electric discharge machining material

1. Accepted Manuscript
Title: State of the art in powder mixed dielectric for EDM
applications
Author: Houriyeh Marashi Davoud M. Jafarlou Ahmed A.D.
Sarhan Mohd Hamdi
PII: S0141-6359(16)30069-1
DOI: http://dx.doi.org/doi:10.1016/j.precisioneng.2016.05.010
Reference: PRE 6407
To appear in: Precision Engineering
Received date: 17-1-2016
Revised date: 14-4-2016
Accepted date: 21-5-2016
Please cite this article as: Marashi Houriyeh, Jafarlou Davoud M, Sarhan Ahmed AD,
Hamdi Mohd.State of the art in powder mixed dielectric for EDM applications.Precision
Engineering http://dx.doi.org/10.1016/j.precisioneng.2016.05.010
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2. 1
State of the art in powder mixed dielectric for EDM
applications
Houriyeh Marashia,b*
, Davoud M. Jafarloua,b
, Ahmed A. D. Sarhana,b,c*
, Mohd
Hamdia,b
a
Department of Mechanical Engineering, Faculty of Engineering, University of
Malaya, 50603 Kuala Lumpur, Malaysia,
b
Center of Advanced Manufacturing and Materials Processing (AMMP),
50603 Kuala Lumpur, Malaysia,
c
Department of Mechanical Engineering, Faculty of Engineering, Assiut
University, 71516 Assiut, Egypt
Corresponding Authors:
Houriyeh Marashi,
Address: Department of Mechanical Engineering, Faculty of Engineering, University of
Malaya, 50603 Kuala Lumpur, Malaysia.
E-mail: houriyeh@marashi.co,
Tel: 0060133730989
Ahmed A. D. Sarhan,
Address: Department of Mechanical Engineering, Faculty of Engineering, University of
Malaya, 50603 Kuala Lumpur, Malaysia.
E-mail: ah_sarhan@yahoo.com,
Tel: 0060379674593

3. 2
Highlights
 Thorough review of powder mixed electrical discharge machining (PMEDM) is
provided.
 This article reviews alternatives for oil dielectric with focus on PMEDM.
 PMEDM spark characteristics and powder’s thermo-physical properties are
discussed.
 According to powder material, the performance of PMEDM is discussed in
detail.
 This review assists in evaluation of PMEDM performance prior to
experimentation.
Abstract
Electrical discharge machining (EDM) is a non-conventional machining technique for
removing material based on the thermal impact of a series of repetitive sparks occurring
between the tool and workpiece in the presence of dielectric fluid. Since the machining
characteristics are highly dependent on the dielectric’s performance, significant
attention has been directed to modifying the hydrocarbon oil properties or introducing
alternative dielectrics to achieve higher productivity. This article provides a review of
dielectric modifications through adding powder to dielectric. Utilizing powder mixed
dielectric in the process is called powder mixed EDM (PMEDM). In order to select an
appropriate host dielectric for enhancing machining characteristics by adding powder, a
brief background is initially provided on the performance of pure dielectrics and their
selection criteria for PMEDM application follow by powder mixed dielectric thoroughly
review. Research shows that PMEDM facilitates producing parts with predominantly
high surface quality. Additionally, some studies indicate that appropriate powder
selection increases machining efficiency in terms of material removal rate. Therefore,
the role of powder addition in the discharge characteristics and its influence on
machining output parameters are explained in detail. Furthermore, by considering the
influence of the main thermo-physical properties and concentration of powder particles,

4. 3
the performance of various powder materials is discussed extensively. Since suitable
powder selection depends on many factors, such as variations in EDM, machining scale
and electrical and non-electrical parameter settings, a thorough comparative review of
powder materials is presented to facilitate a deeper insight into powder selection
parameters for future studies. Finally, PMEDM research trends, findings, gaps and
industrialization difficulties are discussed extensively.
Keywords: PMEDM; dielectric; powder material; powder size; powder concentration.
Nomenclature
A Plate area of capacitor
C Capacitance
d distance between the conductors
E Electrical field in capacitor
𝐸𝑏 Dielectric breakdown strength
𝐸𝑑 Discharge energy
FP Pulse signal frequency
I Current
Ia Average current
Id Discharge current
Q Charge stored in capacitor
Ra Average surface roughness
Rz Peak-to-valley surface roughness
td Discharge duration
tdelay Ignition delay time
toff Pulse off time

5. 4
ton Pulse on time
tp Pulse duration
𝑈𝑜 Open gap voltage
𝑈𝑏 Breakdown voltage
𝑈𝑑 Discharge voltage
V Voltage
Abbreviations
AJM Abrasive jet machining
ECM Electrochemical machining
EDM Electrical discharge machining
Gr Graphite
MQL Minimum quantity lubrication
MRR Material removal rate
PMEDM Powder mixed electrical discharge machining
SQ Surface quality
SR Surface roughness
TWR Tool wear rate
Greek symbols
𝜀0 Permittivity in free space
𝜀𝑟, Relative permittivity,
ĸ Dielectric constant

6. 5
1 Introduction
Electrical Discharge Machining (EDM) was established over sixty years ago by Doctors
B. R. and N. I. Lazarenko as a rudimentary die-sinking machine initially employed for
stock removal [1]. Nowadays, EDM is implemented in various industrial fields, such as
die and mold, aerospace and biomedicine. The outstanding capabilities of this technique
include the ability to machine high-hardness conductive materials and produce complex
geometrical shapes, it involves a simple tool-making process and eliminates the
mechanical stress and chatter phenomena [2]. Despite the advantages, tool wear,
relatively low material removal rate (MRR) and its adverse effects on surface quality
limit EDM application. Adding powder to dielectric, namely powder mixed electrical
discharge machining (PMEDM), is an effective means of enhancing EDM performance
owing to different spark attributes.
Since PMEDM was introduced in 1980 [3], researchers have focused on improving the
machining performance or introducing new PMEDM capabilities like surface
modification to ultimately facilitate unique development for commercialization.
Although several studies evaluate the PMEDM advancements over the years [2, 3], no
review article emphasizes material, size and powder concentration. To fill this gap, an
organized review of PMEDM is provided. First, the spark formation mechanism in
conventional EDM as well as important input and performance parameters are
reviewed. Second, the significance of dielectric for EDM machining characteristics is
discussed. A detailed review of various dielectric categories for PMEDM applications is
presented, since the base medium (for hosting the powder) needs to be selected initially.
Third, the role of powder addition in spark characteristics followed by the influence of

7. 6
various powder particles, materials, sizes and concentrations is comprehensively
reviewed followed by a systematic study of powder materials to ease appropriate
selection of powder parameters for further research studies. At last, the PMEDM
research trends, findings, gaps and technology industrialization are discussed
extensively.
2 Electrical discharge machining
In order to understand the PMEDM spark characteristics, the material removal
mechanism in conventional EDM is described. Conventional EDM undergoes
continuous charging and discharging between the tool and workpiece while immersed in
dielectric fluid. Electrical discharge consists of three phases: (i) preparation, (ii)
discharge and (iii) interval phases, which are described schematically in Fig. 1
according to [4-6].
EDM performance is evaluated considering key performance parameters, i.e. MRR,
surface quality and TWR. These parameters are significantly influenced by the input
machining parameters, which are generally categorized as electrical and non-electrical
parameters [7]. Electrical parameters are related to measurable electrical values that are
regulated by the power supply unit [8], such as discharge current, pulse on time, pulse
off time, discharge voltage and polarity [9], and non-electrical parameters including but
not limited to electrode lift time, reciprocating speed, working time, gain and nozzle
flushing, and tool and workpiece rotation [10]. The time spent on electrode lifting
movements for jump operation (up and down electrode movement during the EDM
process to increase and decrease the discharge gap) is called electrode lift time [11].
Jump operation is performed to enhance machining gap flushing. Electrode working

8. 7
time denotes the time until the jump operation takes place to control the opening/closing
of the discharge gap [12]. The electrode’s reciprocating speed is considered when the
electrode performs machining with reciprocating motion (back-and-forth).
MRR in EDM is directly related to the economic aspect of production and is considered
the highest priority [5]. MRR reflects the process operational speed and is defined in
two ways, as the volume [13] or weight [14] of material removed from the workpiece
over the machining time.
The machined surface quality is evaluated according to roughness, waviness and flaws.
Among these, surface roughness measurement is the most widely employed technique
and is commonly expressed as the average surface roughness (Ra). Ra is the average
deviation from the mean surface by calculating the arithmetic average deviation of the
ordinates of profile height increments from the centerline of that surface [15].
Tool wear rate (TWR) is another performance measure that decreases EDM efficiency
by increasing tool production cost and deteriorating produced component accuracy due
to end, side and corner wear [16]. TWR is defined as the material weight [17] or volume
[18] removed from the tool over the machining time. Moreover, to assess the
simultaneous effect of MRR and TWR, tool wear ratio is introduced as TWR/MRR [3].
Fig. 2 schematically shows the current and voltage waveform for a single spark during
the EDM process. Most electrical parameters affect the MRR mainly through discharge
energy (𝐸𝑑) and pulse signal frequency (𝐹
𝑝). 𝐸𝑑 is the mean value of electrical energy
per one impulse, which is converted into heat and is expressed as [19]:
𝐸𝑑 = ∫ 𝑢(𝑡)
𝑡𝑑
0
𝑖(𝑡)𝑑𝑡 =
̃ 𝑈. 𝐼𝑑. 𝑡𝑑 (1)

