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

• Applicants: Professor Dr.-Ing. Bernd-Arno Behrens; Dr.-Ing. Dominik Brands; Professor Dr.-Ing. Jörg Schröder
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 530125423

In formed metallic components, the avoidance or minimization of residual stresses has so far been the main objective, in order to improve service life and manufacturability. In contrast, the targeted use of forming-induced residual stresses to increase the fatigue strength, for example, has hardly been considered. To improve component properties, the focus of this project is on the analysis and targeted modification of residual stresses in hot bulk formed components. Here, the key aspect is the numerical and experimental consideration of the specifically adapted process control in order to capture the influences of various process parameters on the distribution and stability of residual stresses. In a joint four-year research project, comprehensive characterizations of the thermal, metallurgical and mechanical material properties were carried out. By means of experimental and numerical analyses, the evolution and distribution of residual stresses were investigated and their classification into 1st, 2nd and 3rd type was made, in particular by multiscale simulation models. A specific control of the forming parameters and an active temperature control were integrated. The combination of experiment and simulation enabled calibration and validation of the models. Thus, the design of an experimental demonstrator process for the adjustment of advantageous surface-near residual compressive stresses could already be realized. Based on this, the design, engineering and optimization of the industry-oriented manufacturing process of a shaft as a highly loaded forged component are now performed. The implemented simulation models are extended to additional process areas and expanded to include a calculation approach for lifetime prediction. A macroscopic, phenomenological description within the framework of the Finite Element (FE) Method is used, taking into account the thermomechanical-metallurgical material properties, for the representation of 1st type residual stresses. Multi-scale FE-simulations and Phase field models are applied to model microstructural residual stresses (2nd and 3rd type) and microstructure evolution. In the course of numerical process design, the aim is to maximise the lifetime of the shaft by tailoring residual stress distribution. Subsequently, the optimized process chain is realized experimentally and the property-improving mechanisms of the residual stresses in the component from the optimized process are compared to a conventional reference process in continuous operation. Thus, simulation models on different scales are developed for an efficient design of hot forming process chains aiming at the adjustment of advantageous residual stress distributions as well as a lifetime prediction model depending on the residual stress state in order to make targeted use of the residual stresses for a property improvement of the components.

• DFG Programme: Research Grants