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A Comparative Study on the Surface Integrity of Plastic Mold Steel due to Electric Discharge Machining BüLENT EKMEKCI, OKTAY ELKOCA, and ABDULKADIR ERDEN The violent nature of the electric discharge machining (EDM) process leads to a unique structure on the surface of a machined part. In this study, the influence of electrode material and type of dielectric liquid on the surface integrity of plastic mold steel samples is investigated. The results have shown that regardless of the tool electrode and the dielectric liquid, the white layer is formed on machined surfaces. This layer is composed of cementite (Fe3C) and martensite distributed in retained austenite matrix form- ing dendritic structures, due to rapid solidification of the molten metal, if carbon-based dielectric liq- uid is used. The intensity of cracking increases at high pulse durations and low pulse currents. Cracks on the EDM surfaces have been found to follow the pitting arrangements with closed loops and to cross perpendicularly with radial cracks and continue to propagate when another discharge takes place in the neighborhood. The amount of retained austenite phase and the intensity of microcracks have found to be much less in the white layer of the samples machined in de-ionized water dielectric liquid. The number of globule appendages attached to the surface increased when a carbon-based tool electrode material or a dielectric liquid was used during machining. I. INTRODUCTION ELECTRIC discharge machining (EDM) provides an effec- tive manufacturing technique that enables the production of parts made of hard materials with complicated geometry that are difficult to produce by conventional machining processes. The ability to control the process parameters to achieve the required dimensional accuracy and surface finish has placed this machining operation in a prominent position in industrial applications. The absorbing interest in EDM has resulted in great improvements in its technology, and it has become an important nontraditional machining process, widely used in aerospace, automotive, tool, and die industries. Electric discharge machining can be described as a process for eroding and removing material by transient action of elec- tric sparks on electrically conductive materials immersed in a dielectric liquid and separated by a small (~μm) gap. Thus, electrical energy in the form of short duration impulses with a desired shape is supplied to the electrodes. The required energy is usually in the form of rectangular pulses and can be generated by using spark generators designed for this pur- pose. When such a voltage pulse is applied to the electrodes, an electric spark discharge occurs within the interelectrode gap. It is well known that erosion on the electrode surfaces is mainly due to the thermal effect of an electric discharge. The charge induced on electrodes by a spark generator cre- ates a strong electric field. This field is strongest where the electrodes are closest to each other. Molecules and ions of dielectric fluid are polarized and oriented between these two peaks. When the dielectric strength of the liquid in the gap BüLENT EKMEKCI, Assistant Professor, is with the Mechanical Engi- neering Department, Zonguldak Karaelmas University, 67100 Incivez/ Zonguldak, Turkey. OKTAY ELKOCA, Research Engineer, is with the Research and Development Center, Eregli Iron and Steel Work Co., 67330 Krd. Eregli/Zonguldak, Turkey. ABDULKADIR ERDEN, Professor, is with the Manufacturing Engineering Department, Atilim University, 06836 Incek/ Ankara, Turkey. Manuscript submitted March 4, 2004. exceeds a natural limit, a low resistance discharge channel is formed due to the electron avalanche striking the anode and cathode. This collision process transforms kinetic energy in the form of heat and pressure. The amount of heat generated within the discharge channel is predicted to be as high as 1017 W/m2 and, thus, could raise electrode temperatures locally up to 20,000 K even for short pulse durations.[1] Therefore, melting, vaporization, and even ionization of the electrode materials occur at the point where the discharge takes place. No machin- ing process is known where similar high temperatures can be obtained in such small dimensions. The pressure increase in the plasma channel forces expansion discharge channel bound- aries and decreases the current density across the interelectrode gap. Most of the time, the pressure increase is so high that it prevents evaporation of superheated material on both electrode surfaces. When the pulse voltage ceases, a sharp decrease in the channel pressure triggers a violent erosion process. The superheated molten cavities explode violently into the dielectric liquid. Finally, the surfaces cool instantaneously, where all vaporized and a fraction of melted material in the form of irregularly shaped or hollow spherical particles is flushed away by dielectric liquid. The net result is a tiny crater on both sur- faces of the electrodes, where the remaining part of the melted material has splashed on it. Applying consecutive spark dis- charges with high frequencies and driving one electrode toward the other erode the work piece gradually in a form complemen- tary to that of the tool electrode. A clear characterization of electrodischarge machined surface topography is essential to predict the quality and func- tional behavior of surfaces.