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

Abstract

BÜ LENT EKMEKCICharacteristics 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 white layer thickness to an extent into the parent material, are mainly formed due to contraction of the recast structure joined to the circumferential edge of a crater rim during solidification. This type of crack is common when machining is performed in deionized water and the work material is brittle. Crack penetration depth is found to be proportional to the used pulse energy, and its path has a tendency to form parallel cracks to the machined surface at decreased pulse-on duration.

Metallurgical alterations in the surface of steel cavities machined by EDM

“…This layer is composed of several microscopic metallurgical layers, and its composition is dependent on the machining conditions [4]. The white layer is heavily alloyed with the pyrolysis products from the cracked dielectric liquid as well as tool electrode and has high hardness values.…”

“…Hence, the surface defects, particularly cracks, may reduce the material resistance to fatigue and corrosion, especially under tensile loading conditions. 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 [4]. If these defects are not avoided, the EDM surface needs to be subjected to EDM processes such as grinding and polishing to remove these subsurface defects.…”

“…On the other hand, the third type of microcracks is visualized usually around globular or irregularly shaped attachments on crater rims. Such cracks have very low penetration depth and are randomly distributed on the surface [4].…”

“…[3]. EDM also provides significant improvement in productivity and extends the range and sophistication of tools and dies that can be produced [4]. EDM is widely used for manufacturing of mechanical components made on hard metals such as high alloys, High-Speed-Steels and refract alloys [5].…”

“…It 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 slenderness ratios by using thin electrodes [4]. The absence of direct contact between the tool and the electrode also avoid process problems such as mechanical stresses and vibrations caused by conventional machining processes [6].…”