Abstract: Peripheral coarse grain (PCG) structure is a common microstructural defect appearing in the aluminum alloy extrusion process, which seriously affects the mechanical properties of the profiles. In this work, a series of extrusion experiments and numerical simulations were conducted to investigate the influence of billet temperature and ram speed on the microstructure, mechanical properties and thickness of PCG layers of 6005A aluminum alloy profiles. The mechanism of abnormal grain growth (AGG) occurring on the surface and in the core of profiles was revealed. The result showed that lower ram speed could suppress the formation of coarse grains. The AGG on the surface of the profiles was activated by the shear deformation and lattice distortion derived from the friction on the interface between the profile and die. When the billet was heated to a relatively high temperature, dynamic recrystallization (DRX) was dominant, and the Cube{100}<100> and R-Cube{100}<110> grains underwent abnormal growth to form surface coarse grains. When the billet was heated to a relatively low temperature, the degree of static recrystallization (SRX) became stronger, and the Goss{110}<100> and R-Cube{100}<110> grains underwent abnormal growth to form surface coarse grains. The AGG in the core of profiles was activated by the large grain boundary misorientation and a strain gradient formed because the Cube{100}<100> recrystallized grains were surrounded by the Copper{112}<111> and Brass{110}<112> deformed grains. The second phases in the 6005A aluminum alloy extrusion profiles were mainly β (Mg₂Si) and AlFeMnCrSi. As the billet temperature increased, more β phases dissolved into the aluminum matrix, thus enhancing the strength and hardness of the profiles. As the ram speed decreased, the thickness of PCG layers reduced, thus resulting in higher strength and hardness of the profiles. Due to the integrated effect of solution strengthening and grain refinement strengthening mechanisms, the combination of extrusion parameters for the profile to obtain the best mechanical properties was determined as 540 °C × 0.5 mm/s.

Key words: Recrystallization, Abnormal grain growth, Microstructure, Mechanical properties, Solution strengthening, Grain refinement strengthening