Effect of Tool Polarity on TiN Layer Formation in Dry Electrical Discharge Machining with Nitrogen Dielectric

Abstract

A hard white or recast layer forms during electrical discharge machining (EDM) due to rapid melting and re-solidification. Owing to its high quenching rates and altered composition, this layer often exhibits substantially higher hardness than the base material, offering opportunities for surface engineering applications. In this study, dry EDM was employed to develop a white layer consisting of titanium nitride (TiN) mixed in an iron matrix on an annealed H13 tool steel substrate. An additively manufactured Ti6Al4V was used as the tool electrode, with both polarities explored, and nitrogen gas was used as the dielectric medium. A layer thickness of approximately $11.4\pm 3.8\mu m$ was achieved under positive tool polarity, compared to $32.6\pm 11.7\mu m$ under negative polarity. However, the surface roughness ($S_a$) obtained with positive polarity was 45% lower than that under negative polarity. The titanium content in the white layer increased by 103% under negative polarity, demonstrating a strong polarity-dependent effect on material transfer. A maximum microhardness of $1063\pm 165{\mathrm{HV}}_{0.05}$, over four times that of the base material, was achieved in the white layer. X-ray diffraction (XRD) analysis confirmed the presence of distinct TiN phases in the machined samples that contribute to increased microhardness. Overall, this approach offers an environmentally friendly EDM route for generating ceramic-based surface layers, enabling the deposition of diverse functional white layers through the controlled selection of dielectric gases, tool materials, and polarity.