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Journal of Manufacturing Processes 35 (2018) 244–253
Contents lists available at ScienceDirect
Journal of Manufacturing Processes
journal homepage: www.elsevier.com/locate/manpro
An experimental investigation on the effects of minimum quantity nano
lubricant application in grinding process of Tungsten carbide
S.F. Hosseinia, M. Emamib, M.H. Sadeghia,
a
b
T
⁎
CAD/CAM Laboratory, Manufacturing Engineering Division, Faculty of Engineering, Tarbiat Modares University, Tehran, Iran
Department of Mechanical Engineering, Faculty of Engineering, Behbahan Khatam Alanbia University of Technology, Behbahan, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords:
Minimum quantity
Nano lubrication
Grinding
Tungsten carbide
Minimum quantity lubrication (MQL) technique is an efficient and eco-friendly method of lubricant application
in machining processes. The lubricant type applied in this technique is not only important in relation to environmental aspects but also has a great effect on machining efficiency. Nanolubricant (nanofluid) is composed
of nanoparticles suspended in base oil and shows prominent lubrication performance. In the present study, the
effects of minimum quantity nano-lubrication (MQNL) in surface grinding of Tungsten carbide grade YG8 is
experimentally investigated. The nanolubricants applied in the experiments include MoS2, graphite, and Al2O3
nanoparticles (with varying concentration) dispersed in two different base oils- mineral oil (paraffin) and vegetable oil (sunflower). The grinding outputs such as specific energy, cutting force, and surface quality were used
as measurands for determining the process efficiency. Furthermore, performance evaluation of MQNL in
grinding process of WC material was performed by comparing the grinding outputs at different environment
such as dry, wet, and MQL. The results show that, if nanoparticles are selected properly, MQNL technique is an
effective method to improve the process efficiency by reducing the grinding force, specific energy, and increasing surface quality.
1. Introduction
Minimum quantity lubrication (MQL) is an environmentally friendly
method of using cutting fluid in machining processes. In MQL machining, the high pressurized air sprays a minute amount of lubricant
(10–200 ml/h) in the form of an aerosol onto the tool-workpiece contact zone [1]. MQL has been applied to different machining processes
such as turning, milling, drilling, grinding, and etc. Also, promising
results in MQL machining have been reported [2–5]. Grinding is a
conventional machining process which is mainly applied for finishing
work surface. However, it is inherently associated with high specific
energy requirements, unlike other conventional machining processes
such as turning, milling, drilling, etc., which result in a high grinding
zone temperature and poor surface integrity [6]. The researches conducted on MQL in grinding have depicted that, MQL can considerably
improve the grindability of work material. Hafenbraedl and Malkin [7]
found that utilizing MQL technique in internal cylindrical grinding,
reduces the grinding power as well as specific energy and grinding
wheel wear due to efficient lubrication of the grinding zone. Silva et al.
[8] investigated the effects of MQL in cylindrical plunge grinding of
ABNT 4340 steel (60 HRC). They found that the MQL technique results
⁎
in more effective lubrication, improve the surface roughness, wheel
wear, grinding forces and residual stress. Tawakoli et al. [9] performed
a comparative study of two steel grades namely, hardened steel 100Cr6
and soft steel 42CrMo4 in MQL surface grinding process. Their study
showed that lower tangential grinding force and improved surface
quality are obtained when MQL is applied to grinding 100Cr6 hardened
steel. Furthermore, MQL grinding compared to flood cooling resulted in
higher material removal rates with higher surface quality and lower
grinding force. Barczak et al. [10] investigated three cooling methods:
wet, dry and MQL on the grinding process of steel alloys EN8, M2, and
EN31. Their research showed that MQL can perform comparably to
flood delivery under the experimental conditions applied.
The media used as a lubricant in MQL plays an important role in
improving the grinding efficiency and surface quality. Studies by
Brinksmeier et al. [11] demonstrated that the type of lubricant applied
in MQL grinding (ester oil or emulsion) can significantly affect the
process results. Brunner [12] showed that ester oil, as compared to
mineral oil, during MQL grinding of 16 MnCr5 (SAE-5115) reduces the
tangential and normal grinding forces to one third, but increases the
surface roughness by 50%. Sadeghi et al. [5] investigated the influences
of two types of lubricants (vegetable oil and synthetic ester oil) on MQL
Corresponding author.
