Globularization behavior of ELI grade Ti-6Al-4V alloy during non-isothermal multi-step forging

Abstract

The globularization behavior of ELI grade Ti-6AI-4V alloy having different initial lamellar structures during multi-step forging under non-isothermal condition was investigated. The samples with either a thin or thick lamellar structure, which were produced from P annealing followed by different cooling conditions, were upset and stretched repeatedly at forging start temperatures of 940C and 900C, and subsequently air cooled. The microstructural changes after non-isothermal multi-step forging were analyzed with respect to globularization of alpha lamellae. After multi-step forging at 940C, the initially thin lamellar structure changed to homogeneous equiaxed alpha globules, but elongated a globules with high aspect ratio were obtained from the initially thick lamellar structure. By forging at 900C, the initially thin lamellar structure was changed to a mixed structure of both equiaxed and elongated alpha globules, while severely deformed lamellae were preserved inside distorted colonies with little globularization for the initially thick lamellar structure. Globularization of the samples by forging at 900C with both initially thin and thick lamellar structures was less effective compared to that by forging at 940C. A quantitative microstructural analysis revealed that, in addition to the initial microstructure and forging start temperature, globularization of alpha lamellae was significantly affected by the instantaneous microstructural development by thermal fluctuation during non-isothermal multi-step forging.