Strengthening and toughening of additively manufactured Ti-6Al-4V alloy by hybrid deposition and synchronous micro-rolling

To overcome the disadvantages of inhomogeneous microstructures and poor mechanical properties of additively manufactured Ti-6Al-4V alloys, a novel technique of hybrid deposition and synchronous micro-rolling is proposed. The micro-rolling leads to equiaxed prior beta grains, thin discontinuous intergranular alpha, and equiaxed primary alpha, in contrast to the coarse columnar prior beta grains without the application of micro-rolling. The recrystallization by micro-rolling results in discontinuous intergranular alpha via the mechanism of strain and interface-induced grain boundary migration. The evolution of alpha globularization, driven by a solute concentration gradient, starts from the sub-boundary until the formation of equiaxed primary alpha. Simultaneous strengthening and toughening are achieved, which means an increase in yield strength, ultimate tensile strength, fracture elongation, and work hardening rate. The formation of alpha recrystallization leads to more fine grain boundaries to strengthen the yield strength, and the improvement of ductility is due to the better-coordinated deformation ability of discontinuous intergranular alpha and equiaxed primary alpha. As a result, the fracture mode in micro-rolling changes from intergranular type to transgranular type.