Flank wear after 20min of machining with conventional coolant (v= 100 m/min, f =

0.05mm/rev and t = 0.5 mm) [25].

Fig.10- Flank wear after 20min of machining with cryogenic coolant (v = 100 m/min, f =

0.05mm/rev and t = 0.5 mm) [25].

Discussion

This study shows us to confirm the presence of a basic and important role in more operations

machining of titanium and of various kinds of cooling, which coincide with the operations of the

mechanical operation of the work-piece and that effect on many factors, including the forces of

cutting, surface roughness, temperature distribution in a cutting process and deformation as well.

To prove the validity of verification of all, we have adopted in this research the different results

of the researchers, but those results in the same area and under the working conditions of different

methods of cooling are multiple. We found that Ali Khan could get the results show that cryogenic

cooling reinforced the age of the tool. Cryogenic cooling has increased the tool life by 4.27 times

more than normal cooling.

As it is well known that the temperature rise when tool and work-piece are interfaced, so could

do this case to lead the welding process, but also when the cooling could reduce the intense heat and

improve the specifications required of it and end the phenomenon of welding between the chip and

the work-piece.

Applied Mechanics and Materials Vols. 110-116 1663

However, Palanisamy adopted of pressure cooling, but the different pressure cooling between

lower pressure of 6 bars to the extreme pressure 90 bars. He found that the form of chip will be

spiral and is connected with a long distance according to the “fig.3” Furthermore, a significant rise

in temperature between the work-piece and the tool, however, found that the cooling pressure 90

bars may be produced by Reich intermittent short with a significant decrease in temperature and an

increase in the smoothness of the surface and improve the properties of the work-pieces.

It is also noticeable in the work of researcher Yildiz, who put himself to special conditions for

logging operations of the cutting forces, temperature, specific use of the work-pieces and the tool.

He found that there is a big difference and marked clearly on each of these properties before cooling

and after. The results were the best after cooling.

We can get a result in this a review paper after all this investigation depth of the working

conditions from different researchers. For instance, the cutting forces, and cooling are also different

than the cooling associated with operations that have taken advantage from the right way and using

the distinct under what conditions they will, we can get a smooth surface and a decrease in

temperature resulting in longer life of the tool for three times. These are cases save for us time and

cost and improve product quality as well.

Conclusion

The aim of this study is to analyze and point out the effect types of cooling on cutting performance

in material removal operations and its application methods. Other conventional coolants, heat

generation and temperature distribution in a cutting process have been also discussed. When

compared with dry cutting and conventional cooling, the most considerable characteristics of the

cryogenic cooling, nitrogen gas, and high pressure coolant (HPC) application in machining

operations could be determined as enabling substantial improvement in tool life and surface finish-

dimensional accuracy through reduction in tool wear through control of machining temperature

desirably at the cutting zone. This type of cooling has been executed in cutting operations in

different ways by using liquid nitrogen and nitrogen gas for pre-cooling the work piece, cooling the

chip, cooling the cutting tool and cutting zone. In these studies, comparisons and discussion have

been made between conventional cutting strategies and liquid nitrogen and nitrogen gas cooling

methods and found the importance of cooling during machining titanium alloys. Consequently, the

conclusions described above could change relating to tooling work piece pairs and cutting

conditions.

Acknowledgment

The authors would like to acknowledge the support our University Tenaga Nasional “UNITEN”.

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Effects of Coolants on Improving Machining Parameters while Mach-InabilityTitanium Alloy (Ti-6Al-4V): A Review

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