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

Globular a 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 a phases in alpha+beta titanium alloy Ti-6Al-4V fabricated by laser aided additive manufacturing (LAAM). However, there is no prior research reporting methods for obtaining globular alpha phases in LAAM-built near-a titanium alloys. This work investigated the cyclic heat treatment (CHT) procedures suitable for the LAAM-built near-alpha titanium alloy Ti-6Al-2Sn-4Zr-2Mo (Ti6242) to attain the globular alpha phases. The results show that 980 degrees C is the most suitable upper temperature limit for CHT. However, it is difficult to achieve a high volume fraction of the globular alpha phases in the LAAM-built Ti6242 alloys through CHT, which is ascribed to the low composition gradient caused by more a-stabilizing elements and fewer beta-stabilizing elements. The as-built sample demonstrated elongation of 6.3%, which is lower than the AMS 4919J standard (elongation >= 10%). After 980 degrees C CHT and 980 degrees C CHT with solution heat-treatment, the formation of the globular alpha 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 exhibited a mixed brittle and ductile fracture behavior, while the 980 degrees C CHT and 980 degrees C CHT with solution heat-treated samples displayed ductile fracture.

Automated summary

Cyclic heat treatment (CHT) has been found effective in inducing the formation of globular alpha phases in titanium alloys, improving their ductility. This study investigated suitable CHT procedures for LAAM-built near-alpha titanium alloy Ti-6Al-2Sn-4Zr-2Mo to obtain globular alpha phases. The results showed that 980 degrees C is the most suitable upper temperature limit for CHT. However, it is difficult to achieve a high volume fraction of globular alpha phases in the LAAM-built Ti6242 alloys due to their composition gradient.