Microstructural optimization and mechanical enhancement of SLM Ti6Al4V TPMS scaffolds through vacuum annealing treatment

Ti6Al4V triply periodic minimal surfaces (TPMS) scaffolds having a porosity of 70% and pore size of 500-600 μm were deposited by selective laser melting (SLM), and the effect of vacuum annealing treatment (VAT) on its microstructural optimization and mechanical enhancement were explored. The results showed that only microscopic regular pore defects formed within SLM scaffolds, and VAT had no positive effect on reducing or eliminating porosity. However, VAT induced narrowly weak β(110) and β(200) diffraction peaks and transformed the fully acicular α`/α-martensites into long-rod-shaped α martensites within β matrix. Nanoindentation properties of VAT samples were slightly deteriorated, which is mainly caused by coarsened α and soft β phase formation in absence of acicular α`/α. In contrast with the negligible modified compression elastic modulus and maximum compression strength after VAT, the compression energy absorption capacity and tensile strength were enhanced by 14.5% and 17.3%, respectively. Tensile fracture position shifted from the bulk-lattice boundary with quasi-cleavage failure mechanism to the mid-position of lattice region with ductile fracture mode, exhibiting excellent tensile ductility with an increment of about 200% in elongation. Conclusively, VAT is an effective method to alter the brittle α`/α-martensites into the size-moderate α+β mixture, simultaneously improving the mechanical strength and ductility of Ti6Al4V TPMS scaffolds.