E-mail addresses: s.farshid.hosseini@gmail.com (S.F. Hosseini), dr.Emami@bkatu.ac.ir (M. Emami), dr.sadeghi@gmail.com (M.H. Sadeghi).
https://doi.org/10.1016/j.jmapro.2018.08.007
Received 27 August 2017; Received in revised form 13 July 2018; Accepted 7 August 2018
1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
grinding of Ti–6Al–4 V titanium alloy. Their results showed that synthetic ester oil outperforms vegetable oil in terms of higher surface
quality (lower surface roughness, improved surface texture, no burned
surface) and lower grinding force in the conditions investigated. Emami
et al. [13] explored the performance of four types of lubricants, namely
mineral, hydrocracked, synthetic, and vegetable oils, with regard to the
reduction in cutting force, specific energy and surface roughness during
near dry grinding (MQL grinding) of Al2O3 engineering ceramic. Their
research depicted that synthetic and hydrocracked based oils show
satisfactory environmental and technical performance in MQL grinding
of Al2O3 ceramics. It is clear from the above literature that MQL-media
is a key technical area that can enable the success of MQL grinding
processes. Recently, nanolubricants due to their special tribological
properties have been applied as MQL-media in machining processes.
Nanolubricant is defined as an engineered material that consists of
nanometer sized particles dispersed in base oil [14]. In minimum
quantity nano-lubrication (MQNL) process the nanolubricant is atomized into droplets by an air atomizing nozzle. The oil droplets containing nanoparticles are carried by the air stream and impinge on the
wheel surface. Then the wheel grains impregnated with both oil and
nanoparticles come into the cutting zone and lubrication is performed.
It has been shown that nanoparticles can improve the tribological
properties of base oils by increasing the extreme-pressure and loadcarrying capacity and decreasing the friction coefficient [15,16]. The
effectiveness of the nanolubricants depends on the properties of the
base oil and dispersed nanoparticles characteristics. Type, Morphology,
crystal structure, size, and quantity of nanoparticles are important
factors influencing the lubrication performance (tribological properties)
of nanolubricants [17]. Shen et al. [15] applied water-based Al2O3 and
diamond nanofluids (with varying concentration of nanoparticles) in
the MQNL grinding of cast iron and compared the results with dry, wet,
and MQL methods. Their study showed that lower grinding force, improved surface roughness, and higher G-ratio are obtained when high
concentration nanofluids are used as media. Kalita et al. [18] experimentally investigated the influences of oil-based nanolubricants composed of MoS2 nanoparticles (< 100 nm) dispersed in two different
base oils -mineral oil (paraffin) and vegetable oil (soybean)- in MQNL
grinding process. The experiments were carried out on cast iron and EN
24 steel under different lubrication conditions—MQNL (with varying
concentration of nanoparticles), MQL with pure base oils, and flood
grinding using water-based coolant. The authors concluded that the
MQNL grinding increases process efficiency by reducing the specific
energy, coefficient of friction, and tool wear. Moreover, the nanolubricant effectiveness is also found to increase with increasing nanoparticle concentration. Vegetable and mineral based-nanolubricant
performed best for steel and cast iron, respectively. Furthermore, the
formation of tribo-chemical films on work surface was identified as the
mechanism responsible for process improvements.
The literature review shows that the MQL using nanolubricants have
a great potential to be applied in grinding processes. Therefore, more
investigations are required to elucidate the effects of nanolubricants on
grinding process of engineering materials. On the other hand difficult to
grind materials such as carbides, superalloys, ceramics, and etc have
poor grindability and there is an urgent need to improve their grinding
efficiency and surface quality. Tungsten carbides due to high hardness,
toughness, and wear resistance are extremely difficult to grind. These
materials have been used extensively for various wear-resistant applications, such as cutting bits for machining, coal mining and well boring
as well as wear resistance parts in dies, machinery, and ore crushing
equipment. Therefore applying nanolubricants in MQL grinding of
Tungsten carbide material is an interesting case to be explored. In the
present study nanolubricants with different types of nanoparticles such
as Al2O3, MoS2, and graphite at two concentrations (1 wt% and 3 wt%)
dispersed in two different base oils—mineral oil (paraffin) and vegetable oil (sunflower)— are applied in MQL grinding of tungsten carbide
(WC) grade YG8. For comparing purposes, other lubrication techniques
Table 1
Workpiece Material Specification.
Tungsten
Carbide
Grade
Density [g/
cm3]
Elastic modulus
[GPa]
TRS[MPa]
HardnessHRA
YG8
14.80
698
2200
89.5
Fig. 1. Grinding workpiece specimens (rectangular bars of 42 × 8×4 mm).
such as dry, wet and pure MQL are also tested. Cutting force and surface
quality (including surface roughness and surface texture) are used as
measurands for determining the efficiency of the process. The morphology and of chemical analysis of the ground specimens are studied
using field emission scanning electron microscopy (FE-SEM) and energy
dispersive spectroscopy (EDS).
