Effect of cyclic heat treatment on microstructure and tensile property of a laser powder-bed fusion-manufactured Ti–6Al–2Zr–1Mo–1V alloy

Abstract

Laser powder-bed fusion (L-PBF) is increasingly being employed to fabricate titanium components for aerospace applications. However, L-PBF fabricated titanium alloy parts typically exhibit high strength but low ductility because of fine α′ martensite. Post-heat treatment is essential to compose α′ into stable α, thereby achieving improved ductility. In this study, we put forward a novel post-heat treatment, i.e., a cyclic treatment that included multiple cycles of low-to-high and high-to-low temperature annealing, to improve the ductility of an L-PBF fabricated Ti–6Al–2Zr–1Mo–1V (TA15) alloy. Its effect on the microstructures and tensile properties was investigated. The results showed that the as-fabricated TA15 alloy exhibited fine martensite, leading to high strength (i.e., an ultimate tensile strength of 1168 MPa) and low ductility (i.e., an elongation of 7.1 %). Conventional post-heat treatments introduced lamellar α+β structures and resulted in improved ductility of 9–13.9 % and reduced strength of 896–1070 MPa. The cyclic multistep treatment created trimodal structures of α-laths, equiaxed α, and thin film β. Especially, it contributed to the formation of up to 57 % equiaxed α grains compared to less than 5 % in the conventional treatments, leading to the enhanced ductility. This specific structure displayed superior properties, an ultimate tensile strength of ∼900 MPa, and elongation of ∼16 %, which are comparable to the TA15 alloy wrought counterparts. The optimized strength and ductility also demonstrated the potential of cyclic multistep treatment to enhance the performance of other additive manufactured titanium alloys.