Abstract: To improve the fatigue properties of the 7A52 aluminum alloy VPPA-MIG welded joint, the fatigue life of the welded joint was improved by changing the ultrasonic impact time. In this work, each welded joint was subjected to ultrasonic impact treatment for 2.5 min, 5 min, 10 min, 15 min, 30 min and 75 min. The scanning electron microscopy, the transmission electron microscope and the X-ray diffractometer were used to observe and analyze the surface morphology of welded joints and calculate the surface grain size of welded joints. The hardness, the residual stress and the fatigue properties of welded joints were examined via microhardness test, X-ray stress analysis and fatigue test, respectively. The influence of ultrasonic impact treatment on the fatigue properties of welded joints was analyzed comprehensively. The results show that when the ultrasonic impact time is 2.5—30 min, the defects of the welded joint are obviously reduced, with the grain on the surface of the welded joint refined to the nanometer level. When the ultrasonic impact time is 30—75 min, cracks appear on the impact surface. After ultrasonic impact treatment, the hardness of welded joints can reach 140HV, which is 40% higher than that of untreated welding joint, and the plastic deformation layer can reach 105 μm. Moreover, after ultrasonic impact treatment, the residual stress of welded joint changes from residual tensile stress to residual compressive stress, and the residual compressive stress increases in 2.5—30 min, with the maximum value of -329.2 MPa. When the ultrasonic impact time is 30—75 min, the residual compressive stress decreases. The fatigue strength of the welded joints treated by ultrasonic impact treatment are significantly improved. When the ultrasonic impact treatment time is 30 min, the maximum fatigue strength of the joints are 87.63 MPa, the fatigue strength of untreated welded members are 48.8 MPa and the fatigue strength are improved by 79.57%. Therefore, ultrasonic impact treatment on welded joint can enhance its fatigue performance and prolong the service life of welded components. Through this work, we hope to provide a theoretical basis for the wide use of 7A52 aluminum alloy VPPA-MIG welding components.
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2022, Vol. 36 
