5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

876-1

A STUDY ON THE MINIMUM QUANTITY LUBRICATION IN GRINDING

OF TITANIUM ALLOY (TI-6Al-4V)

1MONITH BISWOJYOTHI, 2A.S.S.BALAN, 3N.ARUNACHALAM,

4*L.VIJAYARAGHAVAN

Manufacturing Engineering Section,

Department of Mechanical Engineering, IIT Madras, Chennai-36

*Email : lvijay@iitm.ac.in

Abstract

Recent developments and chaos on climate change has led to researchers and scientists world over to rethink and

bring impactful changes to the current manufacturing processes. Current manufacturing trends require large usage of

cutting fluids which has led to adverse climatic changes and increased costs to the industries. With the aim to reduce

the consumption of cutting fluids, Minimum Quantity Lubrication (MQL) strategy has been adopted by various

machining and grinding applications. In this work, the grinding of Titanium alloy Ti-6Al-4V with minimum quantity

lubrication conditions were carried out. In order to maintain a good surface integrity and to improve the grindability

of Ti-6Al-4V experiments were conducted by varying different MQL parameters. The MQL parameters such as

coolant concentration, coolant flow rate and air pressure were varied and its influence on the grinding forces, surface

roughness, surface texture data were measured and analyzed. The results showed a decreasing trend in grinding

forces and surface roughness with increasing coolant concentration, increasing air pressure and increasing coolant

flow rate. This indicates that the suitability of MQL for grinding of titanium alloy to use in various applications.

1 Introduction

The stringent environmental regulations limits

the amount of carbon discharge from the manufacturing

industries. Due to this there is an increasing need for

environmentally friendly production techniques. Silva et

al. [1, 2] investigated the effects of grinding parameters

on ABNT 4340 steel using MQL technique. The surface

roughness, diametral wear, grinding forces and residual

stresses are improved with the use of the MQL system

in grinding process due to better lubrication of grinding

zone and providing better slipping of grain at the

contact zone [1]. Balan et al [3] studied the grindability

of Inconel 751 alloy under MQL conditions. The

surface roughness, force and the chip morphology were

shown improvement with the MQL conditions. The

specific function of cutting fluid in the machining

process is to provide lubrication and cooling to

minimize the heat produced between the surface of the

part and the tool [1]. However, by eliminating these

fluids, their positive influence on machining is also lost

since cutting fluid is an important technological

parameter in machining [1].

This paper deals with a investigation of

grindability of Titanium alloy Ti-6Al-4V with MQL

conditions. The small amount of lubricant is atomized in

compressed air stream. The atomized spray cools the

grinding zone and provide lubrication. The coolant

concentration, coolant flow rate and the air pressure of

the MQL setup were varied and its influence on the

surface texture, forces, chip morphology were studied

and presented.

2. MQL grinding of Titanium alloys

The Titanium alloy is the most widely used

material for aerospace and defense applications. Their

unique property of high strength to weight ratio, light

weight, low density, and high temperature strength

makes this alloy most favorable for high temperature

and corrosion resistance applications. But it is one of the

most difficult to machine materials due to its low

thermal conductivity, high chemical reactivity at

elevated temperature and low modulus of elasticity. The

grinding zone temperature is usually much higher than

other machining shear zone for Ti-6Al-4V due to its low

thermal conductivity. The heat accumulation occurs on

the surface which may lead to formation of residual

tensile stress, surface cracks and re-deposition on the

ground surface. This results in reduced fatigue life of

the product during continuous usage. Minimum

Quantity Lubrication (MQL) employs grinding with the

usage of very minute quantities of lubricant or cutting

A STUDY ON THE MINIMUM QUANTITY LUBRICATION IN GRINDING OF TITANIUM ALLOY (TI-6Al-4V)

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fluids to the grinding zone. In MQL, the cooling media

is supplied as a mixture of compressed air and a

lubricating oil in the form of aerosol. An aerosol is a

gaseous suspension into air of solid or liquid particles.

