| Summary: | 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
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