Globularization Behavior of ELI Grade Ti-6Al-4V Alloy during Non-Isothermal Multi-Step Forging

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 fromannealing 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 � lamellae. After multi-step forging at 940 � C, the initially thin lamellar structure changed to homogeneous equiaxedglobules, but elongated � 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 elongatedglobules, 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 oflamellae was significantly affected by the instantaneous microstructural development by thermal fluctuation during non-isothermal multi-step forging.