Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 685-690
Published Online July 2012 (http://www.SciRP.org/journal/jmmce)
Effect of Electrode Materials on Electric Discharge
Machining of 316 L and 17-4 PH Stainless Steels
Subramanian Gopalakannan*, Thiagarajan Senthilvelan
1Department of Mechanical Engineering, Adhiparasakthi Engineering College, Melmaruvathur, India
2Department of Mechanical Engineering, Pondicherry Engineering College, Pondicherry, India
Received February 27, 2012; revised April 5, 2012; accepted May 2, 2012
ABSTRACT
Electric Discharge Machining (EDM) is one of the most efficiently employed non-traditional machining processes for
cutting hard-to-cut materials, to geometrically complex shapes that are difficult to machine by conventional machines.
In the present work, an experimental investigation has been carried out to study the effect of pulsed current on material
removal rate, electrode wear, surface roughness and diameteral overcut in corrosion resistant stainless steels viz., 316 L
and 17-4 PH. The materials used for the work were machined with different electrode materials such as copper, cop-
per-tungsten and graphite. It is observed that the output parameters such as material removal rate, electrode wear and
surface roughness of EDM increase with increase in pulsed current. The results reveal that high material removal rate
have been achieved with copper electrode whereas copper-tungsten yielded lower electrode wear, smooth surface finish
and good dimensional accuracy.
Keywords: Electric Discharge Machining; Material Removal Rate; Electrode Wear; Surface Roughness; Stainless Steel
1. Introduction
Electric discharge machining (EDM) is one of the most
widely employed non-traditional machining processes
because it has been accepted as a standard process to
manufacture mould and dies of aerospace, automotive,
nuclear, surgical, petroleum and marine components.
Since EDM does not make direct contact between the
electrode and the work material, it eliminates mechanical
stresses, chatter and vibration problems during machin-
ing. Hence, very hard and brittle materials can also be
machined easily and also to the desired form. It removes
electrically conductive materials by means of rapid, re-
petitive spark discharges from a pulsating direct-current
power supply with dielectric flow between the work
piece and the electrode. Pulse current is one of the pri-
mary input parameters of an EDM process and together
with discharge duration and relatively constant voltage
for the given tool and workpiece materials. Pulse current
is representative of the energy per pulse expended in the
spark gap region. It thus controls the material removal
rate to the expected level. Considering the challenges
brought by advanced technology, the EDM is one of the
best alternatives for machining an ever increasing num-
ber of high strength, wear resistant and corrosion resis-
tant materials. Further, EDM is relatively simple method
to machine very complex geometry with very fine and
high precision.
In EDM process the performance is determined by
Material removal rate (MRR), Electrode wear (EW),
Surface roughness (SR), Surface quality (SQ) and Di-
mensional accuracy (DA). A series of investigations has
been conducted by Soni and Chakraverthi studied the
surface quality, material removal rate, electrode wear rate,
and dimensional accuracy of die steels and alloy steels in
EDM [1]. The effect of the machining parameters on
material removal rate, relative wear ratio, and surface
roughness in EDM of tungsten carbide had been studied
using different electrode materials and concluded that
copper tungsten yields the better results than copper and
brass [2]. Ho and Newman reported that research areas in
EDM could be classified into three major categories: 1)
Machining performance measures; 2) The effect of proc-
ess parameter; and 3) Design and manufacture of elec-
trode [3]. They also concluded that machining perform-
ance depends on the wear and surface quality. The effect
of machining parameters on material removal rate, elec-
trode wear rate, surface quality and diametric over-cut on
tool steels were investigated in detail and concluded that
copper and graphite electrode resulted in the best ma-
chining rate [4]. Mohd Abbas et al. presented the current
research trends on machining and modeling techniques in
predicting EDM performances [5]. Luis et al. had re-
*Corresponding author.

