a review on current research trends in electrical discharge machining (edm)

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International Journal of Machine Tools & Manufacture 47 (2007) 12141228

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A review on current research trends in electrical discharge machining(EDM)

Norliana Mohd Abbas, Darius G. Solomon, Md. Fuad Bahari

Faculty of Mechanical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor Darul Ehsan, Malaysia

Received 15 November 2005; received in revised form 20 May 2006; accepted 17 August 2006

Available online 17 November 2006

Abstract

Electrical discharge machining (EDM) is one of the earliest non-traditional machining processes. EDM process is based on

thermoelectric energy between the work piece and an electrode. A pulse discharge occurs in a small gap between the work piece and the

electrode and removes the unwanted material from the parent metal through melting and vaporising. The electrode and the work piece

must have electrical conductivity in order to generate the spark. There are various types of products which can be produced using EDM

such as dies and moulds. Parts of aerospace, automotive industry and surgical components can be finished by EDM. This paper reviews

the research trends in EDM on ultrasonic vibration, dry EDM machining, EDM with powder additives, EDM in water and modeling

technique in predicting EDM performances.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: EDM; Ultrasonic vibration; Dry EDM; Powder additives; Dielectric; Modeling

1. Introduction

Electrical discharge machining (EDM) is a non-tradi-tional concept of machining which has been widely used toproduce dies and molds. It is also used for finishing partsfor aerospace and automotive industry and surgicalcomponents [1]. This technique has been developed in thelate 1940s [2] where the process is based on removingmaterial from a part by means of a series of repeatedelectrical discharges between tool called the electrode andthe work piece in the presence of a dielectric fluid [3]. Theelectrode is moved toward the work piece until the gap issmall enough so that the impressed voltage is great enoughto ionize the dielectric [4]. Short duration discharges aregenerated in a liquid dielectric gap, which separates tooland work piece. The material is removed with the erosiveeffect of the electrical discharges from tool and work piece[5]. EDM does not make direct contact between theelectrode and the work piece where it can eliminatemechanical stresses, chatter and vibration problems during

e front matter r 2006 Elsevier Ltd. All rights reserved.

achtools.2006.08.026

ing author. Tel.: +6016 218 6521; fax: +60 3 5543 5160.

ess: [email protected] (N. Mohd Abbas).

machining [1]. Materials of any hardness can be cut as longas the material can conduct electricity [6]. EDM techniqueshave developed in many areas. Trends on activities carriedout by researchers depend on the interest of the researchersand the availability of the technology. In a book publishedin 1994, Rajurkar [7] has indicated some future trendsactivities in EDM: machining advanced materials, mirrorsurface finish using powder additives, ultrasonic-assistedEDM and control and automation.However, the review presented in this paper is on current

EDM research trends carried out by researchers onmachining techniques viz. ultrasonic vibration, dry EDMmachining, EDM with powder additives and EDM inwater and modeling techniques in predicting EDMperformances. The areas are selected because of the noveltechniques employed (ultrasonic vibration and powderadditives), the environmental aspect (dry machining andEDM in water) and effort towards validating andpredicting EDM performance (modeling technique). Eachtopic will present the activities carried out by theresearchers and the development of the area that brings itto the current trends. Wire EDM is also discussed in eachtopic.
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2. Ultrasonic vibration

Introduction of ultrasonic vibration to the electrode isone of the methods used to expand the application of EDMand to improve the machining performance on difficult tomachine materials. The study of the effects on ultrasonicvibration of the electrode on EDM has been undertakensince mid 1980s. The higher efficiency gained by theemployment of ultrasonic vibration is mainly attributed tothe improvement in dielectric circulation which facilitatesthe debris removal and the creation of a large pressurechange between the electrode and the work piece, as anenhancement of molten metal ejection from the surface ofthe work piece [8]. Zhang et al. [9] proposed spark erosionwith ultrasonic frequency using a DC power supply insteadof the usual pulse power supply. The pulse discharge isproduced by the relative motion between the tool and workpiece simplifying the equipment and reducing its cost. Theyhave indicated that it is easy to produce a combinedtechnology which benefits from the virtues of ultrasonicmachining and EDM.

2.1. Machining of microholes

In 1995 Zhixin et al. [10] has developed an ultrasonicvibration pulse electro-discharge machining (UVPEDM)technique to produce holes in engineering ceramicsmaterial. They have confirmed by experiment that thisnew technique is effective in obtaining a high materialremoval rate (MRR). Ogawa et al. [11] proved that thedepth of microholes by EDM with ultrasonic vibrationbecomes as about two times as that without ultrasonicvibration and machining rate increased. Thoe et al. [12]dealt with combined ultrasonic and electrical dischargemachining of ceramic coated nickel alloy. They found thefollowing: when drilling 1mm diameter single hole usingvarious tool materials (tungsten, silver steel, mild steel andcopper) with boron carbide abrasive slurry on ceramiccoated nickel alloy work piece, mild steel is found to be themost resilient whereas the other materials fail as a result offatigue fracture or deformation. Using ultrasonic vibrationduring EDM greatly increased the MRR of the work piece.Wansheng et al. [13] introduces ultrasonic vibration intomicro-EDM: Ti6Al4V as work piece material with32mm thickness, carbide YG6X electrode, 20 kHz ultra-sonic vibration and 2 mm amplitude; holes with diameter of0.2mm and depth/diameter ratio of more then 15 can bedrilled.