[2] Saito[3] tried to define the relation between the shape of a single discharge crater and the dis- charge conditions. He found that the interelectrode gap dis- tance causes the diversity of the size of crater made by the discharge. Lloyd and Warren[4] have shown that the anode craters take the form of a circular depression independent of crystal orientation and characterized by a raised circumfer- ential lip resulting from the upheaval of metal during the liquid dispersion time. In addition, they found that the crater diameter is approximately constant for the same spark condition. The METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, FEBRUARY 2005—117 cathode craters, on the other hand, were not found to be truly circular but tend to reflect the symmetry of the crystal faces on which they occur. Greene and Alvarez[5] used a profilometer imaging technique to accurately measure the volume of the electrode craters on different electrode materials produced by EDM. They showed the effects of high pressure generated during sparking on craters with illustrating radial flow lines near the rim. Radhakrishnan and Achyutha[6] have found, by using the relocation technique, that the general appearance of the craters formed is almost the same for different materials, except for their size and depth. They reported a well-defined ridge and considered that this was due to the deposition of the molten material from the crater. Wong et al.[7] worked on a micro EDM, which has a single spark generator, and found that the shapes of the craters are more uniform with a better defined rim at lower energies (?50 μJ) in contrast to irregular diameters at higher levels. A practical EDM surface is a random superposition of craters formed by the discrete removal of metal due to con- secutive discharges. Various experimental results and empir- ical models of surface finish for different operation types and conditions have been published.[7–25] It has been observed that there are many process variables that effect the surface finish such as peak current, duration of current pulse, open voltage gap, electrode polarity, debris concentration, thermal properties of the tool electrode, work piece, and dielectric liquid. Generally, the power functional trend of curves, rep- resenting an increase in surface roughness with respect to increased pulse energy, was presented. Large roughness val- ues can be explained by the generation of large craters due to high energy levels. A great deal of effort has been made to improve EDM accuracy and surface roughness when using this process as ultra precision machining. The material removal is due to electrostatic force acting on the metal sur- face when short pulse duration is applied. In this case, sur- face roughness values (Ra) less than 0.2 μm are possible and a mirrorlike surface can be obtained.[26,27,28] Studies on various machined surfaces with electron micro- scopy[2,5,10,11,14,22,23,29,30] showed that the surface is observed with globules of debris and chimneys formed by entrapped gases escaping from the redeposited material. Evidently, the surface is frozen, virtually instantaneously, when the discharge ceases. However, the shapes of the pockmarks, and partic- ularly their rims, are indicative of their sudden and simulta- neous rupture, coinciding with the sharp decrease in pressure as the discharge is cut off. Another feature on electrical discharge machined surfaces is the abundance of microcracks. The amount of thermal energy created and the conductivity of the work piece deter- mine the cracking behavior of the machined surface. Cracks formed due to thermal stresses in a single discharge tend to follow the pitting arrangements created in the surface by EDM. They normally form closed loops, instead of crossing the material’s surface.[31] Residual stresses are generated since the melted material contracts more than the unaffected parent material during the cooling process, and cracks are developed when the stress in the surface exceeds the mate- rial’s fracture strength[22,29,32] Earlier studies on electric discharge machined surfaces on pure iron and ferrous alloys revealed a nonetchable white cov- ering layer, which is far harder than the base material. Irregu- lar signs of splashing and alloying effect from the electrode material were found on the surface of the white layer.