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In this study, the phenomenon of gas removal behaviors in the melt pool and the flow behaviors of melt pool were observed using an in-situ X-ray imaging method. Based on the ultrasonic effects, a novel processing method was developed to reduce porosity and decrease diffusible hydrogen content in the deposited metal. The effects of ultrasonic output power on the size and frequency of gas bubble collapsing were studied. Results showed that with the assistance of ultrasonic, the gas bubble size were smaller and the collapsing frequency was significantly increased. The porosity decreased from 1.4% to 0.5% and the hydrogen diffusible content decreased from 24.5 to 18.6 ml/100 g when the ultrasonic power increased to 720 W. The possible hydrogen removal mechanism was proposed by two aspects including the microcosmic and macrocosmic scale. Keywords: Underwater welding, Ultrasonic melt processing, Solidification, Diffusible hydrogen, In-situ X-ray imaging

The dynamic behavior of the underwater melt pool was firstly in-situ observed with the X-ray transmission method. A method for quantitatively characterizing the melt pool stability was pioneered. Comparing with conventional melt pool composing metal liquid, the pool was filled with bubbles in underwater environment. By observing the X-ray images and charactering the weld pool waveform, it is confirmed that the underwater melt pool was more volatile than onshore melt pool, which was mainly caused by the evolution of large bubble in the pool. Violent fluctuation of melt pool was the basic reason gaining poor weld and multiple defects in the underwater wet welding. To mitigate pool fluctuations by reducing the size of bubble, the ultrasonic vibration was applied directly on the workpiece, which improved the melt pool stability, uniformized the weld formation, and decreased the weld porosity. Keywords: In situ X-ray imaging, Underwater wet welding, Melt pool dynamics, Bubble evolution, Ultrasonic vibration