ORIGINAL ARTICLE

Review of research work in sinking EDM and WEDMon metal matrix composite materials

R. K. Garg & K. K. Singh & Anish Sachdeva &

Vishal S. Sharma & Kuldeep Ojha & Sharanjit Singh

Received: 18 September 2009 /Accepted: 11 January 2010 /Published online: 20 February 2010# Springer-Verlag London Limited 2010

Abstract Metal matrix composites (MMCs) are newlyadvanced materials having the properties of light weight, highspecific strength, good wear resistance and a low thermalexpansion coefficient. These materials are extensively used inindustry. Greater hardness and reinforcement makes it difficultto machine using traditional techniques, which has impededthe development of MMCs. The use of traditional machineryto machine hard composite materials causes serious tool weardue to the abrasive nature of reinforcement. These materialscan be machined by many non-traditional methods like waterjet and laser cutting but these processes are limited to linearcutting only. Electrical discharge machining (EDM) showshigher capability for cutting complex shapes with highprecision for these materials. The paper presents a review ofEDM process and year wise research work done in EDM onMMCs. The paper also discusses the future trend of researchwork in the same area.

Keywords EDM .Metal matrix composites .

Process parameters

1 Introduction

Electrical discharge machining is basically a non-conventional material removal process which is widely used

to produce dies, punches, and molds. It is widely used forfinishing parts for aerospace and automotive industry andsurgical components [1]. This process can be successfullyemployed to machine electrically conductive parts irrespec-tive of their hardness, shape, and toughness [2–4].

1.1 Working principle of electrical discharge machining

The working principle is based on the thermoelectricenergy. The thermoelectric energy (in form of spark) iscreated between a workpiece and an electrode submerged ina dielectric fluid with conduction of electric current. Theworkpiece and the electrode are separated by a specificsmall gap, the so-called ‘spark gap’, and pulsed dischargesoccur in this gap filled with an insulating medium [5–8].The insulating effect of the dielectric medium has someimportance in avoiding electrolysis effects on the electrodesduring machining process. The electrode moves toward theworkpiece until the gap is small enough so that the appliedvoltage is high enough to ionize the dielectric fluid [9].Dielectric liquid must be selected to minimize the gap(10-100 μm) to obtain precise machining. However, acertain minimum gap width is needed to avoid shortcircuiting, particularly with the electrodes that are sensitiveto vibration (like wire electrodes) or deformation. Theignition of the discharge is initiated by a high voltage,overcoming the dielectric breakdown strength of the smallgap. Short duration discharges are generated in a liquiddielectric gap, which separate electrode and workpiece. Thematerial is removed with the erosive effect of the electricaldischarges from tool and workpiece [10]. In this process,there is no direct contact between the electrode and theworkpiece thus eliminating mechanical stresses, chatter,and vibration problems during machining [1]. The mirrorimage of electrode is copied with an offset equal to the gap

R. K. Garg :A. Sachdeva :V. S. Sharma :K. Ojha : S. Singh (*)Department of Industrial and Production Engineering,Dr B. R. Ambedkar National Institute of Technology,Jalandhar-144011 Punjab, Indiae-mail: malhi.sharanjit@gmail.com

K. K. SinghDepartment of Mechanical Engineering & Mining MachineryEngineering, Indian School of Mines (ISM),Dhanbad-826004 Jharkhand, India

Int J Adv Manuf Technol (2010) 50:611–624DOI 10.1007/s00170-010-2534-5

size. A channel of plasma (ionized, electrically conductivegas with high temperature) is formed between the electro-des and develops further with discharge duration. As themetal removal per discharge is very small, dischargesshould occur at high frequencies (103-106 Hz). For everypulse, discharge occurs at a single location where theelectrode materials are evaporated and/or ejected in themolten phase. As a result, a small crater is generated bothon the tool electrode and workpiece surfaces. Removedmaterials form several hundreds of spherical debris par-ticles, which are then flushed away from the gap by thedielectric flow. After the end of the discharge duration, thetemperature of the plasma and the electrode surfacescontacting the plasma rapidly drops, resulting in a recombi-nation of ions and electrons and a recovery of the dielectricbreakdown strength.

This technique has been developed in the late 1940s[11]. Trends on activities carried out by researchers dependon the interest of the researchers and the availability of thetechnology. Rajurkar in 1994 has indicated some futuretrends activities in electrical discharge machining (EDM):machining advanced materials, mirror surface finish usingpowder additives, ultrasonic-assisted EDM and control andautomation [12]. Process parameters and performancemeasures of this process are shown in Fig. 1.

1.2 Process parameters

The process parameters can be divided into two categories,i.e., electrical and non-electrical parameters.

1.2.1 Electrical parameters

Major electrical parameters are discharge voltage, peakcurrent, pulse duration and pulse interval, electrode gap,polarity, and pulse wave form. Discharge voltage is relatedto spark gap and breakdown strength of the dielectric fluid.The open-gap voltage before electric discharge increases

until ionization path is created between workpiece andelectrode. Once the current starts flowing, the voltage dropsand stabilizes at the working gap level. Thus a highervoltage setting increases the gap, which in turn improvesthe flushing conditions and helps to stabilize the cut.

Peak current is the amount of power used in dischargemachining and is considered as most significant processparameter. The current increases until it reaches a presetlevel during each pulse on-time, which is known as peakcurrent. Peak current is governed by surface area of cut.Higher peak current is applied during roughing operationand details with large surface area. This is the mostimportant parameter because the machined cavity is areplica of tool electrode and excessive wear will hamper theaccuracy of machining. New improved electrode materialslike graphite, can work on high currents without muchdamage [1].

Pulse duration is commonly referred to as pulse on-timeand pulse interval is called pulse off-time. These areexpressed in units of microseconds. Since all the work isdone during pulse duration, hence this parameters and thenumber of cycles per second (frequency) are important.Material removal rate (MRR) depends upon the amount ofenergy applied during the pulse duration [13]. Increasedpulse duration also allow more heat to sink into theworkpiece and spread, which means the recast layer willbe larger and the heat-affected zone will be deeper. Materialremoval rate tends to decrease after an optimal value ofpulse duration.