9. 8
where td, U and Id are discharge duration, voltage and current, and variables u(t) and i(t)
indicate the instantaneous voltage and current, respectively.
Selecting proper machining conditions leads to instantaneous electrical discharge and is
independent from other electrical values. In this case, the ignition delay time (tdelay) can
be neglected, thus the discharge duration td is equal to the pulse duration or pulse on
time (ton) and 𝐸𝑑 expressed as [19]:
𝐸𝑑 = 𝑈. 𝐼𝑑 . 𝑡𝑜𝑛 (2)
Eq. 2 indicates higher spark energy with the increase of either U, Id or ton, which
subsequently leads to higher MRR, TWR and surface roughness due to higher energy
impacting both the tool and workpiece.
Besides 𝐸𝑑, the pulse frequency, 𝐹
𝑝, is another factor representing the contribution of
electrical parameters and is defined as follows:
𝐹
𝑝 =
1
𝑡𝑝
=
1
𝑡𝑜𝑛+ 𝑡𝑜𝑓𝑓
(3)
where, 𝑡𝑝 is pulse duration and 𝑡𝑜𝑓𝑓 is the off time (charging time).
Shorter toff leads to higher pulse frequency and therefore higher MRR. However, at less
than the optimized toff, the debris flushing efficiency and cutting stability decrease,
prolonging the machining time due to spark instability and erratic cycling [20]. Polarity
is another important parameter that is selected as positive or negative depending on the
tool and workpiece material, current density and pulse length combinations [21].

10. 9
3 Various dielectric categories
The dielectric medium fills the machining gap and acts as a spark conductor to maintain
the machining gap, ensure stable operation, cool and quench the workpiece and
electrode surface, and flush debris particles from the gap [22]. To accomplish these
duties, the dielectric should have certain specifications, such as adequate electrical
discharge efficiency and viscosity, suitable oxidation stability, high flash point,
minimum odor and low cost [23]. In order to select a suitable combination of powder
and dielectric, a background of plain dielectrics that initially need to be selected is
provided in this section. The dielectric that will host the powder can be in the form of
liquid, gas or a mixture of liquid and gas. Therefore, a variety of dielectric categories
and their performance are concisely described in this section. At last, the dielectric
selection criteria for PMEDM application is considered in section 3.5.
3.1 Hydrocarbon oil
Hydrocarbon oil dielectrics, such as mineral oils, kerosene, mineral seal and transformer
oil as the basic dielectric types are still the most common die-sinking EDM dielectrics
[24]. There is extensive evidence that hydrocarbon oils are generally more efficient than
deionized/distilled water in die-sink applications [25], since they offer smaller spark
gaps, facilitating machining of more sensitive shapes [26]. In addition, hydrocarbon oils
offer no corrosion, an easier purifying process and higher aging resistance compared
with water dielectric [26]. However, oil-based dielectric poses fire hazards and a variety
of substances are released by their thermal decomposition including benzene (C6H6) and
benzopyrene (C20H12), which are carcinogenic [25]. These problems and the significant
influence of dielectric on performance parameters highlight the necessity to use

11. 10
alternative dielectrics. In this regard, various alternatives have been proposed and are
summarized in Fig. 3.
The decomposition of hydrocarbon oil dielectrics through machining releases carbon-
based compositions that cause gap pollution and spark instability. Nonetheless,
increasing carbon content in the gap is not always a drawback. Mohri, Fukuzawa [59]
took advantage of this phenomenon and introduced a new method for EDM of
insulating ceramics using kerosene as dielectric by placing a metal plate on the top
surface of the insulator ceramic workpiece as an assisting electrode. Electrical
conductive compounds involving decomposed carbon from the dielectric generated on
ceramic surfaces maintain the workpiece surface electrical conductivity during
machining. In the past decades, this method has gained popularity for machining
insulator ceramics which was reviewed in detail by Schubert, Zeidler [60].
3.2 Water
3.2.1 Deionized water
Deionized water is commonly selected as dielectric in wire-EDM, micro-hole
fabrication in EDM and hole drilling [25]. Jeswani [30] revealed that machining in
deionized water enhances the MRR and surface quality, and reduces the electrode wear
ratio compared to kerosene at high-pulse energy ranges, but diminishes machining
accuracy. Lin, Chow [32] specified that in micro-slit EDM of Ti-6Al-4V, deionized
water produces noticeably lower carbon adhesion to the electrode surface without a drop
in discharge energy or abnormal discharge compared to kerosene. However, a single

12. 11
spark forms a relatively smaller crater with deionized water due to the re-solidification
of most molten material [31].
Kruth, Stevens [33] found that in deionized water dielectric, the carbon element forms
Fe3C (iron carbides) in columnar and dendritic structures on the surface, resulting in de-
carbonization, whereas increasing carbon content was detected when employing oil
dielectric. Moreover, the retained austenite phase quantity and micro-crack intensity
substantially decreased within the white layer when using deionized water [34]. Surface
cracks primarily form due to thermal stress from drastic quenching and non-uniform
temperature distribution over the machined surface [35]. In investigating the
susceptibility to cracks of a nickel-based super alloy, the surface machined in deionized
water oxidized after heat treatment and no crack propagation occurred, unlike kerosene
dielectric [36]. Furthermore, water-in-oil emulsion as dielectric resulted in recast layer
formation with higher hardness than machining in kerosene or deionized water [38].
3.2.2 Low resistivity deionized water
Deionized water as dielectric always results in electrochemical dissolution due to the
slight water conductivity [27]. To benefit from this effect, Masuzawa, Kuo [42] reduced
the resistivity of deionized water to the maximum allowable resistivity (5×104
Ω.cm) to
attain the requirements of both dielectric and electrolyte in order to enable EDM and
ECM simultaneously and achieve lower surface roughness. Kurita and Hattori [43]
reduced the Ra value of a machined hole from 1 μm to 0.07 μm using 2 min of EDM-
ECM complex lapping. Nguyen, Rahman [44] also utilized low-resistivity deionized
water, which improved the finishing and machining accuracy through the transition
from EDM to ECM, which dissolved the heat-affected zone. Besides, covering the

13. 12
electrode walls with a thin insulating film allowed for tool wear compensation during
blind hole drilling with EDM-ECM due to the electrodeposition effect during the
process [27].
3.2.3 Tap water
A comparison of the machining characteristics of tap water, deionized water and a
combination of tap water (25%) and distilled water (75%) indicated that zero electrode
wear is achievable when using a copper tool with negative polarity [39]. Casanueva,
Azcondo [41] developed a new power supply for spark erosion based on a series-
parallel resonant converter that significantly reduced the machine weight and size,
making it especially appropriate for portable operations in tap water dielectric.
Furthermore, it was revealed that under optimum conditions in machining Ti-6Al-4V,
employing tap water increased the MRR by 87.3% and decreased the TWR by 25.7%
and surface roughness by 18.9% [40].
3.3 Gas dielectric
In 1985, NASA publicized that a “dry EDM process” is possible if argon and helium are
involved as dielectric [25], which led to employing oxygen or air gas dielectric. Dry
EDM with gas dielectric has the advantages explained in Fig. 3 [28].
3.3.1 Oxygen
In 1991, Kunieda, Furuoya [48] proposed a new approach of supplying oxygen gas to
the gap when using a water-based dielectric. However, the concept of completely dry

14. 13
EDM was not introduced until 1997, when Kunieda, Yoshida [45] proved that dry EDM
is achievable. They revealed that MRR increased with increasing the oxygen
concentration in air and the tool wear ratio was nearly zero for any pulse duration. In
addition, high-speed EDM milling of 3D cavities benefits from remarkably low tool
wear and substantially increased MRR, specifically if oxygen gas is used [49].
Furthermore, dry EDM milling is more advantageous than oil EDM milling and oil die-
sinking EDM in terms of time and cost during 3D machining of cemented carbide (G5)
[50].
Despite the advantages of EDM in gas, instability and arcing are substantial problems of
dry EDM [29]. Kunieda, Takaya [51] proposed a piezoelectric actuator with high
frequency response to control the EDM gap length and prevent frequent short-circuiting
in dry EDM, which occurs due to shorter discharge gaps than conventional EDM.
Another study [52] proposed deploying a pulsulating magnetic field tangential to the
electric field to enhance electron movement and the degree of ionization in plasma. This
led to 130% productivity improvement and zero tool wear in contrast to dry EDM
without a magnetic field.
3.3.2 Air
Kunieda, Yoshida [45] stated that precise 3D machining is possible with an NC tool
path that supplies uniform high-velocity air flow over the working gap. Furthermore,
Curodeau, Richard [46] observed that a thermoplastic composite electrode accompanied
by air rather than water or oil as dielectric was efficient, since a rough-machined tool
steel part was iteratively flattened by 30 m over three thermoplastic composite
electrode forming and EDM process cycle. Moreover, an abrasive jet machine (AJM)

