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Materials and Manufacturing Processes
ISSN: 1042-6914 (Print) 1532-2475 (Online) Journal homepage: http://www.tandfonline.com/loi/lmmp20
Performance evaluation of Al2O3 nano powder
mixed dielectric for electric discharge machining
of Inconel 825
Amit Kumar, Amitava Mandal, Amit Rai Dixit & Alok Kumar Das
To cite this article: Amit Kumar, Amitava Mandal, Amit Rai Dixit & Alok Kumar Das (2017):
Performance evaluation of Al2O3 nano powder mixed dielectric for electric discharge machining of
Inconel 825, Materials and Manufacturing Processes, DOI: 10.1080/10426914.2017.1376081
To link to this article: http://dx.doi.org/10.1080/10426914.2017.1376081
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Sep 2017.
Published online: 05 Sep 2017.
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MATERIALS AND MANUFACTURING PROCESSES
https://doi.org/10.1080/10426914.2017.1376081
Performance evaluation of Al
2
O
3
nano powder mixed dielectric for electric
discharge machining of Inconel 825
Amit Kumara, Amitava Mandalb, Amit Rai Dixitb, and Alok Kumar Dasa
aDepartment of Mechanical Engineering, Indian Institute of Technology (ISM), Dhanbad, India; bIndian Institute of Technology (ISM), Dhanbad, India
ABSTRACT
Electrical discharge machining (EDM) process is popular for machining conductive and difficult-to-cut
materials, but low material removal rate (MRR) and poor surface quality are major limitations of the process.
These limitations can be overcome by adding the suitable powder in the dielectric. The powder particles
influence electric field intensity during the EDM process which in turn improve its performance. The size
(micro to nano) and properties of the mixed powder also influence the machining efficiency. In this regard,
the objective of the present work is to study the performance of EDM process for machining Inconel 825
alloy by mixing Al
2
O
3
nanopowder in deionized water. The experimental investigation revealed that
maximum MRR of 47 mg/min and minimum SR of 1.487 µm, which are 44 and 51% higher in comparison to
conventional EDM process, respectively, can be achieved by setting optimal combinations of process
parameters. To analyze these observed process behavior, pulse-train data of the spark gap were acquired.
The discharge waveform identifies the less arcing phenomenon in the modified EDM process compared to
conventional EDM. Further, surface-topography of the machined surface was critically examined by
capturing field emission scanning electron microscopy and atomic force microscopy images.
ARTICLE HISTORY
Received 13 July 2017
Accepted 28 August 2017
KEYWORDS
AFM; EDM; FESEM;
nanopowder; pulse-train;
surface-topography
Introduction
Electrical discharge machining (EDM) is a nonconventional
machining process where material removal takes place due
to recurring spark between two electrodes (tool electrode
and work piece), separated by dielectric.
[1]
Due to virtually
forces free and contact-free characteristics, EDM is found to
be an alternative for processing of difficult-to-machine con-
ductive materials. Despite good prospects of the EDM as a
manufacturing process, low efficiency and inability to
generate good surface finish are considered to be the key
drawbacks of the EDM preventing its wider use. Therefore,
further research on the EDM process is required for improving
the material removal rate (MRR) and surface quality of the
machined parts.
In this regard, different approaches, such as use of the
cryogenic cooled electrode, providing rotational motion to
the tools, applying an external magnetic field in the sparking
zone, etc. have been proposed so far.
[2–8]
By tool rotation,
improvement in MRR and surface finish has been reported
as debris’ removal becomes easy due to whirl condition and
centrifugal force at the machining zone. However, after certain
rotary motions of the tool, more bubbles are generated at the
machining area. These bubbles cause ionization with lower
energy, thereby influencing plasma channel and generating
lower energy for evaporation and sublimation of the work-
piece. As a result, the MRR decreases. In the magnetic field-
induced EDM process, the MRR also increases due to the
expulsion of debris from the machining gap. But this process
is limited to the low-intensity magnetic field which improves
the MRR only up to a certain degree.
[2]
The researchers have also improved EDM process by
modifying the dielectric medium. In this connection, the
powder particles are mixed with dielectric medium to study
their influence on machining. While studying the electric field
distribution in powder-mixed dielectric, the value of electric
field intensity in the case of conductive powder-mixed dielec-
tric has been found to be thrice greater than the conventional
EDM, while it is one and half times higher in the case of
nonconductive powder-mixed dielectric.
