Effect of wire-EDM on fatigue of nitinol shape memory alloy

Electrical discharge machining (EDM) is advantageous in machining high strength and high-aspect-ratio components. However, EDM-induced thermal damage would detrimentally reduce fatigue life of critical components, such as biomedical Nitinol implants. As such, this paper focuses on the relationship between surface integrity and fatigue life of the wire-EDMed Nitinol samples. Surface integrity of Nitinol samples machined at main cut (MC) can be significantly improved by subsequent trim cuts. Nitinol samples at finish trim cut (TC) show lower surface roughness and thinner white layer than those at MC. The fatigue testing results show that the samples made by finish TC have 48% more fatigue life than those by MC. The examination of the fractography shows that fatigue cracks initiate from microvoids in the thick white layer. In addition, a thinner white layer or less tensile residual stress benefits fatigue performance, and would lead to longer fatigue life.