Development of microstructural heterogeneities and dynamic restoration activity during ring rolling of Ti-6Al-4V alloy and its tensile response

Ring rolling is one of the advanced metal forming processes to produce seamless rings. Being a bulk forming process, inhomogeneity in the distribution of strain and temperature can be anticipated in the ring cross-sections. This, in turn, is expected to influence the evolution of microstructure and correspondingly the mechanical properties. Processing of Ti-6Al-4V alloy is equally challenging because of its poor thermal conductivity and formability with a limited deformation window. Ti-6Al-4V preforms were rolled at elevated temperature under two different main roll feed rates to obtain approximately 30 % deformation in the lab scale ring rolling facility. The evolution of microstructure, including crystallographic texture, was examined as a function of feed rate at different locations of the rings using electron backscattered diffraction (EBSD) technique, and subsequently the tensile response of the rings at selected locations was evaluated at room temperature. The effect of feed rate was found to be marginal on the microstructure evolution; however, there were substantial differences in grain sizes and texture evolution in the regions from inner to outer side of the ring cross-section. Coarse grains were seen at centre region of the ring, while fine grains were present in the region near inner diameter of the ring. Surface regions which experienced more deformation developed basal texture, while the interior regions possessed shear texture after ring rolling. Detailed analysis of the EBSD results suggested orientation dependent continuous dynamic recrystallization as the restoration mechanism in the alloy during ring rolling. The effect of feed rate on the tensile response of the ring rolled alloy was found to be negligible; however, the tensile strength of the ring varied with the location, in accordance with the microstructural heterogeneity. This study provides proper basis for quality control and better understanding for technical guidance required for designing the ring rolling process, specifically for Ti-6Al-4V alloy.