Surface integrity evolution from main cut mode to finish trim cut mode in W-EDM of shape memory alloy

Shape memory alloys such as Nitinol are widely used in medical, aerospace, actuator, and machine tool industries. However, Nitinol is a very difficult-to-machine material due to the superelasticity, high ductility, and severe strain-hardening. The machined surface should have tailored micro texture to enhance cell adhesion. This study explores the process capability of W-EDM (DI-water based dielectric) in machining Ni50.8Ti49.2 by one main cut (MC) mode followed by four trim cut (TC) modes. Experimental results show that the 6-sigma distributions of Ra are very different between MC mode and finish TC mode. Thick white layers (2-8 μm) with microcracks in MC mode and very thin white layers (0-2 μm) free of those defects in finish TC mode can be observed. However, microcracks would not propagate into the heat affected zone (HAZ) below the white layer. The microhardness of white layer by TC mode is about 50% higher than that by MC mode. In addition, Ni is the dominant element for the measured microhardness.