Aerosols are generated using the process called

atomization, which is the conversion of a bulk liquid

into a spray or mist, often by passing the liquid through

a nozzle. Typically, in MQL the cutting fluid flow rate

of around 50–500 ml/hour is maintained. In

conventional flooded coolant system, the injected

cutting fluid doesn’t reach the grinding zone resulting in

elevated temperatures. The cutting fluid diverges

elsewhere because of the presence of an air barrier

surrounding the grinding wheel. Using pressurized air

helps the mist to penetrate through the air barrier and

reach the grinding zone. Sadeghi et al [4] studied the

MQL grinding of Ti-6Al-4V Alloy and showed that

better surface quality can be achieved with MQL

grinding. Guoqiang Guo et al [5] investigated the use

MQL conditions for grinding of Ti-6Al-4V alloy using

SiC grinding wheel. The MQL grinding provided better

results with less surface damage in comparison with dry

and fluid grinding. These encouraging results calls for

further investigations on MQL parameters and its

influence on grindability of Ti-6Al-4V alloy.

3 Experimental Work

The surface grinding experiments were carried

out on a Schutte type tool and cutter grinding machine

fitted with an automatic table traverse unit. Down

grinding was carried in the experiments to limit the

wheel loading and the sliding heat during grinding.

Table 1 shows the detailed Grinding Conditions. Nine

experiments for each concentration level were

conducted under the same grinding conditions.

Fig.1 MQL Grinding Set-up, 1-Grinding wheel, 2-

Dynamometer, 3-Air compressor, 4- Atomizing spray

Nozzle, 5- Charge amplifier

Fig:2 MQL Set-up, 6- Cutting Fluid Mixture Reservoir,

7. Nozzle 8- Flexible Delivery Hose, 9- Air Pressure

Gauge

The tool and cutter grinder had a spindle motor

rated 0.7/0.9kW. The workpiece was held firmly using a

small vice mounted on the dynamometer on a slide on

top of the electromagnetic rectangular chuck. Resin

bonded CBN grinding wheel was used to perform the

experiments on workpiece with dimensions of 55 x 22.5

x 20 mm. The photograph of the experimental setup was

is shown in Fig.1.The 55 x 20 mm surface was face up.

After the mounting was done the workpiece was ground

several times to ensure flatness and parallel conformity

with wheel feed. Throughout the experiments the speed-

feed-depth of cut was kept constant. Before conducting

every experiment, the grinding wheel was dressed at

10µm down feed using a silicon carbide stone.

A specific water miscible cutting fluid having

high solubility was used for experiments such that it can

provide good lubrication property as well as keep the

cutting temperatures low. The emulsion produced out of

the mixture gave a fine milky brown color. ‘KyrosEpsol

Super S/1’ is a chlorine free, water miscible cutting

fluid with excellent solubility. It is a highly stable, bio-

resistant; oil-in water emulsion developed for use as a

general purpose, cost effective, metal working fluid. It

is free from chlorine, nitrite and phenolic compounds

and has excellent anti-foam characteristics. The MQL

set-up was gravity fed, i.e., the coolant reservoir was

kept at a height higher than the nozzle. The coolant

trickles down the tube into the mixing chamber in the

nozzle due to suction pressure created by the injected

5 2

4

6 7

3

5

1

6

87

9

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

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pressurized air. The coolant mixing with the pressurized

air is regulated by a coolant flow rate valve. An

‘Internal Mix’ type atomizing nozzle was used where

liquid and air are mixed internally to produce a

completely atomized spray. The MQL arrangement was

shown in Fig.2.The experimental design entails the

selection of suitable levels to the most important

parameters for all grinding environments. The most

important MQL parameters that need to be studied are

air pressure, coolant flow rate, and concentration of

coolant in emulsion.

Table 1.Experimental Conditions

Table 2.Parameters and their Levels

Parameters Level 1 Level 2 Level 3

Concentration of

Coolant (%) 2 3 4

Air Pressure (Bar) 2 4 6

Coolant Flow Rate

(ml/hr) 60 80 100

The Table 2 shows the parameters and their

levels that used for the entire set of experiments. A total

of 33= 27 different combinations of experiment

conditions were conducted.

4 Results and Discussions

The cutting force and surface roughness values

showed a decreasing trend with increasing pressure and

increasing coolant concentration. Wheel-work interface

experiences a better lubricating action effectively

reducing sliding friction and adhesion of chip on rake

face of the grit.