Pulse interval mainly affects machining speed andstability of cut. Shorter interval, results in faster machiningoperation. However, if the interval is too short, the ejectedworkpiece material will not be flushed away with the flowof the dielectric fluid and the dielectric fluid will not bedeionized. This results in instability of the next spark.Erratic cycling due to unstable spark results in prolongedmachining time. At the same time, pulse interval must begreater than the deionization time to prevent continuedsparking at one point [14].

Electrode gap is set by tool servo mechanism. Thismechanism is designed to respond well to the average gapvoltage [15]. The most important requirements for goodperformance are gap stability and the reaction speed of thesystem as the presence of backlash is undesirable. Thereaction speed must be high in order to respond to shortcircuits or open-gap conditions.

The electrode polarity may be positive or negative. Ingeneral, polarity is determined by experiments and isdependent tool material, work material, current density,and pulse length combinations. Modern power suppliesinsert an opposite polarity “swing pulse” at fixed intervalsto prevent arcing. A typical ratio is one swing pulse forevery 15 standard pulses [1].

EDM Process

Process parameters

Performance measures

Electrical Non electrical

1. Flushing

2. Electrode Rotation

3. Workpiece Rotation

1. MRR 2. TWR 3. SR

MRR-Material removal rate

TWR-Tool wear rate

SR-Surface roughness

1. Peak voltage 2. Peak current 3. Pulse duration 4. Polarity 5. Electrode gap 6. Pulse interval 7. Pulse wave form

Fig. 1 Process parameters and performance measures of EDM Process

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The pulse wave form is normally rectangular but gene-rators with other pulse shapes have also been developed. Witha generator which can produce trapezoidal pulses, the relativetool wear reduces to a very low value [16]. Other types ofgenerators introduce an initial pulse of high voltage but lowcurrent and of a few microseconds duration, before the mainpulse, which facilitates ignition.

The EDM process is of stochastic thermal nature havingcomplicated discharge mechanism [17]. Hence, it isdifficult to explain all the effect of these parameters onperformance measures. However, researchers now rely onprocess analysis for optimization of parameters to identifythe effect of operating variables on achieving the desiredmachining characteristics. Lin [18] used gray relationalanalysis for solving the complicated interrelationshipsbetween process parameters and the multiple performancemeasures. Taguchi approach has also been applied by manyother researchers [19–21] to analyze and design the idealEDM process.

1.2.2 Non-electrical parameters

Main non-electrical parameters are flushing of dielectricfluid, workpiece rotation, and electrode rotation. Thesenon-electrical parameters play a critical role in optimizingperformance measures.

Researches on flushing pressure reveals that it affects thesurface roughness, tool wear rate, act as coolant and alsoplay a vital role in flushing away the debris from themachining gap [22–24]. Based on the experimental results,Lonardo and Bruzzone [22] reported that flushing pressureduring the roughing operation affects the MRR and toolwear rate (TWR), while in the finishing operation, itinfluences the SR. Both MRR and TWR increased withincrease in flushing pressure. The flushing pressure alsoinfluences the crack density and recast layer, which can beminimized by obtaining an optimal flushing rate based onempirical data [23].

Workpiece rotary motion improves the circulation of thedielectric fluid in the spark gap and temperature distributionof the workpiece yielding better MRR and SR [25].Similarly, electrode rotation results in better flushing actionand sparking efficiency [26]. Hence, improvement in MRRand SR has been reported due to effective gap flushing dueto electrode rotation [27–29].

1.3 Performance measures

Performance measures are material removal rate, tool wearrate, and surface roughness. In MRR research, workfocused on material removal mechanism and methods ofimproving MRR [30–35]. Similarly, research work on toolwear process and methods of improvement in TWR has

been reported [21, 36, 37]. Though EDM is essentially amaterial removal process, efforts have been made to use itas a surface treatment method and/or an additive process.Many surface changes have been reported ever since theprocess established itself in the tool rooms of manufactur-ing industry [38].

1.4 Types of EDM processes

1.4.1 Sinking EDM

In this process, the workpiece can be formed, either byreplication of a shaped tool electrode, or by three-dimensional (3D) movement of a simple electrode like inmilling or a combination of the above. The electrodematerial is normally copper or graphite. The numericalcontrol monitors the gap conditions (voltage and current)and synchronously controls the different axes and the pulsegenerator. The dielectric liquid is filtrated to remove debrisparticles and decomposition products.

1.4.2 Wire EDM

In WEDM, material is eroded from the workpiece by aseries of discrete sparks occurring between the workpieceand the wire separated by a stream of dielectric fluid, whichis continuously fed to the machining zone. The process usesa thin copper wire of diameter about 0.1-0.3 mm as theelectrode and the workpiece is mounted on a computernumeric-controlled (CNC) worktable. Complex two-dimensional shapes that can be cut on the workpiece bycontrolled movement of the x-y worktable [39]. The thinwire is fed continuously through the workpiece by amicroprocessor, which enable parts of complex shapes tobe machined with extraordinary high accuracy. Themicroprocessor also constantly maintains the gap betweenthe wire and the workpiece, which varies from 0.025 to0.05 mm. The process eliminates the need for elaborate pre-shaped electrodes, which are commonly required in sinkingEDM to perform the roughing and finishing operations. Thewire has to make several machining passes along the profileto be machined to attain the required dimensional accuracyand surface quality. This process is widely applied not onlyin tool and die-making industry, but also in the fields ofmedicine, electronics, and the automotive industry [40].

1.4.3 Micro-EDM

This process is capable of machining not only micro-holesand micro-shafts as small as 5 μm in diameter but alsocomplex 3D micro-cavities. This is unlike mechanicaldrilling, which can produce holes just up to 70 μm, or themicro-fabrication process such as laser machining, which

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can only create holes of 40 μm [1]. This process is basicallyof four types—micro-wire EDM, die-sinking micro-EDM,micro-EDM drilling, and micro-EDM milling. In micro-wire EDM, a wire which has a diameter down to 0.02 mmis used to cut through a workpiece. In die-sinking micro-EDM, an electrode is used containing micro-features to cutits mirror image in the workpiece. In micro-EDM drilling,micro-electrodes (of diameters down to 5–10 µm) are usedto ‘drill’ micro-holes in the workpiece. In micro-EDMmilling, micro-electrodes (of diameters down to 5–10 µm)are employed to produce 3D cavities by adopting amovement strategy similar to that in conventional milling.On micro-EDM, a model with numerical simulation andexperimental validation is proposed by the researchers [41].This model predicts reasonable values for current density,crater area, power dissipation, and the rate of channelgrowth. MRR in a single discharge, plasma temperature,and radius of crater at the cathode predicted by using themodels were found to agree well with the experimental datain the literature [42].

