Life estimation of hot press forming die by using interface heat transfer coefficient obtained from inverse analysis

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During the hot press forming process, the die experiences repeated thermal and mechanical loads owing to exposure to the heated workpiece. Such repeated thermal loads may cause deterioration in the mechanical properties of the die, leading to fatigue failure. Therefore, for the successful implementation of the hot press forming process in mass production, it is necessary to estimate the die life under hot press forming conditions. The fatigue life of a hot press forming die is estimated based on the stress history of the die during the forming process. Because accurate understanding of thermal behavior is essential for reliable analysis of fatigue life of the die at elevated temperatures, we characterized the thermal boundary condition, i.e., the heat transfer coefficient at the die-workpiece interface. For this purpose, die temperatures during a hot press forming process were measured as a function of time at select locations on the die. Inverse finite element method (FEM) analysis of the hot press forming process was performed to determine the interface heat transfer coefficient. The interface heat transfer coefficient was applied to the FEM simulation, and the temperature distribution and stress values for the die were determined. Considering the thermomechanical stress history, the fatigue life of the die was estimated based on the stress-life approach.