[9]
Due to this higher
electric field intensity, an electric breakdown in the gap is
initiated at a larger distance compared to the conventional
EDM. This larger gap creates more uniform flushing con-
ditions, stable sparking, and consequently disperses them over
a wider area of a workpiece. Moreover, when the gap is larger,
the stray capacitance inserted in the gap is smaller, which
affects the surface quality. The effect of different powders, such
as copper, aluminum, iron, and carbon, and their concen-
tration also influence the machining rate as well as surface
roughness (SR).
[10–16]
The study revealed that the machining
rate in powder-mixed EDM increases up to a certain level of
powder concentration and by increasing the powder
concentration.
[10]
During the comparison of performance
between the conventional EDM and the powder-mixed
EDM, an improvement in MRR up to 60% and reduction in
Electrode Wear Rate (EWR) up to 28% have been found to
be achievable by mixing a certain amount of graphite powder
none defined
CONTACT Amitava Mandal amitava03@gmail.com Department of Mechanical Engineering, Indian Institute of Technology (ISM), Dhanbad 826004,
India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lmmp.
© 2017 Taylor & Francis
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with kerosene dielectric medium.
[17]
Further, using inorganic
oxide particles and lipophilic surface agent as impurities of
the dielectric medium, improvement in microcrack and the
surface defect has been reported.
[18]
Surface modification by
mixing tungsten powder particles in dielectric enabled 100%
improvement in the microhardness.
[19–20]
Significant
improvement was observed using Al and Si powder particles
in recast layer thickness.
[21]
Using negative polarity, Wong
et al.
[22]
obtained near mirror finish surface in powder mixed
electric discharge machining in which C, Si, and MoS
2
pow-
ders were mixed in the dielectric. Further, Tzeng and Lee
[23]
and Yih-Fong and Fu-Chen
[24]
investigated the role of sizes
of powder particles and concluded that the smaller the particle
size, the better is the surface finish. However, it was found that
smaller size of powder particles forms a thick recast layer.
In this regard, nanopowder of different materials such as
carbon nanotube, graphite nanopowders, nano-TiO
2
powder
were tested to check the performance of EDM. This new
approach of powder (nano-sized) mixed EDM is termed as
nanopowder mixed electric discharge machining
(NPMEDM).
[25]
Prabhu and Vinayagam
[26,27]
investigated this
process using single-wall carbon nanotubes (CNT) having
large surface area and linear geometry by which it can be
easily mixed in the dielectric. Experimental investigations
indicated that with low-pulse energy machining parameters,
nanolevel-machined surface finish can be obtained. Also,
depth of the microcracks and SR was found proportional to
the power input, especially the input current. The EDM pro-
cess, with CNT-mixed dielectric, was reported efficient when
the experiment was performed at low pulse of energy.
[28]
This
process is also capable of providing mirror-like surface.
[29]
Jahan et al.
[30]
investigated the effect of adding graphite
nanopowders with 55-nm average particle size in the dielec-
tric. Experiments established that improved surface finish
and MRR could be achieved as well as EWR reduced and also
improved surface-topography, and crater distribution can be
attained with an increase in the spark gap. Similar results were
also obtained by mixing 1 g/L TiO
2
powder in dielectric.
[2]
Although, the performance of NPMEDM has shown
improvement, but the cost of these nanopowders is still high.
Therefore, it is important to explore the use of other low-cost
nanopowder of different metals and their oxides. This will
make the proposed process more economically viable for the
industry. In view of this, a low-cost Al
2
O
3
nanopowder is used
for the proposed experimental work. In the present work,
nanopowder of size 45–50 nm Al
2
O
3
was mixed with
deionized water (dielectric). Response surface methodology
(RSM)-based Box–Behnken was used to conduct the experi-
ments. Two important performance measures, i.e., MRR and
SR were considered to assess the influence of process
parameters. Finally, the optimal combination of process
parameters was obtained, and a detailed investigation on the
surface-topography was performed.
