Enhancing work hardening and ductility in additively manufactured β Ti: roles played by grain orientation, morphology and substructure

A metastable β Ti alloy was additively manufactured by laser powder bed fusion (LPBF). Tensile testing along the build direction of the as-LPBF material (LPBF-0°) revealed significant work softening immediately following yielding with no uniform deformation. By contrast, substantial work hardening and uniform elongation well over 10% were achieved perpendicular to the build direction (LPBF-90°). Similar effects were obtained in the build direction after super transus heat treatment (LPBF-0°+HT) although the strength was slightly lowered. In addition, the yield drop phenomenon observed in both orientations of the as-LPBF materials disappeared after HT. The enhanced work hardening ability, and thus ductility, can be attributed to increased interactions of slip bands/slip bands owing to additional {112}<111> slip systems becoming operative in LPBF-0°+HT and LPBF-90° while LPBF-0° was dominated by {110}<111> only. The other variations after HT may be related to the coarsening of grain structure and removal of specific substructures in the as-LPBF microstructure.