2. Experimental setup and procedure
The work material used in this study is tungsten carbide (WC) grade
YG8. The physical and mechanical properties of the workpiece material
are shown in Table 1. Grinding workpiece specimens were cut to rectangular bars of 42 × 8×4 mm (Fig. 1) using wirecut Electro Discharge
Machining (EDM). The experimental setup and grinding parameters
used in this study are shown in Fig. 2 and Table 2 respectively. The
grinding machine used for the tests is JUNG F 50 horizontal surface
grinder. A resin bonded diamond grinding wheel with grit size D181
and a 75% diamond concentration was used with a constant peripheral
speed of 30 m/s. A brake-controlled truing device (Norton 4597) with a
vitrified silicon carbide wheel GC60 L at a speed 1250s.f.p.m, depth
10 μm and transverse feed rate 150 mm/min was used for wheel truing.
Prior to each test, wheel dressing was carried out with an alumina stick
Norton 38A150-I8VBE so that the wheel topography consistency could
be maintained. Each grinding experiment was done on the 42 mm ×
4 mm specimen surface in up-cut plunge mode. The nanolubricants
were prepared for the experiments by addition of nanoparticles, such as
MoS2, graphite, and Al2O3 (with 1 wt% and 3 wt% concentration) in
two different base oils: mineral oil (paraffin) and vegetable oil (sunflower) as shown in Fig. 3. Moreover, a sonicator (Hielscher UP4OOS,
probe tip diameter 3 mm) was used at 80% amplitude for 15 min to
disperse the nanoparticles in the base oils. An MQL system equipped to
separately control the oil and air flow rates with a gas-assisted atomization nozzle sprays the nanolubricants to the grinding zone. The MQL
jet spray was targeted at an angle a = 15°, distance L = 30 mm toward
a point on the wheel surface and at a height H = 15 mm from the work
surface. Furthermore, the nanolubricants at a flow rate of 1.5 ml/min
with gas at a flow rate of 30 l/min were applied via the MQL nozzle to
the lubrication target point. Fig. 4 illustrates the schematic of the nanolubricant spray impinges on the grinding wheel. Additionally, pure
MQL, flood cooling as well as dry tests were conducted for comparison
purpose with the nanolubrication tests. The pure MQL tests were carried out using neat base oils: mineral oil or vegetable oil with the same
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Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
Fig. 2. Schematic of experimental setup.
Table 2
Grinding conditions.
Workpiece material
YG8 Tungsten Carbide
(42 mm × 8 mm × 4 mm)
Grinding machine
Horizontal surface grinding machine JUNG
F 50
1A1 200 × 15 × 51 K120 N D181 C75
Plunge surface grinding, up cut
30 m/s
10 m/min
20 μm
Dry, Wet, MQL, MQNL
Mineral oil, Vegetable Oil
1.5 ml/min
2 bar
30 mm
Grinding wheel
Grinding mode
Wheel speed (Vc)
Work speed (Vw)
Depth of cut (ae)
Environments
MQL/MQNL base oil
MQL/MQNL lubricant flow rate
MQL/MQNL gas (air) pressure
MQL/MQNL nozzle distance from
the lubricating zone (L)
MQL/MQNL horizontal angle to
the workpiece (α)
Truing
Dresser
Fig. 4. The schematic of the nanolubricant spray impinges on the grinding
wheel.
Table 3
Physical-chemical properties of base oils.
15 degree, To workpiece
Physical-chemical
properties
Brake controlled truing device
Aluminum oxide stick A150 1/2 × 1×6"
lubricant delivery settings as nanolubrication tests. Also, in flood
cooling tests Mobilcut 100 at a 5% concentration and 5 l/min flow rate
was used. Table 3 provides base oil properties and Table 4 shows test
numbers with the condition of lubricants applied in the experiments.
A 3-component piezoelectric Kistler dynamometer Type 9255B together with a Kistler charge amplifier Type 5019 A measured the
grinding forces. The data acquisition software DynoWare Type 2825 A
was used for visualization, calculation and recording force data. The
specific grinding force was obtained by dividing the grinding force over
the width of the workpiece. Prior to conducting profilometry, the surface of all samples was washed with acetone and dried with hot air.