[2,4,33–35] This observation gives a sense of how the electrode material affects the work piece surface quality. So, it was considered that this alloying effect could be used to enhance the surface quality, such as by reducing residual stresses by a suitable source of alloying element.[2,4,35] The hardness value was found to be high when compared with the hardness value obtain- able by quenching.[4] This layer was observed under all machining conditions, including when water was used as the dielectric material.[2,4,33,34] Lloyd and Warren[4] obtained a fused outer zone consist- ing of dendritic austenite and a cementite-austenite eutectic (ledeburite structure of a hypoeutectic white cast iron), when machining with a graphite electrode and in paraffin dielectric under severe conditions, or a fully austenitic surface followed by an austenite-cementite matrix, when machining with a copper electrode under less severe conditions. Optiz[33] reported a hypereutectic recast layer in hot forging steel. Massarelli and Marchionni[36] reported a similar structure of carbides in an austenite matrix, but stated that different elec- trodes do not change the morphology of the white layer; only the ratio of the carbide and the austenite phases varies. However, Simao et al.[24] have reported an increase in white layer hardness when employing powder metallurgy (PM) green compact and sintered TiC/WC/Co electrodes during electric discharge texturing (EDT). They used glow discharge optical emission spectroscopy (GDOES) to analyze surface enrichment/depletion of the modified/alloyed EDT roll sur- faces, and observed that Ti and W contained in the PM elec- trodes together with C decomposed from the dielectric fluid during sparking were transferred to the AISI D2 roll surface. Similarly, Tsai et al.[37] have reported Cu and Cr migration to the machined surface from Cr/Cu based composite elec- trodes. Rebelo et al.[14] reported a severe increase in carbon intensity of the surface as 9 times greater at the surface than the bulk material by microprobe analysis. Ghanem et al.[23] also detected enrichment in carbon and hydrogen in the outer layer by GDOES depth profiling. An increase in carbon content in the surface and subsurface layers has been attributed by most workers to the pyrolysis of the dielectric, but others have suggested that carbon is assimilated more rapidly from graphite electrodes than from carbonaceous dielectric. Thomson[29] has concluded that carbon was absorbed from the dielectric rather than from the electrode. The near-surface hardening is more important in the austenitic structure than in the ferritic structure due to the solubility of carbon in the fcc structure.[23] Rebelo et al.[14] and Kruth et al.[38] have shown that Fe3C cementite was formed on the surface of martensitic steels, whereas Cabanillas et al.[39] have found two different regimes of carbide formation: s-carbide, austenite, and martensite for sparks of energy below 0.5 J; and cementite, austenite, and traces of marten- site, Fe7C3, or Fe5C2 for higher spark energies on the pure iron in hydrocarbon dielectrics. Lim et al.[40] managed to visualize the recast layer by using unconventional metallographic reagents and showed a variety of microstructures; as a result, they categorized these observations into three main groups according to recast layer thickness. The first type was found to be around 20 to 50 μm and has a multiplayer structure made up of over- lapping layers of similar microstructures. The second type was found to range between 10 and 20 μm and is largely 118—VOLUME 36B, FEBRUARY 2005 METALLURGICAL AND MATERIALS TRANSACTIONS B columnar and dendritic in nature. The last type was found to have a thickness less than 10 μm and to be fairly resis- tant to etching. Thus, it could not be described and is named as featureless. In most cases, a thermally affected layer was found beneath the recast layer.[2,4,33,36,41,42] It is partly affected by carbon drawn by the dielectric. This layer generally has a tempered microstructure. The hardness value of this layer is often found to be less than that of the underlying hardened material. In a number of studies, an intermediate layer between the recast and the tempered layers has also been observed.[2,4,33,36] This layer was found to exhibit a carbon gradient and contami- nation of materials from the tool electrode. It is possible that this layer includes part of the melted layer plus a region beyond which diffusion has occurred in solid state under Table I. Composition of the Plastic Mold Steel (Weight Percent) Material C Cr Mn Mo Ni Si DIN 1.2738 0.38 2.0 1.5 0.2 1.1 0.30 Topographic examinations were performed with a JEOL* *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo. JSM-5600 Scanning Electron Microscope (SEM). Samples were prepared using conventional metallographic techniques on cross sections, in which thermally affected layers can be observed normally with an Olympus* metallographic micro- severe machining condition. The thickness of the thermally affected layer increases proportionally with respect to dis- charge energy. This layer contains a high density of second- phase particles, which are larger in size and more rounded than the carbide particles in the parent material.