2016 STLE Annual Meeting & Exhibition
May 15-19, 2016
Bally’s Las Vegas Hotel and Casino
Las Vegas, Nevada, USA
GRINDING OF INCONEL 718 ALLOY WITH AIR-OIL-WATER MIXTURE
DELIVERED BY MQL TECHNIQUE
Environmentally Friendly Fluids
R.B. Da Silva*1, D.Oliveira2, P.H.C. De Castro3, E.O. Ezugwu4, A.Marques5,
A.R. Machado6,7,
1,2,3,5,6
Federal University of Uberlandia, Brazil; 4Air Force Institute of
Technology, Kaduna, Nigeria; 7Mechanical Engineering Graduate Program,
Pontifícia Universidade Católica do Paraná – PUC-PR, CEP 80215-901,
Curitiba/PR, Brazil
*[email protected]
INTRODUCTION
Grinding is considered a finishing process used to achieve high
dimensional and geometrical tolerances, including superior surface finish. This
is mainly possible due to the small abrasive grains with high hardness and low
radial depth of cut. However, abrasive grains are usually made from ceramic
that are united by abond (resin or vitrified), so conventional grinding wheels are
poor conductor of heat. In addition, grinding of most metals requires high
specific energy, with very few exceptions (such as some ferrous materials).
Since, the specific energy is a measure of machining efficiency (ratio of
machining power to material removal rate), grinding is often referred to as a low
efficient machining process (MARINESCU et al., 2004).
Another point to be considered in grinding is how the surface and subsurface integrity of the machined parts are affected by the high cutting
temperatures generated at the cutting surface. The main problems are
structural changes, oxidation and decarburation, tensile residual stresses,
distortion of the parts and burn of surfaces (MANDAL et al, 2014). These
problems are more critical when grinding superalloys such as nickel and
titanium-based ones. Because those materials are temperature resistant, they
retain hardness at higher temperatures, which can demand higher specific
energy during grinding. As a consequence, grinding wheel experiences rapid
wear rate. If grains become dull, the specific energy increases rapidly, thereby
impairing the grinding of nickel based superalloys.
Nickel alloys are mainly employed in applications such as aggressive
environments because they can maintain high resistance to corrosion and high
strength, mechanical and thermal fatigue, mechanical and thermal shock, creep
and erosion at elevated temperatures (EZUGWU, 2002). On the other hand,
especially the nickel-based superalloy Inconel 718haslow thermal conductivity
(REED, 2006) that will generate very high cutting temperatures during
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machining. According to Ezugwu (2002), the high temperature strength,
toughness and ductility of nickel alloys impair chip segmentation. In addition,
their high tendency to work hardening during machining creates a hardened
surface that usually leads to rapid deterioration of grain edge and consequently
becoming more difficult to maintain tighttolerances or the critical metallurgical
integrity of the machined component.
Among the cooling techniques for grinding nickel alloys, the use of
Minimum Quantity Lubrication (MQL) technique is an alternative due to its
cooling and lubrication effects. Compressed air carries oil droplets in a mixture
that is pumped and directed to the cutting regions which ensures the cooling
action while the lubricating action is ensured by the oil droplets. The present
study aims to investigate the effect of application of MQL technique, compared
to flooding cooling, in grinding Inconel 718,in order to further contribute to the
knowledge of machining this difficult-to-cut alloy.
METHODOLOGY
Experimental trials were carried out on a Mello tangential grinding
machine, model P36. The abrasive wheel used was a conventional straight
white aluminum oxide (Al2O3) with designation AA60K6V and dimensions of 300
mm external diameter x 25mm width x 76 mm internal diameter, from Norton
Abrasives manufacturer. A single-point diamond dresser was used to dress the
grinding wheel before each test at a depth of dressing of 10 µm. The workpiece
material employed was the Inconel 718 with the dimensions of 30 mm length x
20 mm width x 15 mm height. It is a precipitation hardening nickel base
superalloy. Wheel speed (Vs) of 35m/s and workpiece speed (vf) of 0.16m/s
were kept constant. Six conditions were employed in this study involving the
combination of tree depth of cut values (ae) with two cooling techniques
(conventional method and MQL). Variation of depth of cut resulted in three
different grinding cycles: 10 µm (6 cycles), 20 µm (3 cycles) and 30 µm (2
cycles). A total of 60 µm ofthe workpiece height was removed for each cutting
conditions. A vegetable based fluid ME-3, from Tapmatic manufacturer, was
used in all the trials at a dilution rate of 1:19. 545l/h (545,000 ml/h) and 240ml/h
were the flow rates used for conventional and MQL coolant delivery techniques,
respectively.
Surface roughness, Ra Rq and Rz parameters, were recorded at the end
of each testin three different areas of the machined surface with the aid of a
Mitutoyo SJ-201P portable stylus instrument. A cut-off of 0.8 mm and sampling
length of 5 mm were previously selected. The average of all the readings for
each condition represents the surface roughness value of the machined
surface. The workpiece samples were analysed in a Scanning Electron
Microscope (SEM) while microhardness was measured with a HMV Micro
Hardness tester, SHIMADZU, at twelve different depths below the ground
surface: 0.010,0.025, 0.040, 0.055, 0.070, 0.085, 0.100, 0.130, 0.160, 0.190,
0.300 and 0.500 mm. The microhardness measurements were repeated three
times.