Workpiece roughness Ra and Rz were measured along and across the
grinding direction by a portable Mahr (Mar Surf PS1) stylus profilometer with a 1.75 mm cutoff length. Every experiment was repeated
five times with the identical settings, and the average value of the
measurants was calculated as the result. Scanning electron microscopy
(SEM), and energy dispersive X-ray spectroscopy (EDS) were used respectively for assessing the microstructural and chemical analysis of the
ground surface of work samples.
Kinematic viscosity at 40°c
(ASTM D-445)
Flash point (ASTM D-92)
Density at 15°c (ASTM D4052)
Additives
Base oil Type
Mineral Cutting oil
Behran-11
Sunflower Vegetable
oil
24 cSt
29 cSt
170°c
870 kg/m3
170°c
975 kg/m3
Chlorine compounds
–
3. Results and discussion
3.1. Specific grinding force
The results of the specific tangential and normal grinding forces as a
function of lubrication condition, obtained during grinding of YG8
tungsten carbide, are shown in Figs. 5–10. Fig. 5 depicts the results of
the specific tangential grinding force. The base oil applied for pure MQL
and nanolubricants in this part of experiments is vegetable oil. For
comparison purpose, other lubrication methods including dry and wet
grinding are also tested. Correspondingly, in Fig. 6 the specific normal
grinding force results in the specified lubrication condition are shown.
Figs. 5 and 6 illustrate that in conventional wet grinding the specific
Fig. 3. The nanolubricants prepared for the experiments.
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Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
Table 4
The test numbers with lubricants applied in the experiments.
Test number
Lubrication condition
Base oil
Nanoparticle
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Dry
Wet
Pure MQL
Pure MQL
MQNL
MQNL
MQNL
MQNL
MQNL
MQNL
MQNL
MQNL
MQNL
MQNL
MQNL
MQNL
–
–
Vegetable
Mineral
Vegetable
Vegetable
Vegetable
Vegetable
Vegetable
Vegetable
Mineral
Mineral
Mineral
Mineral
Mineral
Mineral
–
–
–
–
1 wt%
3 wt%
1 wt%
3 wt%
1 wt%
3 wt%
1 wt%
3 wt%
1 wt%
3 wt%
1 wt%
3 wt%
Al2O3
Al2O3
Graphite
Graphite
MoS2
MoS2
Al2O3
Al2O3
Graphite
Graphite
MoS2
MoS2
Fig. 7. The specific tangential force results (The base oil applied for pure MQL
and nanolubricants in these experiments is mineral oil).
Fig. 5. The specific tangential grinding force results (The base oil applied for
pure MQL and nanolubricants in these experiments is vegetable oil).
Fig. 8. The specific normal force results (The base oil applied for pure MQL and
nanolubricants in these experiments is mineral oil).
Fig. 6. The specific normal grinding force results (The base oil applied for pure
MQL and nanolubricants in these experiments is vegetable oil).
Fig. 9. Comparison of the specific tangential force results in grinding with
mineral and vegetable base oils.
tangential and normal forces are lower than dry grinding. This is due to
the lubrication effect of an emulsion to reduce friction in the process of
conventional wet grinding. It is also observed that pure MQL grinding
with vegetable base oil leads to lower tangential and normal forces
compared to the conventional wet grinding. It could be attributed to the
higher lubricating effect of the non-soluble vegetable neat oil compared
with water-based emulsion and also the efficient lubricant delivery in
MQL process in comparison with the ordinary flood delivery. Nanoparticles except graphite dispersed in vegetable base oil show a further
reduction in specific force values as a function of an increase in their
concentration. However, graphite nanoparticles dispersed in vegetable
oil has increased grinding forces. Moreover, nanolubricants containing
MoS2 compared to Al2O3 dispersed nanolubricants result in higher lubrication efficiency and therefore lower grinding forces. The specific
tangential and normal forces in grinding using mineral base oil are
shown in Figs. 7 and 8 respectively. It can be seen from Figs. 7 and 8
that dry grinding shows the highest specific (tangential and normal)
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Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
the mineral oil. Therefore the vegetable oil enhances the absorption of
graphite nanoparticles on the surface of the porous grinding wheel,
leads to wheel loading and hence increases the grinding forces.
It can be concluded from Table 5 that MoS2, graphite and Al2O3
nanoparticles that are dispersed in mineral oil, respectively, have the
greatest effect on reducing the specific forces. Moreover, with an increase in nanoparticle concentration from 1 wt% to 3 wt%, the specific
tangential and normal forces decrease.
The main reasons for friction decline with nanoparticle addition to
base oil are as follows [19]:
1- Spherical MoS2 nanoparticles may have a rolling effect, which
changes the sliding friction into rolling friction, leading to decrease of
friction.
