Improving the anisotropic tensile properties of laser-directed energy deposition Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si titanium alloy with subcritical isothermal annealing

The laser directed energy deposition (LDED) process is marked by a low nucleation rate and rapid solidification speed, which tends to form a coarse columnar prior β -grain structure. Heat treatment serves as a crucial method for altering the grain structure of high-strength titanium alloys and mitigating their mechanical property anisotropy. In this work, the influence of subcritical isothermal annealing on the grain structure and anisotropic tensile properties was investigated in Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si titanium alloy fabricated by LDED. The results show that after subcritical isothermal annealing, the coarse prior columnar grains of the as-deposited samples are partially transformed into equiaxed grains. Its occurrence results from dislocation migration and subgrain boundary formation induced by subcritical isothermal annealing. Meanwhile, the anisotropy of mechanical properties has also been significantly improved. The tensile strength of the as-deposited specimens along the horizontal direction and the vertical direction were 1168.2 MPa and 1034.5 MPa respectively. After HT3 treatment, these values are 1043.1 MPa and 1043.2 MPa respectively, reducing the difference from 133.7 MPa to 0.1 MPa, improving the anisotropic tensile properties. This is caused by the sliding of α laths and the aggregation phenomenon of the α phase at the grain boundaries. Analyzing the sample's fracture surface reveals that its fracture mode has transformed from intergranular fracture before annealing to a mixed mode of intergranular and transgranular fracture after annealing. This study provides a foundation for eliminating anisotropy in heat treatment and advances the application of titanium alloy additive manufacturing in aerospace construction.