Effect of Heat Treatment on the Microstructure and Mechanical Properties of Medical-Grade ELI Ti64 Alloy Additively Manufactured by Laser Powder Bed Fusion

Medical-grade extra-low-interstitial Ti64 alloy (ELI Ti64) is a promising material for biomedical implants, and additive manufacturing facilitates the fabrication of complex implant geometries. However, additively manufactured ELI Ti64 typically exhibits insufficient ductility, necessitating heat treatment to tailor its microstructure and properties. In this work, we selected three temperatures near the α′-phase decomposition point (800 °C, 850 °C, 900 °C) to investigate how closely spaced heat treatment temperatures affect the microstructure and properties of manufactured ELI Ti64 alloy via laser powder bed fusion (LPBF). The manufactured ELI Ti64 alloy consisted mainly of acicular α′ martensite with only 0.3% of the β phase. Annealing promoted the α′ → α + β transformation. At 800 °C, α′ decomposed only partially; at 850 °C, α′ fully decomposed into lamellar α and short rod-like β; at 900 °C, the α laths coarsened and the β-phase content increased to 4.6%. The average α/α′ grain size increased from 1.71 to 2.48 μm, corresponding to approximately 45.03% coarsening. With increasing annealing temperature, strength decreased while elongation increased. The LPBF-manufactured ELI Ti64 alloy exhibited a yield strength of 1004 MPa, an ultimate tensile strength of 1054 MPa, and an elongation of 10.4%. The product of strength and elongation (PSE) increased from 10.96 GPa·% for the LPBF-manufactured condition to 13.4, 19.7, and 20.9 GPa·% after annealing at 800, 850, and 900 °C, respectively. The 900 °C/2 h + furnace cooling treatment achieved the best strength–ductility combination for tensile behavior, with an 89.60% improvement in PSE compared with the LPBF ELI Ti64 alloy. These results demonstrate that even within a narrow annealing temperature window, the microstructure and mechanical properties of LPBF-manufactured ELI Ti64 can be effectively tailored.