A strain rate and temperature dependent model for describing nonlinear unloading stress-strain response after warm and hot compression deformation

For an accurate numerical simulation of springback behavior and control of shape and dimensional accuracies of the manufacture component under warm and hot precision plastic forming, the use of an appropriate material model dependent with strain, strain rate and temperature describing the nonlinear unloading process is vital, which is remained to be developed. The thermo-mechanical coupling effect in difficult-to-form material deformed at elevated temperature makes the unloading process more complicated and thus more difficult to model. In this paper, by using face-centered cubic (FCC) aluminum alloys as case study materials, loadingunloading experiments with different strains and strain rates at warm and elevated temperatures of 300 - 500 degree celsius were conducted, the effects and contributions of strain, strain rate and temperature on unloading chord modulus (elastic modulus) and nonlinear unloading stress-strain response were quantified. A unique strain-, strain rate- and temperature-dependent nonlinear unloading coupled model was established. By validating and applying the nonlinear unloading coupled model in the both monotonous loading-unloading and cyclic loadingunloading warm and hot compression deformation, the calculation results agree well with experimental results, indicating that the model can describe perfectly the nonlinear unloading stress-strain responds and springback behavior. From the discussion on calculated results and capabilities of model, it was found that the proposed unified equation incorporating strain rate and temperature is also able to be coupled with the existing strain-dependent chord modulus model such as modified Yoshida-Uemori (Y-U) model. The capability of predicting nonlinear unloading curves and springback strains of aluminum alloys deformed at various strains, strain rates, and temperatures demonstrates the potential applicability of the model to a wide range of warm and hot forming process conditions.