Study on recast layer thickness and residual stress during WEDM of SMAs

Shape memory alloys (SMAs) are difficult to machine by thermoelectric processes due to the affinity of titanium (Ti) for oxygen, and the specific properties of SMAs have been taken into account for the design of SMA actuators. In this investigation, wire electro-discharge machining of a specific SMA is investigated with specific reference to the recast layer thickness and residual stress state. The effects of processing parameters such as pulse on time, pulse off time and servo voltage on the recast layer thickness and residual stress state were evaluated. A scanning electron microscopy investigation on the recast layer showed that its thickness was a strong function of pulse on time and servo voltage, with the thinnest recast layer commensurate with high servo voltage and short pulse on time. The residual stress state was found to follow closely the recast layer thickness – that is, a thick recast layer implied a large tensile residual stress and vice versa. In conclusion, it was demonstrated that a short pulse on time combined with a high servo voltage produced the lowest residual stresses and had the least impact on the surface and subsurface properties of the material.