1.4.4 Powder-mixed EDM

The mechanism of this process is totally different from theconventional EDM [43]. A suitable material in powderform is mixed into the dielectric fluid. When a suitablevoltage is applied, the spark gap filled up with additiveparticles and the gap distance setup between tool and theworkpiece increased from 25-50 to 50–150 µm [44]. Thepowder particles arrange themselves under the sparkingarea and gather in clusters. The chain formation helps inbridging the gap between both the electrodes which causesthe early explosion. Faster sparking within discharge takesplace and causes faster erosion from the workpiece surface.

1.4.5 Dry EDM

In this process, tool electrode is in the form of a thin-walledpipe through which high-pressure gas or air is supplied. Therole of the gas is to remove the debris from the gap andcooling of the inter electrode gap. The technique wasdeveloped to decrease the pollution caused by the use ofliquid dielectric which leads to production of vapors duringmachining and the cost to manage the waste.

1.5 Metal matrix composite materials

Metal matrix composites are composite materials with atleast two constituent parts. One being a metal and the othermaterial may be a different metal or another material, suchas a ceramic or organic compound. These are made bydispersing a reinforcing material into a metal matrix. Thereinforcement surface can be coated to prevent a chemical

reaction with the matrix. For example, carbon fibers arecommonly used in aluminum matrix to synthesize compo-sites showing low density and high strength. However,carbon reacts with aluminum to generate a brittle and water-soluble compound Al4C3 on the surface of the fiber. Toprevent this reaction, the carbon fibers are coated withnickel or titanium boride.

These materials have significant advantages like high-specific stiffness, wear resistance, strength, weight reduc-tion, high thermal conductivity, low coefficient of thermalexpansion and high-dimensional stability. These materialsare now used in automotive and aerospace field due to theirsuperior physical and mechanical properties. These mate-rials have good formability [45, 46]. Typical matrix metalsare Cu, Fe, Al, Mg, Ti, and Pb. Important reinforcementmaterials are SiC, Al4O3, and TiB2. These reinforcementmaterials are applied either in whisker particulate andcontinuous fibrous form. Inspite of many advantages,secondary conventional machining processes like turning,drilling, and milling of materials limits the wide applica-tions of MMCs. This is due to poor machinability,productivity, and machined product quality. Machining ofthese materials includes high tool wear rate, sub-surfacedamage, and cracking [47–49]. These materials can bemachined by water jet and laser cutting, but these processesare limited to linear cutting only. EDM is more capable forthese materials for cutting complex shapes with highprecision [48, 50]. Nowadays, EDM is widely used formachining of materials including high alloy tool steels,conductive ceramics, germanium, alumina subtracts, andpolycrystalline diamonds with accuracy and finishing downto submicron range [47, 51–56].

2 Research in sinking EDM on metal matrix composites

2.1 SiC/aluminum matrix

Ramulu and Taya investigated machinability of 15 vol.%and 25 vol.% SiC whisker/2124 aluminum matrix (SiCw/Al) composites [57]. The material samples were cut atcoarse, medium, and fine conditions using copper and brasstools. It was found that material removal rate increases withincrease in power of electrode. MRR in 15 vol.% SiCw/2124 Al is >25 vol.% SiCw/2124 Al. Material removal rateobtained by using copper electrode is 5-10% less than thatof obtained when using brass electrode. Machining timeappears to be higher in 25 vol.% SiCw/Al than in 15%SiCw/Al composite. The micro-hardness tests on SiCw/Alcomposite have revealed that the machining causes surfacesoftening at slower cutting speed. It was also found thathigher cutting speed results in micro-damage in the surfaceand sub-surface area.

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However, in the study performance, measures wereevaluated only for variation in average current. Effects ofvariation of other parameters have not been taken intoaccount. Also, machinability of 25 vol.% SiCw/2124 Alwas evaluated only for brass electrode, whereas the secondsample material was evaluated for both brass and copperelectrode.

Next study was conducted on particle reinforced alumi-num matrix composite (SiCp/Al). Hung et al. in 1994investigated the feasibility of applying electrical dischargemachining process for cast aluminum MMC reinforced withsilicon carbide particles [49]. Statistical models of theprocess were also developed to predict the effect of processparameters on metal removal rate, re-cast layer, and surfacefinish. It was found that the presence of SiC particles resultsin reduced MRR. This is because these particles shield thealuminum matrix and protect it from being vaporized. Theunmelted SiC particles drop out from the compositematerial together with surrounding molten aluminumdroplets. While some aluminum droplets are flushed awayby the dielectric, others trap the loosened SiC particles thenre-solidify onto the surface to form a re-cast layer (RCL).No crack was found in the RCL and the softened heat-affected zone, which is below the RCL. It was reported thatMRR and depth of recast layer is mainly controlled byinput power and the current alone dominates the surfacefinish of machined surface.

The study is detailed in nature and particularly investigatesthe effect of variation in process parameters on performancemeasures. Explanation of formation phenomenon of recastlayer and its properties is additional information.

De Silva and Rankine studied the electro-erosioncharacteristic of SiC/Al and found that the Al matrixsurrounding the reinforcing particles was melted [58]. TheSiC particles were then dislodged from the matrix andflushed away by the dielectric fluid [59].

