Evolution of Microstructures, Texture and Mechanical Properties of Al-Mg-Si-Cu Alloy under Different Welding Speeds during Friction Stir Welding

The effects of different welding speeds on the microstructures and mechanical properties of Al-0.75Mg-0.75Si-0.8Cu alloys were investigated using optical metallographic microscopy (OM), X-ray diffraction (XRD) analysis, an ETM105D electronic universal testing machine and field emission electron microscopy (SEM). The results reveal that during the friction stir welding process, the welded joint forms the base material (BM), heat-affected zone (HAZ), thermomechanically affected zone (TMAZ) and nugget zone (NZ), under the action of shear force and friction heat. The textures present in the BM are mainly C ({001}<100>) recrystallised cubic texture and P({110}<112>) recrystallised texture, Goss (G) texture, brass (B) texture and {112}<110> rotating copper texture. The organisation of the NZ undergoes dynamic recrystallisation, forming fine isometric crystals with large angular grain boundaries, accounting for more than 75% of the total. The geometrically necessary dislocations (GNDs) in the NZ grow as the welding speed rises. Moreover, {111}<11−0> and {111}<1−10> shearing textures, {001}<110> recrystallisation textures and fibre textures are mainly present in NZs. The average grain size in the NZ was the smallest, and the mechanical properties were the best at a welding speed of 125 mm/min. The grain size and the tensile strength and elongation of the NZ were 2.945 µm, 200.7 MPa and 12.7% for the joint at a welding speed of 125 mm/min, respectively.