AISI 304 stainless steel has low thermal conductivity and a high linear expansion coefficient; conventional welding with this material produces coarse grains and poor joint tensile properties. To address the problems inherent in conventional 304 stainless steel welding, an ultrasonic transducer was brought into contact with a base metal and treated with ultrasonic vibration during welding. The residual stress distribution, microstructure, and mechanical properties of the welded joint were then analyzed. The results showed that under the combined action of thermodynamics and the dynamics caused by ultrasonic treatment, the flow performance of the molten pool was enhanced, heat was transferred from the high-temperature region to the low-temperature region, and the metal elements were constantly diffused during the crystallization of the molten pool. Thus, the transformation rate of columnar crystals to equiaxed crystals was accelerated, the proportion of equiaxed crystals increased, and the grains became refined. Compared with conventional welding, the microstructure of the 304 stainless steel welded joints was better after the introduction of ultrasonic vibration, and its tensile residual stress was significantly lower. The improvement in the microstructure of the weld improved its mechanical properties, and the hardness of the weld zone increased significantly, which was close to that of the base metal. After the ultrasonic treatment, the increase in the equiaxed crystals and the refinement in the grains effectively improved the quality of the welded joints so that the tensile strength of the welded joints was higher than that of conventional welded joints. Under the condition of the same ultrasonic vibration frequency and power, the tensile strength of welded joints with 2-mm steel plate exhibited a notable increase, which was 13 %.