Hocheng et al. have made a preliminary study ofMRR of SiC/Al composite [60]. Heat conduction modelwas applied to interpret correlation between the majormachining parameters such as electrical current, pulseduration, and crater size produced by single spark for therepresentative material SiC/Al. Material removal charac-teristics of single and continuous discharge were investi-gated. The study starts from single discharge andrelationship between crater size and set discharge para-meters was found. Two heat conduction models were usedto calculate crater size formed in a single discharge andresults were compared with experimental data. Theexperimental results follow very well with finite stepsource heat conduction model. The material removal ratewas found proportional to the applied current and on-time.The crater size of SiC/Al is also compared with that ofsteel and found larger than that. In the continuous

discharge analysis, one finds the discharge of SiC/Al ismore irregular and the material removal rate is faster at thebeginning followed by being retarded due to the existenceof SiC particles in the gap. As closed-loop gap control isapplied. The material removal rate is greatly improved asexpected. For effective machining of SiC/Al, large currentand short on-time is recommended.

The research work is important in establishing correla-tion between theoretical and empirical results. However, thecomposition of composite is not mentioned in paper. Thuseffect of SiC percentage cannot be predicted.

Karthikeyan et al. in 1997 worked on mathematicalmodeling for electric discharge machining of aluminum–silicon carbide particulate composites [61]. They investi-gated the effects of the percent volume of SiC, current andpulse duration on the MRR, TWR, and SR. Also, theydeveloped the mathematical models to predict the samewithin the operating region. The analysis of the experimen-tal observations showed that the above-mentioned perfor-mance measures are greatly influenced by the percentvolume of SiC present in material, current, and pulseduration. The MRR increases with an increase in thecurrent and decreases with an increase in the percentvolume of SiC and the pulse duration. The TWR increaseswith an increase in the current and the percent volume ofSiC, but decreases with an increase in the pulse duration.The surface roughness increases with an increase in thecurrent, the percent volume of SiC and the pulse duration.Moreover, an optimal combination of the various processparameters was found in such a way that the TWR and thesurface roughness can be minimized and the MRR can bemaximized.

The paper finds the effect of variation in processparameters and composition of material on performancemeasures and also reports parameters optimization. It alsocontributes in theoretical modeling of the process for thematerial. However, EDM is a complex process whereseveral disciplines of science and branches of engineeringare involved in its theory. Therefore, it is difficult todevelop universal mathematical model for predictingperformance measures. Thus, the developed model is validonly within same operating region of experiment.

Muller and Monaghan presented details and results of aninvestigation into the machinability of SiC particle rein-forced aluminum matrix composites using different non-conventional machining processes such as electro dischargemachining, laser cutting and abrasive water jet [62].Objective of the research work was to investigate differencein surface quality, including surface roughness, surfacetopography, and sub-surface damage of machined work-piece. Furthermore, the effect of the reinforcement on themachining operation was investigated by performingcomparative tests on non-reinforced aluminum alloy sam-

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ples. The results obtained indicate that Al/SiC particlereinforced metal matrix composite (PRMMC) is machin-able by using same non-conventional machining processes.The findings show that EDM process is suitable formachining PRMMCs, but the process is very slow.Machining results in a crater-like surface. The size of thecraters increases with increased discharge energy. Also,relatively small amount of sub-surface damage is found onthe cut surfaces after machining (depending on the chosenmachining settings).

The research work focuses mainly on influence ofreinforcement on surface quality of machined material.Other performance measures have not been taken intoaccount in much detail. Since different machining processeshave different setup and different material removal mech-anism therefore resulted in different surface integrities.

Ramulu et al. carried out experimental investigation onthe effect of surface roughness generated by machiningprocess on mechanical properties of 15 vol.% SiCp/A336aluminum metal matrix composite [63]. Main objective wasto study the fatigue behavior of machined surface. Based onthe investigation, several conclusions were drawn. Surfaceintegrity effects include surface roughness caused bydeposition of recast layer, pitting of surface, surfacemicro-cracks, and slight material softening below recastlayer. The surface pitting is caused by spark penetration andparticle pollution. The yield and ultimate strength ofmaterial were found to reduce with increase in MRR.Vickers hardness tests revealed that material softening takesplace up to depth of approximately 200 µm below recastlayer. The direct physical effects includes high surfaceroughness and disruption of microstructure in form ofrecasting of matrix aluminum and pollution of siliconcarbide particulate which promotes the initiation of surfacecracks and eventual failure. The machining results inclearly observable surface damage of material, resulting ininferior surface integrity. Also, a rougher surface isproduced than polished one which in term contributes todecrease in fatigue strength of material by 15-20% for samelife at high cycle (Nf>10,000).

Prior to this study, little was known about fatiguestrength of EDM-worked MMCs. The thermal damagecaused by sparking results in imperfect surface integrity.Since fatigue strength is influenced by surface quality, itsimpact is needed to be investigated. There are very littlepublished work investigating mechanical properties ofEDM-worked material, and this research work is one ofthem.

Mohan et al. evaluated the machining feasibility of Al-SiCcomposites [64]. The objective was to investigate influenceof process parameters, electrode material, and volumepercentage of SiC particle on performance measures.Electrode polarity, discharge current, pulse duration, and

electrode rotation were taken as parameters. Al-20% SiC andAl-25% SiC composite samples were taken for study.Copper and brass were selected as electrode material. Severalconclusions were made based on experimental results. TheMRR was found high with positive polarity and increasedwith increase in current. It was more with brass electrode incomparison with copper electrode. The increase of either thepulse duration or volume percentage of SiC results indecrease in MRR and it increases with increase in rotationalspeed. The TWR was less when volume percentage ofsilicon carbide particle was less and increased with increasein current. The surface roughness value decreased withdecrease in pulse current and increased with increase involume percentage of SiC. The study optimized processparameters for different volume percentage of SiC andelectrodes for maximum MRR.

The research work was more and less similar to the workdone by Che Chung Wang and Biing Hwa Yan [65]although, in some aspect, it is different. For example,different material was selected for study and also perfor-mance measures were evaluated for different electrodedesign and two different electrode materials.

P. Narender Singh et al. worked on optimization by grayrelational analysis of machining parameters on machiningAl-10%SiCp composites [66]. The multi-response optimi-zation of the process parameters viz., metal removal rate,tool wear rate, taper, radial overcut, and surface roughnesson electric discharge machining of Al-10%SiCp as castmetal matrix composites using orthogonal array with grayrelational analysis was reported. Orthogonal array with grayrelational analysis was employed to optimize the multi-response characteristics of machining of Al-10%SiCp

composites. The experimental result for the optimal settingshows that there is considerable improvement in theprocess. The application of this technique converts themulti-response variable to a single-response gray relationalgrade and, therefore, simplifies the optimization procedure.

