Effect of heat treatment on microstructure and mechanical properties of the selective laser melting processed Ti-5Al-2.5Sn α titanium alloy

In the present work, selective laser melting technology (SLM) has been used to produce Ti-5Al-2.5Sn α titanium alloy and the influence of post-deposition heat treatments on microstructure and mechanical properties of the as-deposited alloy has been in-depth analyzed for the first time. As-deposited Ti-5Al-2.5Sn exhibits a columnar prior-β microstructure with martensitic α′ needles filled inside and a small amount of grain-boundary α (αGB) stripes precipitated at the prior-grain boundaries. After subtransus heat treating at 600 °C to 750 °C for 2 h, incomplete recrystallization of the as-deposited microstructure occurs and the recrystallized α grains with equiaxed morphology nucleate mainly through grain boundary migration of the stripe-like αGB, leading to a duplex microstructure composed of the soft recrystallized α and 75.1-97.6% high-strength α′. After subtransus heat treating at 850 °C to 950 °C for 2 h, complete recrystallization of the as-deposited microstructure occurs and the coarsening of recrystallized α grains becomes more pronounced at 950 °C. By contrasting, an excessively coarsened widmanstatten microstructure composed of α laths with lengths up to several hundred micrometers has been obtained after supertransus heat treating at 1100 °C for 2 h. Both of the as-deposited and heat-treated samples exhibit relatively weak textures, resulting in very little mechanical anisotropies. Because of the dominant α′ microstructure, as-deposited possesses ultrahigh tensile strengths but also a poor elongation (7.7-8.2%) in comparison to the conventionally-fabricated Ti-5Al-2.5Sn. After the heat treatments, tensile strengths gradually decrease whereas elongation increases first and then decreases with the elevating annealing temperature, reaching its peak at 750 °C (14.4-15.2%). Finally, it has been proved that both elongations and tensile strengths of the 650-850 °C/2 h heat-treated samples can exceed those of the conventionally-fabricated Ti-5Al-2.5Sn. These findings provide some basis for the rapid fabrication of high-strength α titanium alloy products with a satisfactory ductility.