Effect of Post-Heat Treatment Process on the Microstructure and Mechanical Properties of TA15 Titanium Alloy Fabricated by L-PBF

TA15 titanium alloy fabricated by Laser Powder Bed Fusion (L-PBF) exhibits high strength but poor ductility due to its fine acicular α′ martensitic microstructure. This study systematically investigates the effects of post-annealing treatments (800–950 °C for 0.5–4 h) on the microstructural evolution and mechanical performance of L-PBF-built TA15. Results show that with increasing temperature and time, the metastable α′ martensite decomposes into a progressively coarser lamellar (α + β) structure. This transformation leads to a decrease in strength and hardness but a significant improvement in ductility, with elongation increasing from (8.5 ± 0.5)% (as-built) to (19.4 ± 1.1)% (900 °C/2 h) as the ultimate tensile strength (UTS) decreased from (1100 ± 29) to (895 ± 37) MPa. However, annealing at 950 °C, which approaches the β-transus temperature, induces a coarse Widmanstätten structure in the alloy. Although this structure yields a relatively high elongation (23.8 ± 3)%, it also leads to excessive strength loss, with an ultimate tensile strength of only (833 ± 23) MPa, rendering it less desirable for structural applications requiring high load-bearing capacity. Moreover, such coarse lamellar structures are generally associated with poor fatigue resistance, as cracks tend to propagate along prior β grain boundaries. An optimal strength-ductility synergy is achieved by annealing at 900 °C for 0.5 h, yielding an ultimate tensile strength of (951 ± 13) MPa and an elongation of (18.8 ± 1.7)%. These findings provide crucial guidance for tailoring the mechanical properties of L-PBF-fabricated TA15 alloy through post-processing heat treatments.