Materials and Methods
Experimentation
In this work, all experiments were conducted with Sparkonix
ZNC/ENC35 EDM machine as shown in Fig. 1. A small tank
was fabricated for the effective utilization of dielectric, and a
submersible pump was used to ensure proper flushing of the
dielectric fluid at the sparking zone. In this setup, a servo motor
moves the tool, i.e., electrode, up and down and maintains a
desirable gap between both the electrodes (i.e., tool and work-
piece). When a voltage is applied between the electrodes, the
electric field becomes greater than the strength of dielectric.
As a result, dielectric fluid breaks down and allows current to
flow between the electrodes. The current in this process flows
in a discreet form. Therefore, some current is bypassed to
achieve a constant current flow between the electrodes. Due
to the generation of spark, the material gets removed from both
the electrodes. Sparking between the electrodes is allowed for a
certain duration, which is termed as spark time. Thereafter, the
tool electrode is lifted to a reference plane for certain duration
of time, which is termed as lift time. During the lift time, debris,
formed by thermal erosion process, are flushed out by dielectric
fluid from the machining zone.
In this experiment, a 100 mm �50 mm �5 mm Inconel
825 plate was taken as workpiece material, which is a nickel,
chromium, and iron alloy with additions of molybdenum
and titanium. This super alloy is widely used in the nuclear
power plant, petrochemical, chemical, missile industries, and
highly corrosive environment due to its resistance toward cor-
rosion and heat. Apart from this, it possesses high resistance
toward oxidation at high temperature. A copper tool with a
cross-sectional dimension of 5 mm �15 mm was used as tool
electrode, and Al
2
O
3
nanopowder-mixed deionized water was
used as a dielectric medium to conduct the experiment.
Spherical and cubic Al
2
O
3
nanopowder with an average
particle size of 45 nm was mixed in deionized water. The
volumetric concentration of Al
2
O
3
nanopowder was kept at
0.25% in deionized water which is milky white in color. Three
input variables, namely, peak current (IP), pulse-on-time
(T
ON
), and gap voltage (GV) were considered as critical pro-
cess parameters and their ranges were taken by performing
pilot experiments. In the present paper, the objective of the
experimentation was to obtain the values of process variables
for improved SR and MRR values. Therefore, the range of
these variables was fixed on the basis of their effect on SR
and MRR. The range of process parameters was taken as IP
as 2–8 A, T
ON
as 8–20 μs, and GV as 10–50 V. Other process
parameters were kept constant during the experiment as 50%
duty cycle, 3 µs spark timing, 1 A by-pass current, 50% sensi-
tivity, and 75 mm
2
tool surface area.
In the present study, Box–Behnken technique was used to
design the experiment. It consists of 2
k
or 2
k1
factorial points,
where “k” is the number of factors. These designs with three para-
meters are provided 15 runs which include eight corner points,
one central point and six-star points. After experimentation,
the data were noted for MRR and SR. The MRR was calculated
using Eq. (1), where W
i
is the initial weight of the workpiece
before machining and W
f
is the final weight of the workpiece
after machining. The measured weights were taken with the help
of Sartorius weighing balance (BSA4202S-CW). Simultaneously,
machining time (t) was noted for each experiment.
MRR ¼ðWiWfÞ
tð1Þ
2 A. KUMAR ET AL.
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The average SR of the machined surface was assessed using
SURFTEST SJ-210 (MITUTOYO) by considering measuring
speed (0.5 mm/s), sampling length (0.8 mm), and measuring
range (auto). The average roughness height was used to evalu-
ate the SR of produced surface. Experimental results are pre-
sented in Table 1, which are the average values obtained
from three sets of the experimental run.
Response surface modeling
Response surface methodology is a technique used for improv-
ing, developing, and optimizing processes. This method is
used to study the relationship between one or more input
parameters and response measure. Nowadays, researchers
extensively use RSM, particularly where several input variables
influence the performance parameters of the process. The
input variables are known as independent variables which
can be controlled. A generalized form of RSM quadratic model
is presented in Eq. (2), where Y is the output response, X
1
, X
2
,
X
3
,…X
k
are the independent variables, b
0
, b
i
, b
ii
, b
ij
are the
coefficients. i and j vary from 1 to k where k is the number
of factors.