2- Nano particles act as the solid lubricant and maintain thermal
stability at higher pressure and temperature. While the lubricating
properties of liquid oils at higher temperatures decline.
3- A tribological film forms on the work surface when using solid
lubricant nano-particles. This tribological film reduces the cutting
friction and grinding forces.
In Figs. 9 and 10 the effects of mineral and vegetable base oils on
specific tangential and normal forces are shown respectively. It is observed that the grinding forces with vegetable base oil are higher than
with mineral base oil. This may be attributed to the lower thermal
stability of vegetable oil over mineral oil. Therefore the film stability of
the vegetable oil in harsh conditions of wheel-workpiece contact (i.e.
high temperature, high pressure and high cutting speed) is poor. Hence,
it is necessary to improve the tribological behavior and thermal stability of vegetable oil applied in nanolubrication in order to achieve
better lubrication performance. Modification of tribological properties
of vegetable oils can be achieved for instance by adding agents such as
antioxidants, EP (extreme pressure), and AW (anti wear) additives.
There are also several methods recommended for improving the
thermal stability of vegetable oils such as: (i) chemical modification of
base oils (e.g., epoxidization of base oils) (ii) reformulation of additives,
(iii) genetic modification of oilseed crop [20,21].
Fig. 10. Comparison of the specific normal force results in grinding with mineral and vegetable base oils.
forces. With flood cooling, a slight reduction in specific forces is observed. Nanolubricants show a further reduction in specific force values
due to the presence of nanoadditive and as a function of the increase in
their concentration. In Table 5 the specific tangential and normal force
decrements compared with dry grinding are calculated and shown. The
negative and positive sign of the values shown in this Table means
decreasing and increasing the force, respectively. As shown in the table,
the largest decrease is related to MoS2 nanoparticle with a concentration of 3 wt% in mineral base oil, which results in a decrease of 51.1%
and 46.99% of tangential and normal forces compared to dry grinding.
However, the case for graphite nanoparticles with 1 wt% concentration
in plant base oil doesn’t give favorable results which depict an increase
of 26.68% and 34.07% in tangential and normal forces, respectively, in
comparison to dry grinding. This could be for the following reasons:
The applied vegetable oil has higher viscosity and polarity compared to
Table 5
Specific tangential and normal force decrements compared with dry grinding.
Lubrication condition
Specific force decrements compared with dry grinding
Wet
Pure MQL (vegetable base oil)
MQNL
(vegetable base oil & 1 wt% Al2O3)
MQNL
(vegetable base oil & 3 wt% Al2O3)
MQNL
(vegetable base oil & 1 wt% Graphite)
MQNL
(vegetable base oil & 3 wt% Graphite)
MQNL
(vegetable base oil & 1 wt% MoS2)
MQNL
(vegetable base oil & 3 wt% MoS2)
Pure MQL (mineral base oil)
MQNL
(mineral base oil & 1 wt% Al2O3)
MQNL
(mineral base oil & 3 wt% Al2O3)
MQNL
(mineral base oil & 1 wt% Graphite)
MQNL
(mineral base oil & 3 wt% Graphite)
MQNL
(mineral base oil & 1 wt% MoS2)
MQNL
(mineral base oil & 3 wt% MoS2)
248
Tangential Force (%)
Normal force (%)
−6.89
−11.60
−16.47
−5.18
−8.11
−10.46
−28.52
−23.65
+26.68
+34.07
−8.64
−.0.02
−37.76
−36.57
−43.91
−40.71
−17.57
−25.01
−18.65
−20.50
−31.22
−25.85
−33.91
−31.16
−43.82
−41.11
−40.95
−38.45
−51.10
−46.99
Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
Fig. 14. Surface roughness results Rz (μm) of ground surfaces across the
grinding direction.
Fig. 11. Surface roughness results Ra (μm) of ground surfaces along the
grinding direction.