Machine Tool and Cutter Grinder

(Schutte, Germany, 1965)

Grinding mode Plunge Surface Grinding (Down

Cut)

Grinding wheel Resin Bonded CBN Wheel;

F10A.100.304

Grinding wheel

specifications

D150-T13-X3-B126-

R100PN47-

H31.75

Wheel speed (Vs) 2826m/min (Constant)

Table feed rate

(Vw) 0.857m/sec (Constant)

Depth of cut (h) 30µm per pass (Constant)

No of passes 2

Coolant flow rate

(Q) 60ml/hr, 80ml/hr, 100ml/hr

MQL cutting

fluid KYROS EPSOL SUPER S/1

Coolant

concentration 2%, 3%, 4% in 100ml of water

Air pressure (P) 2 Bar, 4 Bar, 6 Bar

MQL nozzle

distance from

contact zone

d = 100mm (approx.)

Horizontal angle

to the workpiece

θ = 20° (approx.)

Workpiece

material

Ti-6Al-4V plates of thickness

(55 X 22.5 X 20 mm)

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9

Av

era

ge N

orm

al

Fo

rce F

z (N)

2% 3% 4%

Fig.3. Bar graph showing the variation of average

tangential Force

A STUDY ON THE MINIMUM QUANTITY LUBRICATION IN GRINDING OF TITANIUM ALLOY (TI-6Al-4V)

876-4

When coolant concentration increases, the film forming

tendency of the lubricant increases due to higher

viscosity, energy formation of the work material drops

down due to Rehbinder effect [6] resulting in reduction

in cutting forces. The variation of average tangential

and normal forces with different MQL parameter were

shown in Fig.3 and Fig.4.

The Rehbinder effect results in reduction in the hardness

and ductility of a material by a surface-active molecular

film. When air pressure increases the air flushes out the

chips from the grinding surface hence maintaining a

clean cutting edge by which rubbing of the chips with

workpiece is reduced leading to lesser cutting forces.

Observations show that at high pressure and at

low or moderate coolant flow rates and highest coolant

concentration we get the best grinding performance.

This can be observed from the Fig.5. where the

variation of surface roughness was shown with different

MQL conditions. so at higher air pressure the mist can

easily penetrate the air barrier surrounding the wheel

hence providing better

4.1 Chip Morphology

The chip morphology clearly indicates the

mechanism of grinding at different MQL conditions.

Ductile materials produce long ribbon-like chips

whereas hard and brittle materials such as ceramics

produce small powder-like particles. At low pressure

and concentration there is no sufficient cooling in the

grinding zone, which leads to melting and formation

globules kind of chips as shown in Fig.6

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5 6 7 8 9

Ra

(µ

m)

2% 3% 4%

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9

Av

era

ge T

an

gen

tial F

orc

e F

x(N

) 2% 3% 4%

Fig.4. Bar graph showing the variation of average

Normal force

Fig.5. Bar graph showing the variation of

average surface roughness (Ra (µm)

Fig.6. SEM image showing chip morphology with

MQL parameters of 4% Concentration, 6 Bar Air

Pressure and 100 ml/hr

Fig.7. EDAX profile of the marked area

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

876-5

Fig.6 and 7 showing a SEM image of

chip morphology with MQL parameters of 4%

Concentration, 6 Bar Air Pressure and 100

ml/hr Flow Rate along with EDAX profile for

the marked area. Table 3 shows the

concentration by weight of its constituent

elements. Chip study shows the formation of

oxides, carbides or hydrides. The table

suggests the chips formed were devoid of

oxide/carbide /hydride formation.

Table 3.Concentration by Weight

Round or oval like shaped chips shows the fact that

chips got severely heated and melted (shown in Fig 8

(b)). Such kind of chips appears in the case of either

improper grinding parameters being selected or with

improper lubrication being used.

The chips are melted in a separate process, since a large

amount of grinding heat is generated and conducted into

the chips. With high pressure and concentration, the

chips become serrated with lamella type morphology.

The Ti-6Al-4V alloy’s ductility was improved due to

longer chips formation with increasing coolant

concentration and air pressure as shown in Fig 8. This is

due to the Rehbinder effect, a possible reduction of

surface energy of the work surface, with effective

lubrication there is an increase in elastic-plastic

deformation under the cutting edge.

