Electrical discharge machining is a thermo-electrical material removal process,
in which tool electrode shape is reproduced mirror wise into a work material
(Beri et al., 2008). The mechanism for removing
material primarily turns electrical energy into thermal energy through a series
of successive sparks between the electrode and the work-piece in a dielectric
fluid. The thermal energy is consumed in generating high temperature plasma,
eroding the work-piece material (Tsai et al., 2003).
The recent developments in the field of EDM have progressed due to the growing
application of EDM process and the challenges being faced by the modern manufacturing
industries, from the development of new materials that are hard and difficult-to-machine
such as tool steels, composites, ceramics, super alloys, hastalloy, nitralloy,
waspalloy, nemonics, carbides, stainless steels, heat resistant steels, etc.
being widely used in die and mould making industries, aerospace, aeronautics
and nuclear industries (Singh et al., 2004).
EDM has also made its presence felt in the new fields such as sports, medical
and surgical instruments, optical, dental and jewellery industries, including
automotive R and D areas. The various machining characteristics used to evaluate
the performance of EDM such as material removal rate (MRR), Tool Wear Rate (TWR),
relative wear ratio and surface roughness (SR) (Wu et
al., 2005). The important variables that affect the performance of EDM
are peak current, pulse duration, pulse-off time, the polarity of the electrode,
nozzle flushing etc. (Kansal et al., 2007). The
thermodynamic and physical properties of the tool and the work-piece also influence
the electrical discharge machining performance (Wang and
The following studies have been carried out to investigate the performance
of EDM on different type of steel work-piece. The metal removal rate depends
upon the amount of energy per spark, but too much current can damage the work-piece
or the electrode (Bojorquez et al., 2002). An
experiment revealed that copper electrode gives better surface finish, high
MRR and less electrode wear for tool steel work-piece followed by brass, aluminum
and copper tungsten electrode (Singh et al., 2004).
It was shown that the MRR is also controlled by the frequency of the sparks
and the low discharge currents with higher frequency correspond to low stock
removal using reverse polarity. At all values of current copper provide maximum
MRR and minimum tool wear rate, zero whereas aluminum contributes minimum MRR
and maximum TWR, around 60%. Tsai et al. (2003)
performed the experiment on steel work-piece and they reported that EDM technology
yields higher roughness with negative polarity than that obtained with positive
polarity. Negative polarity machining yields a higher roughness than positive
polarity machining, because the composite electrode was anode, such that a larger
discharge spot results in a wide melting zone. The machining characteristics
of SiC/6025 Al composite was investigated and the results evidenced the more
material removal rate (~40 mm3 min-1) for aluminum alloy
with 20% SiC than that of obtained (~29 mm3 min-1) for
aluminum alloy with 25% SiC using brass as electrode (Mohana
et al., 2004). The increase in volume percentage of SiC caused to
decrease in MRR and increase in Electrode Wear Rate (EWR). Maximum MRR was achieved
at 180 μ sec as pulse-on time. Their results evident that EWR decreases
with pulse duration and increases with peak current. The increase of peak current
and pulse duration cause to increase the surface roughness. An experiment has
been performed to find out the influences of EDM parameters on surface roughness
for machining of 40CrMnNiMo864 tool steel (AISI P20) which is widely used in
the production of plastic mold and die (Kiyak and Cakir,
2007). They investigated that when pulsed current and particularly pulse
time increased, machined work-piece surface exhibited a higher surface roughness.
Pulsed current had an effect on surface roughness at low pulse time, but the
influence of pulse time was more significant than pulsed current at higher pulse
times. Bojorquez et al. (2002) reported that
the surface finish of the machined part depends upon the ability of the electrode
to resist wear. Theisen and Schuermann (2004) investigated
the influence of EDM on pseudo-elastic NiTi shape memory alloys (SMAs). Their
observation revealed that increasing current intensity increases the working
energy, so that discharge craters become deeper and wider, thus resulting more
MRR and high surface roughness. Increase of frequency also raises surface roughness,
since craters depth becomes more profound. The discharge current that principally
has a significantly higher effect, than frequency. The copper and aluminum electrodes
achieve the best MRR with the increase in discharge current, followed by coppertungsten
electrode. Pulse current and pulse-on duration can be utilized to significantly
improve the thickness of the white layer. Experiment was conducted on EDM by
using SKD 61 steel as work-piece and copper as electrode where commercial kerosene
was used as dielectric fluid (Wu et al., 2009).
It was observed the material removal rate increases with peak current till 4A,
but when peak current reaches 6A, the MRR was lower because of larger diameter
of debris and tars due to the higher electrical discharge energy. It was argued
that at high ampere (6A) the debris and tars in larger diameter exist in the
gap between the electrode and work-piece.
