Effect of Electrode Materials on Electric Discharge Machining of 316 L and 17 - 4 PH Stainless Steels

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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

Email: *[email protected]

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



 (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

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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