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

Abstract

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 (αp) and secondary α phase (αs) under different annealing conditions. Results indicate that, with the increase in holding temperature, the content of αp lamellas decreases with the increasing αs content, maintaining the total α phases concentration stabilized. The width of αp lamellas and the nominal specific surface area of α phase both exhibit positive correlation with the temperature, while the increment of αp and the widths of αs lamellas show an increase–decrease tendency. Besides, with the decrease in cooling rate, the total content of α and the width of αp 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 αp lamellas, and the cooling rate influences the nucleation quantity of αs in unit time. During the cooling stage, the αp lamellas grow initially, and then, the nucleation and crab-like structure growth occur followed by those of the αs 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.