grinding direction. As per Figs. 11 and 12, dry grinding leads to higher
roughness Ra (μm) values than wet and pure MQL in both along and
across the direction of grinding. In dry grinding the lack of lubricant
results in higher grinding forces, surface burning remarks, higher wheel
wear, higher wheel loading, higher grinding temperature and greater
surface roughness. Moreover, the effects of nanolubrication on surface
roughness can be viewed in Figs. 11 and 12 indicate that applying MoS2
and graphite nanoparticles respectively reduces roughness values in
both cutting directions (along & across the grinding). It is worth mentioning that MoS2 with respect to graphite nanoparticle shows higher
tribological and lubrication properties and therefore provide lower
friction at tool-workpiece interface. Furthermore, increasing the MoS2
and graphite nanoparticle concentration from 1 wt% to 3 wt% decreases the roughness values. This is due to the fact that MoS2 and
graphite nanoparticles are solid lubricants and reduce friction and
grinding forces, leading to lower surface roughness. However, the addition of Al2O3 nanoparticle to vegetable or mineral base oils results in
increasing roughness values along and across the grinding direction
(Figs. 11and 12). This is probably due to the high wear resistance of
Al2O3 nanoparticles and the resultant wear scratch formation on the
work surface during the grinding process. The surface roughness results
Rz (μm) of ground surfaces along and across the grinding direction are
shown in Figs. 13 and 14 respectively. As can be seen from these figures, a similar trend like that of Figs. 11 and 12 is observed in the
roughness charts of gound samples.
The oil applied in wet grinding is watermissible and has both
cooling and lubrication properties. However, due to high ratio of water
to oil (20/1) in the emulsion, its cooling effect is more than its lubricating properties. On the other hand, the oil used in MQL grinding is
water immiscible and provides more lubrication than cooling. As demonstrated in Figs. 5–14 applying the emulsion in conventional wet
grinding results to lower forces and roughness than dry grinding. While
with a minute amount of neat oil in MQL grinding, lower forces and
roughness than wet grinding is obtained. It is due to that the lubricant
in MQL technique penetrate more effectively into the wheel-workpiece
contact zone and provides greater lubrication than conventional flood
cooling [22]. Investigating the effects of oil type (mineral and vegetable) on surface roughness in MQL and MQNL tests shows that in most
conditions the mineral oil results in lower roughness in respect to vegetable oil as illustrated in Figs. 11–14. This is probably due to the
higher thermal stability of the mineral oil compared to the vegetable oil
applied. Moreover, chlorine compounds which are added to mineral oil
act as Extreme pressure (EP) additive. The EP additive has a function of
reducing chip welding under high pressures of wheel-workpiece contact
zone and improves lubrication. In Table 6, surface roughness decrements are compared against dry grinding. As per Table 6, the pure MQL
Fig. 12. Surface roughness results Ra (μm) of ground surfaces across the
grinding direction.
Fig. 13. Surface roughness results Rz (μm) of ground surfaces along the
grinding direction.
3.2. Surface roughness
In this section, the surface roughness of test samples at different
lubrication conditions are measured and shown. The surface roughness
parameters Ra (μm) and Rz (μm) of the samples along and across the
grinding direction are depicted in Figs. 11–14. Moreover, surface
roughness decrements compared with dry grinding are calculated and
shown in Table 6. Figs. 11 and 12 respectively show the surface
roughness results Ra (μm) of ground surfaces along and across the
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Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
Table 6
Surface roughness decrements compared with dry grinding.
Lubrication condition
Surface roughness decrements compared with dry grinding (%)
Along the grinding
Wet
Pure MQL (vegetable base oil)
MQNL
(vegetable base oil & 1 wt% Al2O3)
MQNL
(vegetable base oil & 3 wt% Al2O3)
MQNL
(vegetable base oil & 1 wt% Graphite)
MQNL
(vegetable base oil & 3 wt% Graphite)
MQNL
(vegetable base oil & 1 wt% MoS2)
MQNL
(vegetable base oil & 3 wt% MoS2)
Pure MQL (mineral base oil)
MQNL
(mineral base oil & 1 wt% Al2O3)
MQNL
(mineral base oil & 3 wt% Al2O3)
MQNL
(mineral base oil & 1 wt% Graphite)
MQNL
(mineral base oil & 3 wt% Graphite)
MQNL
(mineral base oil & 1 wt% MoS2)
MQNL
(mineral base oil & 3 wt% MoS2)
Across the grinding
Ra
Rz
Ra
Rz
−7.62
−40.07
−26.17
−7.49
−26.79
−23.78
−6.39
−36.72
−7.43
−5.82
−27.57
8.02
−42.65
−42.34
−25.13
−16.09
−52.08
−48.44
−49.25
−39.94
−58.12
−52.51
−55.96
−44.39
−54.34
−50.59
−51.94
−42.69
−61.03
−57.59
−56.88
−49.71
−45.73
−33.55
−28.77
−29.57
−40.00
−10.05
−32.84
3.74
−44.77
−42.85
−25.99
−15.31
−55.74
−50.58
−51.88
−46.46
−61.61
−57.68
−58.71
−52.33
−57.65
−53.47
−54.78
−49.80
−65.49
−61.76
−61.46
−57.56
the lower extent of microfracture, microholes and surface defects is
obtained through grinding in pure MQL condition compared with the
conventional wet. Also, comparing Fig. 15(e,f) and (g,h) shows that
applying the mineral oil results in a surface with a higher quality than
the vegetable oil.