Yan et al. [14] adopted a machining method thatcombined micro electrical discharge machining (MEDM)and micro ultrasonic vibration machining (MUSM). Theyshowed that the diameter variation between the entranceand exit (DVEE) could reach a value of about 2 mm inmicroholes with diameters of about 150 mm and depth of500 mm. They obtained microholes with a roundness valueof about 2 mm (the maximum radius minus the minimumradius). A study on the effects of ultrasonic vibration on

the EDM performance for fabricating microholes inNitinol was done by Huang et al. [15]. Fabricatingmicroholes in nitinol increased the machining efficiencymore than 60 times without significantly increasing theelectrode wear. Yeo et al. [16] investigated the limitationsof microholes machining capabilities as well as current,attempts to improve micrhole drilling in UEDM. Based ontheories in fluidization engineering and ultrasonic digas-sing, a method of introducing ultrasonic vibrations intoMEDM processes was conceptualized and developed.

2.2. Vibration, rotary and vibro-rotary

Ghoreishi and Atkinson [17] compared the effects ofhigh and low frequency forced axial vibration of theelectrode, rotation of the electrode and combinations of themethods (vibro-rotary) in respect of MRR, tool wear ratio(TWR) and surface quality (SQ) in EDM die sinking andfound that vibro-rotary increases MRR by up to 35%compared with vibration EDM and by up to 100%compared with rotary EDM in semi finishing.

2.3. Theoretical model and fuzzy logic

An adaptive fuzzy control system of servomechanism forelectro-discharge machining with ultrasonic was studied byZhang et al. [18] to adjust discharge pulse parameter in atimely manner and machining gap to optimize themachining state and to improve the machining efficiency.

2.4. Workpiece vibration

A new method for micro ultrasonic machining has beendeveloped by Egashira and Masuzawa [19]. The work piecewas vibrated during machining and they have succeeded inmachining microholes as small as 5 m in diameter in quartzglass and silicon. In the machining range, high tool wearposed a problem. To solve the problem, a sintered diamondtool was tested and was proven to be effective. Gao andLiu [20] found that the efficiency of the ultrasonic micro-EDM is eight times greater than micro-EDM when workpiece material is stainless steel with 0.5mm thickness andthe electrode is tungsten with 43 mm diameter. Prihandanaet al. [21] have studied the effect of vibrated work piece.They have shown that when the vibration was introducedon the work piece the flushing effect increased. They havefound that high amplitude combined with high frequencyincrease the MRR.

2.5. Tool wear

Yu et al. [22] have studied the tool wear during the 3Dmicro ultrasonic machining. They showed that the toolshape remain unchanged and the tool wear has beencompensated by applying the uniform wear methoddeveloped for micro EDM and its integration with CAD/

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CAM to micro ultrasonic vibration process for generatingaccurate three-dimensional (3D) micro cavities.

2.6. Ultrasonic vibration in gas

Zhang et al. [23] studied the ultrasonic EDM in gas. Thegas is applied through the internal hole of a thin-walledpipe electrode. The result shows that the MRR increasedwith respect to the increase of open voltage, pulse duration,amplitude of ultrasonic actuation, discharge current andthe decrease of the wall thickness of electrode pipe whilethe surface roughness is increased with respect to theincrease of open voltage, pulse duration and the dischargecurrent. Zhang et al. [24] developed a theoretical model toestimate the roughness of finished surface. Before that,Zhang et al. [25] found that heat generation by oxidation ofthe molten and evaporated steel enhances the machiningefficiency and surface roughness is found to be affected bygas medium. For air and oxygen gas the correspondingvalues of Ra was measured to be 0.032 and 0.046,respectively.

2.7. Wire EDM

Guo et al. [26] studied the machining mechanism of wireEDM (WEDM) with ultrasonic vibration of the wire andfound that the combined technology of WEDM andultrasonic facilitates the form of multiple-channel dis-charge and raise the utilization ratio of the energy thatleads to the improvement in cutting rate and surfaceroughness. High frequency vibration of wire improves thedischarge concentration and reduces the probability ofrupture wire. Guo et al. [8] concluded that with ultrasonicaid the cutting efficiency of WEDM can be increased by

Wire

vibrates

[26]

Wire W'piece

vibrates vibrates

[8] [12]

Tool Tool Tool

vibrates vibrates vibrates

[10] [9] [19]

1995 1996 1997 1998 1999 2000 2001

Fig. 1. Progress of method in combining ultraso

30% and the roughness of the machined surface reducedfrom 1.95Ra to 1.7Ra.

2.8. Remarks

Ultrasonic vibration EDM is suitable to produce deepand small holes products. Cu is most frequently selected asthe tool electrode either in gas machining or in dielectricmachining. This is maybe due to the characteristics of thematerial which stable under sparking condition [27]. Therange of the ultrasonic frequency used during the experi-ment is between 17 and 25 kHz. Most of the experimentsare evaluating on the performance of steel based workpiece materials since these materials are widely used inindustries. However the harder material such as aluminaceramic is also evaluated.Fig. 1 shows the progress of method in combining

ultrasonic vibration with EDM from year 1995 to 2006.The method starts with vibrating the electrode followed byvibrating the work piece in year 1999, which gainspopularity from year 2003 and continues until year 2006.

3. Dry machining

In dry EDM, tool electrode is formed to be thin walledpipe. High-pressure gas or air is supplied through the pipe.The role of the gas is to remove the debris from the gap andto cool the inter electrode gap. Fig. 2 shows the principle ofdry EDM. The technique was developed to decrease thepollution caused by the use of liquid dielectric which leadsto production of vapour during machining and the cost tomanage the waste.Yu et al. [28] investigated the capability of the technique

in machining cemented carbide material and compared the

Workpiece

vibrates

[14]

Tool: rotates

vibrates &

vibro-rotary

[17]

Tool rotates

vibrates

up&down

[13]

Tool

vibrates

[18]

Tool W'piece W'piece W'piece

vibrates vibrates vibrates vibrates

[25] [20] [22] [21]

2002 2003 2004 2005 2006

nic vibration with EDM from 1995 to 2006.