Y¼b0þX
k
i¼1
biXiþX
k
i¼1
biiX2
iþX
k
j>1
bijXiXjð2Þ
The value of coefficients in the RSM can be estimated from
a set of experimental data. In this regard, Minitab 17 statistical
software was used for the regression analysis. The developed
RSM models for MRR and SR in coded form are presented
in Eqs. (3) and (4), respectively.
MRR ¼17:67 þ10:563IP þ4:717 TON 5:179GV 0:10IP
�IP 0:44TON �TON þ1:35GV �GV þ4:54IP
�TON 3:67IP �GV 0:92TON �GV
ð3Þ
SR ¼2:5518 þ0:6053IP þ0:0995TON þ0:1695GV 0:1896IP
�IP 0:1839TON �TON þ0:1206GV �GV 0:0294IP
�TON ¼0:1395IP �GV þ0:0207TON �GV
ð4Þ
Figure 1. Experimental setup.
Table 1. Experimental results.
Std order
Peak current (IP) Pulse on time (T
ON
) Gap voltage (GV)
MRR (mg/min) R
a
(µm) Coded values Machine values (A) Coded values Machine values (µs) Coded values Machine values (V)
1 –1 2 –1 4 0 30 6.333 1.4870
2 1 8 –1 4 0 30 18.667 2.7244
3 –1 2 1 10 0 30 6.500 1.6910
4 1 8 1 10 0 30 37.000 2.8110
5 –1 2 0 7 –1 10 8.500 1.8560
6 1 8 0 7 –1 10 36.667 2.8198
7 –1 2 0 7 1 50 8.500 1.8670
8 1 8 0 7 1 50 22.000 3.3886
9 0 5 –1 4 –1 10 19.548 2.1888
10 0 5 1 10 –1 10 31.000 2.4002
11 0 5 –1 4 1 50 8.000 2.5356
12 0 5 1 10 1 50 15.782 2.8298
13 0 5 0 7 0 30 18.667 2.5755
14 0 5 0 7 0 30 17.667 2.5200
15 0 5 0 7 0 30 16.667 2.5600
MATERIALS AND MANUFACTURING PROCESSES 3
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To find the adequacy of the developed RSM models,
analysis of variance (ANOVA) was performed for SR at 95%
confidence level. The ANOVA results for MRR and SR are
presented in Table 2. The table shows detailed contribution
of the process parameters and their interactions. By analyzing
the tables, it is found that all factors are significant individually
Table 2. Analysis of variance for fitted models.
Source DF Seq. SS Adj. SS Adj. MS F P
For MRR
Regression 9 1432.73 1432.73 159.193 38.76 0*
Linear 3 1285.12 1285.12 428.374 104.3 0*
Square 3 7.96 7.96 2.655 0.65 0.618
Interaction 3 139.65 139.65 46.549 11.33 0.011*
Residual Error 5 20.54 20.54 4.107
Lack-of-fit 3 18.54 18.54 6.179 6.18 0.142
Pure error 2 2 2 1
Total 14 1453.27
S ¼2.02661, PRESS ¼301.073, R
2
¼98.59%, R
2
(pred) ¼79.28, R
2
(adj) ¼96.04%
For SR
Source DF Seq. SS Adj. SS Adj. MS F P
Regression 9 3.64382 3.64382 0.40487 141.68 0*
Linear 3 3.24067 3.24067 1.08022 378 0*
Square 3 0.32019 0.32019 0.10673 37.35 0.001*
Interaction 3 0.08296 0.08296 0.02765 9.68 0.016*
Residual Error 5 0.01429 0.01429 0.00286
Lack-of-Fit 3 0.01265 0.01265 0.00422 5.14 0.167
Pure error 2 0.00164 0.00164 0.00082
Total 14 3.65811
S ¼0.0534575, PRESS ¼0.206064, R
2
¼99.61%, R
2
(pred) ¼94.37%, R
2
(adj) ¼98.91%
MRR, material removal rate; PRESS: predicted residual error sum of squares.
*Significant at 95% confidence level.
Figure 2. Effect of process parameters on MRR. Note: MRR, material removal rate.
