On the effect of the complexity of the constitutive model in simulating superplastic forming

Accurate constitutive material models are essential for the realistic simulation of metal forming processes. However, for superplastic forming (SPF), mostly the material models found in literature are based on the fitting of the simple power law equation. In this study an investigation was carried out on the effect of the complexity of the constitutive model on the accuracy of the SPF simulation results. This was achieved by following an experimental-numerical investigation of the SPF of the AZ31B magnesium alloy. High temperature bulge forming tests and microstructural analyses were carried out to generate the data required to fit two different constitutive models. The first is the simple power law. The second model takes into account grain growth and cavity formation in addition to the strain and strain rate hardening. The two models were then implemented in the simulation of SPF of a car-shaped geometry and the results were compared with those obtained from actual forming experiments. Results show that both models are capable of predicting the thickness distribution and the shape of the formed part to an acceptable degree. However, the more complicated model shows a better capability in predicting the forming time required to achieve the part geometry.