Electrical discharge machining was done on stainless steel (SUS304) work-piece
in the case of copper electrode and conventional EDM oil, electro-rheological
(ER) fluid with different cons (Tsai et al., 2008).
The discharge pulse number and the surface roughness were found maximum using
conventional EDM oil. Using ER fluid and the abrasive Al2O3
the roughness was improved 85%. Mirror-like surface can be obtained when using
the smaller capacitance of 0.001 μF. It was also found that for all values
of pulse duration (12.8-200 μ sec) the MRR rises with peak current up to
48 A and then dropped when machining with higher current due to inferior discharge
caused by insufficient cooling of the work material (Lee
and Li, 2001). Material removal is directly proportional to the amount of
energy applied during the pulse-on time (Puertas and Luis,
2003). This energy is really controlled by the peak current and the length
of the on time.
Stainless steel is selected as engineering materials mainly because of their
excellent corrosion resistance in many environments. The corrosion resistance
of stainless steels is due to their high chromium contents. Austenitic stainless
steel has better corrosion resistance than ferritic and martensitic stainless
steel (Smith and Hashemi, 2006). Stainless steels are
used abundantly in engineering applications especially stainless steel 304 is
used in chemical and food processing equipment. To enhance its machining facility
it is essential to identify the performance characteristics on electrical discharge
machining on SUS 304. Although various parameters influence the performance
of EDM, such as peak current, pulse-on time, polarity of electrode, properties
of the tool and work-piece, etc., it is perceived that peak current and pulse-on
time retain great impact on EDM performance. Thus, this study has been accomplished
to analyze the performance of electrical discharge machining on stainless steel
304 material considering the peak current and pulse duration variables.
MATERIALS AND METHODS
This study was conducted at Automotive Excellence Centre, Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Kuantan. The duration of the project is April 2009 to August 2010.
Pulse duration or on-time refers the duration of time (μ sec) in which
the current is allowed to flow per cycle (Puertas and Luis,
2003). Pulse interval or off-time is the duration of time (μ sec) between
the sparks and the percentage of the on-time relative to the total cycle time
(on-time plus off-time) is specified as duty factor or cycle.
|| Chemical composition of stainless steel 304 work piece
|| Experimental conditions
||Typical EDM pulse current and pulse time train for controlled
Characteristic of pulse-on time, off time, peak current and average current
are shown in Fig. 1. This experimental work is conducted utilizing
die sinking electrical discharge machine of AQ55L model. Cylindrical copper
electrode having a size of Ø 19x37 mm and positive polarity for electrode
(reverse polarity) is used to machine austenitic stainless steel 304 materials.
The work material holds tensile strength of 580 MPa and 290 MPa as yield strength
(Smith and Hashemi, 2006). The size of the work-piece
was Ø 22x30 mm. The chemical composition of the work-piece is tabulated
in Table 1. Commercial kerosene is used as dielectric fluid
and during machining kerosene is circulated in the tank. In this study, pulse
duration and discharge pulse-off time were of 50, 100, 150 and 200 μ sec.
The list of experimental parameters is also listed in Table 2.
The MRR is expressed as the weight of material removed from work-piece over
a period of machining time in minutes (Wu et al.,
2005). The MRR is calculated from the difference of weight of work-piece
before and after machining as expressed in Eq. 1 (Mandal
et al., 2007):
where, Wi is the weight of work-piece before machining in g; Wf is the weight of work-piece after machining in g; t is the machining time in minutes; ps is the density of steel (7.8x10-3 g mm-3).
The electrode wear rate is calculated from the weight difference of electrode before and after machining as expressed in Eq. 2:
where, Ei is the weight of electrode before machining in gm; Ef is the weight of electrode after machining in gm; t is the machining time in minutes; ρcu is the density of copper (8.9x10-3 g mm-3).
The SR of the work-piece can be expressed in different ways including arithmetic average (Ra), average peak to valley height (Rz), or peak roughness (RP), etc.
Generally, the SR is measured in terms of arithmetic mean (Ra) which according
to the ISO 4987: 1999 is defined as the arithmetic average roughness of the
deviations of the roughness profile from the central line along the measurement
(Wu et al., 2005). Arithmetic mean or average
surface roughness is considered in this study for assessment of roughness.
RESULTS AND DISCUSSION
Influence of peak current and pulse on time on MRR: The influence of
peak current and pulse on time in terms of material removal rate is discussed
in this section. The contour plot and 3-D surface plot illustrate the experimental
result as shown in Fig. 2 and 3. The results
evidence that at all values of pulse duration the material removal rate increases
as pulse current is increased. This is due to the fact that as peak current
increases the discharge energy is increased consequently erodes more material
from the work-piece. Thus increasing peak current causes material removal more.