The research work is focused on parameter optimizationby applying gray theory. Gray analysis provides excellentsolution to uncertain, multi-input, and discrete data prob-lems. Since EDM process is of similar nature, therefore, themethod is highly useful in parameter optimization of suchexperimental work.

P. Narender Singh et al. in the same year worked on Al-10%SiCp as-cast metal matrix composites [67]. Theobjective of the work was to investigate the effect ofcurrent, pulse on-time and flushing pressure on metalremoval rate, tool wear rate, taper, radial overcut, andsurface roughness of machined material. Many conclusionswere drawn by experimentation. MRR was found higher forlarger current and pulse on-time settings at the expense oftaper, radial overcut, and surface finish. Electrode wear wasalso found to be higher, even larger than the material

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removal rate for larger current settings. The dimensionalaccuracy is affected at higher current and pulse on-timeratings. Both material removal rate and electrode wear areconsiderably influenced by flushing pressure.

Two more performance measures radial overcut andtaper were taken into account along with three majorperformance measures.

Mohan et al. investigated the machining characteristicsof SiC/6025 Al composite using rotary electro-dischargemachining with a tube electrode [68]. The objective ofresearch work was to investigate effects of dischargecurrent, pulse duration, SiCp percentage, tube electrodehole diameter, and speed of electrode rotation on perfor-mance measures. Also, mathematical modeling and para-meter optimization was performed for experimental results.Many conclusions were drawn from their study. The effectof drilling with the rotating tube electrode resulted in highermaterial removal rate than the rotating solid electrode. Theelectrode tube hole diameter significantly affects theperformance measures. The MRR and SR improve withthe decrease in hole diameter but electrode wear increases.The increase in volume percentage of SiC has resulted indecrease in material removal rate, surface roughness, andincrease in electrode wear. The increase in rotational speedof the tube electrode has produced higher material removalrate, electrode wear, and better surface quality. The materialremoval rate and electrode wear were higher for theinjection flushing than the side flushing.

The results obtained are similar in line with the previousstudy. In previous research works, it was reported thatelectrode rotation results in higher material removal ratedue to improved flushing condition. Also, tube electroderesulted in better performance measures than solid one.

Seo et al. in year 2006 investigated on machining offunctionally graded 15-35 vol.% SiCp/Al composites [69].The material was drilled to assess the machinability andworkpiece quality as a function of SiCp percentage,resulting dimensional quality of hole, surface topography,and metallurgical integrity of subsurface. Some aspects ofmaterial removal mechanism were also investigated. Basedon results, several conclusions were made. For producingmaximum MRR, optimum combination of the peak currentand pulse on-time was found. MRR showed an increasingtrend with increase in product of peak current and pulse on-time up to the optimum value and then decreases. Materialremoval rate also increases with increase SiCp percentage.Larger tool wear is reported with a combination of highpeak current and low pulse on-time. The average diametererror was found proportional to pulse on-time and peakcurrent. Higher energy results in rougher surface. Underrapid and coarse cutting condition, negatively skewedmachined surface was found. This was due to presence ofwell-defined discontinuous patches of recast material, deep

vacancies, and cracks on recast layer. At machined sub-surface layer, the fragmented and melted SiC particles werereported under scanning electron microscopy and energydisperse spectroscopy-X-ray testing.

Response surface models are developed in this researchwork to find the correlations between machining measuresand process parameters. No non-electrical parameter wastaken into account in this research work.

Sushant Dhar et al. worked on mathematical modelingof cast Al–4Cu–6Si alloy–10 wt.% SiCp composites [70].The objective of the work was to evaluate effect ofcurrent, pulse on-time and air gap voltage on materialremoval rate, tool wear rate, and radial overcut. Themathematical model developed can be used to predict theoptimal conditions suitable for machining of the worksamples. Linear programming was used to find theoptimum conditions for maximum MRR with reducedTWR and radial overcut. All the three performancemeasures increased significantly in a nonlinear fashionwith increase in current. The material removal rate andradial over cut were found to increase with increase inpulse duration. Gap voltage was found to have little, butsome effect on the three responses.

Al–4Cu–6Si alloy finds widespread application automo-tive industry. Therefore, research work may also haveindustrial application along with application for futureresearch work.

Akshay Dvivedi et al. investigated the machinability ofAl6063 SiCp metal matrix composite and obtained optimalsetting of process parameters [71]. The material wasdeveloped using melt stir-squeeze-quench casting routeand characterized for density, porosity and electricalconductivity. These properties may significantly affect thebehavior of material subjected to EDM. During pilotexperimentation, an optimal setting was found for maxi-mum MRR with acceptable electrode wear and satisfactorydimensional accuracy. Increase in electrical parametersbeyond optimum setting resulted not only in a decrease inMRR but also increased TWR with dimensional inaccuracy.Also, it was found that high value of pulse current, pulse-ontime, and pulse offsetting results in maximum MRR withinselected range of process parameters. The statisticalinfluence of pulse current on MRR is more than effect ofany other parameters. Also, machining with optimumflushing pressure and gap control setting will achievehigher material removal rate. The process variable affectingmaterial removal rate according to relative importance arepulse current, pulse offsetting, flushing pressure, pulse onand gap control setting. The optimum value of theseparameters was determined by experimentation.

The research work is related to influence of processparameters on performance measures and parameter opti-mization, however, the material selected for study is new

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for investigation on EDM and no previous published workis found for same.

