Determination of Flow Curves by Stack Compression Tests and Inverse Analysis for the Simulation of Hot Forming

Due to the increasing number of body-in-white parts that are manufactured by hot forming of boron alloyed sheet metal (22MnB5), the demand for a virtual representation of this specific manufac- turing process is evident. For a realistic simulation of hot stamping processes, the accurate modeling of the flow stress as function of strain, strain rate and temperature is essential. In the last years a large varity of empirical-analytical as well as physically based models for the yield stress has been proposed. Three existing models that have shown a good capability to represent the flow behavior of 22MnB5 in recent publications are presented and fitted to the experimental data. The underlying experimental data for the determination of the flow stress is obtained by stack compression tests, which were conducted in a high-speed deformation dilatometer. In a first step the model parameters are fitted to the experimental flow curves without considering the friction, which inherently is present in a compression test. Since the friction between die and specimen has significant influence on the state of stress within the specimen, an inverse, simulation-based ap- proach for the determination of the model parameters is employed. For this purpose a simple 2D FE model for each test configuration is set up. The resulting “friction-free” yield stress is up to 15% lower than the one without considering friction. Regarding the models considered, the approach developed by TONG and WAHLEN, which is based on the Z ENER -H OLLOMON parameter and a H OCKETT-S HERBY type formulation, provides the best fit of the experimental data.