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Fig. 2. The principle of dry EDM [25].

Fig. 3. Work removal rate [28].

Fig. 4. Electrode wear [28].

Fig. 5. Machining time [28].

Fig. 6. Electrode wear ratio [28].

N. Mohd Abbas et al. / International Journal of Machine Tools & Manufacture 47 (2007) 12141228 1217

machining characteristics between oil EDM milling and oildie sinking EDM. They found that for machining the sameshape oil die sinking EDM shows shorter machining time.But because oil die sinking requires time for producingelectrodes, dry EDM should be more useful in actualproduction. The information given in this paper isinteresting and they are reproduced here for better clarity.Figs. 3 and 4 show the work removal rate and electrodewear ratio in groove machining. According to the results,work removal rate of dry EDM milling is about six timeslarger than that of oil EDM milling, and electrode wearratio one-third lower. In Fig. 5, it is shown that the EDMmethod with the shortest machining time was oil diesinking EDM, dry EDM milling was second, and oil EDM

milling third. The lowest electrode wear ratio machiningwas dry EDM milling (see Fig. 6).The following sections present the progress in dry EDM

based on topics: MRR and tool wear, polarity and surfaceroughness and improvement techniques.

3.1. MRR and tool wear

In 1991 Kunieda et al. [29] has revealed a new method toimprove EDM efficiency by supplying oxygen gas into gap.They found that the stock removal rate is increased due tothe enlarged volume of discharged crater and morefrequent occurrence of discharge. Then in 1997 Kuniedaet al. [30] discovered a 3D shape can be machined veryprecisely using a special NC tool path which can supply auniform high-velocity air flow over the working gap andMRR is improved as the concentration of oxygen in air isincreased.The mechanism for minute tool electrode wear in dry

EDM was studied by Yoshida and Kunieda [31]. The toolelectrode wear is almost negligible for any pulse durationbecause the attached molten work piece material protectsthe tool electrode surface against wear. From observationof the cross-section of the tool electrode surface, it wasfound that the tool electrode wore by the depth of only2 mm during the early stage of successive pulse dischargessince the initial surface of the tool electrode was notcovered with the steel layer.ZhanBo et al. [32] studied the feasibility of 3D surface

machining by dry EDM to investigate the influence ofdepth of cut and gas pressure, pulse duration and pulseinterval and the rotational speed of the tool electrode. Theresult shows that optimum combination between depth of


cut and gas pressure and when pulse duration 25 mm it isleads to maximum MRR and minimum tool wear.As the rotational speed increases the tool wear increasesmoderately.

3.2. Polarity and surface roughness

A paper entitled Discussion of electrical dischargemachining in gas written by Li et al. [33] recommendpositive polarity to be employed in dry EDM becauseelectrodes play main roles in collision and ionization and inorder to ensure machining process stable at the sparkdischarge state, a certain gas pressure is necessary tostrengthen deionization in dry EDM and to keep dischargepoints dispersed in gap.

Zhang et al. [34] developed a theoretical model toestimate the roughness of the finished surface. Experimentswhich have been carried out using AISI 1045 steel as workpiece material and copper as the electrode show that theroughness of finished surface increases with an increase inthe discharge voltage, discharge current and pulse dura-tion. Curodeau et al. [35] proposed a new EDM processinvolving the usage of a thermoplastic composite electrodeand air as dielectric in order to perform automatedpolishing of tool steel cavity. The process can reduce44 mm Ra surface finish to 36 mm Ra.

3.3. Improvement of the technique

3.3.1. Dry ultrasonic vibration electrical discharge (dry

UEDM)

Zhang et al. [25] initiated a new method which isultrasonic vibration electrical discharge (UEDM) machin-ing in gas. The experimental result shows that the increasein open voltage, pulse duration, amplitude of ultrasonicvibration and decrease of wall thickness of the pipe cangive an increase of the MRR. He also found that oxygengas can produced greater MRR than air. Then theories ofultrasonic vibration in increasing MRR were introduced[36]. On further investigation, Zhang et al. [23] found thatMRR with the same surface roughness UEDM in gas isnearly twice as much as EDM in gas but less than theconventional EDM.

3.3.2. Dry EDM milling

The dry EDM technique is improved by Kunieda et al.[37] when they introduced the high speed 3D milling by dryEDM. The MRR increased when the discharge powerdensity on the working surface exceeds a certain thresholddue to thermally activated chemical reaction between thegas and work piece material. The maximum removal rateobtained was almost equal to that of high speed milling ofquenched steel by a milling machine. Most researchers dealwith steel as work piece when investigating the perfor-mance of dry EDM [30,37,38].

3.3.3. Using piezoelectric actuator

Kunieda et al. [38] introduced an improvement of dryEDM characteristics using piezoelectric actuator to help incontrolling the gap length. To elucidate the effects of thepiezoelectric actuator an EDM performance simulator wasdeveloped to evaluate the machining stability and MRR ofdry EDM.

3.4. Wire EDM

Furudate and Kunieda [39] conducted studies in dryWEDM. The process reaction force is negligibly small, thevibration of the wire electrode is minute and the gapdistance in dry WEDM is narrower than in conventionalWEDM using dielectric liquid which enables the dryWEDM to realize high accuracy in finish cutting. Nocorrosion of the work piece gives an advantage to dryWEDM in manufacturing high precision dies and molds.Wang and Kunieda [40] agreed that WEDM is applicablefor finish cut especially for improving the straightness ofthe machined surface. Traveling tool electrode can removedebris from the working gap even in atmosphere and byutilizing this process as finish-cut the straightness obtainedalong the works thickness direction is better than thatmachined in water [41].Kunieda and Furudate [42] found some drawbacks of

dry WEDM which include lower MRR compared toconventional WEDM and streaks generated over thefinished surface during the studies in high precision finishcutting by dry EDM. The drawbacks can be resolved byincreasing the wire winding speed and decreasing the actualdepth of cut.