15. 14
with integrated dry EDM achieved higher MRR and lower surface roughness at 6 and 9
A current than dry EDM [47].
3.4 Liquid and gas mixture
Near-dry EDM entails using a mixture of liquid and gas as dielectric by means of a
minimum quantity lubrication (MQL) dispenser. Kao, Tao [54] used a mixture of water
and air with MQL and attained higher MRR, a sharper cutting edge, and less debris
deposition than dry EDM. Additionally, mixing water and nitrogen and using a graphite
(Gr) electrode resulted in fine surface finish and high stability under low discharge
energy input [55]. Tao, Shih [56] observed that among kerosene + air, kerosene +
nitrogen, water + air and water + nitrogen dielectrics, employing kerosene + air and a
copper-infiltrated Gr tool produced the best overall surface finish. In near-dry EDM
milling, Fujiki, Ni [57] considered the effects of electrode lead and tilt angles, and
dielectric fluid flow rate. Furthermore, a new gap control strategy for five-axis near-dry
milling was applied that increased the MRR by 30% while the TWR and Ra were not
affected [58].
3.5 Dielectric selection criteria for PMEDM application
Regarding the selection of a suitable dielectric to host the powder, the performance of
various plain dielectrics was discussed in sections 3.1 through 3.4. Obviously, adding
impurities does not affect the performance of all dielectric types equally. A limited
number of studies have concentrated on the performance of various pure dielectrics after
powder addition. Kibria, Sarkar [61] and Ekmekci and Ersöz [62] indicated the different
influence of powder addition to oil vs. water as base dielectric in PMEDM.

16. 15
Investigating the performance of water and kerosene after the addition of powder
showed that boron carbide (B4C) mixed in deionized water significantly improved the
MRR during micro-EDM due to the efficient discharge distribution and increasing
machining efficiency, while B4C mixed in kerosene did not considerably affect the
MRR [61]. Furthermore, adding powder to kerosene and water showed higher values of
overcut compared to pure dielectric. The addition of silicon carbide (SiC) powder to tap
water and hydrocarbon oil revealed that unlike SiC mixed in hydrocarbon oil, SiC
particles were evenly distributed on the machined surface when SiC mixed in tap water
was employed as dielectric. This was attributed to the boiling of the molten pool after
discharge, especially during machining with hydrocarbon oil [62]. The carbon in
dielectric liquid initiated the violent boiling process [34] that altered the paths of
suspended particles rushing to the melted cavity with the dielectric liquid [62].
Regarding the financial aspect, according to the review article by Leão and Pashby [25]
on the use of environmentally-friendly dielectric fluids, machining with commercial
water-based dielectric (Elbolub) increased the total operating cost by ∼36% compared
with hydrocarbon oil (BP180) due to the costs of equipment depreciation (21%),
electrical energy (15%), dielectric losses (78%) and filtering aids (57%). However, the
operating cost per part was ∼42% lower, since the MRR achieved with Elbolub was
2.33 times higher than oil dielectric.
Lin, Chen [47] used gas dielectric to carry the powder particles in AJM integrated with
EDM. Gas as dielectric imposes lower tool cost than hydrocarbon oil dielectrics due to
extremely lower TWR, which enables attaining zero electrode wear under specific
machining conditions. Yet the occurrence of arcing and debris reattachment on the

17. 16
machined surface are still problematic [28, 29]. Therefore, mixing liquid and gas has
been practiced in the past decades in a few studies, indicating that near-dry EDM can
increase the MRR compared to dry EDM. Adding powder to the mixture of air and
mineral oil (near-dry EDM) has been considered to investigate the performance of
different electrode and workpiece materials [63, 64]. However, there remains a chance
for considering the role of powder addition to mixtures of liquid and gas to a great
extent.
Fig. 4 indicates the popularity of various types of dielectric fluid use in PMEDM studies
considered in this article. More than 80% of the studies have applied hydrocarbon-based
dielectrics in PMEDM with around 50% kerosene. The main dielectric for die-sinking
EDM is hydrocarbon-based, which has shown higher efficiency than deionized/distilled
water [25] due to the appropriate combination of properties, including small spark gap,
a corrosion-free environment, easier purification, higher aging resistance and lower
thermal conductivity than water [26]. The second most popular dielectric is water with
around 12% of the studies have employed water dielectric for PMEDM, since water
results in higher MRR levels according to some situations [25]. In addition, water as
dielectric exhibits lower viscosity, less gap contamination, and a safer and healthier
operational environment than hydrocarbon oil dielectric [25]. Deionized water in EDM
results in low carbon content and smaller heat-affected zone. The low resistivity of
deionized water facilitates achieving a simultaneous EDM-ECM process, which reduces
recast layer thickness. However, it is stated that electro-chemical dissolution always
occurs [27], which can be unfavorable.

18. 17
There are very few investigations that consider the role of base dielectric in the
PMEDM process, yet available studies indicate diverse influence of powder addition in
various dielectrics on machining performance. A range of factors should be taken into
consideration in the selection of base dielectric in PMEDM applications in future
studies, such as the density of powder compared to the dielectric. For instance, Chow,
Yang [65] observed that owing to the lower density of Aluminum (Al) than water, the
stirring effect was not sufficiently good for mixing pure water with Al powder.
Another important property of dielectric that affects the role of added powder in the
process is polarity. All different types of dielectrics used in EDM can be categorized as
polar and non-polar [66]. From a molecular point of view, polar dielectrics such as
water consist of randomly distributed permanent electric moments (Fig. 5(a)), which get
aligned according to the electric field imposed (Fig. 5(b)). On the other hand, non-polar
dielectrics such as kerosene exhibit random distribution in the absence of an electrical
field without demonstrating permanent pole-pole interaction (Fig. 5(c)). By inducing an
external electrical field, the non-polar dipoles are temporarily aligned (Fig. 5(d)) [26].
Therefore, the density, polarity and other dielectric fluid properties such as thermal
conductivity and viscosity may be effective parameters that should be thoroughly studied
further to reveal the behavior of various dielectrics after adding impurities.
4 Influence of powder addition on the EDM process
In conventional EDM, normal pulse discharges regularly cause arcing due to
insufficient pure dielectric deionization and excessive local debris [67]. However,
adding sufficient powder to the dielectric decreases the electrical resistivity and expands
the gap, subsequently stabilizing the process through better flushing and servo-hunting

19. 18
[68]. A wider discharge gap also decreases the heat flux [69], which reduces the
material removal of a single spark and enhances the surface quality. However, such gap
expansion is not possible with all powder materials, since powder density, electrical
resistivity, and thermal conductivity along with particle size and concentration are
highly determinative.
Upon addition of conductive powder, the reduced electrical resistivity of the dielectric
enhances the ionization and spark frequency between the tool and workpiece (Fig. 6).
The increased spark frequency appears to overcome the lower MRR of a single spark
resulting from gap expansion. Therefore, multiple discharge patterns are evident from a
single input pulse due to multiple discharge paths created, leading to discharge energy
dispersion [70, 71] in contrast to pure dielectric that produces a consistent waveform
pattern (Fig. 7). Moreover, discharge dispersion reduces the emergence of surface ridges
due to the formation of shallower craters with lower borders [72]. Subsequently, smaller
discharge craters and debris particles ease gap exhaust and accelerate the MRR [65].
Therefore, as a result of using powder added dielectric, the heat flux magnitude incident
on the workpiece by a single spark reduces accordingly. Consequently, a smaller crater
forms on the surface from a single spark, which is compensated by higher spark
frequency.
To describe the effect of powder addition on the dielectric’s breakdown voltage, the
electrode and workpiece resemble a capacitor with potential difference of V between
them. V is correlated to the system charge through the surrounding medium properties
and is expressed as:

20. 19
𝑉 =
1
𝐶
𝑄 (4)
where, Q refers to the charge separation of the system and the capacitance C depends on
the geometry of the electrode and workpiece as well as the properties of the medium
between them (dielectric). The value of C for two parallel plates with surface area A and
separation distance d is expressed as [73]:
𝐶 =
𝜀0𝜀𝑟𝐴
𝑑
(5)
where, d is the distance between the two plates, 𝜀0 is the permittivity in free space (flux
density of the field in vacuum) and 𝜀𝑟 is the relative permittivity, which is equal to
dielectric constant (ĸ) and is defined as the flux density of the field in dielectric 𝜀𝑑 over
𝜀0.
Assuming that the field line between the plates is straight, the magnitude of the potential
difference between the plates is [66]:
|∆𝑉| = 𝐸𝑑 (6)
where, E is the electric field in capacitor.
Above a critical magnitude of the electric field at which dielectric breakdown takes
place is called the breakdown strength, where the dielectric in capacitor becomes
conductive. In case of a homogeneous electric field, the field intensity is the same at full
breakdown trajectory length; therefore, electric breakdown strength is defined as [74]:
𝐸𝑏 =
𝑈𝑏
𝑑
(7)
where, 𝐸𝑏 is the breakdown strength and 𝑈𝑏 is the breakdown voltage.