In this section, the effects of nano-enhanced lubricant on work
surface texture in MQNL grinding is investigated. The samples ground
with MQL oil mixed with 3 wt% nanoparticle were chosen for SEM
micrography. The surface texture of samples ground with nanolubricant
containing 3 wt% Al2O3 are shown in Fig. 16(a,b) and (c,d) for the
vegetable and mineral base oils respectively. As it is clear in the images,
adding Al2O3 nanoparticles in lubricating oil doesn’t improve the surface quality of work samples. Actually, due to high hardness, Al2O3
nanoparticles tend to scratch the work surface and increase the
roughness. Moreover, in this process, the mineral oil compared to the
vegetable oil results in a surface with a less roughness.
The SEM images of samples prepared under graphite-enhanced and
MoS2-enhanced lubricant MQL grinding are shown in Fig. 16(e–h) and
(i–l) respectively. The results illustrate that the samples ground with
nanolubricants containing graphite or MoS2 generate surface textures
with higher quality than the previous tests. Additionally, MoS2 compared to graphite generates a surface with higher quality in MQNL
grinding. On the other hand, mineral oil is more susceptible to reduce
surface defects than vegetable oil. Therefore, in the experimental condition specified in this study, applying nanolubricant containing 3 wt%
MoS2 dispersed in mineral oil during the grinding process of Tungsten
carbide material produces a surface texture with the highest integrity
(Fig. 16 (k,l)).
For further investigation and ensure the performance of nano-particles in the grinding process of samples, EDS chemical analysis has
tests with mineral oil result in 45.73% & 40% decrements in surface
roughness value Ra (μm) along and across the grinding direction respectively. The greatest roughness decrement is observed with nanolubricant containing 3 wt% MoS2 dispersed in mineral oil. In this condition, the roughness value Ra (μm) is reduced 65.49% & 61.76% along
and across the grinding direction respectively.
3.3. SEM micrography
In this section, the SEM micrographs from the surface of some selected samples ground at different lubrication conditions are shown and
compared. The samples depicted in Figs. 15 and 16 are ground at dry,
wet, pure MQL and MQNL (with 3 wt% nanoparticle concentration)
conditions. All the SEM micrographs are prepared in two magnifications (500x and 1000x). Fig. 15(a) and (b) show the SEM micrograph of
a sample ground in dry condition. As per the work surface texture
shown in this figure, the chip removal process has not performed
evenly, therefore, avulsion, microfracture, and microholes symptoms
are evident. In the lack of lubrication, dry grinding results in higher
frictional heat generation at wheel – workpiece interface and therefore
a surface with integrity defects and poor appearance is obtained.
In Fig. 15(c) and (d) the result of micrography of a sample ground in
conventional wet condition is shown. As per these figures, the existence
of areas with fracture symptoms on the work surface and also microchips adhered to the surface indicate that material is mostly removed in
the form of chips by microfracture mechanism. Therefore, the surface
ground in conventional wet conditions contains integrity defects, adhered microchips, and poor appearance.
Fig. 15(e,f) and (g,h) show the results of pure MQL grinding with
vegetable and mineral oil respectively. As can be seen in these figures,
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Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
of the ground work surface in dry, wet, MQL and MQNL conditions are
shown respectively. The results of Fig. 17 depicts that in dry grinding
only Tungsten (W), Cobalt (Co) and Carbon (C) elements are detected
on the work sample. The result of EDS analysis of the sample ground
with Al2O3-enhanced vegetable oil is shown in Fig. 18. It is evident that
here Aluminium (Al) and Oxygen (O) elements are added to the elements detected on the surface of the dry grinding sample. In other
words, this experimental finding confirms the trace elements of the
nano material Al2O3 on the work surface. In Fig. 19. the EDS spectra of
work surface ground with MoS2-enhanced mineral oil is illustrated. The
elemental traces of molybdenum (Mo), sulfur (S) and oxygen (O) in EDS
spectra of this figure are visible. The detection of trace elements of
nanoparticle in EDS spectra of MQNL grinding samples might be due to
the physical deposition of nanoparticles or the formation of chemical
tribofilms on the work surface. The physical deposition can be formed
by the adhesion of thin MoS2 nanoparticles to the surface in the form of
a lubricating boundary film [23]. This physically based lubrication
concept of the MoS2 nanoparticles can be explained using rolling, deformation, exfoliation, and delivery of the exfoliated nanosheets to the
work surface under the extreme pressure of the grinding process. Additionally, chemical tribofilms may also be formed under severe conditions of high temperature and pressure of the grinding zone. In this
regard, the presence of oxygen (O) in the spectrum of Fig. 19. might be
due to the formation of MoO3 compound in the transfer film as reference [24] has noted. However, more information about element
bindings and probable formation of chemical tribofilms on the work
surface can be obtained by XPS analysis of the work surface.
