Pressure rate controlled unified constitutive equations based on microstructure evolution for warm hydroforming

Abstract

A viscoplastic model considering the influence of microscopic evolution and macroscopic deformation has been developed to represent the deformation behavior of material in warm sheet hydroforming process. Based on a hydroforming environment, a set of rate dependent constitutive equations, which is constructed by using the pressure rate, the evolution of dislocation density and kinematic isotropic hardening, has been proposed to predict stress–strain response of material. The non-linear Chaboche’s isotropic hardening criterion is also modified to characterize the instantaneous state of hardening considering microstructure evolution. Different from traditional sheet plastic forming process, the rate of fluid pressure variation is a significant factor that can determine the forming speed. Therefore, the pressure rate is applied to be one factor that can influence the deformation of material in warm hydroforming. The hydraulic bulge experiments on aluminum alloy at warm temperature indicated that the deformation behavior of material is sensitive to pressure rate. The genetic algorithm optimization technique was used to determine the optimum values of a set of free material constants associated with the proposed constitutive model. The computed data has been in a good agreement with the test data on the basis of the optimized material constants.