Finite element analysis of thermomechanical behavior and residual stresses in cold flowformed Ti6Al4V alloy

Ti6Al4V alloy is very hard to flowform at room temperature due to its limited ductility. However, it retains excellent adiabatic heat trapping and thermal softening abilities, which can play an important role in improving the formability. In this paper, a 3-D finite element model of backward flowforming with three staggered rollers has been developed in Abaqus/explicit to study the thermomechanical behavior and the residual stress evolution. The model has been validated via flowforming experiments. The role of the heat generation due to plastic deformation is highlighted by comparing a thermomechanical analysis to a purely mechanical analysis without incorporating the additional heat input due to the plastic work. The maximum predicted temperature rise during cold flowforming is 911 degrees C, which significantly reduces the flow stress in the deformation zone. The two most important factors, which affect the temperature rise in the deformation zone, are friction coefficient and coolant heat transfer coefficient. Hence, a study has been done to assess the sensitivity of the thermomechanical behavior and the residual stresses towards these factors. The friction between the mating surfaces is helpful, but a friction coefficient higher than 0.1 causes through-thickness strain heterogeneity. An increase in the friction coefficient reduces the residual stresses, while an increase in the convective heat transfer coefficient causes a transition from compressive to tensile residual stress along the thickness of the tube.