White Layer Composition, Heat Treatment, and Crack Formation in Electric Discharge Machining Process

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NTRODUCTION

TRADITIONAL manufacturing processes are increasingly being replaced by advanced techniques, such as electric discharge machining (EDM), for machining of engineering materials with high strength and hardness. Electric discharge machining provides significant improvement in productivity and extends the range and sophistication of tools and dies that can be produced. EDM can be described as a process for eroding and removing material by transient action of electric sparks on electrically conductive materials submerged in a dielectric liquid and separated by a small (~lm) gap. It is well known that the main mode of erosion is caused by the thermal action of an electrical discharge. The amount of heat generated within the discharge channel is predicted to be as high as 1017 W/m2 and, thus, could rise electrode temperatures locally up to 20,000 K even for short pulse-on (1 to 2000 ls) durations.[1] Therefore, melting, vaporization, and even ionization of the electrode material occur at the point where discharge takes place. When the pulse voltage ceases, the superheated molten cavities explode violently into the dielectric liquid and cool instantaneously, where all are vaporized and a fraction of melted material is flushed away by the BU¨LENT EKMEKCI, Assistant Professor, is with the Department of Mechanical Engineering, Zonguldak Karaelmas University, Zonguldak 67100, Turkey. Contact e-mail: [email protected] Manuscript submitted August 20, 2008. Article published online January 21, 2009. 70—VOLUME 40B, FEBRUARY 2009

dielectric liquid. Application of consecutive discharges results in a random superposition of craters formed by the discrete removal of the metal. The application area of EDM is not limited by the hardness or strength of material to be machined. EDM can be used to machine any conductive material. Since there is no mechanical contact between the electrode and the work material, it is possible to machine complex geometries with high aspect ratios by using thin electrodes. However, a heat-damaged layer is left on the machined surface due to rapid heating and cooling cycles. The structure of the heat-affected layer is different from the parent material. Although it is favorable in terms of improved wear resistance, defects such as voids, cracks, residual stresses, etc. result in an overall deterioration of the component mechanical properties. Components machined by EDM, such as tools and dies, are often subjected to severe cyclic pressure and temperature loadings. Hence, the surface defects, particularly cracks, may lead to shortened service life due to reduction in material resistance to fatigue and corrosion, especially under tensile loading conditions.[2–4] Therefore, surface cracks became a fundamental consideration when evaluating the performance of the EDM technique, and the prime objective of EDM must be to establish the conditions that suppress their formation.[5] The formation of surface cracks was attributed to the differentials of high contraction stresses exceeding the material’s ultimate