Abstract: The gradient-distributed peripheral coarse-grain (PCG) layer, resulting from abnormal grain growth (AGG) on the surface of Al-Zn-Mg-Cu alloy profiles during solution treatment (ST), significantly reduces mechanical properties and fatigue life. In this paper, 12 groups of orthogonal extrusion experiments were carried out to study the effects of extrusion parameters and die structure on PCG thickness and distribution in profiles. Combined with TKD-EBSD and TEM analyses, the PCG formation mechanism was clarified by considering the synergistic effects of strain, stored energy, and grain boundary. Results revealed that PCG thickness correlates linearly with ram speed and nonlinearly with billet temperature. At the ram speed of 0.1 mm/s, the thickness of PCG layers decreases parabolically with increasing temperature, while at 0.3-0.5 mm/s, it exhibits a rightward-inclined "S-shaped" variation. The optimal extrusion parameters that effectively suppress coarse grains (CGs) during ST are 450 °C × 0.1 mm/s, controlling the thickness of PCG layers at the hundred-micron level and maximizing the strength of solution-treated profiles. Extruded profiles display a gradient distribution with {111}<112> orientation in the surface layer (twin dominant), Copper{112}<111> in the intermediate layer (slip dominant), and Goss{110}<100> in the core (stable state). Higher ram speed and longer bearing promote the accumulation of shear strain and formation of Σ3 twin boundaries (Σ3 TBs) between {111}<112> grains, driving abnormal growth along {111}<110> orientation during ST. The {111}<110> grains swallow {111}<112> grains and grow abnormally up to the Goss{110}<100> interface, then stagnate due to reduced driving force. The fibrous grain structure formed at lower ram speed inhibits coarse-grain growth during ST. It was found that the PCG formation mechanism can be divided into three stages according to different driving forces: I) stored energy gradient + high mobility grain boundary, II) curvature + residual stored energy, and III) curvature + Σ3 TBs induction. This work provides theoretical guidance for regulating PCG structure in high-strength aluminum alloy profiles.

Key words: Peripheral coarse grain, Al-Zn-Mg-Cu alloy, Extrusion parameters, Σ3 twin boundary, Stored energy gradient