Experimental as well as numerical modelling and analysis of microstructural residual stresses of hot formed components with targeted cooling

Efforts are currently being made to avoid or reduce residual stresses in formed metallic components, since residual stresses are supposed to have negative effects on fatigue strength and manufacturability. This ignores the fact that residual stresses can also be deliberately used to positively influence component properties, e.g. with regard to fatigue strength. Therefore, the aim of this project is to analyse the residual stresses in hot formed components under specific process control, both experimentally and numerically, in order to determine the influences on their distribution and stability.

For this purpose, cylinder specimens with an eccentric bore were thermo-mechanically treated in different process scenarios in order to investigate the resulting inhomogeneous residual stress states. For the experimental and numerical analysis of the evolution and distribution of residual stresses in the material, a comprehensive characterisation of the thermal, metallurgical and mechanical properties was carried out during the first funding period. The classification into residual stresses of 1st, 2nd, and 3rd kind was considered by multi-scale simulation models. The close interaction between experiment and numerical simulation allowed calibration and validation of the models and material descriptions. With these models, a qualitatively good prediction of the development of residual stresses in the reference process was already achieved.

In the second funding period, these simulation models are used, extended and optimised with regard to the design of hot forming processes with integrated controlled heat treatment, which allow the targeted adjustment of residual stresses. A macroscopic, phenomenological description within the framework of the finite element method (FEM), taking into account thermo-mechanical-metallurgical effects, is used for the representation of the residual stresses of the 1st type. Phase field models and multi-scale FEM simulations are used to model the microstructural residual stresses (2nd and 3rd type) and the microstructure evolution. After validation of the material data, the main goal is to investigate the adjustability of residual stresses with respect to property improvement in the hot-formed components. In addition to the targeted control of the forming parameters, a spray field cooling system for active temperature control is integrated into the forming process. Subsequent numerical and experimental studies are performed to analyse the interactions between process parameters and resulting residual stresses.

The results obtained provide an elaborate understanding of the relationships between thermal, mechanical, metallurgical material effects, process parameters and the resulting residual stresses. The developed numerical methodology allows the efficient design of forming processes with integrated heat treatment for the specific adjustment of advantageous residual stress distributions.