Surface modification of TiNi-based shape memory alloys by dry electrical discharge machining

Abstract

This study attempts to decrease the martensite transformation temperature of Ti50Ni50 shape memory alloy (SMA) for its use in biomedical applications by Cr addition. In addition, surface modification of Ti50Ni50 and Ti50Ni49.5Cr0.5 SMAs using electrical discharge machining (EDM) is proposed. Nitrogen gas is used as a dielectric medium, and a pure titanium pipe is used as the tool electrode. The machining characteristics and surface properties of Ti50Ni50 and Ti50Ni49.5Cr0.5 SMAs after EDM in nitrogen gas were investigated. Many electrical discharge craters and recast materials are observed on the EDMed surface of these SMAs. Material removal rate, electrode wear rate, and surface roughness increase with increasing pulse current and duration, and they share an inverse relationship with the thermal conductivity of these SMAs. After EDM, the SMAs continue to exhibit good shape recovery, and even the recast layers have high surface hardness. The recast layers, comprising TiN and CrN, with high hardness and good adhesion are expected to improve the SMAs’ wear and corrosion resistance.