To improve the loose structure and serious porosity of (Al–Zn–Mg–Cu) 7075 aluminum alloy laser-welded joints, aging treatment, double-sided ultrasonic impact treatment (DSUIT), and a combination of aging and DSUIT (A–DSUIT) were used to treat joints. In this experiment, the mechanism of A–DSUIT on the microstructure and properties of welded joints was analyzed. The microstructure of the welded joints was observed using optical microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD). The hardness and tensile properties of the welded components under the different processes were examined via Vickers hardness test and a universal tensile testing machine. The results showed that, after the aging treatment, the dendritic structure of the welded joints transformed into an equiaxed crystal structure. Moreover, the residual tensile stress generated in the welding process was weakened, and the hardness and tensile strength were significantly improved. After DSUIT, a plastic deformation layer of a certain thickness was generated from the surface downward, and the residual compressive stress was introduced to a certain depth of the joint. However, the weld zone unaffected by DSUIT still exhibited residual tensile stress. The inner microhardness of the joint surface improved; the impact surface hardness was the largest and gradually decreased inward to the weld zone base metal hardness, with a small improvement in the tensile strength. Compared with the single treatment process, the microstructural and mechanical properties of the welded joint after A–DSUIT were comprehensively improved. The microhardness and tensile strength of the welded joint reached 200 HV and 615 MPa, respectively, for an increase of 45.8% and 61.8%, respectively. Observation of the fractures of the tensile specimens under the different treatment processes showed that the fractures before the aging treatment were mainly ductile fractures while those after were mainly brittle fractures. After DSUIT of the welded joints, a clear and dense plastic deformation layer was observed in the fracture of the tensile specimens and effectively improved the tensile properties of the welded joints. Under the EBSD characterization, the larger the residual compressive stress near the ultrasonic impact surface, the smaller the grain diameter and misorientation angle, and the lower the texture strength. Finally, after A–DSUIT, the hardness and tensile properties improved the most.

To improve the low-cycle fatigue properties of Al–Zn–Mg–Cu 7075 aluminum alloy laser-welded joints, a post-weld treatment method for double-sided ultrasonic impact treatment (DSUIT) was used to treat the joints. The mechanism of different DSUITs on the microstructure and fatigue properties of welded joints was analyzed. The results showed that DSUIT reduced the welding defects and roughness of the joint surface, and the dendrite structure at the upper and lower surfaces of the joints was broken to form a plastic deformation layer (PDL) with an approximate thickness of 100 μm. Furthermore, the texture strength and grain size at the PDL were reduced. A beneficial residual compressive stress was introduced into the upper and lower surfaces of the joints after the DSUIT. When the number of cycles was 2 × 10⁶, the maximum fatigue strength of the joints was 103.02 MPa after DSUIT, indicating an increase of 111.8% of the untreated joints of 48.62 MPa. Moreover, observance of the fatigue fracture of the joints revealed that the PDL produced by the DSUIT inhibited the initiation and propagation of cracks at the surface. Therefore, the DSUIT effectively improved the low-cycle fatigue properties of the joints.