Controlling martensitic transformation characteristics in defect-free NiTi shape memory alloys fabricated using laser powder bed fusion and a process optimization framework

Laser Powder Bed Fusion (L-PBF) was utilized to fabricate fully dense, near-equiatomic (Ni50.1Ti49.9) and Ni-rich NiTi (Ni50.8Ti49.2) shape memory alloy (SMA) parts which exhibited tensile ductility up to 16%, shape memory strain of 6%, and tensile superelasticity up to 4%. Annealing heat treatments marginally improved the superelasticity of the as-fabricated Ni-rich NiTi parts due to the formation of a small volume fraction of Ni4Ti3 precipitates. The selection of optimum processing parameters that yielded fully dense parts was guided by a process optimization framework based on a computationally inexpensive analytical model used to predict the melt pool dimensions. The framework also included single-track experiments to validate the model predictions and a criterion for the maximum allowable hatch spacing to prevent the formation of lack of fusion porosity. This framework allowed for constructing L-PBF processing maps for the present NiTi SMAs and revealed that fully dense parts could be printed over a wide range of process parameters. By controlling the L-PBF process parameters, in particular laser power, laser scan speed, and volumetric energy density, in the processing space that result in fully dense parts, it was demonstrated systematically that the composition of the printed parts could be precisely changed by controlling the evaporation of Ni. The flexibility of parameter selection to print defect-free NiTi SMAs and composition control by preferential evaporation of Ni opens the possibility to print functional NiTi SMA parts or devices without post-processing.