Microstructural evolution in Al-Mg-Sc alloy (AA5024): Effect of thermal treatment, compression deformation and friction stir welding

Effect of thermal annealing and deformation in both compression and friction stir welding conditions on microstructural evolution in the cold rolled Al-4.36Mg-0.26Sc-0.09Zr (wt.%) alloy was investigated. To evaluate the thermal stability of the alloy, differential scanning calorimetry and static annealing experiments were carried out as a function of temperature. Microhardness measurements and quasi-static compression testing were performed on the as-received and annealed alloy samples. Friction stir welding was carried out on the as-received alloy sheet in butt configuration at two different tool traverse speeds of 250 and 500 mm/min. Upon annealing, the alloy showed continuous recrystallization with transformation of the elongated grain structure possessing strong rolling texture to coarse equiaxed microstructure with random orientation. The annealed alloy exhibited reduced hardness and compressive strength at room temperature. Detailed microstructural investigation of hot compression deformed and friction stir welded samples revealed formation of subgrain structure and followed by fine recrystallized grains with nearly random orientation. The analysis suggests that continuous dynamic recrystallization involving progressive subgrain rotation is the possible mechanism for microstructural changes occurring during hot deformation of Al-Mg-Sc alloy.