3.5. Remarks

To the best of our knowledge, the earliest papermentioning about the technique was published in 1991.However, paper entitled Electrical discharge machining ingas which was published in CIRP Annals in 1997 wasreferred for 18 times [43]. Researchers from Japan andChina give major contribution for the progressing researchin this area. The characteristics of dry EDM list byKunieda [44] are:

(1)
Tool electrode wear is negligible for any pulse duration.(2) The processing reaction force is much smaller that in
conventional EDM.
(3) It is possible to change supplying gas according to
different applications.
(4) The residual stress is small since the melting resolidi-
fication layer is thin.
(5) Working gap is narrower than in conventional EDM.(6) The process is possible in vacuum condition as long as
there is a gas flow.
(7) The machine structure can be made compact since no
working basin, fluid tank and fluid circulation systemneeded.

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Fig. 7. Research studies conducted in dry EDM.


The development of the technique concerns more on toincrease the MRR since the MRR is lower compared toconventional machining. Fig. 7 shows the proportion ofresearch studies conducted in this area. It can be seen thatdry EDM can be applied to EDM, UEDM, WEDM, EDMmilling and a few other techniques. 34% of the papersreferred deal with EDM technique. This technique shouldbe supported and more investigation should be made sinceit helps to save the environment. The progress in dry EDMresearch is shown in Fig. 8. It can be seen that dry EDMhas many advantages over oil EDM. The history of thedevelopments taking place in dry EDM can be observed.

4. Powder additives

Fine abrasive powder is mixed into the dielectric fluid.The hybrid material removal process is called powdermixed EDM (PMEDM) where it works steadily at lowpulse energy [45] and it significantly affects the perfor-mance of EDM process. Electrically conductive powderreduces the insulating strength of the dielectric fluid andincrease the spark gap between the tool and the work piece.EDM process becomes more stable and improves machin-ing efficiency, MRR and SQ. However, most studies wereconducted to evaluate the surface finish since the processcan provide mirror surface finish which is a challengingissue in EDM. The characteristics of the powder such asthe size, type and concentration influence the dielectricperformance [46].

4.1. Surface quality (SQ)

Ming and He [47] indicated that some conductivepowder and lipophilic surface agents can lower the surfaceroughness and the tendency of cracks in middle finish andfinish machining but the inorganic oxide additive does nothave such effect. Wong et al. [48] compares the near-mirror-finish phenomenon using graphite, silicon (Si),

aluminium (Al), crushed glass, silicon carbide (SiC) andmolybdenum sulphide with different grain size. Al powderhas been reported to give mirror finish for SKH-51 workpieces, but not on SKH-54 work pieces. They suggestedthat it is important to have the correct combination ofpowder and work piece materials and an understanding ofthe fundamental mechanisms affecting such combinationswill promote the applications of PMEDM to feasiblyproduce superior surface finish and properties of compo-nents using EDM.When investigating the surface modification of SKD 61

with PMEDM, Yan et al. [49] revealed that the corrosionresistance and surface hardness were improved by addingthe proper powder into dielectric. Silicon powder was usedby Pecas and Henriques [46] to assess improvementthrough quality surface indicators and process timemanagement over a set of different processing area. Theresult shows that 2 g/l of Si concentration, smooth and highreflective craters were achieved with average surfaceroughness (Ra) depends on the area and varies between0.09 mm for 1 cm2 and 0.57 mm for 64 cm2 electrode. Thepolishing time has a greater effect on decreasing the surfaceroughness. Furutani et al. [50] studied a deposition methodof lubricant during finishing EDM to produce parts forultrahigh vacuum such as space environment usingPMEDM. Smoother surface can be obtained by addingaluminum powder to the mixture of molybdenum disulfide(MoS2) powder and working oil and it has smaller frictioncoefficient than that with normal working oil. Yih-fongand Fu-chen [51] investigates the effect of powder proper-ties on SQ of SKD-11 work piece using Al, chromium (Cr),copper (Cu), and SiC powders. The smallest particle(7080 nm) generates best surface finish and Al powderproduces the best surface finish.

4.2. Material removal rate

Jeswani [52] revealed that the addition of about 4 g/l offine graphite powder in kerosene increases MRR by 60%and tool wear by 15%. Yan and Chen [53] describes theeffect of dielectric mixed with electrically conductivepowder such as Al powder on the gap distance, surfaceroughness, material removal rate, relative electrode wearratio, and voltage waveform. It is shown that the dielectricwith suspended electrically conductive powder can enlargethe gap distance and can improve the energy dispersion,surface roughness, and material removal rate.Machining efficiency and surface roughness of rough

PMEDM in rough machining was studied by Zhao et al.[54] using Al with 40 g/l and 10 mm granularity and theydiscovered that machining efficiency was improved from2.06 to 3.4mm3/min with an increasing rate of 70%. Themachining efficiency can be highly increased by selectingproper discharge parameter (increasing peak current andreducing pulse width) with better surface finish incomparison with conventional EDM machining. Tzengand Lee [55] indicated that the greatest MRR is produced

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1991 Kunieda

et al [29]: oxygen gas introduced in discharge gap

based dielectric

in water

increased removal rate.

