The study investigates the use of graphite powder as an additive in additive mixed electrical discharge machining (AEDM) to enhance the machining rate of Inconel 718. Using Taguchi methodology, the researchers determined optimal process parameters, finding that the machining rate improved by 26.85% with an optimal addition of 12g/l of graphite at best settings. The significant parameters affecting machining performance were identified as polarity, peak current, pulse on time, and concentration of the powder.

1. AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010
© 2010 AMAE
DOI: 01.IJPIE.01.01.51
48
Realizing Potential of Graphite Powder in Enhancing
Machining Rate in AEDM of Nickel Based Super
Alloy 718
Anil Kumar1
, Sachin Maheshwari2
, Chitra Sharma3
, Naveen Beri1
1
Research scholar, University School of Engineering and Technology, GGSIPU, Delhi, India
(ak_101968@yahoo.com)
2
Division of Manufacturing Process & Automation Engineering, NSIT, Dwarka, New Delhi, India
3
Department of Mechanical andAutomation Engineering, IGIT, Kashmiri Gate, Delhi, India
Email: {ssaacchhiinn@redifmail.com, chitrabisht@rediffmail.com, nav_beri74@yahoo.co.in}
Abstract— Additive mixed electric discharge machining
(AEDM) is a recent innovation for enhancing the capabilities
of electrical discharge machining process. The objective of
present study is to realize the potential of graphite powder as
additive in enhancing machining capabilities of AEDM on
Inconel 718. Taguchi methodology has been adopted to plan
and analyze the experimental results. L36
Orthogonal Array
has been selected to conduct experiments. Peak current, Pulse
on time, duty cycle, gap voltage, retract distance and
concentration of fine graphite powder added into the dielectric
fluid were chosen as input process variables to study
performance in terms of material removal rate. The ANOVA
analysis identifies the most important parameters to maximize
material removal rate. The recommended best parametric
settings have been verified by conducting confirmation
experiments. From the present experimental study it is found
that addition of graphite powder enhances machining rate
appreciably. Machining rate is improved by 26.85% with 12g/
l of fine graphite powder at best parametric setting.
Index Terms— Electrical discharge machining, Additive mixed
electrical discharge machining, Machining rate, Spark gap,
Graphite powder concentration, Taguchi methodology
I. INTRODUCTION
Electrical discharge machining (EDM) is a common
nonconventional material removal process. This technique
has been widely used in modern metal working industry for
producing complex cavities in dies and moulds, in press tools,
aerospace, automotive and surgical components
manufacturing industries which are otherwise difficult to
create by conventional machining methods [1-2]. This
machining process involves removal of material through
action of electrical discharges of short duration and high
current density between tool electrode and the work piece.
However, its low machining efficiency and poor surface
finish restricted its further applications [3]. To overcome
these problems, one relatively new innovation used to
improve the efficiency of EDM in the presence of additives
suspended in the dielectric fluid. This new hybrid machining
process is called additive mixed electrical discharge
machining (AEDM) [4-5]. Fig.1. depicts principal ofAEDM.
The machining mechanism of AEDM is different from
conventional EDM process [3-4]. In AEDM when a voltage
of 80-320V is applied across work piece and electrode
electrical intensity in the range of 105
to 107
V/m is generated
[3]. Under the influence this electric intensity additives
powder particles get energized and behave in a zigzag
fashion. These additives particles arrange themselves in the
form of chain at different places under the sparking area
(Fig.2). The chain formation helps in bridging the gap
between both the electrodes. This bridging effect results in
lowering the breakdown strength of the dielectric fluid which
causes early explosion in the gap. As a result, the series
discharge starts under the electrode area. Due to increase in
frequency of discharging, faster erosion takes place from
the work piece surface. Therefore gap contamination
facilitates ignition process and increases gap size thereby
improving process stability. The absence of debris may
results in arcing due to absence of precise feeding mechanism
with highly position resolution. However excessive
contamination may increase spark concentration i.e. arching
leading to unstable and inefficient process [6].Corresponding author: Anil Kumar, Department of Mechanical
Engineering, Beant College of Engineering & Technology Gurdaspur
143521, Punjab, India.