21. 20
Breakdown voltage limits the maximum energy that can be stored in capacitor, which is
particularly depends on the dielectric material. It was shown that adding impurities (in
our case powder particles) alter the magnitude of electrical field [75] and subsequently
according to equation (7) reduces the breakdown voltage as a result of bridge formation.
In order to explain the formation of bridge in discharge gap in detail, assume that the
additive particles are spheres with higher permittivity than dielectric. The powder
particles in the gap get polarized in an electric field and experience force equal to
toward the place of maximum stress, and in a uniform electric field that can usually be
developed by a small sphere gap, the field is the strongest in the uniform field region
[76]. Thus, the force on the particle is zero and the particle remains in equilibrium.
Therefore, the particles will be dragged into the uniform field region. Since the
permittivity of the particles is higher than that of the liquid, the presence of particles in
the uniform field region will cause flux concentration at its surface. Other particles also
tend to move towards the higher flux concentration. If the present particles are large,
they become aligned due to these forces and form a highly conductive bridge across the
gap. The electric field in the liquid in the gap will increase and if it reaches a critical
value Eb breakdown will take place. If the number of particles is not sufficient to bridge
the gap, the particles will give rise to local field enhancement; if the field exceeds the
dielectric strength of the liquid, local breakdown will occur near the particles and
consequently, gas bubbles will form, which have much lower dielectric strength. Hence,
this will ultimately lead to easier liquid breakdown [75, 76].

22. 21
5 Powder parameters and PMEDM performance
The powder parameters, including particle material, size and concentration have a
decisive role in PMEDM performance. Furutania, Saneto [77] observed that the
combination of particle size and material results in dissimilar particle movement
patterns in the discharge gap. Besides, optimum powder concentration selection is
important to assure the efficiency of performance parameters and avoid discharge
instability. This section explicitly describes the powder parameters’ effects. Note that
the influence of powder parameters is highly dependent on EDM derivation, dielectric
type and machining scale.
5.1 Powder material
Powder thermo-physical properties, mainly particle density, and electrical and thermal
conductivity highly influence the PMEDM discharge mechanism. Wong, Lim [68]
examined the effect of Gr, Silicon (Si), Al, SiC, molybdenum disulfide (MoS2) and
crushed glass on PMEDM performance parameters and observed that excluding crushed
glass, all powders at least doubled the gap width. Among the powders, Al was found to
generate a mirror finish in machining SKH-51 by forming a surface with well-formed
and small overlapping craters. The largest gap was associated with Al powder, which
was expanded by a factor of almost 12 due to its high electrical and thermal
conductivity. The gap expansion value is in agreement with the measurements obtained
by Chow, Yan [70], proving that increasing ton leads to an even wider gap when Al
powder is added to dielectric; however, this is not the case for SiC powder additive due
to its extremely high electrical resistivity. To clarify, when adding SiC and Al powder
to kerosene dielectric during micro-slit EDM, the SiC powder enhances the MRR and

23. 22
increases TWR, whereas Al powder provides the best surface quality with the highest
micro-slit expansion.
Tzeng and Lee [78] indicated that among Al, SiC and chromium (Cr) powders, using Cr
leads to the highest MRR followed by Al and SiC; the most deteriorative effect on tool
wear was observed for SiC, Al and Cr, respectively. The discrepancy between this
research and Chow, Yan [70] regarding the performance of Al and SiC powders is
correlated to the machining scale, powder concentration or prescribed process
parameters. Furthermore, among Al, Cr, SiC and Cu, Al powder generated the best
SKD-11 surface finish and the thinnest recast layer [79]. However, copper (Cu) powder
did not contribute in the process due to high density, which caused this powder to
deposit at the bottom of the tank [78, 79].
A comparative study on micro-sinking PMEDM by Klocke, Lung [80] revealed that for
various I values, adding either Al or Si powder led to optimized performance
parameters. They found that particular combinations of powder and workpiece and
appropriate electrode polarity and pulse parameter settings can produce mirror-finish
surfaces [68]. Another powder material for PMEDM applications is Cr, which appeared
to generate the second highest surface quality following Al. This is due to the higher Cr
density than Al, which increases the particle impact on the melted zone, causing slightly
higher surface roughness [79]. Furthermore, Cr addition was found to cause a thicker
recast layer compared to Al powder, which is attributed to the lower thermal
conductivity of Cr; Cr took less heat and applied more thermal input to the workpiece,
which also resulted in higher MRR, because Al powder suspended dielectric absorbed
more energy [78].

24. 23
Gr powder is another widely used additive powder that increases the electrical
conductivity of dielectric and provides excellent lubricity. Gr powder addition resulted
in increasing MRR and decreasing TWR [81]. Furthermore, Gr has shown to have a
promising application in micro-PMEDM [82, 83]. Adding Gr, MoS2 and Si to dielectric
seemed to provide glossy and mirror-like surfaces to varying degrees in PMEDM of
SKH-54 [68]. MoS2, which is normally used as a commercial solid-state lubricant is
another material used to produce high surface quality by increasing the wetting effect
[84].
Titanium (Ti) and tungsten (W) are other metallic powders employed as additives in
PMEDM. Addition of Ti powder to dielectric appeared to increase the MRR [85],
micro-hardness [86] and machined surface hydrophilicity and also increase the surface
quality by reducing the Ra and surface crack density [87]. Additionally, W powder
mixed dielectric was shown to increase the machined surface micro-hardness by 100%
[88].
Beside SiC, ceramic powders such as titanium dioxide (TiO2), titanium carbide (TiC)
and B4C have been utilized in PMEDM. Adding B4C to deionized water was found to
significantly increase the MRR and fabricate accurate micro-holes in machining Ti-6Al-
4V [61, 89]. Chen and Lin [90] applied TiC powder mixed kerosene dielectric in
machining Al–Zn–Mg. The results indicated that the decomposed Ti migrates to the
machined surface and forms a TiC external layer with higher hardness than the base
material.

25. 24
5.2 Particle size
Powder particle size is emphasized as an important powder parameter to obtain
desirable results in the PMEDM process. It is observed that larger particles result in
greater gap expansion; this effect is more pronounced for higher ton values, probably
owing to greater contamination and lower deionization between the workpiece and tool
[78]. According to Tzeng and Lee [78], the smallest particles (70-80 nm) generated the
smallest discharge gap increase, the highest MRR and the least TWR. In another study,
Yih-fong and Fu-chen [79] claimed that additive particle size is determinative in
machined surface quality. The smallest particles (70–80 nm) produced the best surface
finish while simultaneously increasing the recast layer thickness.
5.3 Particle concentration
The appropriate particle concentration leads to process efficiency and stability. Note
that the optimum concentration for maximizing performance is dependent on powder
characteristics. Increasing the concentration beyond the optimum value causes short
circuiting, arcing and unstable machining due to the existence of excessive particles
similar to debris particles, whereby too much debris is generally deemed the dominant
cause of spark concentration [78]. Furthermore, the presence of excessive particles in
the discharge gap causes a settling problem and bridging effect, which lead to surface
deterioration [91].
6 Investigations of various powder materials in PMEDM
A wide range of materials are addressed in available PMEDM research articles. The
main question raised in this regard pertains to what powder material would maximize

26. 25
the machining performance parameters. Since the performance of powder in PMEDM
varies depending on EDM derivation, machining scale, electrical parameter settings,
workpiece material and many other factors, answering this question becomes difficult.
Accordingly, in this section significant attention is directed to presenting a thorough
review of powder materials used in PMEDM investigations and their influence on
machining performance parameters. The popularity of PMEDM among different
variations is presented in Fig. 8. Evidently, powder addition has been most often
implemented in conventional EDM followed by micro-EDM.
6.1 Graphite
Among the earliest investigations on PMEDM, Jeswani [81] introduced the addition of
4 g/l of Gr powder (10 µm) to dielectric, which enhanced the MRR by 60% and
decreased TWR by 15%. Furthermore, Wong, Lim [68] revealed that addition of Gr
powder (38±3 µm) leads to mirror-finish surfaces for SKH-54 workpieces up to peak
current of 2 A. Kumar, Maheshwari [92] employed cryogenically-treated copper tools in
PMEDM of Inconel 718 in the presence of Gr powder in dielectric, which appeared
effective in reducing both TWR and tool wear ratio during machining. Singh, Kumar [93]
investigated the addition of Gr powder to dielectric for improving the micro-hardness
characteristics of superalloy Co 605 during EDM. They specified that a significant
amount of powder becomes alloyed with the machined surface. Micro-hardness is
influenced by peak current, polarity and pulse on time for EDM and PMEDM.
Other than the investigation conducted by Wu, Yan [71] in which they used Al and
surfactant added to dielectric, Kolli and Kumar [94] investigated the influence of
surfactant and Gr (average size of 20 µm) in PMEDM of Ti-6Al-4V. The MRR was

