Effects of Ultrasonic-Frequency Pulse Current on Arc Stability, Acoustic Signal Emission and Weld Quality in Underwater Wet Multi-Layer Multi-Pass Welding with Q345 Steel

ITo address limitations of poor arc stability and inadequate weld quality in conventional underwater wet welding for marine structural repairs, this study developed an ultrasonic-frequency pulsed (UFP) assisted underwater wet welding (UFP-UWW) process by coupling a constant-voltage power source with an ultrasonic-frequency pulse current generator. The dynamic features of arc voltage, welding current, and underwater acoustic signal were systematically investigated under varying excitation voltages (10–60 V) and frequencies (20–90 kHz). The microstructure, mechanical performance and corrosion resistance of underwater multi-layer multi-pass welds on Q345 low-alloy steel were evaluated. Results indicated that ultrasonic-frequency pulsed current modulation improved overall arc stiffness and enhanced arc combustion stability. The arc acoustic signal pressure and electrical signals exhibited consistent synchronization with external ultrasonic excitation frequency. The arc acoustic signal pressure amplitude increased with elevating excitation voltage but decreased with increasing frequency. The UFP-UWW process successfully produced defect-free multi-layer multi-pass welds. Ultrasonic-frequency electromagnetic stirring in the pool enhanced thermal convection and mass transfer, which disrupted columnar grain growth and refined the weld microstructure. Compared with conventional UWW, the UFP-UWW weld metal showed a significant improvement in tensile strength (an increase of ~7.4%) and impact toughness (by 22.7%), alongside enhanced corrosion resistance. This study confirmed the applicability of UFP-UWW in multi-layer multi-pass welding of thick plates, providing a reliable technical solution for high-quality marine engineering repairs.