Abstract: In order to solve the problem of severe softening of duralumin alloy welded joints, 7075 aluminum alloy was selected as filler material for high assembly welding of 6061 aluminum alloy. The hardness of welded joints did increase, but hot cracks occurred during welding. In this study, an ultrasonic assisted platform is designed to introduce ultrasonic vibration into the traditional tungsten argon arc welding to solve the problem of welding hot crack under the condition of ensuring the hardness of welded joint. The results show that the hot crack disappears when the ultrasonic power is low, but the hot crack occurs again when the ultrasonic power reaches 480 W. With the increase of ultrasonic power, the grain size and shape of the welded joint change, the columnar crystal in the fusion zone is broken and transformed into fine equiaxed crystal, and the equiaxed crystal in the weld center is refined. When P = 640 W, the average grain size in the weld center is the smallest. With the increase of ultrasonic power, the number of precipitates in the welded joint gradually increases and the distribution is gradually uniform, which has an impact on the welding hot crack. Therefore, there is a range of ultrasonic power, which can solve the problems of welded joint softening and welding hot crack at the same time.