Microstructure evolution and mechanical properties of high-strength 7075 aluminum alloy during multi-pass hot radial forging process

The 7075 aluminum alloy has exceptional strength, but refining its microstructure and improving its mechanical properties presents considerable challenges. In this study, a hot radial forging process comprising preheating, multi-pass radial forging, and T6 heat treatment, was implemented on 7075 aluminum alloy rods. The microstructure evolution mechanisms in the edge and core regions of the workpieces were investigated through observations on the changes in grain morphology (grain boundary, size, and orientation) and second phase precipitation. The microhardness and tensile properties of the workpiece were assessed at varying cross-section reduction rates (CSRRs) during radial forging and distinct solution durations during T6 heat treatment. Results demonstrate that preheating considerably enhances the plasticity of the workpiece. During multi-pass radial forging, the key mechanisms of refining microstructure are geometrical dynamic recrystallization and continuous dynamic recrystallization induced by the radial compression of grains and interweaving between adjacent grains. The implementation of T6 heat treatment on the radial forged workpiece facilitates the formation of continuous equiaxed subgrains, the proliferation of fine recrystallized grains and the uniform precipitation of η′ phase. Radial forging combined with T6 heat treatment considerably strengthens the edge region of the workpiece (↑6.7 % in ultimate tensile strength [UTS], ↑33.3 % in elongation) at 30.6 % CSRR, and improves the overall mechanical performance of the workpiece (Edge: ↑5.7 % in UTS, ↑32.3 % in elongation; Core: ↑4.9 % in UTS, ↑40.4 % in elongation) at 55.6 % CSRR. This study may provide an expeditious and flexible manufacturing process for multi-step 7075 aluminum alloy shafts that withstand high loads in lightweighting applications.