Effect of Mn Content on Microstructure and Tensile Properties of Friction Stir Welded Joints of Al-Mg-Si-Cu Alloy

Friction stir welding tests were carried out on Al-Mg-Si-Cu alloys with different Mn content (0, 0.1%, 0.4%, 0.7%, mass fraction); the alloys were treated by solution at 510 ℃ and aging at 195 ℃. The effect of Mn content on microstructure and tensile properties of the joints were studied. The results show that the nugget zone of the joint with different Mn content underwent the combined effect of continuous dynamic recrystallization and geometric dynamic recrystallization, forming a large number of fine recrystallized grains, and the serrated grains were elongated and divided. With the increase of Mn content, the average grain size in nugget zone increased and then decreased, and the number of precipitates in the joint increased. When the mass fraction of Mn was 0.7%, precipitates coarsened. A/$\overline{\text{A}}$ and C-type shear textures were formed in the nugget zone of joint with different Mn content. The tensile strength of the joint increased first and then decreased with the increase of Mn content, and the elongation after fracture decreased first and then increased. When Mn mass fraction was 0.4%, the tensile strength of the joint was the highest and the elongation after fracture was the smallest.There were dense dimples in the fracture of the joint with Mn, and the fracture mode was ductile fracture. With the increase of Mn content, the size of dimples increased.