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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 : [email protected]
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)
876-2
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
876-3
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.