RESULTS AND DISCUSSIONS
The values of all the surface roughness parameters versus depth of cut
values are shown in Figure 1. It can be noticed that in general roughness values
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increased with depth of cut, as expected, irrespective of the cooling technique
employed. Since contact area increases with radial depth of cut during grinding,
the workpiece area being deformed is also extended, thereby leading to
increase in heat generation in the cutting zone. Heating of the workpiece
surface is accompanied by thermal expansion and outward surface movement
towards the wheel in the pointwhere the high temperature occurs. As a result, a
poorer surface texture is generated (MARINESCU et. al, 2007). Also Ra values
increased with the depth of cut, regardless of the coolant technique employed.
All the Ra values obtained are lower than 0.45 µm, well below the stipulated
rejection limit of 1.6 µm for grinding process. In general, lower Ra values were
obtained after machining with flooding and the major benefit was observed after
grinding at the ae=20 µm. This demonstrates that, even with the cooling
function of compressed air in MQL, the conventional method (flooding) ismore
efficient for better finishing of the workpiece. This may be attributed to the
greater quantity of fluid delivered onto the surface being machined which
enhanceheat dissipation from the cutting zone. On the other hand, although the
Ra results obtained after machining with the MQL technique were about 10%
higher than those obtained after machining with the conventional technique, it is
important to note that the amount of fluid was reduced by a factor of 2,250 and
the Ra parameter was kept below the stipulated limit of 1.6 µm.
Surface roughness, Ra (µm)
Flooding
MQL
0,5
0,4
0,3
0,2
0,1
0
10
20
Radial depth of cut (µm)
30
Figure 1 – Surface roughness (Ra) values after grinding Inconel 718 at different
cutting conditions.
Results of the microhardness valuesmeasured for all the machined subsurfaces are shown in Figure 2. Since microstructure of the Inconel 718 alloy
presents high heterogeneity, the microhardness values measured prior to
grinding were different for each sample, so the values shown in Figure 2 were
treated and normalized (reference line) for comparison. From this figure it can
be noticed that there is a tendency of reduction in hardness after machining with
the MQL technique up to 300 µm, irrespective of the depth of cut employed,
unlike for conventional method in which all the values, with exception of the
ae=30 µm, are around the reference line. Grinding the Inconel 718 with flooding
and the highest depth of cut of ae=30 µm provided the lowest values of
hardness (480 HV) that remain constant up to 200 µm below the machined
surface. This demonstrates that grinding such superalloy at depth of cut in
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excess of 20 µm, even in presence of abundant cutting fluid, is not
recommended because it can lead to reduction of hardness and possible
metallurgical alterations.
MQL 10
MQL 30
Flooding 20
Normalized microhardness
MQL 20
Flooding 10
Flooding 30
Microhardness (HV)
550
525
500
475
450
0
100
200
300
400
500
Distance below machined surface (μm)
Figure 2– Microhardness values below the machined surface (HV).
Images of the machined surfaces with magnification of 1000 times for the
different cutting conditions are shown in Figures 3a-f. It can be observed that
the marks left by the abrasive grains are well defined. The width of marks
generallyincreased with the depth of cut,as expected, because as the grain
contact area increases and hence more material is being deformed. As a
consequence the surface texture is similar to those observed in Figures 3c-3f
(after machining with MQL technique). In addition, it can be noticed that poor
machined surfaces were generated after machining with the MQL technique.
There is an evidence of clogging of the abrasive grain after machining with the
MQL technique at more severe conditions, i.e., the workpiece material has been
re-deposited during the passage of the abrasive grain. This phenomenon
occurs when the grain scratches the workpiece surface, but cannot be able to
remove the material. Inconel 718 is a material with high mechanical
resistanceand ductile, thus, increasing depth of cut values will increase the
tendency of clogging.
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a) ae = 10 µm, Flooding
b) ae = 20 µm, Flooding
c) ae = 10 µm, MQL
d) ae = 20 µm, MQL
e) ae = 30 µm, Flooding
f) ae = 30 µm, MQL
Figure 3 - Images of the workpiece surfaces after grinding with different cutting
conditions.
CONCLUSIONS
i)
The conventional coolant technique outperformed the MQL technique in
terms of surface finish and hardness variation after machining with radial depth
of cut below 30 µm with alumina abrasive wheel.
ii)
All the surface roughness (Ra parameter) values were below 0.45 µm,
demonstratingthat even the air-oil-water MQL technique can be an
environmentally friendly alternative for grinding Inconel 718 alloy when good
finishing is the main requirement, although flood cooling can provide even better
finishing.
iii)
Evidence of reduction in hardness was observed after machining with the
MQL technique up to 300 µm, irrespective of the depth of cut employed, unlike
for conventional method where all the values clustered around the reference
hardness values obtained prior to machining.
iv)
It is not recommended to grind Inconel 718 alloy with depth of cut in
excess of 20 µm under the conditions investigated.
AKNOWLEDGEMENTS
One ofthe authors thanks the Brazilian foundationagency, FAPEMIG (Process
no. PEE-00423-16), and PROPP-UFU for financial support. All authors are
grateful to CAPES and Post Graduate Program of Mechanical Engineering of
Federal University of Uberlandia for sponsoring this work.
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