Microstructural Inheritance and Tensile Behavior of LPBF-Fabricated TA15 Titanium Alloy After Sequential Annealing

Laser powder bed fusion (LPBF)-fabricated TA15 titanium alloy commonly exhibits a fine acicular lath morphology, which has frequently been interpreted as martensitic in previous studies and is generally associated with high strength and limited plastic accommodation. In this study, LPBF-TA15 specimens were annealed at 800, 900, and 950 °C for 2 h, followed by furnace cooling, and subsequently subjected to secondary annealing at 550 °C for 4 h. Each sequentially annealed condition was compared with its corresponding single-step condition to distinguish retained microstructural differences from the tensile-property changes associated with the subsequent treatment. Microstructural evolution and monotonic tensile properties at room temperature and 300–600 °C were investigated. Annealing at 800 °C retained a relatively fine lamellar morphology. Increasing the initial annealing temperature to 900 and 950 °C produced progressively larger apparent lath and colony scales, with the most pronounced coarsening observed at 950 °C. Tensile results are reported as mean ± standard deviation. After secondary annealing, A800-S550 exhibited the highest mean room-temperature strength among the three secondary-annealed conditions, with a yield strength of 1045.0 ± 2.6 MPa, an ultimate tensile strength of 1116.7 ± 2.3 MPa, and an elongation of 14.3 ± 0.7%. From 300 to 600 °C, its yield strength decreased from 701.0 ± 3.2 to 493.6 ± 9.8 MPa, while its ultimate tensile strength decreased from 823.5 ± 3.8 to 585.6 ± 7.1 MPa; the elongation at 600 °C was 19.0 ± 1.8%. In this study, microstructural inheritance refers to the persistence, after the common 550 °C treatment, of differences in lath and lamellar-colony scales and EBSD boundary characteristics established during initial annealing. The secondary-annealed conditions retained distinct microstructural scales and exhibited different tensile responses; however, a unique causal relationship between the retained morphology and the magnitude of the property changes was not established. Residual stress, post-heat-treatment oxygen variation, and quantitative texture evolution were not independently evaluated. The conclusions are limited to the heat-treatment schedules and monotonic tensile conditions examined in this study.