Effect of build distribution and particle properties on the physical characteristics of laser powder bed fusion of in situ alloyed nitinol

The impact of powder flow characteristics on in situ nickel (Ni)-titanium (Ti) alloy formation within the laser powder bed fusion (LPBF) process is poorly understood. In this study, flow segregation patterns of Ni-Ti powder blends within the LPBF build chamber were examined and were found to be influenced by the substrate surface, build layout distribution, and particle size distribution. These segregation patterns significantly impacted relative density (RD) and elasto-caloric properties of in situ alloyed nitinol components, with regions of lower RD correlated with lower Ni content and higher phase transformation enthalpies than regions with higher Ni content. It was found that powder segregation rates between Ti and Ni particles were higher for rougher substrates, which also contained higher amounts of unmelted powder than smoother substrates. Furthermore, the position of the unmelted powder relative to the deposition arm sweep impacted powder segregation patterns throughout the build chamber. Powder segregation patterns in the LPBF deposition bed were also affected by differences in material density between Ni and Ti and interparticle cohesive forces. The insights gained from this work provide a route to achieving improved microstructural and chemical homogeneity of in situ alloyed nitinol, with tailored thermo-mechanical performance.