27. 26
found to be at a maximum with 6 g/l surfactant, 13.5 g/l Gr powder concentration and
20 A current. However, the surface roughness was achieved at 4 g/l surfactant and 4.5
g/l Gr powder concentration and 10 A current.
The addition of nano-size Gr powder to dielectric for enhancing micro-EDM was
studied by Jahan, Rahman [91], Prihandana, Mahardika [82] and Prihandana, Mahardika
[83]. Jahan, Rahman [91] studied Gr powder (55 nm) addition to dielectric oil for micro-
EDM sinking and milling of cemented tungsten carbide (WC-Co), which was found to
provide smooth and defect-free nano-surfaces. PMEDM milling showed a wider gap
(Fig. 9(a)) and lower MRR (Fig. 9(b)), tool wear ratio (Fig. 9(c)) and surface roughness
(Fig. 9(d)). The spark gap in both milling and sinking demonstrated an increasing trend
with increasing powder concentration, but MRR in PMEDM sinking appeared to have a
peak at around 0.8 g/l and the optimum surface roughness value in both sinking and
milling was at 0.2 g/l concentration (Fig. 9(e) and Fig. 9(f)). For concentrations over 0.2
g/l, Ra decreased due to settling and bridging. In order to increase the accuracy of micro-
scale PMEDM, Prihandana, Mahardika [82] proposed ultrasonic vibration of dielectric
(43 kHz frequency and 0.4 μm amplitude) and found that machining time reduced up to
35% by introducing 2 g/l of Gr powder (55 nm) to kerosene dielectric and micro-crack
appearance on the machined surface diminished. Nano-Gr powder addition also improved
the Ra from 1.37 to 1.17 µm at 15 g/l concentration compared to pure dielectric in
micro-EDM of a Ti workpiece, and the machining time reduced by a factor of 20 at 10
g/l concentration [83]. Furthermore, the combination of added Gr nanopowder and
workpiece vibration (1 kHz frequency, 1.5 µm amplitude) reduced the machining time
from ~72 to ~3 min. This enhancement was greater than when using either one
separately.

28. 27
Compared to the abovementioned investigations, Liew, Yan [110] employed lower Gr
nanofiber concentrations (up to 0.3 g/l) in micro-EDM of reaction-bonded SiC ceramic
workpieces in time-controlled (10 min) and depth-controlled (20 µm) methods.
Apparently, the electrical resistivity concern of this ceramic was resolved better during
micro-EDM. A wider discharge gap (Fig. 10(a)), higher MRR (Fig. 10(b)), and lower
surface roughness (Fig. 10(c)) and tool wear ratio (Fig. 10(d)) were observed at
optimum powder concentration. Furthermore, the shape of the machined profile
improved significantly (Fig. 10(e)).
6.2 Aluminum
Zhao, Meng [95] reported that adding 40 g/l Al powder (10 µm) to dielectric during
machining of steel workpieces optimized the MRR at ton=10 µsec and Ra at I=19 A.
Fig. 11(a) through Fig. 11(i) show the influence of adding powder to kerosene dielectric
on MRR, TWR, Ra and recast layer thickness at various ton, particle concentration and
size ranges. Tzeng and Lee [78] and Yih-fong and Fu-chen [79] revealed that among
various Al powder sizes, adding 70-80 nm (the smallest) to kerosene dielectric provided
the highest MRR at 0.5 cm3
/l powder concentration and ton=25 µsec (Fig. 11(a, b, c)).
Additionally, increasing the powder concentration or ton led to reduced tool wear ratio
(Fig. 11(d) and Fig. 11(e)). Moreover, the lowest Ra and thickest recast layer were
observed for the smallest Al powder particles (Fig. 11(f) through Fig. 11(i)).
Addition of Al powder (1 µm) to dielectric through micro-slit EDM of Ti-6Al-4V was
found to increase the discharge gap and improve the material removal depth by around
30% at 5 g/l powder concentration [70]. However, Al powder mixed dielectric
deteriorated the machined slit accuracy as a consequence of excessive overcut (Fig. 12).

29. 28
Introducing surfactant (Polyoxythylene-20-sorbitan monooleate) to dielectric with Al
powder increased the homogeneity of Al particles in dielectric [71]. Adding surfactant
(0.25 g/l) and Al powder (0.1 g/l) led to increased gap distance (Fig. 13(a)), mirror-like
surface in machining SKD-61, increased material removal depth by 60% (Fig. 13(b))
and a thinner recast layer (Fig. 13(c)). Wong, Lim [68] found that adding 2 g/l of Al
powder (45±3 µm) to dielectric also produced a mirror finish of SKH-51 -- a finishing
quality not evident for SKH-54.
Al powder was added to mineral oil dielectric to improve the machining efficiency of a
WC-Co workpiece [96], and the optimal powder concentration to maximize MRR was
17.5 g/l. The electrode wear ratio tended to decrease with reduced Al powder
concentration to a minimum of 15 g/l, after which it tended to increase. Another study
carried out by Hu, Cao [97] indicated that adding Al powder (<2 µm) to kerosene
dielectric in machining A1 matrix composites improved the Ra by approximately 31.5%
in 10 measurement trials. Additionally, this composite’s microhardness and wear
resistance improved by 40 and 100% respectively, as a result of using Al powder.
6.3 Silicon
Kansal, Singh [101] specified the important parameters and their effects on PMEDM of
AISI D2 with Si powder in kerosene dielectric. The optimum machining condition to
obtain the highest MRR found was: I=16 A, ton=100 μs, toff=15 μs, feed rate=0.83 mm/s
and particle concentration=4 g/l. Si powder concentration and I were the most
influential parameters on MRR. Molinetti, Amorim [108] observed that addition of Si
powder to dielectric (average size of less than 5 µm) reduces the surface roughness by a
factor of 5 at peak current of 2 A compared to pure dielectric (Fig. 14). The presence of

30. 29
Si powder in dielectric resulted in the adherence of Si particles to the surface, and in the
case of negative polarity at low discharge current the machined surface properties could
be modified through the formation of silicon carbides.
In PMEDM parametric optimization using response surface methodology, adding up to
2 g/l of Si powder to dielectric improved both MRR and Ra [10]. However, these
performance parameters were still expected to increase for higher particle
concentrations in dielectric according to the response surface. Confirmation tests
showed that the error between experimental and predicted MRR and surface roughness
values was within ±8% and -7.85% to 3.15%, respectively. Peças and Henriques [102]
added Si powder (10 µm) to dielectric at a concentration of 2 g/l. This method provided
the lowest peak to valley surface roughness (Rz) (Fig. 15(a)) and crater depth (Fig.
15(b)). Moreover, increasing the powder concentration decreased the crater width and
white-layer thickness (Fig. 15(c) and Fig. 15(d)). Fig. 15(e) and Fig. 15(f) show that the
roughness increased for larger tool areas, but had a different trend after adding 2 g/l of
Si powder to dielectric. In accordance to the above study findings [102], Peças and
Henriques [103] showed that although Si powder in dielectric noticeably decreased the
surface roughness, in larger areas the surface quality slightly dropped due to the
capacitive effect. The capacitive effect implies higher peak current than the one set on
the discharge generator, causing the formation of deeper and more irregular craters.
Si powder has also been used in powder mixed near-dry EDM. Bai, Zhang [63]
considered the influence of I, ton, toff, flow rate, Si powder concentration, air pressure
and tool rotational speed on MRR. They showed that under all machining conditions, a
W18Cr4V workpiece and brass electrode resulted in higher MRR, whereas the addition

