Laser powder bed fusion of Ti-rich TiNi lattice structures: process optimisation, geometrical integrity, and phase transformations

The use of Laser Powder Bed Fusion (LPBF) in fabricating TiNi-based lattices enables tailoring the mechanical and physical properties of the structure, in addition to the functionality associated with the shape-memory effect. In this work, TiNi lattice structures were fabricated using LPBF, following an optimisation study for LPBF parameters investigating the geometrical integrity of the lattices, microstructural evolution, and phase transformation behaviour. A process map for TiNi lattices was constructed to visualise the influence of LPBF parameters on the build density, elemental evaporation, and impurity pick-up. The optimum LPBF processing window was found to be ∼60–90 J/mm ³ volumetric energy density, achieving >99% relative density. Optimisation of the geometrical integrity of the LPBF-fabricated lattices, including the pore and strut sizes, was performed by considering the influence of the laser track width (LTW), beam compensation (BC), and contour distance (CD). As a result, when CD-BC = LTW/2, the deviation in strut size from the target design size was reduced to <2%. The build microstructure was affected by LPBF parameters, where the dendritic cell size increased with the increase in LPBF heat input, also resulting in a change in the solidification structure morphology from cellular to columnar dendritic structures. The phase transformation behaviour was investigated using Differential Scanning Calorimetry to understand the effect of LPBF parameters on the formation of Ti ₂ Ni intermetallics and impurity pick up (oxygen and carbon), and the resulting impact on the phase transformation temperatures.