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

Micro laser powder bed fusion (mu-LPBF) additive manufacturing presents enormous potential in fabricating complex metallic micro-components (i.e. the component with a feature size < 100 mu m). 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 mu-LPBF prepared micro-components. The fabricated NiTi thin-wall structures not only achieve a minimum feature size of 52 mu m and low surface roughness of < 2 mu m, 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 mu m 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 mu-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 mu-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 mu-LPBF processing which typically employs only single-track scanning mode.