1997 Kunieda et al [30]: EDM can be achieved in gas.

2001 Kunieda et al [42]: narrower gap, no corrosion of work piece and high finish cutting in dry EDM.

2003 Wang et al [41]: dry EDM removes environmental pollution due to liquid dielectric. Better straightness with dry

EDM. Kunieda et al [37]:oxidation of work pieces due to the usage of

oxygen electrode wear is almost negligible increase MRR. Curodeau et al [35]: a thermoplastic composite electrode used in

dry EDM using air as dielectric medium. Z. B. Yu et al [28]: dry EDM is suitable for 3D milling of difficult

to cut materials such as cemented carbide.

2004 Kunieda et al [38] improvement of dry EDM by controlling the discharge gap using a piezoelectric actuator.

Wang T. et al [40]: the explosive force and electrostatic force acting on wire electrode decrease in dry WEDM.

Zhang et al [36]: ultrasonic vibration improves MRR in gas by increasing the effective discharge.

Li L.Q [33]: discharge passage extends rapidly in the gas medium of dry EDM.

2005 Zhang et al [34]: a theoretical model of surface roughness in ultrasonic vibration assisted EDM (UEDM) in gas.

Zhang et al [23]: in UEDM, MRR increased with the increase of open voltage, pulse duration, amplitude of ultrasonic actuation, discharge current and with the decrease of the wall thickness

of electrode pipe.

1999 Yoshida et al [31]: tool electrode is almost negligible for any pulse duration in dry EDM.

2002 Zhang et al [25]:EDM with ultrasonic aid (UEDM) can be achieved in gas medium.

Fig. 8. Progress in dry EDM research.

N. Mohd Abbas et al. / International Journal of Machine Tools & Manufacture 47 (2007) 121412281220

by chromium and 7080 nm of grain size. Kansal et al. [56]established optimum process conditions for PMEDM inthe rough machining phase using the Taguchi method withgraphite powder and found out that addition of anappropriate amount of the graphite powder into thedielectric fluid caused discernible improvement in MRRand reduction in tool wear as well as in surface roughness.

4.3. Recast layer

Influence of powder particle in micro EDM especially inthe gap, thermal spread in dielectric and influenced zonewere investigated on 718 inconel alloy by Klocke et al. [57].Al and Si powder were added with 2 and 10 g/l concentra-tion, respectively. The result shows that Al powder leads to

thinnest rimzone and highest MRR. Wu et al. [58] addedsurfactant with Al powder and found that it has moreapparent effect to distribute electrical discharging energy toachieve thinner recast layer around 12 mm in thicknesscompared to 58 mm under pure dielectric.

4.4. Modeling

Sharif et al. [59] established empirical relation ofmachining responses using response surface methodologytechnique while machining Ti-6246 work piece by addingSiC powder in the dielectric. The result shows that MRR,tool wear and SQ are greatly influenced by the current,voltage and pulse on time and the surface roughness washighly influenced by the presence of SiC additives. Kansal


et al. [60] used the same method as [59] to optimize theprocess parameter of PMEDM. Pulse on time, duty cycle,peak current and concentration of SiC powder were chosenas variables. Combination of high peak current and highconcentration yields more MRR and smaller surfaceroughness. The error between experimental and predictedvalues of MRR and surface roughness are within 78%and 7.85% to 3.15%, respectively.

4.5. Other methods

An effective method for promoting the MRR of SKD11was found by Yan and Chen [61] when rotating theelectrode and machining in the dielectric suspended withalumina powder under optimum conditions. Resultsindicated that the best performance from the viewpointof surface finish is achieved using the powder particles of1 mm diameter, concentration of 4 g/l, and the electroderotation speed of 60 rpm. A surface modification methodby EDM with a green compact electrode has been studiedto make thick TiC layer using titanium alloy powder inkerosene-like dielectric was proposed by Furutani et al.[62].

TiC layer grows with thickness of 150mm with hardnessof 1600Hv on carbon steel with an electrode of 1mm indiameter. Chow et al. [63] studied the added powder inkerosene for the micro-slit machining of titanium alloyusing electro-discharge machining and discovered that SiCpowder in kerosene can produce better material removaldepth than Al. Kozak et al. [64] have published a paperentitled Study of electrical discharge machining usingpowder suspended working media which compared thecharacteristics obtained using kerosene and a mixture ofdeionized water with abrasive powder.

The effect of finishing in abrasive fluid machining onmicroholes by EDM [65] shows that using SiC abrasive, theoptimal combination for the finishing of titaniumalloy is 8 mm grain size and 0.5 g/ml concentration whilefor stainless steel it is 20 mm grain size and 0.5 g/mlconcentration

0 2 4

Al

SiC

Si

Graphite

Cr

Others

Fig. 9. Distribution of type of powder

4.6. Remarks

The earliest paper mentioning about PMEDM waspublished in 1981. A number of researchers worked in thisarea and investigated on various types of powders withdifferent concentration and grain sizes on various types ofwork pieces. The effect of adding powder in dielectric wasstudied and many researchers have shown that it improvessurface finish to a great extent. From the reported papers,it is observed that the additions of Cu powder make almostno difference to the pure kerosene dielectric.From the collected literature a bar chart is made in Fig. 9

to show the type of powder used and to compare thenumber of works dealing with each type of powders. It isseen that aluminium powder suspension attracts manyresearchers since it contributes well for improving MRR aswell as the surface finish.

5. EDM in water

Water as dielectric is an alternative to hydrocarbon oil.The approach is taken to promote a better health and safeenvironment while working with EDM. This is becausehydrocarbon oil such as kerosene will decompose andrelease harmful vapour (CO and CH4) [23]. Research overthe last 25 years has involved the use of pure water andwater with additives.