S. GOPALAKANNAN, T. SENTHILVELAN
686
ported that the material removal rate and electrode wear
in die sinking EDM of silicon carbide and conductive
ceramics using copper electrode by applying design of
experiments [6]. While studying the machining charac-
teristics of tungsten carbide-cobalt composite and ce-
ramics by EDM process, it is observed that increasing the
pulse time enhances the machining instability which has
significant effect on the surface finish of the workpiece
[7]. Experiments which were carried out on AISI P20
tool steel as work material and copper as electrode show
that the roughness of finished surface increases with an
increase in the discharge voltage, pulse current, and pulse
duration [8]. Wear of copper electrode was studied by
employing Taguchi’s standard orthogonal array in die-
sinking EDM of tool steel used in moulds and dies [9].
Muttamara et al. investigated the effect of electrode po-
larities in copper, graphite and copper-infiltrated graphite
electrodes in generation of conductive layer formation in
EDM of alumina [10]. Jahan et al. experimented micro-
EDM of tungsten carbide using different electrode mate-
rials of tungsten, copper tungsten and silver tungsten and
reported that silver tungsten electrodes are capable of
producing smooth and shiny surfaces with negligible
amount of surface defects [11]. Marafona and Araujo
investigated the influence of the hardness of the alloy
steel on material removal rate and surface roughness of
the work material [12].
From the earlier investigations, it has been observed
that no extensive work has been carried out to study the
effect of pulse current using copper, copper tungsten and
graphite electrode materials on the corrosive resistant
stainless steels as work material. Therefore, this research
envisage to investigate the effect of various electrode
materials and pulse current on material removal rate,
electrode wear rate, diameteral overcut and surface
roughness in EDM of 316 L and 17-4 PH stainless steels.
The above said work materials have been chosen by con-
sidering its wide range of applications in chemical proc-
essing industries, refineries, petroleum, shipping and nu-
clear industries due to its superior corrosion resistance.
The main objective of this study is to identify better elec-
trode material which results in greater MRR and lower
EW, lower surface roughness and good dimensional ac-
curacy.
2. Experimental Details
2.1. Machine Tool
The experiments were performed on Roboform 40 die-
sinking EDM supplied by Charmilles Technologies which
is shown in Figure 1. It is energized by a 64 A pulse
generator. A commercial grade EDM Oil (SAE 30) was
used as dielectric fluid during the experiments. An im-
pulse jet flushing system was used to flush away the
eroded material from the sparking zone is shown in Fig-
ure 2.
2.2. Work Piece Material
The work materials used in this study are stainless steel
316 L and 17-4 PH. The chemical composition of the
work materials are given in Table 1. The dimensions of
the work materials are 25 mm in diameter and 20 mm in
length.
2.3. Electrode Material
The prime requirement of any electrode material is that it
must be electrically conductive and maintain less elec-
trode wear. In principle, the materials best suited should
have a very high melting point and a very low resistance
to electricity. Electrode tool materials perform with vary-
ing degree of success on different workpiece materials.
The selection of particular electrode material depends
primarily upon the specific cutting application and upon
the material being machined. In EDM of steels, copper,
copper-tungsten and graphite electrodes are most widely
used [2]. The electrodes of 10 mm diameter were selected
for the purpose of this research. The physical properties
of all the electrode materials are given in Table 2.
Figure 1. Die-sinking EDM machine used to carry out the
experiments.
Figure 2. Impulse jet flushing syste m used.

S. GOPALAKANNAN, T. SENTHILVELAN 687
Table 1. Chemical composition (wt%) of work materials.
Elements 316 L (wt%) 17-4 PH (wt%)
C 0.026 0.06
Si 0.37 0.81
Mn 0.16 0.30
Cr 16.55 17.22
Cu 0.16 3.01
Ni 10.0 3.91
Nb - 0.16
P 0.029 0.023
S 0.027 0.03
Mo 2.02 0.76
N 0.036 -
Fe Balance Balance
Table 2. Physical properties of electrode material s.
Material Graphite Copper Copper tungsten
Composition 100% 99.9%
75% tungsten,
25% copper
Density (g/cm3) 1.811 8.96 15.2
Melting point (˚C) 3675 1084 3410
Electrical resistivity
(µΩ·cm) 14 9 5.5
Hardness HB 10 HB 100 HB 200
2.4. Experimental Procedure
The pulse current is normally selected on the basis of the
maximum removal rate possible within the allowable
mean current, electrode wear and surface integrity. The
experiments were carried out for a depth of cut of 2 mm
for all electrode materials with five different pulse cur-
rent settings of 6 A, 12 A, 18 A, 24 A and 30 A. Material
removal rate (MRR) is expressed as the ratio of differ-
ence of weight of the work piece before and after ma-
chining to the machining time [13].