4 A. KUMAR ET AL.
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for both MRR and SR. Also, interactions between IP and T
ON
as well as IP and GV are found significant for MRR, whereas
the only interaction between IP and GV is found significant
for SR. R
2
values of the developed empirical models were
examined. A higher value of R
2
indicates that model fits data
in a better way signifying a good relationship between input
parameters (independent variable) and responses. ANOVA
results presented in tables below show that the values fit well
in both models. The regression models were designed at
95% confidence level. The value of “p” higher than 0.05 is con-
sidered to be statistically nonsignificant. Value within 0.05, on
the other hand, is considered to be statistically significant. The
lack-of-fit term in the ANOVA table is nonsignificant, which
is desired. A higher value of “F” also indicates that parameter
and interaction terms significantly affect the process output.
Results and Discussion
To study the influence of process parameters on responses of
Al
2
O
3
nanopowder-mixed EDM process, the developed RSM
models were used to obtain the values of MRR and SR in
different combinations of the process parameters. In this
regard, a MATLAB program was designed to generate a set
of data using Eqs. (3) and (4). Thereafter, the generated data
set was used to plot graphs to study the effects of process
parameters on MRR and SR as shown in Figs. 2 and 3, respect-
ively. Further, to investigate the improvement in the surface-
topography, field emission scanning electron microscopy
(FESEM) and atomic force microscopy (AFM) analysis were
performed.
Effect of working parameters on the material
removal rate
Material removal rate in EDM is an important parameter for
assessing the efficiency of the process. Figure 2(a) shows the
effect of peak current on MRR at various T
ON
when GV was
kept constant. It reveals that as IP increases, MRR increases
as well. It is well known that higher IP leads to greater melting
of metal, thereby resulting in higher MRR. T
ON
decides the
time for which pulse remains ON. Therefore, as the value of
T
ON
increases, pulse energy increases as well and this results
in higher MRR as shown in Fig. 2(a). A similar trend was
observed in Fig. 2(b), where the effect of IP on MRR was
investigated, when GV varies by keeping T
ON
constant.
Figure 2(c) and 2(d) shows the effect of GV on MRR at various
Figure 3. Effect of process parameters on surface roughness (SR).
MATERIALS AND MANUFACTURING PROCESSES 5
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IP and T
ON
, respectively. From Fig. 2(c), it is clear that as the
gap voltage between the workpiece and electrode increases, the
value of MRR decreases. This phenomenon is significant at
higher IP as it causes high pulse energy. A similar trend
between MRR and GV was observed at various T
ON
(Fig. 2(d)). This is due to lower discharge efficiency at higher
GV, where discharge energy efficiency is the percentage of
total energy that is responsible for material removal from
the workpiece. The energy loss is caused due to the accumu-
lation of debris in the working zone and inadequate gap
deionization.
[31]
By optimizing process parameters at IP 8 A, T
ON
20 µm and
GV 10 V applying the desirability function approach, the
maximum MRR of 48.063 mg/min was predicted. The experi-
mental validation result found MRR at 47 mg/min, which was
very close to the predicted one. However, the obtained MMR
in the same combination of process parameters during con-
vention EDM process was 32.75 mg/min. This is due to the
powder particles come in contact with the electrons and ions
present in the plasma channel. Therefore, powder particles
get energized and arrange in a chain-like structure between
the electrodes. This chain-like structure makes a bridge which
decreases the insulating strength of the dielectric due to which
easy short circuiting takes place. This phenomenon causes
early sparking between the electrodes due to which series dis-
charge takes place, and therefore results in higher MRR.
[32]
Further, in NPMEDM, the MRR rises as powder particles
increase the gap between the electrodes. This increase in gap
enables proper flushing which easily expels the melted metal.
Effect of working parameters on the surface roughness
Surface roughness of the generated surface by EDM process is
also a vital performance measure for assessing the developed
method. Effect of process parameters on SR has been studied
from Fig. 3(a–d). Figure 3(a) and 3(b) shows the effect of IP
on SR at various T
ON
and GV, respectively. SR is significantly
influenced by IP as it increases with increasing value of IP. It
also increases rapidly with higher T
ON
. All of these due to
higher discharge energy at higher IP and T
ON
. The higher dis-
charge energy causes violent spark and impulsive force, which
result in formation of larger crater size on the machined sur-
face. As surface roughness of EDMed surface is mainly
depends on crater size and distribution of recast layer, it causes
higher surface roughness. However, the surface roughness
increases up to a certain value of T
ON
and thereafter it
decreases. This is due to lower pulse intensity and larger diam-
eter of plasma channel at longer pulse ON time.