5.1. Pure water

The first paper about the usage of water as dielectric waspublished by Jeswani [66] in 1981. He compared theperformances of kerosene and distilled water over the pulseenergy range 72288mJ. Machining in distilled waterresulted in a higher MRR and a lower wear ratio than inkerosene when a high pulse energy range was used. Withdistilled water, the machining accuracy was poor but thesurface finish was better. Tariq Jilani and Pandey [67]investigated the performance of water as dielectric in EDMusing distilled water, tap water and a mixture of 25% tapand 75% distilled water. The best machining rates have

6 8 10

used based on the collected papers.


been achieved with the tap water and machining in waterhas the possibility of achieving zero electrode wear whenusing copper tools with negative polarities.

Konig and Siebers [68] explained the influence of theworking medium on the removal process. They indicatedthat working medium has a sustained influence on theremoval process. The erosion process in water-based mediaconsequently possesses higher thermal stability and muchhigher power input can be achieved especially under criticalconditions, allowing much greater increases in the removalrate. A considerable difference between conventional oil-based dielectrics and aqueous media is specific boilingenergy of aqueous media is some eight times higher andboiling phenomena occur at a lower temperature level.During investigating on white surface layer, Kruth et al.[69] found that the use of an oil dielectric increases thecarbon content in the white layer and appears as ironcarbides (Fe3C) in columnar, dendritic structures whilemachining in water causes a decarbonization. Whileinvestigating the influence of kerosene and distilled wateras dielectric on Ti6A14V work pieces Chen et al. [70]found that carbide is formed on the work piece surfacewhile using kerosene while oxide is formed on the workpiece surface while using distilled water. The debris size ofTi6Al4V alloy in distilled water is greater than that inkerosene and compared with kerosene, the impulsive forceof discharge in distilled water is smaller but more stable.

Ekmekci et al. [71] presented an experimental work tomeasure residual stresses and hardness depth in EDMsurfaces. Stresses are found to be increasing rapidly withrespect to depth, attaining to its maximum value aroundthe yield strength and then fall rapidly to compressiveresidual stresses in the core of the material since the stresseswithin plastically deformed layers are equilibrated withelastic stresses. In 2005, Sharma et al. [72] investigated thepotential of electrically conductive chemical vapor depos-ited diamond as an electrode for micro-electrical dischargemachining in oil and water. While doing a comparativestudy on the surface integrity of plastic mold steel,Ekmekci et al. [73] found that the amount of retainedaustenite phase and the intensity of microcracks havefound to be much less in the white layer of the samplesmachined in de-ionized water.

A new application in EDM power supply was designedto develop small size EDM systems by Casanueva et al.[74]. The proposed control achieves an optimum and stableoperation using tap water as dielectric fluid to prevent thegeneration of undesired impulses and keep the distancebetween the electrode and the work piece within theoptimum stable range. Studies conducted by Kang andKim [75] in order to investigate the effects of EDM processconditions on the crack susceptibility of a nickel-basedsuper alloy revealed that depending on the dielectric fluidand the post-EDM process such as solution heat treatment,cracks exist in recast layer could propagate into substratewhen a 20% strain tensile force was applied at roomtemperature. When kerosene as dielectric, it was observed

that carburization and sharp crack propagation along thegrain boundary occurred after the heat treatment. How-ever, using deionized water as dielectric the specimen afterheat treatment underwent oxidation and showed no crackpropagation behavior.

5.2. Water with additives

Koenig and Joerres [76] reported that a highly concen-trated aqueous glycerine solution has an advantage ascompared to hydrocarbon dielectrics when working withlong pulse durations and high pulse duty factors anddischarge currents, i.e. in the roughing range with highopen-circuit voltages and positive polarity tool electrode.Leao and Pashby [77] found that some researchers havestudied the feasibility of adding organic compound such asethylene glycol, polyethylene glycol 200, polyethyleneglycol 400, polyethylene glycol 600, dextrose and sucroseto improve the performance of demonized water.The surface of titanium has been modified after EDM

using dielectric of urea solution in water [78]. The nitrogenelement decomposed from the dielectric that containedurea, migrated to the work piece forming a TiN hard layerwhich resulting in good wear resistance of the machinedsurface after EDM.

5.3. Wire EDM

Levy [79] experimented an environmentally friendly andhigh-capacity dielectric regeneration for wire EDM. Heused a filtration unit based on membrane technology to getan efficient and long-term autonomy regeneration system.Diane [80] emphasize on proper mix and type of resin usedin deionization of water for wire EDM. An articlepublished in 1993 [81] mentioned that Mitsubishi Electrichas developed an anti-electrolysis power supply that canimproved the surface finish with water-immersion di-electric. Takeshi and Yutaka [82] claimed that Mitsubishihas developed DWC90PA, a wire EDM machine to meetthe high-end die-machining requirements of the semicon-ductor and precision electronic component industrieswhich uses water as the machining medium. Minami etal. [83] proposed a coloring method of titanium alloy usingWEDM during the finishing without any other posttreatment. When machining in water, a molten andresolidified surface created by electrical discharge processwas colored directly by the interference phenomenon oflight in the anodic oxide film formed with electrolysisreaction. The thickness of the oxide film controlled by theaverage working voltage determined the color tone.

5.4. Remarks

Water-based dielectric can replace hydrocarbon oilssince it is environmentally safe. When comparing theperformance of water-based dielectric with hydrocarbon oilit shows that surface finished in distilled water is better

ARTICLE IN PRESS

1981 Jeswani M.L [66]: Machining in distilled water resulted in higher MRR and lower wear ratio.

1984 S. Tariq Jilani et al [67]: The best machining rates have been achieved with tap water as the dielectric medium; zero electrode wear possible.

1987 Koenig W. et al [76]: Use aqueous solution of organic compounds as medium for EDM sinking almost completely excludes any fire hazard, permitting safe operation of plant.

1993 Koenig W. et al [68]: EDM sinking process can be made more cost effective through the use of water based media, significantly improving competitiveness with other process.