MRR jb ja ww t

(1)
where wjb and wja are weights of the work piece before
and after machining, and t is the machining time. Elec-
trode wear rate (EWR) is expressed as the ratio of dif-
ference of weight of the tool electrode before and after
machining to the machining time [13].

EWR eb ea ww t (2)
where web and wea are weights of the tool electrode be-
fore and after machining, and t is the machining time.
Percent electrode wear is calculated as the ratio of vol-
ume of material eroded from the tool electrode per unit
time to the volume of material eroded from the work
piece in the same time [2].




EW%EWR MRR100
(3)
Since there exists many ways of measuring MRR and
EW, in this work the material removal rate and electrode
wear values have been calculated by weight difference of
the work material and the electrode before and after ma-
chining using a digital weighing scale and recorded. The
density values of work materials 316 L and 17-4 PH and
electrode materials of graphite, copper and copper tung-
sten were used to calculate the MRR and EW [6].
The machined cavity will always be larger than the
electrode and the difference between the electrode and
the work material gap is called the “overcut”, or “dia-
meteral overcut” as shown in Figure 3. The diameteral
overcut (DOC) are due to the presence of side sparks
found to occur in the work material (Sing et al., 2004).
The amount of overcut will vary according to the amount
of pulse current, pulse on time, type of electrode and
work material. The primary factor that affects the DOC is
the quantity of pulse current that exists in the gap and
pulse duration [13]. The DOC is always measured per
side. The DOC is expressed as the difference of diameter
of the hole produced to the tool diameter as stated in
Equation (4).
DOC
j
e
dd
e

 (4)
where de is the diameter of the tool electrode and dje is
the diameter of the hole drilled. An optical microscope
was used to measure the diameter of the eroded hole. The
parameter used for surface roughness is Ra, which is the
arithmetic mean of the departures of the roughness pro-
file from the mean line [2]. A surface roughness tester
(Kosaka Surfcoder SE 1200) was used to measure the Ra
values.
The same experiment was repeated with all three elec-
trode materials. During experiments, pulse current was
raised by keeping the voltage at 40 V, pulse duration at
Figure 3. The diameteral overcut.

S. GOPALAKANNAN, T. SENTHILVELAN
688
25 µs, pulse interval at 200 µs, and dielectric fluid pres-
sure at 20 kPa for all machining conditions and the val-
ues are presented in Table 3. The polarity of electrode
materials copper, graphite and copper tungsten were
negative [6].
3. Results and Discussion
3.1. Effects Pulse Current on Material Removal
Rate
Figure 4 shows the effect of pulse current on MRR of
316 L work material. The trend shows that as the pulse
current increases, the MRR also increases and similar
trend has been observed for all the three electrode mate-
rials. The copper electrode yields the highest MRR of
27.95 mm3/min followed by graphite of 22.46 mm3/min
and copper tungsten of 18.62 mm3/min for 316 L stain-
less steel. The increase in MRR with the increase in pulse
current is due to the enhancement of spark energy that
facilitates the action of melting and vaporization. More
so, this action results in advancing the impulsive force in
the spark gap and thereby increasing the MRR.
Figure 5 reveals similar trend of increasing MRR
while increasing the pulse current for 17-4 PH. However,
there is a significant increase in MRR between the pulse
current ranges 18 A - 30 A for all the three electrode ma-
terials compared to 6 A and 12 A. This is because of the
higher pulse current that causes rapid erosion of work
material which has low hardness value. Therefore 17-4
PH material gives higher MRR as compared to 316 L.
3.2. Effects of Pulse Current on Electrode Wear
Electrode wear is mainly due to high density electron
impingement generated during machining from work and
electrode materials. The electrode wear vs pulse current
for 316 L material is shown in Figure 6. Copper has the
highest EW of 50.74% as against 39.62% for graphite
and 17.86% for copper tungsten. In copper electrode, the
EW increases as the pulse current is increased due to its
low melting point where as in graphite and copper tung-
sten the EW is less because of their high melting point.
For all the three electrode materials, it shows that the
Table 3. Experimental machining conditions.
Sparking voltage 40 V
Discharge current in steps 6 A, 12 A, 18 A, 24 A, 30 A
Servo system Electro hydraulic
Electrode polarity Positive
Dielectric used Commercial grade EDM oil
Dielectric flushing Jet flushing system
Figure 4. Effect of pulse current on material removal rate of
316 L.
Figure 5. Effect of pulse current on material removal rate of
17-4 PH.
Figure 6. Effect of pulse current on electrode wear of 316 L.
trend of EW graph remains relatively constant after about
24 A. The EW of copper tungsten is very low when
compared with other two electrode materials because of
its high resistance to spark.
Figure 7 shows the effect of pulse current on electrode
wear for 17-4 PH work material similar to 316 L
stainless steel, copper has the highest EW 62.08% while
comparing with graphite (44.18%) and copper tungsten
(16.36%) for 17-4 PH as well. The behavior of the three
electrode materials remains same as that of 316 L work
material.
3.3. Effects of Pulse Current on Surface
Roughness
Figures 8 and 9 depict the effect of pulse current on the
surface roughness of the work materials 316 L and 17-4
PH respectively. It has been observed that as the pulse

