Simultaneous improvement of strength and elongation of laser melting deposited Ti-6Al-4V titanium alloy through three-stage heat treatment

Abstract

This study reported a strategy of three-stage heat treatment to simultaneously improve yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of laser melting deposited (LMDed) Ti-6Al-4V titanium alloy . After the three-stage heat treatment, the basketweave microstructure consisting of acicular α laths in the as-built sample was transformed to a multiscale-α microstructure consisting of the coarse-plate α, fine-plate α and ultrafine α in the heat-treated sample. The effect of microstructure on the tensile properties was revealed via in-situ tensile test combined with the analysis of the deformation and fracture mechanism . Although the coarse-plate α in heat-treated sample is larger than the acicular α in as-built sample, the combination of multiscale α grains with a certain relative content aroused an increment of the YS from 879 ± 28 MPa in as-built sample to 973 ± 9 MPa in heat-treated sample. The good deformation compatibility among the multiscale α grains impeded the crack propagation effectively, which resulted in the improvement of the EL from 14 ± 2% to 16 ± 1%. Furthermore, the improvement of the EL enhanced the dislocation multiplication and strain hardening in the multiscale α microstructure to induce the increment of the UTS from 945 ± 40 MPa to 1092 ± 20 MPa. The results could provide a guidance for tailoring microstructure and improving the strength and ductility of additive manufacturing titanium alloy.