2.2 Al2O3/Aluminum matrix

Yan and Wang investigated the machining characteristics ofAl2O3/6061Al composite using rotary electro-dischargemachining with a tube electrode [72]. They concluded thatthe machining process of the Al2O3/6061Al composite byEDM-drilling is feasible in comparison to other machiningprocesses. Other machining techniques such as water jetmachining, laser machining and wire EDM can be appliedbut these processes are mainly limited to linear cutting [65].Since the cost for using laser machining is generallyprohibitive and EDM wire-cut process is not appropriatefor a metal matrix composite workpiece due to excessivebreakage of the electrode wire, sinking EDM becomes anoptimal choice in the machining of Al2O3/6061Al compos-ite owing to its easy control in operation and precisecriterion of high complex-shape demand. The observedvalue of MRR with rotating hollow tube electrode is foundhigher than that of with solid electrode. Although the toolwear rate is also higher for hollow tube electrode, theoverall advantage still makes this revised technology anacceptable tool. The peak current and volume fractionsignificantly affect the MRR, TWR and SR. In contrast, therotating speed and flushing pressure of the electrode haveminor effects on the same performance measures. EDM-drilling with an optimized flushing pressure and electroderotation can achieve an approximately constant cutting feedrate. Also empirical expressions have been developed topredict the value of performance measures.

This research work investigates two innovative featuresthan previous papers on MMCs. These are electroderotation and change in electrode design by using hollowtube electrode. So, the paper opened new areas of focus forresearchers in field of EDM on MMCs.

Che Chung Wang and Biing Hwa Yan worked onoptimization of the blind-hole drilling of Al2O3/6061Alcomposite using rotary electro-discharging machining [65].Taguchi methodology was used for experimental design.Based on the results, several conclusions were made. Themachining process of the Al2O3/6061Al composite byblind-hole drilling is feasible. The blind-hole drilling witha rotational eccentric through-hole electrode results inhigher MRR and electrode wear. The overall performancefor this new EDM blind-hole drilling is still acceptable. Theelectrical parameters much significantly influence themachining process than the non-electrical parameters.The polarity of the electrode largely affects either theMRR or the SR, whereas the peak current mainly influencethe electrode wear. The increase of either the rotationalspeed of the electrode or the injection using pressure of the

dielectric fluid or the presence of two eccentric through-holes in the electrode may result in a higher MRR;however, the modification is still limited within a certainrange. Semi-empirical expressions have also been developedto simplify the evaluation of the material removal rate,electrode wear, and surface roughness under various machin-ing conditions.

The paper investigates influence of electrode rotationalong with other electrical parameters on performancemeasures. One new electrode design of eccentric through-hole electrode has also been tried. This was a new aspect ofthis research work.

2.3 Powder-mixed EDM

Kansal et al. have done an experimental study of themachining parameters in powder mixed electric dischargemachining of Al-10%SiCP metal matrix composites [73].The aim of this experiment was to establish optimumprocess conditions by an experimental investigation. Re-sponse surface methodology was used for experimentaldesign. They mixed aluminum powder into the dielectricfluid. A modified powder-mixed dielectric circulation wasdeveloped in the laboratory for experimentation. Relation-ships were developed between various input processparameters and output characteristics. The various inputparameters taken into account were concentration ofaluminum powder in dielectric, peak current, and pulseduration. Output characteristics taken into account weremachining rate and surface roughness. Based on experi-mental results, the most important parameters were decided.The parameters were also optimized for maximum machin-ing rate and minimum surface roughness. The recommen-ded optimal process conditions were verified by conductingconfirmation experiments.

The results of experimentations can be compared withthe results of previous research work with the same materialin sinking EDM to find effectiveness of powder mixeddielectric.

Singh et al. in 2008 have carried out a comparativemachinability study on stir-casted 6061Al/Al2O3P/20p workspecimens by using plain dielectric and silicon carbideabrasive powder-suspended dielectric fluid [74]. Copperelectrode was used for experimentation. They evaluatedmachinability in terms of surface roughness. The results ofboth the processes have been analyzed using Lenth’smethod to find the significant parameters and to obtainoptimum machining parameter settings. It was foundexperimentally, that abrasive particle size, abrasive particleconcentration and pulse current are the most significantparameters that affect the surface characteristics.

The research work is a comparative investigationbetween machining with plain and powder-mixed dielectric

618 Int J Adv Manuf Technol (2010) 50:611–624

on composite. It is important to determine the effectivenessof powder mixed electric discharge machining, but onlysurface roughness is taken into account as performancemeasure.

2.4 Other MMCs

One recent work by Ahamed et al. has been found onhybrid-type MMC [75]. Hybrid metal matrix compositesare a class of materials, having two or more discreteparticulate reinforcements. The objective of the researchwork was to investigate the effect of machining parametersnamely current, pulse on-time, pulse off-time, and flushingpressure on the material removal rate and surface roughnesswhile machining hybrid composites Al–5%SiC–5% B4Cand Al–5%SiC–5% glass prepared by stir casting. Theeffect of inclusion of B4C and glass on machining ofaluminum–SiC composite was investigated. Presence ofceramic particulate reinforcements impedes the machining.A trade-off has to be made between the levels of parametersfor achieving the combined objective of maximizingmaterial removal and minimizing surface roughness. Afairly long spark is required to remove material which hasembedded in it hard particles such as B4C and SiC. Longerspark duration is essential to remove the SiC and glassparticles which are, however, easily flushed away by thefluid at a fairly lower pressure of flushing. This is becauseof the lower density of glass when compared to B4C. Thewhite layer, which is a characteristic of machined surfaces,is seen prominently on both materials.

The research work is devoted to find the effect ofmachining parameters on material removal rate and surfaceroughness. Electrode wear has not been taken into accountin this research work. There is a lot of scope for futureresearch work in EDM of such kind of materials.

3 Research in wire EDM on metal matrix composites

Among the different material removal processes, wireelectrical discharge machining (WEDM) is considered asan effective and economical tool in the machining ofmodern composite materials [76]. But there is not so muchresearch work on MMC in this process [58, 77, 78].

Gatto and Iuliano performed WEDM tests under oneroughing and two finishing conditions on two composites.The materials selected for investigation were SiC/2009 Alalloy with 15% whiskers and with 20% particles reinforce-ment [79]. For understanding the reinforcement and thebehavior of the matrix during the machining of bothcomposites, the machined surfaces, their sections, andprofiles were examined by scanning electron microscopyand energy dispersive semi-quantitative analyses of X-rays.

