Repair Welding of Tunnel Defect in Friction Stir Weld of Al-Zn-Mg Alloys
The tunnel defect formed in friction stir welding dramatically, which deteriorated mechanical properties of joints owing to its larger size in lack of fusion. By welding the Al-Zn-Mg aluminum plates with specific process parameters, the defective joints with tunnel defect were obtained. Then, the repair welding and twice repair welding were carried out to use friction stir welding on the defective joint. Experiment results showed that the joint with tunnel defect could be repaired by the friction stir repair welding. Microstructure of repaired joints showed that grain size in the nugget zone decreased slightly while inhomogeneity of grain size was increased, and grain presented recrystallization phenomenon in the retreating side of thermo-mechanically affected zone; Mechanical properties testing results showed that the yield strength and tensile strength increased obviously. The tensile fracture scanning results showed that repaired joints presented layered in different positions. Compared with the repair welding joint, the mechanical properties of twice repair welding joint lowered slightly, and the thickness of welding reduced gradually with the repair welding.