Deformation mechanisms and mechanical properties of the high-strength and ductile Al-Zn-Mg-Cu alloys processed by repetitive continuous extrusion forming process with different heat treatments

Continuous severe plastic deformation (SPD) processes, which overcome the limitations of economic efficiency and sample size for classical SPD approaches, are promising industry technologies for manufacturing aluminum alloy products with refined microstructures and enhanced properties. In the present work, the repetitive continuous extrusion forming (R-Conform) process combined with auxiliary thermal treatments was implemented in Al-Zn-Mg-Cu alloys, which had poor workability. By combining microstructural observation, mechanical testing and finite element simulation, the influence of auxiliary heat treatments on the mechanisms of grain refinement and strengthening for Al-Zn-Mg-Cu alloys processed by R-Conform were systematically investigated. The results indicated that the grains of Al-Zn-Mg-Cu alloys were significantly refined by a continuous dynamic recrystallization (CDRX) during the R-Conform process with special thermomechanical conditions. The increased supersaturated solid solution concentration and constituent precipitate fraction caused by prehomogenization were found to accelerate CDRX for grain refinement through the pinning effect on dislocation motion. Due to the simultaneous effect of shear deformation and online solution, the R-Conformed alloys contained massive lattice distortions and high concentrations of solute atoms. As a result, the processed Al-Zn-Mg-Cu alloys combined with prehomogenization and direct aging treatments possibly possess a good combination of strength and ductility due to the integrated functions of fine grains, massive lattice distortion, and evenly distributed nanosized precipitates.