Effect of volume fraction and unit cell size on manufacturability and compressive behaviors of Ni-Ti triply periodic minimal surface lattices

Ni-Ti shape memory alloys with triply periodic minimal surface (TPMS) lattice structures have aroused increasing interests due to their great application potential in bone tissue engineering. In this study, the effect of volume fraction and unit cell size on the manufacturability, compressive mechanical properties, and shape memory behaviors after compression of Ni-Ti Gyroid TPMS lattice structures, which were fabricated by laser powder bed fusion (L-PBF), were systematically investigated. A mathematical model was established to analyze the geometric influence factors on manufacturing fidelity of Ni-Ti Gyroid TPMS lattice structures fabricated by LPBF. It was revealed that the manufacturing fidelity decreased as the volume fraction and unit cell size decreased due to the aggravation of powder adhesion caused by the increase of specific surface area and overhanging rate of struts. Three power-law equations based on the Gibson-Ashby models were established to describe the dependences of compressive modulus, nominal yield strength, and ultimate strength of Ni-Ti Gyroid TPMS lattice structures fabricated by L-PBF on the volume fraction and unit cell size. All the above mechanical properties showed positive dependences on the volume fraction but negative dependences on the unit cell size. Both the volume fraction and unit cell size had little influence on the martensitic transformation temperatures as well as the total shape recovery ratio of Ni-Ti Gyroid TPMS lattice structures prepared by L-PBF. All the as-built Ni-Ti lattices exhibited high total shape recovery ratios of above 96.5% after 8% compressive deformation at room temperature (RT). But noteworthily, as the volume fraction increased, the pseudoelastic response (i.e., the recovered strain during unloading) rapidly decreased, followingly the recovered strain during heating increased. This was ascribed to the enhanced stability of stress-induced martensite at RT caused by the increased actual strain of local struts at the same overall compressive deformation in the Ni-Ti lattices with higher volume fractions. In contrast, the Ni-Ti lattices with different unit cell sizes exhibited similar strain distribution and consequently similar deformation recovery behaviors.