Figure 1. Principal of AEDM [4]
TABLE I
PROCESS PARAMETERS AND THEIR LEVELS

2. AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010
© 2010 AMAE
DOI: 01.IJPIE.01.01.51
49
Figure 2. Mechanism of AEDM.(a) Bridge formation, (b) Spark
initiation because of breakage of chain, (c) Explosion leading to zigzag
particle motion, (d) Re- bridging [4]
Erden and Belgin [7] were the first who studied the effect of
impurities (copper, aluminium, iron and carbon) in electrical
discharge machining of brass steel and copper steel pair and
obtained increase in machining rates with increase in
concentration of impurities. It was further observed that
machining becomes unstable at an excessive powder
concentration due to the occurrence of short-circuits.
Thereafter Jeswani [8] investigated the effect of adding fine
graphite powder in dielectric of EDM and reported that
addition of about 4g of fine graphite powder (10 µm in
average size) per liter of kerosene improved machining
stability thereby increasing metal removal rate (MRR) by
60% and tool wear rate (TWR) by15%. The WR was reduced
by about 28%. This effect was attributed to increase in
interspaces for electric discharge initiation and reduction in
the breakdown strength of dielectric fluid. Later many
researchers Zhao, Meng and Wang [9], Singh, Maheshwari
and Pandey [10] Kumar, Maheshwari, Sharma and Beri
[11,12] investigated the process capabilities of additive
powder mixed EDM. They reported that that machining
efficiency of powder mixed EDM can be increased by
selecting proper discharging parameters. Very little research
work is reported on effect of machining parameters on
machining characteristics in AEDM of Inconel 718. Nickel
based super alloy are extremely useful in gas turbine, space
vehicles, aircraft, nuclear reactors, submarine, petrochemical
equipments and other high temperature applications. The
electrical parameters (like pulse frequency, duty cycle, pulse
on time, spark gap, current and gap voltage, polarity etc.),
material properties of electrode, work piece and dielectric
fluid, properties of additive powders (like melting point,
specific heat, thermal conductivity, grain size and
concentration etc.) are the main factors which influence
additive mixed electrical discharge machining. Hence there
is need to investigate machining parameters to obtain
optimum machining rate. Therefore, promoting the quality
of the process by developing a thorough understanding of
the relationship between these parameters for better machined
surfaces has become a major research concern [6, ­12]. The
aim of the present research work was to identify the
significant parameters affectingAEDM process and find best
parametric settings to maximize MR of Inconel 718 super
alloy.
II. EXPERIMENTAL SET UP AND PROCEDURE
In the present study experiments were carried out on
Electronica make electrical discharge machine; model
SMART ZNC (S50). The dielectric flow system was
modified for circulation of aluminum powder suspended
dielectric medium in small quantities to prevent
contamination of whole of dielectric fluid. Inconel 718
(specimen 65mm X 25mm X 4mm) was selected as work
piece. Cylindrical copper electrode ( 8.0 mm) was used as
an electrode. Based on literature survey and preliminary
investigations, the parameters chosen as inputs are polarity,
peak current, types of electrodes, pulse on time, gap voltage
duty cycle, retract distance and concentration of graphite
additive powder. The Inconel 718 work piece was eroded
with fine graphite powder (325 mesh) in AEDM. The
machining process parameters set up as shown in Table 1
keeping all other parameters constant. Preliminary
experiments were conducted in the given range of different
input parameters to select their levels shown in Table 1. L36
(21
×36
) orthogonal array has been used which contains 36
experimental runs at various combinations of eight input
variables [13]. Machining rate is measured in term ofvolume
of material eroded from work piece per minute by weight
loss method as per following equation.
Machining rate =
(1)
Weight loss from work piece (gms)
Density of work piece (g/mm3
) × machining time (min)
III. RESULTS AND DISCUSSIONS
Taguchi methodology has been used for the design and
analysis of the experiment [13, 14]. The Taguchi method
uses signal-to-noise (S/N) ratio to quantify the variation in
data. Higher response characteristics i.e MR is desirable, so
The selected response characteristic in the present work is
machining rate and will be considered as ‘higher the best’.