[11] The hard- ness of this layer is found to be comparable to or, sometimes, slightly higher than that of the recast layer.[40] A zone of plas- tically deformed material has been reported[41] for single-phase materials, which do not undergo complex phase transforma- tions during EDM. This plastically deformed layer has been found to be from a few tens to a few hundred micrometers in *OLYMPUS is a trademark of Japan Olympus Co., Tokyo. scope. These sections were etched with nital reagent in order to reveal thermally affected zones. Microhardness depth pro- file measurements were made on a Future-Tech* FM-700 *Future-Tech is a trademark of Japan Future-Tech Co., Tokyo. hardness tester using a Vickers indenter with a load of 10 g and an indentation time of 15 seconds. X-ray diffraction pat- terns were obtained with a Shimadzu* XRD-6000. Data were thickness in the underlying metal. Cleavage and grain bound- ary cracks, penetrating into the underlying material, have been observed in brittle materials under severe machining conditions.[4,11,33] The bulk of the material beyond these zones remains unaffected by machining. Technological advances have led to an increase in the usage of high-strength, high-hardness materials in manufacturing industries. Thus, the use of this process has increased in recent years since it has the capability of machining hard materials with complicated forms as fine slots and microholes. How- ever, fracture and fatigue failures generally nucleate at or near the surface of the component, and the frequency of surface defects reduces the strength of the material due to the rapid heating and cooling effects induced by the machining process. These properties determine the resultant operational behavior of the machined parts. In this study, the influence of electrode material and type of dielectric liquid on the surface integrity of plastic mold steel samples is investigated. II. EXPERIMENTAL PROCEDURE Plastic mold steel (DIN 1.2738) samples were stress relieved prior to EDM to ensure stress-free condition. They were heated to 600 °C for 1 hour and cooled slowly. One of the surfaces was electric discharge machined with a FURKAN* EDM 25 industrial machine on a rectangular *FURKAN is a trademark of Turkish Furkan Technologies Co., Istanbul. working area of 10 × 50 mm. The generator produced rectan- gular pulses at average currents of Iav = 1, 2, 4, 8, and 16 A and at durations tp = 6, 12, 25, 50, 100, 200, 400, 800, and 1600 μs. Commercial kerosene and deionized water were used as the dielectric liquids. Copper and graphite were selected as the tool electrodes. The chemical composition of the sample material is given in Table I. *SHIMADZU is a trademark of Japan Shimadzu Co., Kyoto. collected using Cu Ka radiation (h = 1.5405) in the range 10 ? 20 ? 120. The phases were identified from searches in the JPDS (Joint Committee on Powder Diffraction Stan- dards) database. III. RESULTS A. Surface Topography It is well known that the surface roughness is a function of released energy, which is controlled by power supply set- tings. High peak current and long pulse duration produce a rough surface. Conversely, it is also true that lower peak current and pulse duration produce a finer surface, since each pulse removes a small quantity of material proportional to the energy of the pulse from the electrode. Scanning elec- tron micrographs (Figures 1 and 2) show that an electric dis- charge machined surface observed with overlapping craters, globules of debris, and chimneys formed by entrapped gases escaping from the redeposited material. The effect of dielectric liquid and tool electrode on sur- face topography is not clearly stated in the literature. Only a small variation in surface roughness has been reported. Surfaces produced under similar operating conditions by using different dielectric liquid and toll electrode material combinations (Figures 1 and 2) have shown that the topo- graphical features of the surfaces change with respect to the number of globular or irregularly shaped appendages that are attached to the crater rims. No or few appendages could be observed when copper is used as the tool electrode and deionized water as the dielectric liquid (Figure 1(a)). Chang- ing the tool electrode material with graphite resulted in an increased number of such appendages (Figure 1(b)). The METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 36B, FEBRUARY 2005—119 (a) (b) Fig. 1—SEM pictures of electric discharge machined plastic mold steel surfaces, Iav = 16 A, tp = 25 μs. Dielectric liquid: deionized water, electrode: (a) copper and (b) graphite. surface has been found to be densely infiltrated with such features when