The Dependence of Properties on Structural Features in Macro/Micro Levels for PBF-LB Fabricated Porous NiTi Alloy

Porous NiTi alloys are widely used in biomedical and engineering fields. Using the PBF-LB technique, the TPMS and Voronoi porous parts with a porosity of ∼67% and a minimum feature size of 0.15 ± 0.02 mm were fabricated. Although the bulk sample fabricated with the same parameters is defect-free, μ-CT reconstruction on the porous sample revealed inevitable defects with a volume fraction of 0.23%–0.43% and sizes ranging from 15 to 65 μm. For the two structures, a significant increase in thickness of the horizontal walls or struts was observed, with obvious fluctuations at the bottom contours. The limited epitaxial growth of B2 grains due to heat conduction constraints and weakened crystallographic texture were observed. Considering the optimized low-temperature aging (450°C/1 h) that promotes the formation of a high-volume fraction of R phase leading to superior elongation of 18.8% in bulk sample, this treatment was applied to the porous structures to prevent premature fracture during compression. Based on compressive cyclic tests, the mechanical properties as well as the functional properties were investigated. Results revealed that the designed porosity indeed affects the ultimate strength and Young's modulus, but the spatial configuration as TPMS or Voronoi gives a limited influence on the recovery property for the porous NiTi alloy. The mechanical recoverable strain presently peaks at 4.2% when the applied compressive strain reached at 21.3%. This study provides theoretical evidence for tailoring structures at both macro and micro levels to achieve the desired mechanical and recovery properties for porous NiTi alloys.