Effect of ultrasonic vibration on the pores and properties of the laser-arc hybrid welding joint of high nitrogen steel

High nitrogen steel welded joints are prone to problems such as porosity defects and coarse grain size. To address these issues, ultrasonic-assisted laser-arc hybrid welding experiments were conducted on 8 mm-thick high nitrogen steel plates. The effect of ultrasonic vibration on the porosity defects, microstructure, and properties of welded joints was studied. The results indicate that with the increase of ultrasonic power, the pore ratio of the weld first decreases and then increases. The effects generated due to ultrasound such as cavitation and sound flow caused grain refinement while weakening the directionality of grain growth. The refined microstructure of the weld increased the microhardness. However, when the ultrasonic power was 240 W, the microhardness of the welded joint decreased due to the low nitrogen content. As the ultrasonic power was increased, the tensile strength and impact toughness initially exhibited an enhancement followed by a declining trend. The best mechanical properties were obtained when the ultrasonic power was 180 W. In contrast, the worst performance was obtained when the ultrasonic power was 240 W, which was due to higher porosity and severe nitrogen loss. Ultrasonic impact can reduce welding residual stress and improve the corrosion resistance of welds, and the effect becomes more pronounced with the increase of ultrasound power.