Comparative Study on Electrical Discharge Machining of Ultrafine-Grain Al, Cu, and Steel
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UCTION
ULTRAFINE-GRAIN (UFG) materials are lightweight and high-strength materials with a grain size below 1 lm produced through severe plastic deformation (SPD) methods.[1] These materials undergo extreme strain during the SPD process, resulting in locking dislocations and grain refinement.[1] This leads to an increase in the yield strength, hardness, corrosion resistance, and diffusivity, while the ductility and elongation decrease.[1–4] Nevertheless, the ductility can be improved by a heat treatment process.[5] Equal channel angular pressing (ECAP) is a widespread and appropriate method among SPD processes. In this method, the billet is extruded through two equal intersected channels (ECAP die) undergoing large strain. This results in grain refinement of the billet. Figure 1 shows the schematic of ECAP die. According to the Iwahashi survey, the internal (F) and external angles (W) of the ECAP die determine the total equivalent strain, which are imposed on the billet.[6] ECAP has no restriction in producing UFG metallic billets, and a large number of different alloys can be fine grained by the ECAP process.
MOHAMMAD SAJJAD MAHDIEH, Ph.D. Candidate, and RAMEZANALI MAHDAVINEJAD, Associate Professor, are with the School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran 1417466191, Iran. Contact e-mails: [email protected], [email protected] Manuscript submitted March 16, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
Manufacturing of industrial parts, such as a turbine blade[7] and automobile components, out of UFG material has become prevalent in recent years. On the other hand, to manufacture industrial parts precisely with high surface quality, applying the machining process such as burnishing,[8] milling, grinding, and electrical discharge machining (EDM) are essential. EDM is an electrothermal process, eroding the surface of the workpiece by successive electrical discharges.[9] These sparks that occur between tool and workpiece make a very high-temperature area, melting and evaporating the surface of the workpiece.[10] Due to the high-temperature discharges, a melt pool is created on the surface of the workpiece, in which a portion of molten material is flushed away by dielectric liquid and the remaining portion solidifies and forms the recast layer. The recast layer is very hard and brittle, which decreases the fatigue strength of the workpiece due to its high cracks density. Furthermore, the existence of the recast layer reduces the surface quality. Therefore, many attempts have been made to decrease the thickness of the recast layer and its undesirable effects. Decreasing the pulse energy results in decreasing the recast layer thickness and the micro-cracks.[11,12] Applying metallic powder-mixed dielectric liquid such as aluminum and tungsten also improves the surface quality.[13–15] Using cryogenically cooled electrode enhances the surface integrity as well.[16–18] Research shows that producing the tool electrode via powder metallurgy technique increases the surface quality.[1
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