Effects of annealing temperature and cooling rate on microstructures of a novel titanium alloy Ti–6Al–2V–1.5Mo–0.5Zr–0.3Si manufactured by laser additive manufacturing

The microstructure changes of Ti–6Al–2V–1.5Mo–0.5Zr–0.3Si alloy manufactured by laser additive manufacturing (LAM) are systematically investigated with statistical analysis of primary α phase (α_p) and secondary α phase (α_s) under different annealing conditions. Results indicate that, with the increase in holding temperature, the content of α_p lamellas decreases with the increasing α_s content, maintaining the total α phases concentration stabilized. The width of α_p lamellas and the nominal specific surface area of α phase both exhibit positive correlation with the temperature, while the increment of α_p and the widths of α_s lamellas show an increase–decrease tendency. Besides, with the decrease in cooling rate, the total content of α and the width of α_p lamellas increase, while the nominal specific surface area of α phase shows no significant change. The results indicate that, in the annealing process, the holding temperature determines the surplus and growth interfaces of α_p lamellas, and the cooling rate influences the nucleation quantity of α_s in unit time. During the cooling stage, the α_p lamellas grow initially, and then, the nucleation and crab-like structure growth occur followed by those of the α_s lamellas. The time intervals among them are influenced by cooling rate. The mechanism of microstructure formation of the LAMed titanium alloy during annealing stage was discussed, which would guide for the heat treatment method to achieve required microstructure.