Microstructure evolution of Ti-6Al-2Zr-1Mo-1V alloy and its mechanism in multi-pass flow forming

Multi-pass flow forming processes are one of the major approaches to producing thin-walled titanium cylindrical parts with large cross-sections in which microstructure characteristics have a significant effect on the tailoring of desirable properties. In this study, the microstructure evolution of the Ti-6Al-2Zr-1Mo-1V alloy and its mechanism in a multi-pass flow forming process are systematically studied based on the deformation characteristics during the process. The results show that waved grains formed in the process are due to the compressive stress in the rolling direction (RD) and the non-uniform stress field in the deformation area. With the compression and shear deformation induced in multi-pass flow forming, non-standard {0002} basal plane texture is formed whose {0002} poles are tilted from the normal direction (ND) to the RD and the circumferential direction (CD) with an angle of 20° and 30° respectively. It is found that α grains are refined by two mechanisms, viz., continuous dynamic recrystallization (CDRX) and deformation-induced intense and localized shearing (DILS). CDRX mainly promotes secondary α grain refinement, while DILS has significant effects on primary α grain refinement under the larger deformation and on secondary α grain refinement during the entire process. Meanwhile, the grains refined by DILS with different orientations facilitate further deformation. This effect of grain refinement leads to improvement of the yield strength in the RD and the CD by 12.42% and 19.05%, respectively, compared annealed samples of the cylindrical part with the original one. These findings provide a basis for microstructure control and property tailoring of Ti-6Al-2Zr-1Mo-1V cylindrical parts in flow forming processes.