[33]
Figure 3(c)
Figure 4. Waveforms of voltage and current during (a) conventional EDM and (b) NPMEDM. Note: EDM, electrical discharge machining; NPMEDM, nanopowder mixed
electric discharge machining.
6 A. KUMAR ET AL.
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and 3(d) reveals that higher GV enhances roughness of surface
due to a reduction in the gap between workpiece and elec-
trode. These are because of larger craters, which are possibly
formed due to the melting and vaporization of a minute
amount of workpiece material, caused by the intense heat of
a single spark. By optimizing the process parameters with IP
at 2 A, T
ON
at 8 μs and GV at 30 V applying the desirability
function approach, minimum SR of 1.444 µm was predicted.
The experimental result validated the predicted value at
1.487 µm, which was very close to the predicted one.
On the other hand, the experimental result showed that
minimum SR 2.245 µm could be achieved in conventional
EDM process by setting the same process parameters (i.e., IP
at 2 A, T
ON
at 8 μs, and GV at 30 V). This can be explained
with the help of captured discharge waveform (Fig. 4) for
NPMEDM and conventional EDM. Series of discharge takes
place in the case of NPMEDM, whereas, in the case of conven-
tional EDM, asymmetric waveform was observed during the
experiment. This series discharge produced a higher number
of crater, which are uniform and smaller in size.
[20,27]
More-
over, more wide plasma channel is created by the addition
of powder particles, due to which uniform spark occurs
between the electrodes and causes shallower crates.
Discharge waveform analysis
In this study, conventional EDM and the NPMEDM processes
were investigated by capturing the discharge wave forms.
A Tektronix TDS2012C oscilloscope was used to capture
the waveform with bandwidth 100 MHz, 2 channel mode,
2.0 GS/s sample rate on each channel, and 8-bit vertical
resolution. Figure 4 shows the waveforms of voltage and
current for conventional EDM and the NPMEDM process
under the same discharge level. The selected process para-
meters were 8 A peak current, 20 μs pulse ON time, and
30 V gap voltage. Figure 4(a) explains that the ignition delay
time (T
d
), which is necessary to provide insulation and resting
period between the electrode gaps, is very irregular in conven-
tional EDM. Therefore, all phenomena, such as short T
d
, long
T
d
, very short T
d
or arcing as well as slope during discharging,
are visible in this process. On the other hand, in the case of
NPMEDM, a normal ignition delay time (T
d
) and waveforms
can be found [Fig. 4(b)]. The obtained abnormal discharge
waveforms in the conventional EDM could be due to the pres-
ence of debris in the gap. However, in the NPMEDM, the gap
between the electrodes lengthens. As a result, proper flushing
can be done to eliminate the chance of arcing.
Figure 5. FESEM images of the generated surface by (a) NPMEDM and (b) conventional EDM with different magnification, i.e., 10 K �and 25 K�. Note: FESEM, field
emission scanning electron microscopy; NPMEDM, nanopowder mixed electric discharge machining; EDM, electrical discharge machining.
MATERIALS AND MANUFACTURING PROCESSES 7
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Surface-topography
In industrial applications, surface-topography is considered to
be the most essential factor in making components which are
expensive, which are working under high stress and which are
needed to maintain the safety of the whole system. In this
regard, the topography of the surface, generated by the EDM
process, heavily influences the performance of the products.
Therefore, besides SR, the topography of the machined sur-
faces was examined for conventional EDM and the NPMEDM.
In this study, machining was performed applying optimum
process parameters (IP at 2 A, T
ON
at 8 µs, and GV at 30 V)
for the better surface finish and topography of the surface.
FESEM images were taken as well as AFM analysis was done
for this purpose. Further, by examining sample surface, which
was obtained by conducting NPMEDM with the same combi-
nation of machining parameters as conventional EDM, a
significant improvement in surface-topography was noticed.
The surface morphology of all the samples was captured using
FESEM (Supra 55) as shown in Fig. 5. Figure 5(a) exhibits the
surface-topography of a sample, obtained with the NPMEDM
process, whereas Fig. 5(b) shows the surface-topography,
obtained by conventional EDM. It is distinctly visible that
the microcracks, which are very common on the surface,
generated by the conventional EDM process have almost dis-
appeared in the surface generated by the NPMEDM. This is so
due to uniform sparking between the workpiece and the tool
electrode in the case of NPMEDM. Further, observing the
surface-topography at higher magnification, no irregular
deposition of debris on the machined surface can be found.
