Micro laser powder bed fusion of NiTi alloys with superior mechanical property and shape recovery function

Micro laser powder bed fusion (µ-LPBF) additive manufacturing presents enormous potential in fabricating complex metallic micro-components (i.e. the component with a feature size <100 µm). However, such fabricated parts often exhibit poor essential mechanical and functional properties. In this work, through rationally scaling down and coupling key processing parameters, various NiTi micro-components such as thin-wall structures and thin-strut lattices/stents are fabricated, which exhibiting superior manufacturing qualities and mechanical/functional properties to reported µ-LPBF prepared micro-components. The fabricated NiTi thin-wall structures not only achieve a minimum feature size of 52 µm and low surface roughness of < 2 µm, but also exhibit comparable tensile properties and better shape memory effect than conventional LPBF NiTi. The fabricated NiTi micro-lattices/stents with a strut diameter of ≤ 100 µm is shown to sustain up to 50% compressive deformation without mechanical failure and exhibits > 98% shape recovery upon heating. This study also reveals that across the thin wall, the µ-LPBF NiTi possesses unique melt pool morphology consisting of fan-shaped grains in the middle and near-vertical grains on side shoulders. At the microstructure level, it contains Ni₄Ti₃ precipitates of generally smaller size than and dislocations of comparable volume density to the conventional LPBF NiTi. Besides, the µ-LPBF NiTi exhibits a wider phase transformation peak width and lower transformation latent heat than the conventional LPBF NiTi. These characteristics are resulted from the weak thermal cycle history experienced by the material during µ-LPBF processing which typically employs only single-track scanning mode.