Effect of Subcritical Annealing on the Mechanical Anisotropy of Selective Laser Melting-Formed TC4 Alloys

In this paper, to investigate the mechanism of anisotropy generation of SLM-formed TC4 alloy by subcritical annealing, the compressive mechanical properties, microhardness, microstructure and crystal orientation of SLM-formed TC4 alloy were systematically studied. The results show that after subcritical annealing, a compression test was carried out at room temperature with a strain rate of 1 × 10⁻³ s⁻¹, while the compressive yield strength was 1003 MPa perpendicular to the deposition direction, and 947 MPa parallel to the deposition direction. The anisotropy of SLM-forming TC4 alloy was somewhat improved after subcritical heat treatment. Specifically, the difference between the yield strength in the XOY direction and YOZ direction is reduced to 5.58%, and the difference in microhardness is reduced to 1.74%. The subcritical heat treatment of SLM-formed TC4 alloy transforms the internal needle-like α′ phase into the bimodal organization of α + β phases, and the slaty α phase has a lower aspect ratio and is distributed in a staggered manner. This is mainly due to the gradual transformation of the columnar crystal grain boundaries into α-phase grain boundaries, the width of the lath-like α phase is larger than that of the needle-like α′ phase, and the effective slip length increases, among other reasons.