Friction stir processing of Niti shape memory alloy: microstructural characterization

Abstract

Shape Memory Alloys (SMA) are metal materials that, after being strained, come back to their original shape at a designated temperature. Welding NiTi alloys is not simple because when the material is melted, due to the high reactivity of the alloy elements, the typical shape memory properties may disappear. The solid state welding process, such as friction stir welding, is thus attractive for SMA joining and it exhibits potentials for achieving welded joints affected by microstructural changes that preserve the shape memory properties. The present study investigates the feasibility of friction stir welding process to join NiTi shape memory alloys; in order to analyse the welding process, it has been determined the effect of the process on the functional properties of material. Microscopy observations of Friction Stir Processed (FSP) material have been used to highlight processed zone microstructures and the austenitic and martensitic transformation temperatures of the processed material were investigated using a stress applied method. A post processing thermal treatment has been applied in order to investigate the possibility to recover the partial losing of the shape memory capability of material subsequent the FSP.