Effects of “free flow” coupled with severe shear strain on the volume of the refined microstructure in a 7075 aluminum alloy under rotary extrusion

Abstract

Hot deformation processing of aluminum alloys usually leads to a coarse fiber microstructure and strong deformation texture. In this study, rotary extrusion (RE) at 480 ℃ using a 7075 aluminum alloy hollow billet (HB) was studied and compared with a conventional extrusion (CE) HB. Simulation results obtained using DEFORM-3D indicated a unique vortex velocity field during RE. The equivalent strain for RE was higher than that for CE. RE exhibited weaker texture intensity and better grain refinement than CE. New vortex flow lines comprising fine equiaxed grains for the 7075 aluminum alloy were observed for the RE billet, whereas elongated coarse grains were the main features of the CE billet. The equivalent strain, grain size, and texture intensity for RE and CE were 6.47, 7.25 µm, and 1.36 and 1.08, 35.39 µm, and 1.93, respectively. Both continuous dynamic recrystallization and discontinuous dynamic recrystallization contributed to grain refinement and texture weakening, regardless of RE or CE. A 7075 aluminum alloy solid billet (SB) deformed by RE is known to exhibit a low severe deformation depth (< 1 mm) with fine microstructure. Herein, high-volume severe deformation areas consisting of refined grains with a thickness of 5 mm and a diameter of 6 mm were obtained due to the “free flow” combined with severe shear strain during hot deformation, in contrast to the “constrained flow” observed in the SB. Additionally, these findings offer a potential method for overcoming the limited volume and depth of refined microstructures in torsion-related deformations.