S. GOPALAKANNAN, T. SENTHILVELAN 689
Figure 7. Effect of pulse current on electrode wear of 17-4
PH.
Figure 8. Effect of pulse current on surface roughness of
316 L.
Figure 9. Effect of pulse current on surface roughness of 17-
4 PH.
current is increased the surface roughness increased. The
results of the above mentioned two work materials indi-
cate that for the range of pulse current used i.e., between
6 A and 30 A, copper tungsten exhibits better surface
finish while the graphite shows the poorest. The surface
roughness value of copper electrode is in between the
graphite and copper tungsten. The surface roughness of
copper and graphite are high due to the fact that higher
MRR is accompanied by larger and deeper craters. This
causes low pulse currents and spark energy which leads
to the formation of small craters on the machined surface
and thereby improving surface finish [14]. Therefore,
formation of small craters results in good surface finish.
The only difference between these two materials is that
for same pulse currents values, 316 L offers better sur-
face finish than 17-4 PH because of higher MRR in 17-4
PH material than 316 L.
3.4. Effects of Pulse Current on Diameteral
Overcut
Dimensional accuracy becomes more important when
close tolerance components are required to be produced
in aerospace industries, marine industries, and also in
tools, dies and moulds for press work, plastic molding
and die casting. The diameteral overcut is low at low
pulse current and hence the erosion is low. Figures 10
and 11 show the diameteral overcut of work materials
316 L and 17-4 PH respectively. The copper tungsten
electrode gives low and consistent diameteral overcut of
0.1 mm for both materials at high pulse current. In con-
trast, graphite electrode yields the poor dimensional ac-
curacy which results in higher diameteral overcut for
both materials at higher pulse current. The diameteral
overcut of copper is between 0.12 mm (316 L) and 0.16
mm (17-4 PH) in the pulse current range of 12 A to 24 A
and good dimensional accuracy also has been obtained.
The graphite results in high overcut due to its high spark
dispersing effects. The overcut not only depends on the
pulse current and also depends on gap voltage and chip
size.
4. Conclusions
An experimental study has been conducted to investigate
the effect of electrode materials on machining character-
istics in EDM of corrosive resistant stainless steels 316 L
Figure 10. Effect of pulse current on diameteral overcut 316
L.
Figure 11. Effect of pulse current on diameteral overcut of
17-4 PH.

S. GOPALAKANNAN, T. SENTHILVELAN
690
and 17-4 PH. The following inferences are arrived at:
1) For 316 L and 17-4 PH work materials copper elec-
trode gives the better MRR than graphite whereas the
copper-tungsten yields the lowest MRR value. The MRR
obtained by three electrodes in 17-4 PH is higher than
316 L because of its low hardness;
2) Copper-tungsten offers comparatively low electrode
wear for the tested materials whereas the performances
of copper and graphite electrodes are high and almost
similar;
3) Graphite and copper electrodes produce compara-
tively high surface roughness for the tested materials at
higher values of pulse current. The copper-tungsten elec-
trode offers low values of surface roughness at high dis-
charge current which yields good surface finish for both
work materials;
4) Copper-tungsten and copper electrodes performed
consistently at high values of pulse current and copper-
tungsten offers low diameteral overcut and good dimen-
sional accuracy than copper;
5) Copper electrode has been preferred for higher
MRR whereas copper-tungsten is preferred for low elec-
trode wear, good surface finish and good dimensional
accuracy.
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