Integrated physically based modeling for the multiple static softening mechanisms following multi-stage hot deformation in Al-Zn-Mg-Cu alloys

Modern aluminum industries need an in-depth understanding and a more accurate prediction of the flow stress and microstructure evolution during multi-stage thermomechanical processing. The underlying mechanisms of Al alloys are distinct from that of other metallic materials (e.g., steels and copper) owing to the very high stacking fault energy. In the hot forming process of ultra-high strength Al-Zn-Mg-Cu alloys, the high alloying element additions have a more complex effect on multiple static softening mechanisms during post-deformation, i.e. the coupled recovery, recrystallization, precipitation and their interactions, which have not been well revealed and included in the existing plasticity models. In the present work, an integrated physically based model based on the observed microstructural characteristics and static softening behavior was developed to unravel the multiple static softening mechanisms following multi-stage hot deformation of Al-Zn-Mg-Cu alloys. By incorporating the multicomponent effects, i.e. process variables and chemical compositions, into static recovery, static recrystallization and precipitate coarsening models, the evolutions of stress, microstructure and static softening fraction could be accounted reasonably during post-deformation holding. A special attention was paid to model the functions of various alloy solute contents (i.e. Zn, Mg, Cu and Zr) on the precipitation thermodynamic, recovery and recrystallization kinetics in Al-Zn-Mg-Cu alloys. After validating by experimental data, the integrated physically based model could predict the effects of alloying elements on microstructural evolution, recrystallization and static softening kinetics of Al-Zn-Mg-Cu alloys. It was found that the different precipitation behaviors due to the addition of various alloying elements have a considerable influence on recovery, recrystallization and coupled static softening process. The solid solution atoms remaining in the matrix basically contributed to grain boundaries mobility and then slow recrystallization process, which depended on the interaction parameter, binding energy and diffusion rates of various alloying elements. This work offers an in-depth understanding of static softening mechanisms and provides a potential way for future development of advanced models and design strategies for multi-stage thermomechanical processes in Al-Zn-Mg-Cu alloys.