Accurate prediction and compensation of springback in advanced high strength steels after sheet metal forming operations

The main aim of this study is to predict and then compensate the springback of advanced high strength steels by using finite element method. In order to analyze the springback behavior, three different forming tools have been examined. In U-channel forming problem, finite element analyses (FEA) have been performed for isotropic, Hill48 and Barlat89 yield criterion. The best agreement with the experimental results has been observed when the yield locus of the material was defined by Barlat89 yield criteria. A 7-flange tool composed of typical flange conditions with various flange radii has been designed and experiments have been conducted. In order to increase the accuracy and the precision of FEA results of this forming problem, effects of such mathematical parameters as number of integration points through thickness have been investigated. FEA results of the 7-flange forming process have been validated with the experimental results. Besides of the springback, other simulation results such as forming quality, thickness reduction, and punch reaction force of the S-rail process have been studied. Subsequently, springback behavior of the DP, TRIP and TWIP steels have been examined, effect of hardening coefficient and hardening exponent on the springback have been investigated. Effects of tooling parameters of the U-channel forming process have been analyzed in an attempt for springback compensation. Then, two different blank holder force (BHF) applications have been implemented to the 7-flange tool. Compensation effectiveness of these two BHF applications has been compared in line with the new BHF algorithm proposed in this study. Finally, stepped-draw bead application, frequently referred to the help in the industry, adapted to U-channel forming problem and effectiveness of the geometrical parameters on compensation of springback have been investigated. Parametric analyses for 38 design points have been performed. Guiding results for the stepped-draw bead design have been obtained with the surface fitting method.