Development and processing of Ti-Ni-based shape memory alloys using laser melting techniques

This thesis focused on the development of Ti-Ni-based shape memory alloys using a novel alloy development process based on laser melting technique, suspended droplet alloying (SDA), and on development of a manufacturing route Ti-Ni-based structures using selective laser melting (SLM), specifically for Negative Poisson’s ratio (NPR) auxetic structures. To assess the SDA process, a series of Ti-Ni-based binary, ternary and quaternary SMA have been built to analyse the chemical and microstructural homogeneity of the builds produced by the SDA process. The shape memory performance in terms of the transformation temperature, thermal stability and thermo-mechanical stability was also investigated and compared with commercial SMA and literatures. It proved that the SDA built sample was comparable to the bulk SMA built by other alloy development processes. To assess the processability of Ti-Ni-based alloys, SLM process parameters were optimised for Ti-Ni SMA and the SMA NPR structure was built for mechanical testing. It was found that the SLM process parameters can severely alter the microstructure and shape memory properties due to different cooling rate and Ni vaporisation. Finally, TiNiCuNb and TiNiHfCuNb alloys were developed using SDA to improve the shape memory properties by altering the precipitates. The analysis of these alloys indicated that the addition of Cu, Hf and Nb altered the precipitate types and morphology altogether, resulting in a unique shape memory behaviour during thermal cycling