Surface Integrity Evolution from Main Cut to Finish Trim Cut 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. This study explores the process capability of W-EDM (DI-water based dielectric) in machining Nitinol Ni50.8Ti49.2 by one main cut (MC) followed by four trim cuts (TC). Experimental results show that the 6-sigma distributions of Ra are very different between MC and finish TC. Thick white layers (2-8 μm) with microcracks in MC and very thin white layers (0-2 μm) free of those defects in finish TC can be observed. However, microcracks would not propagate into the heat affected zone (HAZ) below the white layer. The white layer by TC is about 50% higher than that by MC. In addition, Ni is the dominant element for the measured microhardness.