Quick prediction of thinning in stretch forming of hat-shaped profiles during multi-stage hot sheet metal forming

Abstract

A novel quick model predicting the thinning distribution in hot stretch forming of hat-shaped profiles as a building block for a closed-loop control of a multi-stage forming process operated at a stroke rate of up to 12 strokes/min (5 s per stroke) is presented. The key characteristic of this new model is that it considers both a spatially as well as time-variant temperature distribution within the blank during forming. This is achieved by using temperature sensors and applying the information as input for the model, which is based on force equilibria and the theory of plasticity. The model predicts the geometry of the formed part which can be used as crucial input information to the process control. For the quick model, an element-based time-discrete approach was chosen. By assuming a plane strain condition and a decoupling of thermal and mechanical computation as well as by adapting further assumptions, the calculation time on a current desktop PC is 4 s. The achieved calculation time is sufficiently short to realize the control at the given stroke rates. The model is validated by experiments using a hot stretch forming setup designed to simulate the multi-stage process. The model successfully replicates the influence of various stroke rates on the final thinning distribution and predicts the effects of diverse pre-cooling scenarios.