Experimental and numerical modeling and analysis of microstructural residual stresses in hot bulk forming parts under specific cooling

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In formed metallic components, the avoidance or minimization of residual stresses has so far been the main objective, with the aim of improving service life and manufacturability. A goal-oriented use of residual stresses for improvement of properties e.g. the operational strength in the field of forming technology has received rare attention until now. The aim of this project is to analyse the influences on distribution and stability of the residual stress development in thermo-mechanically processed components by means of a targeted process control using both experiments as well as numerical simulations. For this purpose, cylindrical specimen with eccentric holes are thermo-mechanically treated in various process cases in order to investigate the resulting inhomogeneous residual stress states. For the experimental as well as numerical analysis of the evolution and distribution of residual stresses in the used material a comprehensive characterisation of the thermal, metallurgical and mechanical properties was carried out in the first funding period. Due to the classification of residual stresses in 1st, 2nd and 3rd type multiscale numerical models were taken into account. The close interaction between experiment and numerical simulation allowed a calibration and validation of models and material descriptions. With these models a good prediction on the development of residual stresses in the reference process was gained. Furthermore, the developed numerical models are used during the second funding period and enhanced as well as optimized with respect to the new work program. Thereby, a macroscopic, phenomenological description in the framework of the Finite Element Method (FEM) based on thermo-mechanical and metallurgical properties is used for the representation of the residual stresses of the 1st type. For modelling microstructural residual stresses (2nd and 3rd type) and microstructure evolution phasefield models and multiscale FEM simulations are applied. After the validation of material data, the current aim is to investigate the controllability of residual stresses with regard to improve the properties of the hot-formed components. Besides specific control of the forming parameters, spray field cooling for active temperature management will be integrated into the forming process, which makes also further modification of the numerical models necessary. Subsequent numerical and experimental studies are used to analyse the interactions between process parameters and residual stresses. In the long term, a methodology should be developed which allows a deeper understanding of the thermo-mechanical material phenomena in connection with the resulting residual stress evolution occuring during the hot forming. Based on this knowledge, it is aimed for a simulation-aided process design with regard to the goal-oriented adjustment of defined, stable residual stresses, which positively influence the component properties.