Such improvement in the surface-topography could be
explained by a higher gap between the workpiece and tool
electrode, which leads to proper and easy flushing of debris.
However, microholes and crater marks on the surface are vis-
ible in both the samples.
To measure the depth and diameter of the microholes
formed during the EDM process, AFM investigation was
performed in both the samples. The images of the samples,
generated by EDM and NPMDEM, were taken by AFM
(Dimension Icon and MultiMode-8, Bruker). RTESPA probe
was used for tapping, where the thickness of the cantilever
was measured as 3.75 µm, width of the cantilever was mea-
sured as 35 µm, length of the cantilever is measured as
125 µm, oscillation frequency was measured as 300 kHz, spring
constant was selected at 40 N/m, and scan size was measured
as 10 µm. Figure 6(a) and 6(b) shows the AFM images of the
samples obtained by the NPMEDM and conventional EDM,
respectively. The right corner of each graph indicates the
variation of depth of the microholes. The darker contrast in
these images indicates deeper areas and brighter contrast
shows the top surface. The depth and diameters of all the
visible microholes in both the samples have been measured.
The average depth and diameter of the microholes in the
Figure 6. AFM analysis of microhole in the sample obtained (a) NPMEDM at (b) conventional EDM. Note: AFM, atomic force microscopy; NPMEDM, nanopowder
mixed electric discharge machining; EDM, electrical discharge machining.
8 A. KUMAR ET AL.
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NPMEDM were found as 538.152 nm and 1.357 µm, respect-
ively, whereas these values in conventional EDM were 1.279
and 1.497 µm. Therefore, it is clear that the depth of the
microhole is drastically reduced in the NPMEDM process.
Past literature established that machined surface, produced
by EDM, contains microholes, which depend on applied
energy and bubble formation during machining.
[27]
As a
result, lower discharge energy may lead to the formation of
minor size of the hole, whereas deeper hole may be formed
in the case of higher discharge energy. Moreover, low and
uniform discharge energy in NPMEDM and larger gap
between electrodes in this process also create sufficient scope
for bubbles to escape instead of getting trapped in the
generated surface.
Conclusion
The machining of Inconel 825 by NPMEDM has been
examined in this study. Online process monitoring (sparking
conditions) was also performed by analyzing the voltage and
current wave forms. Three process parameters (IP, T
ON
, and
GV) and two performance measures (MRR and SR) were
considered for comparing the performance of conventional
EDM and NPMEDM. Moreover, surface-topography images
obtained by FESEM and AFM were used to compare the
microgeometry/surface analysis. In view of the above investi-
gations, the following conclusions have been made:
1. Waveform analysis revealed that the tendency of arcing was
higher in the case of conventional EDM as compared to
NPMEDM. This may be due to an increase in the gap
between electrodes by mixing Al
2
O
3
nanopowder in the
dielectric. It also resulted in better flushing, less short
circuiting, and longer machining time. As a result, higher
machining stability.
2. The statistical analysis of experimental results confirmed
that the peak current, pulse-on-time, and gap voltage were
significant process parameters. By setting the optimal
values of process parameters (i.e., IP 8 A, T
ON
20 µm, and
GV 10 V), the maximum MRR of 47 mg/min was achieved
by the NPMEDM process. The surface roughness value of
1.487 µm was measured at optimal values of process
parameters (i.e., IP 2 A, T
ON
8 μs, and GV 30 V).
3. A significant improvement in surface-topography of the
machined surface was achieved using the NPMEDM. The
microcracks were reduced drastically in the case of
NPMEDM as compared to conventional EDM. The average
depth (538.152 nm) of microholes formed during
NPMEDM was also less (1.279 µm) as compared to conven-
tional EDM. Therefore, the better surface is obtained using
NPMEDM.
4. The Al
2
O
3
nanopowder was used successfully with deionized
water as a dielectric for machining of Inconel 825. Therefore,
these low-cost nanopowder particles can be utilized for the
industrial application of the NPMEDM process.
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