White Layer Composition, Heat Treatment, and Crack
Formation in Electric Discharge Machining Process
BU¨ LENT EKMEKCI
Characteristics of electric discharge machined (EDM) surfaces of normalized, quenched, and quenched and tempered-treated steels in kerosene and deionized-water dielectric liquids are investigated. Optical microscopy, scanning electron microscopy (SEM) and X-ray diffractometry are employed to analyze the machined surface. Surface cracks are examined in terms of white layer composition, heat treatment of the workpiece material, and operational parameters used, such as average discharge current and pulse-on duration. The present results reveal that base material properties and white layer composition have a distinctive function on crack formation that results in different crack network layouts on the surface and penetration depths in the substrate. Surface cracks, which initiate at the surface, travel down perpendicularly toward the interferential zone, and terminate at this interference, are mainly formed due to an increase in nonhomogeneities of metallurgical phases within the white layer. Such cracks are usually encountered on the surfaces when machining is performed in a hydrocarbon-based dielectric liquid using high pulse-on duration and low average discharge current. On the other hand, penetrating cracks, which penetrate the entire