This phenomenon is also supported by the studies of Lee
and Li (2001) and Singh et al. (2004). On
the other hand, it is observed in this study that material removal rate is decreased
with pulse on time at low discharge current. Increasing the pulse duration reduces
the energy density of the discharge spots by expanding the plasma channel and
therefore reduces the material removal rate. Rise in pulse on time and pulse
interval (off time) decreases frequency and the higher frequencies with low
discharge currents correspond to low stock removal. This outcome is also supported
by the study of Singh et al. (2004).
|| Effect of peak current and pulse on time on MRR
|| 3-D plot of the effect of peak current and pulse on time
|| Effect of peak current and pulse on time on SR
It is also investigated that combination of high discharge ampere (12A) and
high pulse on time (200 μ sec) originates more MRR. The reason can be explained
as the discharge current comprised electron and ion currents. The ion current
represents a large fraction of the total current when the pulse duration is
long and that causes more MRR (Chow et al., 2008).
|| 3-D plot of the effect of peak current and pulse on time
Thus, 12 A with long pulse duration allows more ion current and produces more
MRR. Another reason is that long pulse duration decreases frequency and the
low frequency with high power results more metal removal (Dorf
and Kusiak, 1994). So, it can be reported that the influence of pulse duration
is more significant on MRR than pulse current at long pulse duration. This phenomenon
is also supported by the study of Kiyak and Cakir (2007).
Influence of peak current and pulse on time on SR: Figure
4 and 5 exhibit the variation of surface roughness against
peak current and pulse on time. It can be noticed that the surface roughness
increases almost linearly with peak current for different pulse on time. The
increase of discharge current increases discharge energy that promotes melting
and vaporization of the work-piece material and generate larger and deeper craters,
thus contributing to a greater surface roughness. In previous studies it is
also found that surface roughness increases with peak current (Mohana
et al., 2004; Singh et al., 2004; Kiyak
and Cakir, 2007). The results reveal that low peak ampere stimulate fine
surface finish. The surface roughness of the machined work-piece turns down
as the pulse on time increased. Rise in pulse on time reduces frequency that
produces a decreased surface roughness, since discharge craters become superficial
and thinner. The results can be supported by several studies (Theisen
and Schuermann, 2004; Kansal et al., 2007;
Wu et al., 2009). The long pulse duration decreases
frequency and the low frequency creates rough surface. Dorf
and Kusiak (1994) report the similar phenomenon. It is obvious in this research
that the combination of low ampere and high pulse duration provides finest surface
finish. It can be concluded also that the pulse on time has less impact on surface
roughness at high peak current.
|| Effect of peak current and pulse on time on TWR
|| 3-D plot of the effect of peak current and pulse on time
Influence of peak current and pulse on time on TWR: The effect of tool
wear rate versus peak current at various pulse duration is presented in Fig.
6 and 7. Tool wear rate is increased as peak current increases,
till about 175 μ sec pulse on time. Increasing the peak current increased
discharge energy and sparking, removing more melted material from both work-piece
and tool which increases the TWR. This result coincides with the outcome of
Mohana et al. (2004). It is seemed that at all
values of peak ampere, as the pulse on time increase the TWR trends to decrease.
The minimum TWR, zero is found at 200 μ sec pulse duration for all amperes.
This can be argued as the and high pulse duration facilitates no tool wear for
copper electrode remaining the polarity as reverse and work-piece as steel material.
Singh et al. (2004) also claimed the same circumstance.
It was attempted to investigate the effect of the peak current and pulse duration on the performance characteristics of the EDM. In roughing operations using positive polarity, the following conclusions can be drawn:
||The pulse on time and peak current greatly influenceon material
removal rate, tool wear rate and surface roughness
||It is found that at all values of pulse duration the material
removal rate increases almost linearly with increases of discharge current.
The combination of long pulse on time and high discharge current permits
more material removal. It can be represented that the impact of pulse duration
is more significant on MRR than pulse current at long pulse on time
||This experiment exhibits that the surface roughness increases
linearly with peak current for different pulse on time. Conversely surface
roughness decreases as the pulse duration is increased. The product of long
pulse duration and high discharge current causes rough surface. Finest surface
finish can be achieved by utilizing low peak ampere and long pulse on time
||As peak current increases, the TWR increases and the impact
of pulse on time on tool wear is contrary of peak current. As the pulse
on time increase the tool wear rate decrease and the TWR reaches minimum
(zero) at 200 μ sec spark on time for all values of peak amperes. Accordingly,
it can be recommended that long pulse duration with reverse polarity provoke
no tool wear on stainless steel work-piece retaining copper as electrode
The authors would like to express their deep gratitude to Universiti Malaysia Pahang (UMP) for provided the laboratory facilities and financial support under project No. RDU 090394 and Doctoral Scholarship scheme (GRS 090335).