Yoshiro et al [81]: A machine tool maker has established technologies for water-immersion machining, greatly improved the surface finish so that post-process manual polishing is not required.

1995 Kruth J.P et al [69]: Water dielectric cause decarbonisation in the white layer at the surface of a work piece caused by EDM.

1999 Chen S.L. et al [70]: The MRR is greater and the relative wear ratio is lower when machining in distilled water rather than in kerosene.

2004 Leao & Pashby [77]: Water based dielectrics may replace oil-based fluids in die sinking applications.

2005 Yan B.H. et al [78]: TiN was synthesized on the machined surface bychemical reaction that involved elements obtained from the work piece and the urea solution in water as dielectric during EDM: the surface modification of pure titanium metals exhibited improved friction and wear characteristics.

Ekmekci. B. et al [73]: The amount of retained austenite phase and the intensity of microcracks has been found to be much less in the white layer ofthe samples machined in deionized water dielectric liquid.

Kang & Kim [75]: In the case of the kerosene electrical discharge (ED) machined specimens, it was observed that carburization and sharp crack propagation along the grain boundary occurred after the heat treatment. However, the deionized ED machined specimen after the heat treatment underwent oxidation and showed no crack propogation behaviour.

Fig. 10. Advantages of using water-based dielectric in EDM.


compared to kerosene. Investigation should be made toevaluate the performance of water-based dielectric inmachining advanced materials such as carbide andcomposite materials. There are many advantages whenwe use water-based dielectric in EDM reported byresearchers from different parts of the world during thelast 25 years. Fig. 10 presents a few important advantages.

6. Modeling

EDM process is influenced by many input factors.Various techniques viz. dimensional analysis, artificialneural network and thermal modeling are employed topredict the output of the process mainly the surface finish,tool wear and MRR.

6.1. Dimensional analysis

In 1979 Jeswani [84] used the dimensional analysis topredict the tool wear in EDM. The equation relates thevolume of material eroded from the tool electrode to theenergy of the pulse and density, thermal conductivity,

specific heat and latent heat of vaporization of electrodematerial. Yahya and Manning [85] applied the method inanalyzing MRR. The electrical and physical parametersrelated to the process are discharge pulse on time, gapvoltage, sparking frequency, gap current and materialproperties. Results from the model show good agreementwhen compared to experimental finding.A semi empirical model of surface finish [86], MRR and

tool wear [87] for various materials have been establishedby employing dimensional analysis. Using design ofexperiment method, the process parameter viz. peakcurrent, pulse duration, electric polarity and materialproperties are identified. The final results show that theaverage error between the experiment and prediction wasless than 10% for surface finish model and less than 20%for MRR. However, the relations between tool wear anddischarge time under different electric polarity are seen tohave inverse effect.

6.2. Mathematical models

Dibitonto et al. [88] presented a simple cathode erosionmodel for EDM. The compton original energy balance forgas discharges is amended and the model uses the photo-electric effect as the dominant source of energy supplied tothe cathode surface. Then, Patel et al. [89] developed theanode erosion model which accepts power as boundarycondition at anode interface and assumed to produce aGaussian-distributed heat flux on the surface of anodematerial. The area upon which the flux is incident isassumed to grow with time. A model on variable mass andcylindrical plasma was introduced by Eubank et al. [90]. Itconsists of three differential equation-one each from fluiddynamics, an energy balance and the radiation equation-combined with plasma equation of state. Problems with thezero-time boundary conditions are overcome by anelectron balance procedure.McGeough and Rasmussen [91] proposed a model for

electro discharge texturing based on the effect of dielectricfluid and in particular the influence of change in theresistance in the dielectric during each voltage pulse. Thetheoretical predictions confirm the practical findings thatthe surface roughness in texturing is determined primarilyby the peak current used and the length of the voltage on-time. Cogun et al. [92] investigated on surface profile of2080 tool steel machined under varying machining para-meters. It is found that surface roughness increases withincreasing discharge current, pulse duration and dielectricflushing pressure. Surface profile information obtained istransferred to computer, digitized and then modeled in theform of Fourier series.Perez et al. [93] presented a model for relative power

dissipation by taking into account the different currentemission mechanism and cathode space-charge character-istics valid for refractory and non-refractory materials.Tantra et al. [94] tested the validity of model proposed byHeuvelman for erosion strength of material to predict tool


wear and their applicability to EDM process such asdrilling of deep holes in turbine blades. The experimentalresults show the Heuvelman model does not show a directcorrelation with the observation.

6.3. Artificial neural network (ANN)

An attempt of modeling EDM process through ANNwas carried out by Gopal and Rajurkar [95]. Withmachining depth, tool radius, orbital radius, radial step,offset depth, pulse on-time, pulse off-time and dischargecurrent as the input parameters 9-9-2 size back propagationneural network was developed, experiments have beenperformed to check the validity of the ANN model and itcan be concluded that the ANN model provides faster andmore accurate results. Tsai and Wang [96] have comparedthe ANN model on MRR. Six neural network models andneuro-fuzzy model for MRR have been established andanalyzed. Results show that adaptive-network fuzzyinterference system (ANFIS) is more accurate with anerror 16.33%.

In their further investigations, Tsai and Wang [97] haveapplied the same method to predict the surface finish.Results show that tangent sigmoid multi-layered percep-tron (TANMLP), radial basis function network (RBFN),Adaptive RBFN and ANFIS models have shown consis-tent results. Wang et al. [98] combine the ANN and geneticalgorithm to find an integrated solution to the problem ofmodeling and optimization of manufacturing processes.The error of the model is 5.6% for MRR and 4.98% forsurface roughness. The modeling system established betterknowledge about interaction between tool (graphite) andwork piece (nickel alloy). An artificial feed forward neuralnetwork based on the LevenbergMarquardt back propa-gation technique has been developed by Panda and Bhoi[99] to predict MRR. The model provides faster, moreaccurate results and performs well under the stochasticenvironment of actual machining conditions withoutunderstanding the complex physical phenomena exhibitedin electro-discharge machining.