From the experimental results, many conclusions weredrawn. The machining rates in millimeter/minute of bothcomposites were found equal. The roughness values of themachined surfaces of the 15%SiCw/2009 Al alloy compos-ite are less than the corresponding values for the surfaces ofthe 20% SiCp/2009Al alloy composite. The surface rough-ness values of the 20%SiCp 2009Al alloy composite WED-machined surfaces and then glass bead peened were foundless than the corresponding values for the I5%SiCw/2009Alalloy composite. SiC reinforcement and Cu precipitateswere not found in outer re-cast layer. The glass-beadpeening resulted in disappearance of the layer withoutreinforcement. The thickness of the outer layer withoutreinforcement was found less than 5 µm under finishingconditions and for both of the composites.

The paper specially focuses on surface quality ofmachined surface. Investigation into effect of particlereinforcement and whisker reinforcement is one of theimportant aspects of this research work. Another importantaspect is investigation into glass bead peening of machinedsurface.

Rozenek et al. reported experimental investigation forthe effects of machining parameters (discharge current,pulse on-time, pulse off-time, and voltage) on the machin-ing feed rate and surface roughness for machining of metalmatrix composite AlSi7Mg/SiC and AlSi7Mg/Al2O3 [80].The feed rate and surface roughness clearly follows theincreasing trend with increasing discharge energy. Theresult of investigation showed that for the input parametersof 1.6 µs pulse-on time, 8 µs pulse-off time and 80 V gapvoltage, the value of feed rate, and surface roughnessparameter Ra increases with increase in current. With thedecrease in voltage, the values of feed rate V and surfaceroughness parameter Ra drop slowly. Increase in pulse-ontime resulted in higher values of feed rate and surfaceroughness parameter. The maximum cutting speed ofAlSi7Mg/SiC and AlSi7Mg/Al2O3 composites are foundapproximately three times and 6.5 times lower than thecutting speed of aluminum alloy.

The machining rate of composite is significantly affectedby the kind of reinforcement. Therefore, the research workis important because it investigates the influence of type ofreinforcement on performance measures. However, theinfluence of percent volume of reinforcement cannot bepredicted.

Guo et al. investigated into shaping particles reinforcedmaterial by wire-EDM with high-traveling speed [81].Selected for experimentation was 6061 alloy with 20%Al2O3 particle reinforcement. The electrical parameterswere found to have little influence on surface roughness.The experiment resulted in coarse surface irrespective ofhigh energy or low energy is used. The selection ofelectrical parameters had an important effect on cutting

Int J Adv Manuf Technol (2010) 50:611–624 619

rate. Use of low energy resulted in wire breakage due toblind feeding especially for low pulse duration and lowmachining voltage. It was found that at high pulse duration,high voltage, large machining current, and at proper pulseinterval high machining efficiency can be attained.

The research work is based on comparative experimentsinvestigating machining mechanism and technique to shapethe material with WEDM and also optimize the electricalparameters.

Yan et al. investigated machining of Al2O3p/6061Alcomposite on WEDM [82]. The materials selected forexperimentation were 10 and 20 vol.% Al2O3 particlesreinforced 6061Al alloys-based composites and 6061Almatrix material itself. In experiments, machining parame-ters of pulse on-time were changed to investigate theireffects on machining performance, including the cuttingspeed, the width of slit, and surface roughness. Since thewire electrode breaks easily during the machining ofcomposite, therefore, this work also comprehensivelyinvestigates into the locations of the broken wire and thereason of wire breaking. Many conclusions were drawnfrom experimental results. The cutting speed for 6061Alalloy was found the highest than the two Al2O3P/6061Alcomposites and both composite materials yielded similarcutting speeds. Additionally, increasing the volume fractionof reinforcing Al2O3 particles resulted in wire breakage.Cutting of the 6061Al alloys and the 10 vol.% Al2O3P/6061Al composite resulted in smoother surface of work-piece, than cutting the 20 vol.% Al2O3P/6061Al compo-sites. The width of slit of the cut for 20 vol.% Al2O3p/6061Al composite was found much narrower than that ofthe 6061Al alloys and the 10 vol.% Al2O3p/6061Alcomposite. Protruding Al2O3 particles within the dischargegap impeded the advance of the brass wire along themachining path, resulting the brass wire to shift; therefore,clear bandings were found on the machined surface whenthe 20 vol.% Al2O3p/6061Al composite was machined.These bandings appeared under low wire tension cuttingconditions, but these features were generally not found onsurfaces formed under high wire tension. Increasing thepercentage of reinforcing Al2O3 particles deepened andwidened discharge craters of the wire surface, facilitatingthe wire breakage during machining. Particularly, whilemachining the 20 vol.% Al2O3P/6061Al composites, thewire electrode surface exhibits the largest craters, due toabnormal arc discharge. Softening due to high temperatureduring machining quickly reduces the tensile strength of thebrass wire, causing wire breakage. A very low wire tension,a high flushing rate, and a high wire speed are requiredparameters to prevent wire breakage for machining thesecomposite materials. Moreover, an appropriate servovoltage, a short pulse-on time, and a short pulse-off time,normally associated with a high cutting speed, does not

affect much on the surface roughness for compositematerials.

Wire breakage is a prominent problem in WEDM ofMMCs. So an important aspect of the research work was aninvestigation into wire breakage mechanism. The study isuseful for future wire electrode design with creep resistanceat high-temperature resulting in less breakage.

Patil and Brahmankar investigated the performance ofAl/SiCp composites with wire electro-discharge machining[83]. They studied the effect of various control parameterssuch as pulse on-time, off-time, ignition pulse current, wirespeed, wire tension, and flushing pressure on cutting speedand surface finish in WEDM of Al/SiCp composite.Taguchi method was used for experimental design. Basedon experimental results, mathematical models relating themachining performance and machining parameters weredeveloped. Optimal settings for each performance measurehave also been investigated. A comparative study onunreinforced alloy revealed the effect of reinforcement.Cutting speed for unreinforced alloy was found highercompared to composites. But surface finish in compositeswas found superior compared to the unreinforced alloy.Wire breakage posed limitations on the cutting speed ofcomposite.

The research work is related to influence of parametersand parameter optimization. It also contributes to theoreti-cal work by mathematical modeling of experimentation.