It is worth mentioning that, in MQNL grinding process, the nanoparticles are delivered into the grinding zone more effectively. In other
words, in this technique, the nanoparticles dispersed in oil are carried
by MQL jet and targeted to the wheel-workpiece interface. Here the
MQL jet not only transmits the nanoparticles and oil droplets but also
overcomes the air boundary layer around the wheel periphery and
penetrates effectively to the grinding contact zone. This leads in higher
amounts of nanoparticle/oil droplet impinge on the wheel surface and
more efficient lubrication takes place. Also, sliding friction due to the
continuous robbing of abrasive grain wear flats over the work surface,
results in shearing of nanoparticles and delivery of the exfoliated nanosheets to the work surface in the form of boundary lubricating film.
4. Conclusion
In this investigation, different types of lubrication including dry,
wet, MQL and MQNL was explored in the grinding process of Tungsten
carbide material and the following results were obtained:
• Minimum quantity lubrication (MQL) and minimum quantity nano-
Fig. 15. SEM micrograph of the samples ground in dry, wet and pure MQL
conditions.
•
been taken. In this analysis, the detected elements on the ground surface of each sample are shown. Therefore by comparing the EDS spectra
of the samples, the trace elements of the nanolubricant on work surface
due to grinding lubrication is detected. In Figs. 17–19 the EDS spectra
•
251
lubrication (MQNL) compared with wet grinding, are effective
techniques which can considerably reduce the cutting fluids consumption. Furthermore, the application of biodegradable vegetable
oils in MQL and MQNL techniques leads the machining processes
towards eco-friendly and sustainable cutting condition.
The use of graphite and MoS2 nanoparticles in MQNL grinding reduces the friction, cutting force and roughness of the samples due to
the formation of lubricating boundary film on the surface of the
samples and the greater stability of solid nanoparticles than liquid
oil at high pressure and temperature of the wheel-workpiece contact
zone.
Investigation of nanoparticles applied in vegetable oil shows that
the MoS2 and Al2O3 nanoparticles, respectively, have the greatest
effect on reducing grinding forces, while graphite nanoparticles
Journal of Manufacturing Processes 35 (2018) 244–253
S.F. Hosseini et al.
Fig. 16. SEM micrograph of the samples ground with nanolubricants.
•
•
•
•
increase the forces. Also, with mineral oil, the MoS2, graphite and
Al2O3 nanoparticles, respectively, have the greatest effect on the
reduction of grinding forces.
The mineral cutting oil Behran-11 compared with the sunflower
vegetable oil shows better performance in reducing forces and improving surface quality due to its EP additive, which is based on
chlorine compounds and has higher thermal stability and oxidation
stability in comparison with vegetable oil.
SEM microstructure analysis images show better performance of
mineral oil than vegetable oil in both MQL and MQNL processes.
Moreover, it can be concluded that the best surface quality is
achieved by the application of the MoS2 nanoparticle dispersed in
mineral oil.
Metallographic studies with SEM - EDX on the grinding samples
reveals the deposition of nano materials on the surface could be the
possible mechanism for the reduction in friction, force, and improvement in surface integrity with the addition of nano materials.
The results of this study show that, if nanoparticles are selected
properly, minimum quantity nanolubrication (MQNL) method is an
Fig. 17. EDS spectra of the sample ground in dry condition.
effective method to reduce the adverse environmental effects of
cutting fluids, reduce the cutting force and energy, improve the
quality of the grinding surface, and in other words, achieve a sustainable grinding.
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Journal of Manufacturing Processes 35 (2018) 244–253
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Fig. 18. EDS spectra of the sample ground with nanolubricant containing 3 wt
% Al2O3 dispersed in vegetable oil.
Fig. 19. EDS spectra of the sample ground with nanolubricant containing 3 wt
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Acknowledgments
The authors would like to thank the CAD/CAM laboratory of Tarbiat
Modares University for providing testing facilities.
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