Effect of Build Distribution and Particle Properties on the Physical Properties of Laser Powder Bed Fusion In-situ Alloyed Nitinol

The impact of powder flow characteristics on in-situ nickel-titanium alloy formation within the Laser Powder Bed Fusion (L-PBF) process is poorly understood. In this study, flow segregation patterns of nickel-titanium powder blends within the L-PBF 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 and elasto-caloric properties of in-situ alloyed nitinol components, with regions of lower relative density correlated with lower nickel content and higher phase transformation enthalpies than regions with higher nickel content. It was found that powder segregation rates between titanium and nickel particles were higher for rougher substrates, which also contained higher amounts of unmelted powder compared to 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 L-PBF deposition bed were also affected by differences in material density between nickel and titanium 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.