4.2 Surface Texture

Any ground surface would usually consist of

grooves produced by ‘ploughing’. Under examination,

four types of area on the ground surfaces were

observed: (1) ploughing area, (2) burnt area, (3)

smeared area, (4) debris covered area.

Fig 9 Surface texture obtained at 2 bar, 2%

concentration, 100ml/hr,

Fig.10 Surface texture obtained at 4 bar, 3%

concentration, 60ml/hr.

Element Wt% At%

AlK 06.78 11.45

TiK 89.65 85.35

VK 03.57 03.20

Matrix Correction ZAF

3

2

4

1

Fig.8 SEM image of chip morphology

with MQL parameters of 4%

concentration, 2 bar air pressure and 100

ml/hr flow rate

A STUDY ON THE MINIMUM QUANTITY LUBRICATION IN GRINDING OF TITANIUM ALLOY (TI-6Al-4V)

876-6

The surface texture images were shown in Fig.

9,10and11 clearly show the effect of higher coolant

concentration and higher air pressure on the

surfactexture produced.

Fig.11 surface texture obtained at 4 bar, 3%

concentration, 80ml/hr.

Ploughing energy is expended by deformation of

workpiece material without removal. It is usually

associated with side flow of material from the cutting

path into ridges, but it can also include plastic

deformation of the material passing under the cutting

edge. Burnt area shows that for that grinding condition

the lubrication was not enough. Smeared areas are

unwanted marks which occur when a removed chip rubs

against the grinding surface. Debris covered area occurs

when the chips are smeared on to the surfaces.

Sometimes the burn marks may occur due to the

instantaneous cracking and burning of lubricant oil

while grinding. The burn marks are shown by the bluish

and blackish temper color. The possibility of resin bond

of grinding wheel melting and getting embedded on to

workpiece surface cannot be avoided.

5 Conclusions

• The results show us that amongst all the MQL

conditions, medium order coolant flow rate and

higher pressure has better grinding

performance due to boundary lubricated

environment that develops around the wheel-

work interface.

• A higher order coolant concentration (4%)

provides better grinding performance as shown

lower grinding forces, lower surface roughness,

and relatively better surface texture. At low air

pressures, the grinding forces and surface

roughness increases. Chip morphology study

reveals that mostly big chunks of chips were

produced but a change in the trend was

observed for higher coolant concentration and

higher pressure when relatively longer lamella

type chips was seen.

• High coolant concentration with high pressure

and medium coolant flow rate produced good

surface texture without any burn marks, smear

marks or debris covered area.

References

1. L. R. Silva, E. C. Bianchi, R. E. Catai, R. Y. Fusse,

T. V. França, P. R. Aguiar (2005), Study on the

behavior of the minimum quantity lubricant - MQL

technique under different lubricating and cooling

conditions when grinding ABNT 4340 steel, Journal of

the Brazilian Society of Mechanical Sciences and

Engineering, 27, No.2, 192-199.

2. Silva LR, Bianchi EC, Fusse RY, Catai RE, FranÇa

TV, Aguiar PR (2007), Analysis of surface integrity for

minimum quantity lubricant-MQL in grinding.

International Journal of Machine Tools Manufacture,

47, 412:418.

3. Balan A. S. S, Vijayaraghavan L, Krishnamurthy R

(2013), Minimum Quantity Lubricated Grinding of

Inconel 751 Alloy, Materials and Manufacturing

Processes, 28: 430–435.

4. M. H. Sadeghi, M. J. Haddad, T. Tawakoli, M.

Emami, Minimal quantity lubrication-MQL in grinding

of Ti–6Al–4V titanium alloy(2009), International

Journal of Advanced Manufacturing Technology, 44,

487–500.

5. Guoqiang Guo; Zhiqiang Liu; Qinglong An; Ming

Chen(2012), Investigation on surface grinding of Ti-

6Al-4V using minimum quantity lubrication, Int. J. of

Abrasive Technology, Vol.5, No.3, pp.187 - 201.

6. Malkin A. I, ‘Regularities and Mechanisms of the

Rehbinder’s Effect’, ISSN 1061_933X, Colloid Journal,

2012, Vol. 74, No. 2, pp. 223–238.