Manna and Bhattacharyya offered an experimentalinvestigation to determine the parameters setting duringthe machining of aluminum-reinforced silicon carbide metalmatrix composite (Al/SiC–MMC) [84]. On the basis of theexperimental results, several conclusions were drawn forthe effective machining of Al/SiC- metal matrix compositesby the CNC wire-cut EDM. Open-gap voltage was found asthe most significant influencing machining parameters, forcontrolling the MRR. Pulse-on period was second impor-tant influencing parameter. The open-gap voltage influencesthe cutting speed significantly. Wire tension and wire feedrate are the most significant and significant machiningparameters influencing the surface roughness. Similarly,wire tension and spark gap voltage setting were found asthe most significant and significant parameters for control-ling spark gap. Open-gap voltage and gap current werefound as the most significant and significant parameters,respectively, for gap current. Mathematical models weredeveloped and also verification tests for developed modelswere carried out. The test results were analyzed for theselection of an optimal combination of parameters for theproper machining of Al/SiC-MMC.

The research work investigated the relative importanceof parameters affecting different machining characteristics.

Saha et al. in 2009 worked on machinability of 5 vol.% TiC/Fe in situ metal matrix composite [85]. Pulse on-

620 Int J Adv Manuf Technol (2010) 50:611–624

time, pulse off-time, wire feed rate, and average gapvoltage were considered as input parameters. Cuttingspeed and kerf width were taken as measure of perfor-mance due to presence of TiC particles and formation ofFe2O3 while machining results in the unstable machiningprocess. Thus, the process modeling becomes extremelydifficult. In this research work, modeling of wire electrodischarge machining process by normalized radial basisfunction network (NRBFN) with enhanced k-meansclustering technique has been done. Both the models havebeen used to carry out the parametric study and werecompared with experimental results to measure theeffectiveness of this approach. From the experimentalresults and model predictions, several conclusions weredrawn. They found that an increase in the average gapvoltage results in decrease of the cutting speed but itresults in increase in kerf width within the range underconsideration. The pulse on-time had less effect on cuttingspeed and kerf width compared to the average gap voltage.Also, an increase in pulse on-time increases both thecutting speed and kerf width. Among several networkarchitectures, a 4–21–2 network was found as providingthe best prediction.

The research work investigated TiC/Fe composite whichis relatively new. The applied NRBFN technique hasseveral advantages like less complexity, requirements fewertraining samples, easy input-output mapping, and lesschance of getting local least convergence. So, the techniqueis suitable for such kind of study.

In 2009, Liu et al. studied the behavior of wireelectrochemical discharge machining of Al2O3 particle-reinforced aluminum alloy 6061 [86]. The influence ofmachining voltage, current, pulse duration, and electrolyteconcentration, on material removal rate were evaluated inthe light of the contribution of the wire electrical dischargemachining and electrochemical machining actions. Therelative strength of the WEDM and ECM activities inthe machining process under different conditions wasinvestigated with the aid of the voltage waveforms. Itwas found that the conditions of high current or highconcentrations of electrolyte would promote the ECMactivity and result in a high material removal rate.Orthogonal analysis was applied to study the relativeimportance of the various cutting parameters on MRR.The results suggested that for achieving the highestMRR, the applied current is the most influential amongcurrent, pulse duration, and electrolyte concentration.This outcome was supported by experimental results andis explained in terms of the surface area of the matrixphase and the spark gap size.

In this paper, authors compared two types of machiningprocesses with same material. Only material removal rate isconsidered as performance measures.

4 Discussion and future trends

After an elaborate scrutiny of the published work, thefollowing conclusions can be drawn.

& As evident from Fig. 2 most of the published researchwork has been carried out in sinking EDM process. Notso much work has been reported on wire EDM.

& Most of the published work belongs to SiC reinforcedmetal matrix composites. Not so much work is reportedAl2O3 reinforced and other MMCs types.

& Many useful MMCs have not been tried as work materialon EDM process. Table 1 gives the name of some MMCswith their applications which are yet to be tried.

& Very little work has been reported on MMCs with powdermixed EDM. Also, only aluminum powder and SiCpowder have been tried in MMCs. Many other powderslike boron carbide, powders of important alloyingelements such as manganese, chromium, molybdenum,and vanadium are yet to be tried in the dielectric medium.

& Most of the research work has been carried out onoptimization of process parameters for improvement ofperformance measures. In most of research work, mainlyelectrical process parameters and flushing pressure as non-electric parameter have been taken into account. Very littlework has been reported on effect of non-electricalparameters like workpiece rotation and electrode rotation.

& Only one research paper is there to find change inmechanical properties of EDM worked material. So,this area is still open for future research work.

29%

71%

Wire EDM

Sinking EDM

Fig. 2 Research studies conducted in EDM on MMCs

Table 1 Some important MMCs yet to be tried on EDM

S. No. MMC system Industrial application

1 Cobalt matrix with hardtungsten carbide particles

Carbide drills

2 Steel reinforced with boronnitride

Tank armors

3 Aluminum boron carbidematrix

Driveshaft

4 Monofilament silicon carbidefibers in a titanium matrix

F-16 fighting falcon(jet’slanding gear)

5 Al2O3-SiO2/AC4C Vane, Pressure side plate ofoil pressure vane pump

6 SiCw/7075 Joint of aerospace structure

7 Al2O3,CF/Al-alloy Cylinder liner

8 SiCw/Al-17%Si-4%Cu alloy Rotory compressor vane

Int J Adv Manuf Technol (2010) 50:611–624 621

& Many MMCs are yet to be explored for suitableelectrode material and electrode design since very littleresearch work has been reported in these areas.

& Little work has been reported on theoretical models forsimulating the input and output parameters for MMCson EDM processes. Much research work is needed inthis area in future.

& The research works on dry EDM and EDM using purewater as dielectric fluid is emerging area for the future.MMCs are yet to be explored in these areas.

5 Summary

A review of the research work on MMCs with sinking EDM,WEDM, and powder-mixed EDM is presented in this paper.The research work of the last 20 years has been discussed. Foreach and every method introduced and employed in EDMprocess, the objectives are the same: to enhance the capabilityof machining performance, to get better output product, and tohave better working conditions.

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