Abstract
The globularization behavior of ELI grade Ti-6Al-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 β annealing followed by different cooling conditions, were upset and stretched repeatedly at forging start temperatures of 940°C and 900°C, and subsequently air cooled. The microstructural changes after non-isothermal multi-step forging were analyzed with respect to globularization of α lamellae. After multi-step forging at 940°C, the initially thin lamellar structure changed to homogeneous equiaxed α globules, but elongated α globules with high aspect ratio were obtained from the initially thick lamellar structure. By forging at 900°C, the initially thin lamellar structure was changed to a mixed structure of both equiaxed and elongated α 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 900°C with both initially thin and thick lamellar structures was less effective compared to that by forging at 940°C. A quantitative microstructural analysis revealed that, in addition to the initial microstructure and forging start temperature, globularization of α lamellae was significantly affected by the instantaneous microstructural development by thermal fluctuation during non-isothermal multi-step forging.
