Consistent physically-based modeling of dynamic recrystallization under hot working conditions

• Applicants: Professor Dr.-Ing. Markus Bambach; Professor Dr. Franz Roters
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2016 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 315419526

For the design of hot working processes, a thorough understanding of the evolution of the yield stress and microstructure evolution during dynamic recrystallization (DRX) plays a vital role. In metal forming mainly semi-empirical DRX models are still being used today. These models are not based on internal state variables and are therefore not able to reproduce the physical processes underlying DRX. Thus, an extrapolation beyond the conditions applied in the laboratory during model calibration is inherently fraught with uncertainty. Although physically-based models have the potential to close the existing gaps, significant advancements seem necessary. None of the currently available DRX models takes the current state of metal physics research regarding the initiation and progression of DRX sufficiently into account. Often, it is still assumed that the nucleation of DRX requires a critical dislocation density. Insights into the formation of dislocation density gradients at the grain boundaries, into the formation of cell structures in the grain as a precursor for the formation of mobile sub-grain boundaries, and the interaction of sub-grain boundaries and the high angle grain boundaries of the parent microstructure as nucleation mechanism of DRX have not yet found their way into DRX models. Also, in existing models, the dynamic recovery is not modeled as a function of the stacking fault energy. Even more unsatisfactory is the lack of thermodynamic consistency of existing DRX models. It can be shown that Avrami kinetics (which are often used in DRX models) with an Avrami exponent less than or equal to 3 are inconsistent to the thermodynamically based Poliak-Jonas-criterion for the critical conditions of DRX. However, the usual model assumptions for the nucleation of DRX yield an inconsistent exponent of 3. Both from the viewpoint of metal forming as well as from metal physics, the need for a fundamentally enhanced modeling of DRX arises. The objectives of the present research proposal are therefore, to expand the physical understanding of the initiation and progression of DRX under hot working conditions, to develop a physically-based, stacking-fault-energy-dependent and thermodynamically consistent model for dynamic recrystallization, to prove at the example of a high temperature material (Alloy 800H) that the coupled evolution of the microstructure and yield stress can be described quantitatively correct under hot working conditions by the new model, to implement the model in a finite element software for forming simulation after abolishing the time step dependence of the microstructure evolution as well as to validate the model using a practice-oriented example. With this research existing knowledge gaps in the physically-based modeling of dynamic recrystallization shall be closed and a practice-oriented model for the design of metal forming process shall be created.

• DFG Programme: Research Grants