Evolution of transformation behavior and tensile functional properties with process parameters for electron beam wire-feed additive manufactured NiTi shape memory alloys

Abstract

Electron beam wire-feed additive manufacturing (EBAM) has been shown to be an ideal method to additively manufacture NiTi alloys with good tensile functional properties. However, the influences of process parameters on the microstructure and thermomechanical response of EBAM processed NiTi alloys have not been revealed adequately up to now. In this study, the evolution of formation quality, microstructure, phase transformation behavior, and functional properties with the variation of process parameters were comprehensively studied for the NiTi alloys fabricated by EBAM. The results show that the process parameters may significantly affect the surface formation quality of the as-built NiTi alloy parts. All samples exhibit continuous and coarse columnar grains in the building direction in spite of the large variation of EBAM process parameters. One-stage A↔M transformation behavior was found for all as-built samples, while the phase transformation temperatures vary for different process parameters. The change in phase transformation temperature mainly results from different amount of Ni-evaporation, on which the beam power has the most significant effect. Most of the samples, which have low transformation temperatures, exhibit good tensile superelasticity after post heat treatment, while the sample with high transformation temperature shows good shape memory effect at as-built state. This study shows that by regulating process parameters, the EBAM can be used to fabricate NiTi alloys with different thermomechanical responses according to the demands of practical applications, while no need to change the raw material.