31. 30
of 9 g/l of Si powder to the three-phase dielectric was necessary to obtain the highest
MRR with around 40% enhancement compared to pure dielectric. An additional study
on powder mixed near-dry EDM using Si powder was reported by Bai, Zhang [64]. The
results indicated that I, ton, toff, flow rate and powder concentration are more influential
on the MRR of powder mixed near-dry EDM than tool rotational speed and air pressure.
6.4 Silicon carbide
Tzeng and Lee [78] observed that SiC powder mixed kerosene dielectric leads to a very
small machining gap increase and about 30% increase in MRR at I=4 A and ton=25 µsec
with
2
3
duty cycle. However, the MRR did not notably change at lower I (1.5 A).
Additionally, larger SiC particles (2.36 ± 0.08 mm) failed to form a mirror-finish surface
due to the formation of very distinct and deep craters with high density of global
appendage [68]. In micro-slit EDM of Ti-6Al-4V with a copper diskette tool, adding 25
g/l of SiC powder to kerosene resulted in the greatest material removal depth at ton=10
µsec [70]. Nevertheless, electrode wear and slit expansion increased at all pulse
durations and particle concentrations, respectively. Similar to the optimum concentration
obtained by Chow, Yan [70], 25 g/l of SiC mixed deionized water dielectric resulted in
the greatest material removal depth for both average particle sizes of 3 and 5 µm in
micro-slit EDM [65]. Moreover, the SiC particles reduced the gap distance, increased the
electrode wear and material removal depth, and enhanced the surface roughness compared
to pure water dielectric.
According to Yih-fong and Fu-chen [79], SiC powder produced greater explosion force
for normal pulse discharge, leading to the formation of deeper craters owing to the high

32. 31
electrical resistivity and low thermal conductivity. Although SiC did not considerably
change the Ra, it reduced the recast layer thickness. Conversely, Ekmekci and Ersöz [62]
studied the effects of SiC powder mixed in tap water and oil dielectric on the surface
topology and structure of interstitial free steel. They found that suspended particles
around the discharge column accelerated and gained sufficient velocity to penetrate to the
molten pool before solidification, producing a surface embedded with suspended
particles. SiC powder in both water and hydrocarbon oil also increased the recast layer
thickness, but this was more pronounced when the powder was mixed with water.
6.5 Titanium
Ti is another metallic powder used in PMEDM for performance measure enhancement
as well as surface modification. Marashi, Sarhan [85] observed that Ti particles (40-60
nm) added to dielectric in machining AIDI D2 improved MRR and Ra by ∼69% and
∼35% at 6 and 12 A peak current, respectively. Furthermore, the AISI D2 surface
morphology was enhanced as a result of shallower craters and the formation of low
ridges. Furutania, Saneto [77], Janmanee and Muttamara [86] and Chen, Lin [87]
considered Ti powder suspended in dielectric fluid for machined surface modification.
Furutania, Saneto [77] studied the accretion of Ti on a carbon steel surface machined by
EDM with 50 g/l of Ti powder (<36 µm) suspended in oil dielectric. A good condition
specified for accretion was 2≤ton≤5 µsec, toff=1024 µsec and 1≤I≤7 A with negative
polarity. Ti powder suspended in oil dielectric produced a TiC layer with 150 µm
thickness and 1600 Hv hardness.
Janmanee and Muttamara [86] enhanced the machined surface hardness by 76.76%
from 990 Hv to 1750 Hv with improved surface quality and TiC surface completeness

33. 32
using PMEDM. At 20 A current and 50% duty factor, a 5m coated layer of Ti and C
with lower Ra and less surface cracks was achieved. Additionally, fewer micro-cracks
were observed on the surface, since the micro-cracks were filled and substituted by Ti
particles and C that decomposed from the dielectric and acted as a combiner (TiC).
Chen, Lin [87] added Ti powder at various input parameters to deionized water in
micro-current EDM of a Ti workpiece, which led to the formation of a recast layer
containing TiO. A concentration of 6 g/l of powder demonstrated no micro-crack
formation on the modified surface at I=0.1 A for short-pulse durations (≤50 s), and a
thinner recast layer 4-11 m thick was attained. Machining a Ti workpiece at I=0.1 A
for 30 and 50 µsec in dielectric with 6 g/l Ti powder concentration generated a
hydrophilic surface. Hence, an appropriate Ti powder concentration not only prevents
surface crack formation but can also promote machined surface wettability.
6.6 Copper
Cu powder added to dielectric is ineffective in terms of MRR, TWR, Ra and recast layer
thickness due to its high density, which causes settling [78, 79]. However, Sidhu, Batish
[104] employed Cu powder mixed dielectric and copper electrolyte for surface
modification of a metal matrix composite, which increased the machined surface micro-
hardness.
6.7 Chromium
Cr powder mixed dielectric significantly reduced the tool wear ratio and increased the
MRR by about 50% at 0.5 cm3
/l concentration, ton=25 µsec and 2/3 duty factor [78].
Furthermore, addition of Cr to dielectric also enhanced the Ra and recast layer thickness

34. 33
(Fig. 15(g) and Fig. 15(i)) [79]. Ojha, Garg [105] used response surface methodology for
parametric optimization of MRR and TWR. They employed Cr powder mixed dielectric
for machining EN-8. The results indicated that increasing the powder concentration to 6
g/l and I to 8 A significantly increased the MRR, while TWR slightly varied with
increasing concentration.
6.8 Tungsten
W powder has been mainly applied for surface modification purposes. Kumar and Batra
[88] investigated the influence of W powder mixed dielectric in machining OHNS, D2
and H13. The effect of W particles and rising amount of carbon on the machined surface
indicated that suspended powder particles can react with carbon (from the hydrocarbon
dielectric breakdown) at high plasma channel temperatures to form WC, increasing the
microhardness of all three workpiece materials by over 100%. The rise in surface
hardness directly influences abrasion resistance, which increases the life of dies and
other press tools. Bhattacharya, Batish [106] stated that employing brass and W-Cu
electrodes with W powder mixed dielectric results in good surface finish and higher
microhardness, respectively.
6.9 Molybdenum disulfide
Prihandana, Mahardika [84] studied the addition of MoS2 powder (2 µm) mixed
dielectric and ultrasonic vibration of the machining tank with 43 KHz frequency in
micro-PMEDM. They reported superior surface quality and increased MRR by ~85%.
Furthermore, the profile depth and machined surface quality substantially improved.
Wong, Lim [68] claimed that adding MoS2 to dielectric led to a mirror-finish SKH-54

35. 34
surface. Furutani and Shiraki [107] observed that under friction testing, a surface
machined with MoS2 powder mixed dielectric had a smaller friction coefficient than one
machined in pure dielectric.
6.10 Boron Carbide
B4C is an extremely hard ceramic, which Kibria and Bhattacharyya [89] used as an
additive powder. They mixed 4 g/l of B4C powder (~13-16 μm) with kerosene and
deionized water dielectrics. The results indicated that B4C mixed in deionized water
dielectric generated more accurate micro-holes in Ti-6Al-4V with respect to taperness
and circularity compared to deionized water. Although B4C mixed in kerosene did not
significantly increase the MRR, B4C mixed in deionized water increased the MRR
remarkably owing to the efficient discharge distribution and increased machining
efficiency [61].
6.11 Titanium carbide
TiC powder mixed kerosene with ultrasonic tool vibration appeared to be a significant
factor in improving the Al-Zn-Mg machining characteristics [90]. Substantial MRR
increase along with surface roughness and TWR decrease were noticed at high current
values. Decomposed Ti element from the TiC mixed dielectric medium migrated to the
Al-Zn-Mg surface, producing fine grain consolidation during solidification.
Furthermore, some Ti particles penetrating the machined surface during the process
enhanced the hardness and wear resistance through alloying of the workpiece’s external
layer.

36. 35
6.12 Titanium dioxide
Baseri and Sadeghian [109] investigated the addition of TiO2 powder (average particle
size of 20 nm) to kerosene in machining H13 steel with a rotary copper tool. It was
found that increasing the rotational speed up to 200 rpm caused the centrifugal force to
remove debris from the machining gap and the MRR to increase. Increasing the speed to
over 200 rpm created bubbles which affect plasma channel and subsequently decrease
the MRR. Adding up to 1 g/l of TiO2 powder to dielectric escalated the MRR due to
increasing the gap and facilitating the expulsion of debris from the machining gap.
6.13 Manganese
Molinetti, Amorim [108] used manganese (Mn) powder with particles smaller than 10
µm to enhance the machined surface quality of AIDI H13 steel. They found that adding
Mn to dielectric reduced the surface roughness by a factor of two and enhanced the
machined surface hardness by about 40% compared with pure dielectric.
7 Research trends, findings and gaps in PMEDM
The trend of the key research studies on PMEDM from the past years is indicated in
Fig. 16. Thirty-five years ago, Jeswani [81] introduced the addition of impurities (Gr
powder) to dielectric to enhance machining performance. In 1995, Ming and He [112]
observed that adding conductive and inorganic oxide particles to dielectric led to higher
MRR, lower TWR and superior surface quality. The indistinct stage of this technology
continued until 1998 when Wong, Lim [68] compared the utilization of various powder
materials in dielectric. They explicitly disclosed the influence of powder properties on
the performance parameters of EDM. Their research revealed the great potential of