6.4. Other methods

Tariq and Pandey [100] employed heat transfer model tostudy the effects of EDM input parameters: pulse duration,pulse energy and material properties on MRR and cratershape. Pandit and Rajurkar [101] illustrates thermalmodeling using data dependent system (DDS) which is astochastic approach during the time. Altpeter et al. [102]proposed a model for die-sinking controller synthesis forlarge range operating conditions is illustrated with experi-mental data taken from commercial die-sinker, designedand manufactured by Charmilles Technologies. In a paperentitled Modeling of surface finish in electro-dischargemachining based upon statistical multi-parameter analysis,Petropoulos et al. [103] emphasized the interrelationshipbetween surface texture parameters and process para-

meters. A close correlation was revealed between certaintexture parameters and the EDM input variables and singleand multiple statistical regression models are developed.

6.5. Wire EDM

Rajurkar and Wang [104] reported on sparking fre-quency monitor which is developed to detect thermal loadfor on-line monitoring control to prevent wire rupture. Thewire rupture phenomenon is analyzed with thermal modeland the relationship between the machining rate andsurface finish under optimal machine setting was deter-mined by means of a multi-objective model. Puri andBhattacharya [105] reports an account of the vibrationalbehavior of the wire and presents an analytical approachfor solution of wire-tool vibration equation consideringmultiple discharges. The higher the thickness of the workpiece, the larger the maximum amplitude of the vibrationfor a given span of the wire between the guides.Furthermore Puri and Bhattacharya [106] modeled thewhite layer depth through response surface methodology.The input variables are pulse on time during rough cutting,pulse on time, wire tool offset and constant cutting speedduring trim cutting. The optimum speed of cutting is 3mm/min.

6.6. Micro EDM

Katz and Tibbles [107] proposed a micro EDM modelwith numerical simulation and experimental validation.The model has predicted reasonable values for currentdensity, crater area, power dissipation and the rate ofchannel growth. Dhanik and Joshi [108] predicted MRR ina single discharge and the plasma temperature and radiusof crater at the cathode predicted using the model werefound to agree well with the experimental data in theliterature. Dhanik et al. [109] provide a review of modelingefforts in the field of EDM processes and propose thefuture direction for the modeling of micro-EDM process.

6.7. Remarks

Modeling in EDM helps us to get a better understandingof the complex process. The knowledge, the modeling wasfirst introduced by Jeswani in 1979 using dimensionalanalysis to predict the tool wear. After that, variousmethods were introduced to predict the output of EDMprocess. A simple presentation is made in Fig. 11 to showthe theoretical models available in literature for simulatingthe input and output parameters.

7. Summary

EDM has brought many improvements in machiningprocess in recent years. The capability of machiningintricate parts and hard material has made EDM as oneof the most popular machining processes. The contribution

ARTICLE IN PRESS

Modeling EDM input and

outputparameters

DimensionalAnalysis

[84], [88], [86], [87],

[85] & [107] Multiple Statistical

RegressionModel [103]

ResponseSurface

MethodologyModel[106]

Artificial Neural

Network Model

[95], [96] [97]& [99]

Partial Differential

Equation Model [105]

Variable Mass Cylindrical

Plasma Model(VMCPM)

[90] Thermal Model

[100], [101]& [104]

Fourier Series Model[92]

Fig. 11. Theoretical models available in literature for simulating the input and output parameters.


of EDM to industries such as cutting new hard materialsmake EDM technology remains indispensable.

The review of the research trends in EDM on ultrasonicvibration, dry EDM machining, EDM with powderadditives, EDM in water and modeling technique inpredicting EDM performances is presented. In each topic,the development of the methods for the last 25 years isdiscussed. The progress of development in each area ispresented using charts. The ultrasonic vibration method issuitable for micro machining, dry machining is costeffective, EDM in water is introduced for safe andconducive working environment, EDM with powderadditives is concerning more on increasing SQ, MRR andtool wear using dielectric oil and EDM modeling isintroduced to predict the output parameters which leadstowards the development of precise and accurate EDMperformance.

For each and every method introduced and employed inEDM process, the objectives are the same: to enhance thecapability of machining performance, to get better outputproduct, to develop technique to machine new materialsand to have better working conditions.

Acknowledgment

The authors would like to acknowledge the support ofUniversiti Teknologi MARA, Shah Alam, Malaysia forfunding the current research grant for performing thiswork. All those who contributed directly or indirectly arethanked.

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http://ISBN:983-42876-0-7A review on current research trends in electrical discharge machining (EDM)IntroductionUltrasonic vibrationMachining of microholesVibration, rotary and vibro-rotaryTheoretical model and fuzzy logicWorkpiece vibrationTool wearUltrasonic vibration in gasWire EDMRemarksDry machiningMRR and tool wearPolarity and surface roughnessImprovement of the techniqueDry ultrasonic vibration electrical discharge (dry UEDM)Dry EDM millingUsing piezoelectric actuatorWire EDMRemarksPowder additivesSurface quality (SQ)Material removal rateRecast layerModelingOther methodsRemarksEDM in waterPure waterWater with additivesWire EDMRemarksModelingDimensional analysisMathematical modelsArtificial neural network (ANN)Other methodsWire EDMMicro EDMRemarksSummaryAcknowledgmentReferences

a review on current research trends in electrical discharge machining (edm)

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