Achieving strength-ductility synergy in a laser-powder bed fused near-a titanium alloy through well-crafted heat treatments

The design of a two-step heat treatment (solutionizing and aging) protocol for laser-powder bed fused Ti-6Al2Sn-4Zr-2Mo (L-PBF-Ti-6242) and its effect on microstructure and mechanical properties are studied. The heat treatment is designed considering the kinetics of & alpha;' to & beta; transformation and nanotwin annihilation with an emphasis on preserving the unique, ultrafine, and hierarchical microstructure. The solutionizing temperature of 900 degrees C for 10 and 20 min is selected based on the & alpha;' to & beta; transformation percentage. The aging temperatures of 300 degrees C and 350 degrees C for 12-72 h are chosen considering the kinetics of nanotwin annihilation. A total of 15 conditions are evaluated from these solutionizing and aging parameters including as-built, only solutionized, solutionized-aged, and direct aging. The as-built microstructure has the highest strength and lowest ductility due to the ultrafine acicular & alpha;' martensite and dense dislocation network. The solutionized microstructure has & alpha;/& alpha;'and & beta; phases with different dislocation densities. This condition results in the lowest strength and highest ductility, governed by the presence of the bcc phase (& beta;), and high mean effective slip length in the & alpha;/& alpha;' phase. The aging process (following the solutionizing step) results in changes in dislocation substructure in the & alpha;/& alpha;' phase which leads to an increase in strength, controlled by a reduction in mean effective slip length. The formulated two-step heat treatment leads to the best strength-ductility synergy in this study.