Effect of cyclic heat treatment on the microstructure and mechanical properties of laser aided additive manufacturing Ti-6Al-2Sn-4Zr-2Mo alloy

Abstract Globular α phases can significantly improve the ductility of titanium alloys. Cyclic heat treatment (CHT) has been proved to be an effective way to induce the formation of globular α phases in α+β titanium alloy Ti-6Al-4V fabricated by laser aided additive manufacturing (LAAM). However, there is no prior research reporting methods for obtaining globular α phases in LAAM-built near-α titanium alloys. This work investigated the cyclic heat treatment (CHT) procedures suitable for the LAAM-built near-α titanium alloy Ti-6Al-2Sn-4Zr-2Mo (Ti6242) to attain the globular α phases. The results show that 980 °C is the most suitable upper temperature limit for CHT. However, it is difficult to achieve a high volume fraction of the globular α phases in the LAAM-built Ti6242 alloys through CHT, which is ascribed to the low composition gradient caused by more α-stabilizing elements and fewer β-stabilizing elements. The as-built sample demonstrate elongation of 6.3%, which is lower than the AMS 4919J standard (elongation≥10%). After 980°C CHT and 980°C CHT with solution heat-treatment, the formation of the globular α phases significantly increased the elongation to 13.5% and 12.9%, respectively. Although the mechanical strength is reduced after heat-treatment, the room-temperature tensile properties still exceed the AMS 4919J standard. Fractography examination showed that the as-built sample exhibit a mixed brittle and ductile fracture behavior, while the 980°C CHT and 980°C CHT with solution heat-treated samples displayed ductile fracture.