Study on the NiTi shape memory alloys in-situ synthesized by dual-wire-feed electron beam additive manufacturing

In this study, NiTi shape memory alloys were in-situ synthesized by dual-wire-feed electron beam additive manufacturing (dual-wire EBAM), which uses electron beam as energy source working in the vacuum environment, and uses pure Ni and Ti wires as feedstock. Microstructures, chemical composition distribution and mechanical properties of the as-built NiTi alloys were studied. Results showed that the as-built NiTi part exhibited continuous coarse columnar grains all along the building direction. Uniform composition distribution was obtained away from the substrate in the upper and middle regions of the part, which consisted of dominant NiTi phase and exhibited typical one-stage martensitic phase transformation. Good static compressive property with an ultimate compressive strength of 2.9 GPa and a fracture strain of 34.2% were obtained. The as-built NiTi samples also demonstrated good shape memory response, which had recovery rates of 88%, 83%, 81%, 80% and 63% for pre-compressive deformation strains of 2%, 3%, 4%, 5% and 6%, respectively. This study showed that the dual-wire EBAM has great potential to in-situ synthesize NiTi alloys with typical thermomechanical response and to flexibly control the chemical composition and corresponding functional performance of target NiTi alloy components by regulating the feeding speeds of Ni wire and Ti wire independently.