(2)
The results are analyzed using MINITAB 15.1.1 software.
The S/N ratios of the MR for each trial run through have
been calculated from experimental data and response table
for larger the better are summarized in Table 2 giving relative
importance of each parameter on desired response. Higher
the delta value, higher is contribution of parameter on
response. From the Table 2 it is clear that polarity, peak
current, and concentration of powder significantly affect the
process performance. The main effect plots are plotted in
Fig.3. Although cryogenic treatment of copper electrode
increases conductivity however its effect on machining rate
is negligiblePulse on time affects the machining rate
significantly. Increased pulse on time produces crater of
higher diameter, therebyincreasing machining rate. Middle
value of duty cycle 0.8 i.e 80% of available energy produces
higher machining rate. At higher duty cycle there is
possibility of accumulation of more debris within sparking
area leading to unfavorable flushing conditions, thereby
reducing machining rate. A gap voltage of 80V produces
higher machining rate.

3. AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010
© 2010 AMAE
DOI:1.IJPIE.01.01.51
50
TABLE I.
RESPONSE TABLE FOR SIGNAL TO NOISE RATIOS (LARGER THE BETTER)
Figure 3. Main Effect plot for S/N ratios for machining rate
Higher retract distance leads to better flushing conditions.
Increase in concentration of graphite fine powder in dielec-
tric fluid increase material removal rate. This observation
suggests that addition of an appropriate amount of additives
into the dielectric fluid of EDM causes greater erosion of
the material. The reason for the enhancement ofMR is mainly
attributed to reduction in breakdown strength of the dielec-
tric fluid leading to earlyspark, and increase in frequencyof
sparking within the discharge. Fig.3 suggest that +ve polar-
ity, copper electrode , 6 amps peak current , 150 µs pulse on
time , 0.8 duty cycle, 80V gap voltage, 3mm retract distance
and 12g/l concentration of graphite powder produce the
maximum machining rate in Inconel 718. ANOVA analysis
performed on the S/N data suggest that polarity , peak cur-
rent , pulse on time and graphite as additive powder in di-
electric medium significantly affect response characteristic
of machining rate at 5% level of significance.
TABLE II.
ANALYSIS OF VARIANCE FOR S/N DATA (LARGER THE BETTER )
To verify the improvement in MR using the best
parametric settings ofcutting parameters, three confirmation
experiments were performed, and the data from the
confirmation runs and their comparisons with the predicted
value for MR is listed in Table 5. The error is less than 10 %
and the maximum improvement of MR at best parametric
settings is about 26.85%.
IV. CONCLUSIONS
The result of the present work identifies the significant
process parameters and optimizes the machining conditions
in the presence of graphite powder in the dielectric fluid
toget maximum MR from Inconel 718 super alloy. Within
the range of parameters selected for the present work, the
following conclusions are drawn:
1. Selected EDM process parameters and fine graphite
powder concentration affects the machining rate in
AEDM.
2. Additive mixed EDM enhance machining rate
appreciably.
3. Peak current contribute maximum to machining rate
i.e 81.3%
4. Contribution of fine graphite powder is 2.4%
5. Overall machining rate after addition of graphite
powder improves by 26.85%.
6. In the present study within the experimental
conditions the best parametric settings are
A1B1C3D3E2F3G3H3.
7. 12g/l of graphite in EDM medium produces
maximum MR.

4. AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010
© 2010 AMAE
DOI:1.IJPIE.01.01.51
TABLE III
ANALYSIS OF VARIANCE FOR S/N DATA (LARGER THE BETTER
ACKNOWLEDGEMENT
The authors would like to acknowledge the support of
department of Mechanical Engineering, Beant College of
Engineering and Technology, Gurdaspur, Punjab, India and
All India Council for Technical Education New Delhi, India
for supporting and funding the research work under research
promotion scheme (F.No.: 8023/BOR/RID/RPS-129/2007-
2008, F.No.: 8023/BOR/RID/RPS-144/2008-2009 and 8023/
BOR/RID/RPS-86/2009-10) in this direction.
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