A micro-structural model for prediction of void initiation in superplastic forming

Superplastic forming has been used to produce complex parts without local thinning. Cavitations that occur during this procedure can reduce the quality of products. In this paper, a model is proposed to predict void initiation in superplastic forming based on the study of dominant micro-mechanisms. For this purpose, a constitutive model is proposed that considers micro-mechanisms of superplastic forming including: grain interior deformation, grain boundary sliding and grain boundary diffusion. The possibility of void initiation around a second phase particle is evaluated by calculating relative velocity between two adjacent grains and mass transportation around the particle due to diffusion. The proposed model is calibrated for an Aluminum alloy at 500℃ and 550℃. Then the model is used to predict void numbers in various strains and strain rates at these temperatures. Model predictions are in excellent agreement with experimental data. It is shown that grain boundary sliding has a significant role in void initiation of superplastic materials.