Electrical discharge machining of TiNiCr and TiNiZr ternary shape memory alloys

This study investigates the influence of the machining characteristics on TiNiX ternary shape memory alloys (SMAs) using electro-discharge machining (EDM). Experimental results show that the material removal rates (MRRs) of Ti50Ni49.5Cr0.5 and Ti35.5Ni49.5Zr15 alloys in the EDM process exhibit a reverse relationship to the product of the alloy's melting temperature and thermal conductivity. The surface roughness (Ra) of the EDMed TiNiX alloys is found to obey the empirical equation of Ra = β(IP × τP)α. Having a less TΘ × KT value, Ti50Ni49.5Cr0.5 alloy has a larger Ra value than that of Ti35.5Ni49.5Zr15 alloy after electro-discharge machining. Besides, a lower discharge current IP and a shorter pulse duration τP should be selected to have a precise EDM machining of TiNiX SMAs. The hardening effect near the outer surface for EDMed TiNiX alloys originates from the recast layer. The thickness of the recast layer varies with the pulse duration and exhibits a minimum value at the maximal MRR. The EDMed TiNiX alloys still exhibit a nearly perfect shape recovery at a normal bending strain, but slight degradation of shape recovery occurs at a higher bending strain due to the constrained effect on the TiNiX matrix by the recast layer.