37. 36
adding powder to dielectric for machining performance improvement, which led to an
increase in the number of research works on PMEDM in subsequent years. The
cognition phase of PMEDM technology can be considered from 1998 to 2008. In this
period, Gr, Al, Si or SiC were used as additives to improve EDM performance. Since
2008, PMEDM investigations have become even more attractive, because the cognition
phase indicated that adding suitable powder to dielectric increases the dielectric’s
electrical conductivity and subsequently the spark frequency, which leads to superior
machining characteristic. The increasing trend of PMEDM studies since 2008 is evident
with a higher variety of powder materials employed and is expected to continue in the
coming years. In addition, utilizing nano-size powder has recently become popular,
since smaller particles seem to yield better performance in terms of higher MRR, lower
TWR [78] and better surface finish [79].
The variety and popularity of powder materials used in PMEDM studies are
demonstrated in Fig. 17. Among the considered additives, Gr and Al powders, which
were also among the earlier research studies, have captured the most research attention.
Although there is a large number of publications on PMEDM studies, the unique answer
to the question of what the most effective powder material for enhancing machining
characteristics is remains disputed still. Incoherency in the selection of machining input
parameters, machining scale, tool or workpiece material among the available research
studies impedes an appropriate comparison of findings.
Under different machining conditions, adding Gr, Al, Ti, W, TiC, Si or SiC powders to
dielectric results in surface modification. Through external surface alloying, the
existence of Al, Ti, W or TiC increases the surface hardness and wear resistance. These

38. 37
powder materials find substantial application in surface modification, which was
investigated in detail by Kumar, Singh [21]. It is recommended in future investigations
to consider both metallurgical surface compounds and the influence of powder addition
on EDM performance parameters to realize the effect of powder addition on the process
and machined surface properties. This will clarify whether adding powder to dielectric
always results in surface modifications and what the contributing factors of powder in
the occurrence of surface modification are. For instance, adding Cu powder particles
makes no difference to the process [113], because Cu powder deposits at the bottom of
the tank and does not contribute to the discharge process. However, in another research
study, Cu powder was mixed with dielectric to attain surface modification [104].
Based on the collected literature, Fig. 18(a) and Fig. 18(b) are displayed in order to
demonstrate the concentration, size, and powder material ranges used in PMEDM
investigations. The figures indicate that up to 75 g/l powder concentration in dielectric
has been investigated, and the majority of concentrations applied and investigated were
less than 20 g/l. Also for nano-size powders, the maximum concentration employed was
found to be 20 g/l. Furthermore, the graphs show that the 1 to 55 µm and 20 to 150 nm
particle size ranges have been used in PMEDM investigations. Nano powders were used
in a few studies that showed great potential and the need to investigate them to a greater
extent. Besides the material properties stated in this research, such as powder material,
size and concentration, shape has been addressed as an influential property of powder
material [101]. According to [114] as well, very little or no investigation has been
undertaken to realize the effect of particle shape on performance parameters. Moreover,
it is recommended for future studies to establish comparisons using various powder
materials, sizes and concentrations to clearly reveal a piece of the puzzle from the big

39. 38
picture of PMEDM performance. In addition, findings to date indicate that the smallest
particles cause less gap expansion, higher MRR, lower TWR [78], better surface finish
and a thicker recast layer [79], which overall increase EDM performance and surface
finish. However, the influence of particle size of different materials on EDM
performance and the relationship between particle size and optimum concentration
necessitate further attention.
Fig. 19 shows the improvement in performance parameters of EDM according to ton, I,
powder material and concentration. Based on the machining condition (ton and I), a
suitable powder can be selected depending on the most important performance
parameters which are considered for designed machining outputs. Al powder is found to
enhance the MRR and SR in wide range of 0.01≤I≤4 A and ton≤50 µsec which also at
concentration of 15 g/l enhances the SR. Comparing the performance of Al, Si and SiC
at Fig. 19(a) points out that lower concentration of Al is required to improve the
performance parameters compared to SiC and Si, which can be attributed to higher
thermal and electrical conductivity of Al. In addition, Al concentration of higher than 35
g/l leads to higher micro-hardness and wear resistance of machined surface. Adding Ti
powder to dielectric specifies the surface with less micro-cracks and higher
hydrophilicity. At high discharge energy (through increasing ton or I), Ti powder
improved MRR and surface roughness, where no significant deposition of Ti occurred
on the machined surface. It worth mentioning that the influence of toff should be
considered, especially for surface modification.
Employing ultrasonic vibration to the tool, machining area or machining tank in
PMEDM has been investigated in order to improve powder addition effectiveness.

40. 39
Ultrasonic vibration induces stirring and cloud cavitation effects, resulting in better
debris ejection from the gap and preventing tool material deposition on the surface
[111]. Similarly, workpiece vibration with low frequency of 1 KHz has shown to boost
the effect of powder addition to dielectric [83]. The difference between the effect of
applying ultrasonic vibration or vibration at low frequencies on PMEDM performance
and the relationship between powder size and concentration with the influence of
vibration frequency and amplitude on PMEDM performance is still indistinct.
Powder particle deposition at the bottom of the tank reduces powder participation in the
process. Using ultrasonic vibration of the machining tank can be a key solution to
constantly prevent powder suspension and agglomeration throughout the process. In
order to present EDM as a viable machining technique, it is necessary to investigate the
performance parameters under more realistic machining conditions such as fabricating
cavities for longer machining time. Unlike available studies that mainly consider shorter
machining times, it is recommended to consider this point in prospect studies.
8 Challenges of PMEDM industrialization
The advantages of PMEDM have been presented in numerous research studies, but
there are still problems challenging the industrialization of this technique. The problems
start with cost effectiveness and unclear lifetime of a powder mixed dielectric [3]. Fig.
20 schematically illustrates a basic PMEDM circulation system that contains a
machining tank, including a workpiece holder, pump, stirrer, magnetic filter, pipes,
nozzle, etc. This system is equipped with a magnetic filter that enables the constant
reuse of powder mixed dielectric.

41. 40
The first question regarding PMEDM is how to effectively supply the powder particles
to the dielectric. Injecting the powder particles through the tool electrode is possible, but
maintaining a constant powder concentration in dielectric is problematic with this
method. Other main concerns are the circulation of dielectric fluid containing powder
considering the deposition of particles of the elements and the filtration of debris
particles from the suspended powder. In order to address the filtration issue, a magnetic
filter is either placed in the machining tank or the secondary dielectric reservoir and the
distance between the dielectric suction point and nozzle outlet is kept as short as
possible to ensure the complete suspension of powder in the discharge gap [3].
Assessing the powder concentration uniformity and amount of debris after filtration and
through the process is a problem that was investigated using environmental scanning
electron microscopy [85]. Furthermore, powder agglomeration and deposition in the
machining tank is costly and varies the powder concentration during machining. In
order to avoid powder aggregation and deposition, and to ensure homogeneous
suspension of particles in the dielectric fluid, the powder is usually mixed with
dielectric by conventional methods like implementing a stirrer in the machining tank
[90] or blending the powder with dielectric in advance [110]. The addition of surfactant
to powder mixed dielectric was proposed to increase powder homogeneity in the liquid
[71] and ultrasonic vibration of the tank to prevent powder deposition at the bottom of
the tank [84]. In some cases, both the pump and stirrer are located in the machining tank
(closed system) [115], which considerably increases pump abrasion.
Other than the abovementioned points regarding equipment, the selection of suitable
powder material, size and concentration considering optimum machining condition is a
crucial point for commercializing this process. Furthermore, if PMEDM is not

42. 41
commercialized to perform surface modification, powder deposition on the machined
surface and the formation of unfavorable compounds on the machined surface need to be
taken into account. Additionally, powder deposition on the tool declines the tool’s
electrical conductivity, which reduces the machining performance over long machining
hours. Plus, the change in dielectric color after adding powder averts workpiece
visualization during machining, which needs to be considered. Another serious concern
regards health, safety and environmental aspects, such as the exposure of operators to
powder added dielectric, the explosibiliy of some powder materials like Al, and dielectric
disposal [3]. In addition, producing micro and nano powder itself is challenging and costly
besides being harmful to the environment. Therefore, in order to put this technology into
operation, a variety of complications primarily require further attention.
9 Conclusion
With recent progress in material science, such as the introduction of high hardness
alloys, nano-structured materials, ceramics and composites, machining processes face
increasing demand to attain high productivity, high-dimensional and geometrical
accuracy and environmental safety. EDM, which was introduced 60 years ago, is found
to be one the most successful non-traditional techniques for stock removal of high-
hardness conductive materials. However, low MRR and the requisite for post-surface
treatment due to low surface quality limit the vast application of this technique. Among
various solutions to overcome such limitations is PMEDM, which appears to
predominantly attain high surface quality over a short time. Since the introduction of
PMEDM, high numbers of powder materials in combinations of various dielectrics have
been used by researchers. Nevertheless, selecting appropriate additive powder materials

