Robust optimization of springback in dual phase steels during sheet metal forming operations

Nowadays, automobile companies have been focused on manufacturing light-weight and fuel efficient automobiles without jeopardizing the strength and crashworthiness of the structural parts. For that purpose, automotive manufacturers are actively exploring the use of high strength dual phase steels as an alternative to aluminum and magnesium alloys due to their low cost and durability. However, dual phase steels have a tendency to springback more than other structural steels in a forming operation due to their high tensile strength. In addition, variations in manufacturing process parameters and material properties cause springback variation over different manufactured parts. Therefore, it is an important task to reduce the magnitude of springback as well as its variation within to produce robust and cost-effective parts. This work investigates minimization of the magnitude and variation of springback of DP600 steels in both U-channel forming and 7-flange design within a robust optimization framework. Metamodels are constructed to reduce the computational cost of robust optimization and also predict the springback and its variation computed by Monte Carlo simulations. Sensitivity analysis is performed to determine the most important random variables that affect the springback. Yield stress (?y) is found to be most influential parameter. Finally, a double-loop Monte Carlo simulation method is proposed to calculate part-to-part and batch-to-batch springback variations. It is found that as the batch-to-batch variation of yield stress increases, the batch-to-batch springback variation increases while the part-to-part springback variation remains unchanged. For 7-flange design, considerable reductions were obtained for the mean values of different springback angles in amounts varying between 7% and 85% when compared to nominal mean springback results. It was observed for most cases that the optimizer pushed the design variables to lower and upper bounds.