Abstract: In order to utilize the mechanical constraint to implement the control of floating arc bubble during underwater wet welding (UWW), the experimental system of the mechanical constraint assisted underwater wet welding(MC-UWW) is proposed and analysed under water. The dynamic behaviors of arc bubble under the mechanical constraint are observed by high-speed video camera and welding stability is characterized by real-time capturing welding electrical signal according to sensor module. Weld cross-section size and microstructure of the joints in MC-UWW are also detected by optical microscope. They are all compared to those in conventional UWW under identical welding parameters. The results show that the mechanical constraint realizes the control of arc bubble, and overcome the problem of arc instability caused by arc bubble burst. With the decreasing mechanical constraint height, the fluctuations of electrical signals reduce considerably and hence arc stability is improved. Furthermore, the applied mechanical constraint enlarges the size of weld cross-section, reduces the weld reinforcement, and restricts the formation of brittle microstructure in weld metal and heat affected zone. It is inferred that the exerted mechanical constraint enlarges the arc bubble size and reduces the heat loss between the weld pool surface and surrounding water, therefore decreasing the cooling speed of weld joint.

Key words: arc bubble, mechanical constraint, microstructure, underwater wet welding, welding stability