Research on surface microstructures and properties of NiTi shape memory alloy after wire electrical discharge machining

Abstract

This paper used wire electrical discharge machining (WEDM) to machine Nickel-titanium shape memory alloy (NiTi-SMA). Since NiTi-SMA is very sensitive to temperature, the surface of WEDM will form an altered-metal zone (AMZ), which affects the material performance. For investigating the effects of WEDM on the surface microstructures and properties, the method of experimenting clarified the thermal damage mechanism of NiTi-SAM surface formed by WEDM. After WEDM, an obvious three-layer structure (spattered layer, recast layer, and heat-affected layer) was formed on the machined surface, the grains in the recast layer were refined, and the microhardness of the recast layer was significantly improved, the formed hardened layer inhibits the deformation recovery of NiTi-SAM. For machining defects, multi-cutting methods are used to reduce the impact on the machining of NiTi-SAM. Results show that increasing the cutting passes reduces the size of craters and hard convex edges, producing a smoother surface, the roughness Ra value was reduced from 2.79 µm to 0.12 µm. After main cutting, the spattered and recast layers' thickness reached 18.16 µm, and many microcracks appeared in the AMZ. After the fifth trimming, the thickness of the recast layer gradually decreased to less than 0.3 µm. Due to the excellent surface state and enough thinned recast layer, the sample exhibited good corrosion resistance and shape recovery capability. Experimental results provide a reliable processing method for applying NiTi-SMA in micro-electro-mechanical systems and biomedical fields.