43. 42
remains disputed due to the intermingled effect of machining scale, electrical and non-
electrical parameters, and tool and workpiece material, with the effect of additive
powder. Accordingly, this article provided a thorough review of the influence of powder
addition on the EDM mechanism, the most influential powder parameters,
comprehensive insight into available powder materials and future trends in this
technology. The following conclusions are drawn based on the presented literature:
1. Among various types of dielectric used in PMEDM, hydrocarbon dielectric is still
the most efficient one; however, water and gas have found increasing application in
recent years due to:
i. Healthier and safer operational environment, which limits the production of
cariogenic components such as benzene (C6H6) and benzopyrene (C20H12).
ii. Deionized water can be a favorable choice in some EDM applications owing
to lower viscosity, higher thermal conductivity and higher flow rate
compared with hydrocarbon dielectrics. However, only few investigations
have considered the addition of powder to water dielectric.
iii. Deionized water results in low carbon content in the gap and smaller heat-
affected zone. In addition, low-resistivity deionized water as dielectric
facilitates achieving a simultaneous EDM-ECM process, which reduces the
recast layer thickness and prevents the necessity for post-processing.
2. The discharge characteristics in the presence of suitable additive powder remarkably
vary from pure dielectric in terms of induced energy intensity, spark frequency,
discharge gap and spark size as:

44. 43
i. Lower amounts of energy are induced to the workpiece by a single spark in
powder added dielectric than with pure dielectric; however, higher spark
frequency overcomes this effect and leads to higher material removal.
ii. Wider discharge gap and smaller spark size decrease the crater size, which
remarkably enhances the surface roughness.
3. Powder properties, including type, size, shape and concentration in dielectric
remarkably influence the efficiency of machining as follows:
i. Gr and Al powders are the most widely used materials, which improve the
machining efficiency in terms of MRR and surface quality.
ii. Gr, Al, Ti, W, TiC, Si or SiC powders have shown to result in surface
modification during machining.
iii. MoS2 and Gr powders are introduced as materials that improve lubricity.
iv. Ceramic powders, such as SiC and B4C produce smaller gap expansion due
to their lower conductivity than metallic powders.
v. Some powder materials such as Cu and crushed glass have no influence on
machining efficiency, while Cu powder is effective in surface modification.
vi. Most researchers have used powder concentrations below 20 g/l and micro-
size powders ranging from 1 to 55 µm or nano-size powders ranging from 20
to 150 nm.
4. By considering PMEDM over the past 35 years, the main trends and suggestions for
future research are summarized below considering the data collected in this review:
i. The spark characteristics in the presence of additive powder are heavily
affected by machining scale, input parameters (electrical and non-electrical)
and tool and workpiece materials. Therefore, a comprehensive study that

45. 44
considers the same machining conditions is suggested for the selection of
appropriate powder for a unique manufacturing process.
ii. Available studies mostly consider short machining times in evaluating
PMEDM performance. PMEDM will, however, prove to be viable for
industrialization if performance parameters enhancements are achieved
under more realistic machining conditions, like the fabrication of cavities
over longer machining time.
iii. Among powder-related properties, such as powder material, size and
concentration, shape has not been investigated although it is known as an
influential property of powder. However, there is insufficient evidence on
the influence of powder shape on performance parameters.
iv. Findings to date indicate that the smallest particles cause less gap expansion,
higher MRR, lower TWR and a thicker recast layer, which overall increase
EDM performance. However, the influence of particle size of different
materials on EDM performance and the relationship between particle size
and optimum concentration necessitate further attention.
v. Powder particle deposition at the bottom of the tank is a problem that
significantly restricts PMEDM application for mass production. Ultrasonic
vibration of the machining tank may be a key solution to constantly prevent
powder deposition and agglomeration throughout the process. Studies on the
vibrating unit in the PMEDM setup can be considered in the future to
simplify PMEDM for industrialization. Furthermore, the integration of the
ultrasonic vibration effect with EDM has shown to improve powder addition
effectiveness due to the stirring and cloud cavitation effects. The influence of

46. 45
vibration frequency and amplitude on PMEDM performance and the link
between powder size and concentration, and the vibration effect on PMEDM
performance are still indistinct.
vi. Under different machining conditions, adding Al, Ti, W, TiC, Si or SiC to
dielectric results in surface modifications. Through external surface alloying,
the existence of Al, Ti, W and TiC increases surface hardness and wear
resistance. It is recommended to consider investigating surface metallurgical
properties after PMEDM to realize the contributing factors to the incidence
of surface modification.
vii. Although findings to date have shown that the smallest particles appear to
exhibit the highest overall performance improvement, the influence of the
particle size of different materials on EDM performance and the relationship
between particle size and optimum concentration has not been considered.
Acknowledgements
The authors would like to acknowledge the University of Malaya for providing the
necessary facilities and resources for this research. This research was fully funded by the
Ministry of Higher Education, Malaysia with the high impact research (HIR) grant
No.HIR-MOHE-16001-00-D000001 and University of Malaya, Malaysia with the
University of Malaya Research Grant (UMRG) Program No. RP039B-15AET.

47. 46
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List of Figures captions:
Fig. 1. Schematic illustration of electrical discharge phases according to [4], [5] and [6].
Fig. 2 Characteristic values of (a) voltage and (b) current impulses according to [19].
Fig. 3. Alternatives for hydrocarbon oil dielectric.
Fig. 4. Popularity of various dielectric fluids for hosting powder additive among the
PMEDM studies considered in this article from 1981 to 2015.
Fig. 5. Polar molecules (a) without and (b) in electric field; non-polar molecules (c)
without and (d) in electric field [26].
Fig. 6. Typical waveforms of voltage and current for (a) kerosene and (b) kerosene with
Al powder [70].
Fig. 7. (a-g) Effect of suspended Al powder on the sparking mechanism and (h) voltage
waveform [70].
Fig. 8. Popularity of powder addition in various EDM techniques.
Fig. 9. Influence of additive Gr powder on (a) spark gap, (b) MRR, (c) electrode wear
ratio, (d) Ra, and typical images of surface in (e) sinking and (f) milling [91].
Fig. 10. Influence of adding Gr nanofiber to dielectric on (a) spark gap, (b) MRR, (c)
surface roughness, (d) TWR and (e) profile of cross section under time-controlled and
depth-controlled regimes for variable powder concentration in micro-EDM [110].
Fig. 11. Influence of adding (a-f) Al powder on EDM performance [78], (f-i) various
powders on Ra and recast layer [79].
Fig. 12. Micrograph of micro-slits machined with (a) kerosene and (b) Al powder mixed
kerosene dielectric [70].
Fig. 13. Effect of Al powder and surfactant addition on (a) gap distance, (b) Ra, and (c)
recast layer [71].

57. 56
Fig. 14. Ra for surface machined by EDM and PMEDM [108].
Fig. 15. Effect of Si powder concentration on EDM performance including (a) surface
roughness, (b) crater depth, (c) crater width (d) white layer thickness, and effect of
dielectric flow rate on surface roughness of surface machined in (d) pure dielectric, (e)
Si powder mixed dielectric [102].
Fig. 16. Distribution of collected research studies on PMEDM from 1981 to 2015.
Fig. 17. Popularity of powder materials in collected PMEDM studies from 1981 to
2015.
Fig. 18. Additive particle material, size and concentration used in collected PMEDM
studies for (a) micro- and (b) nano particle sizes from 1981-2015.
Fig. 19. Effectiveness of powder materials and concentration on improvement of
machining characteristics (MRR, Surface roughness, Surface quality, micro-hardness,
wear resistance, micro-cracks, hydrophilicity) under various ton, (a) I ≤0.5 A and (b)
0.5< I <20 A from 1998-2015.
Fig. 20. Schematic illustration of PMEDM circulation system [79].

58. 57
Fig. 1

59. 58
Fig. 2

60. 59
Fig. 3
Alternatives
of
hydrocarbon
oil
dielectric
Water
Water as dielectric propose
lower viscosity, less gap
contamination, safer and
healthier operational
environment [25]. However,
electro-chemical dissolution
always occurs [29].
Deionized water
[32], [33], [34], [35], [36], [37],
[38], urea solution in water [39],
water in oil emulsion [40]
Tap water
[41], [42], new power supply
[43]
Low resistivity deionized
water
[29], [44], [45], [46]
Gas
Dry EDM propose low tool
electrode wear, thin recast
layer and its safer and healthier
operational environment.
However, occurrence of arcing
and debris reattachment on the
machined surface are
problematic [30, 31].
Oxygen
[50], [47], [51], [52], [31], [53],
[54], [55]
Air
[47], [48], abrasive jet hybrid
EDM [49]
Mixture of liquid and gas
[56], [57], [58], [59], [60]

61. 60
Fig. 4
Fig. 5
a b c d

62. 61
Fig. 6
Fig. 7

63. 62
Fig. 8

64. 63
Fig. 9
Fig. 10

65. 64
Fig. 11
Fig. 12

66. 65
Fig. 13
Fig. 14

67. 66
Fig. 15
Fig. 16

68. 67
Fig. 17
Fig. 18

69. 68
Fig. 19
Fig. 20