Tailoring the microstructure, tensile anisotropy and strength–ductility balance of laser melting deposited TA15 alloy by multi-step heat treatments

Laser metal deposited (LMD) TA15 titanium alloy typically exhibits high strength but limited ductility and pronounced tensile anisotropy. In this study, solution and aging treatment (STA), duplex annealing (DA), and three-stage heat treatment (THT) were systematically compared to clarify their effects on microstructural evolution and tensile behavior. Differential scanning calorimetry estimated the β-transus temperature to be approximately 997 °C. Under STA, increasing the solution temperature toward and above the β-transus promoted extensive β formation and subsequent α′ martensitic transformation during water quenching, leading to increased strength but reduced ductility. DA promoted α-lath coarsening and secondary α precipitation, thereby improving deformation compatibility and ductility with a moderate reduction in strength. THT produced a microstructure mainly composed of primary α laths and fine secondary α, while the second-stage cooling condition affected the local α-lath morphology and tensile anisotropy. The air-cooled THT condition achieved a yield strength of 942 ± 4 MPa, an ultimate tensile strength of 1067 ± 3 MPa, and an elongation of 13.27 ± 0.99% in the Z direction. Fractographic observations further indicate that the improved strength–ductility balance after DA and THT is associated with enhanced strain transfer and crack deflection by coarser α laths. These results provide a comparative basis for selecting post-deposition thermal routes for LMD-fabricated TA15.