Microstructural evolution of a Ti-6Al-4V alloy produced by forging process combined with torsional motion

The present study identifies the microstructural conversion mechanism and the homogeneity of grain size evolution of the Ti-6Al-4V alloy produced by forging process in combination with torsional motion and alternating rotation of the lower die. Plastic flow behaviors under all forged conditions exhibit a steady state behavior which is indicative of the occurrence of continuous dynamic recrystallization (CDRX). A decrease of flow-stress of sigma(Z) by a ratio of approximately 0.3 is observed in the forging with torsional motion as compared to the uniaxial forging, which is almost good accordance with an estimation by the Mises yield criterion of Y-2 - sigma(-2) - sigma(2)(2) + 3 tau(2)(2 theta). The forging process combined with torsional motion results in an uniform strain distribution throughout the forged product, thereby leading to the enhanced CDRX as microstructural conversion mechanism. Regarding the effect of torsional two-ways motion on grain size evolution, the alternative rotation at the optimum rotation angle in a one pass leads to the enhanced CDRX and grain growth behavior. The pile-up immobile dislocations accumulated under forging can be operated again by the applying of opposite stress (accompanied by alternating rotation of the lower die). Hence, the forging process combined with torsional two-ways motion contributes to a higher accumulation of dislocations and a resultant enhancement of DRX and grain growth. Thus, this work can suggest the optimum forging process combined with torsional motion contributing to not only a reduction of forging load but also a homogeneous fine-grained microstructural formation throughout the forged product.