| Underwater structures such as oil platforms, subsea pipelines are used widely with developing offshore oil and gas resources. These structures are working in the long-term under the environment with water flow and sea erosion. As time goes on, underwater welding repair work for the structures will be increased. Due to its good performance and wide depth of application, dry hyperbaric gas metal arc welding (GMAW) is considered to be a deep-water welding method with practical significance. In this thesis, based on dry hyperbaric GMAW experiments within2MPa ambient pressure which equals to the water depth of200m, the arc behavior, metal transfer characteristics and spatter generation mechanism are the research emphasis. And then the butt welding of X65plate at the water depth of80m using dry hyperbaric GMAW method is studied.Based on the characteristics of underwater welding process, experimental equipment is developed for onshore simulating. The equipment consists of high pressure environment system and automatic welding test system, which can satisfied the welding test requirements within5MPa ambient pressure.A typical arc shape model of dry hyperbaric GMAW is established. As ambient pressure increased, electric field intensity of arc column increases. At high ambient pressure, electrode spot and arc column area expanded with increasing welding current. The relationship between welding voltage and arc length is approximate to linear, which is consistent with GMAW at normal pressure. Rising height of cathode arc root along the wire side in direct current electrode negative (DCEN) decreases with the increasing ambient pressure. The arc stability with DCEN has obvious improvement at ambient pressure more than0.4MPa. In pressure environment, with DCEN, the cathode spots can be seen under the droplet and the anode spots is not generated on the molten pool. This phenomenon is different from direct current electrode positive (DCEP) welding process, whose anode spots and one or several cathode spots can be seen. Arc static characteristic curve of dry hyperbaric GMAW is still rising characteristic. The curve moves downward with increasing ambient pressure. The relationship between wire melting rate and wire extension is approximate to linear. The wire melting efficiency with DCEN is higher than that with DCEP at ambient pressure below0.4MPa. After0.6MPa, the wire melting efficiency with DCEN is nearly the same to that with DCEP.The metal transfer mode in dry hyperbaric GMAW has been investigated by using high speed camera system with infrared laser as backlight. The metal transfer mode with DCEP can be classified to large droplet repelled transfer, projected repelled transfer and hyperbaric streaming transfer. Droplet size and transfer frequency are mean differences of the former two. The transfer tracks of the former two deviate from the axial direction of welding wire and the spatter with near droplet size is easy to generation. With hyperbaric streaming transfer, welding arc is burning around the wire end and the droplet size is very small which transfer inside the welding arc. The process is nearly no spatter generated. According to the different welding voltage, the metal transfer mode with DCEN is short-circuit transfer and droplet repelled transfer. Through the analysis, it has been found that anode arc root rising along the droplet surface is the precondition of droplet repelled transfer forming with DCEP. In the pressure environment, arc root burning contracts to one side of the wire instead of wrapping around the droplet. It results in the direction changing of spot force and electromagnetic force, which cause the asymmetric side force on the droplet. So that the droplet deviates from the axial direction of the wire, and the droplet repelled transfer formed. In addition, the reason of anode arc root rising along the droplet has been analyzed. The critical ambient pressure for metal transfer mode changing has been expounded. Distribution diagram of metal transfer modes with different welding parameters has been obtained by the experiments. Metal transfer mode of dry hyperbaric GMAW with DCEP will be changed into droplet repelled transfer as the increasing ambient pressure.The spatter generation mechanism of dry hyperbaric GMAW has been investigated. Two spatter types, droplet deviated spatter and droplet rebounded spatter, are observed. The former is generated in the process of droplet repelled transfer, which is due to the high horizontal speed of the droplet detachment. This spatter type begins to appear at the ambient pressure reached0.2MPa. In both DCEP and DCEN process, droplet deviated spatter can be generated and its size is close to the droplet. The latter, droplet rebounded spatter, is a new spatter generated process. The spatter is always generated after the cathode root burning on the droplet. And then the droplet rebounds to the free space and changes into the spatter instead of adhering to the workpiece. Droplet rebounded spatter is generated at the ambient pressure over0.4~0.6MPa with DCEP only. The reason that the electromagnetic force is the driving force of droplet rebounded was analyzed. Two basic conditions of droplet rebounded spatter generation is proposed. First, after the droplet contacting with the workpiece, welding arc root burning on the droplet surface is easier than that on the molten pool. Second, the current flowing through the drop reaches a certain value, which caused by the arc root contracting to the droplet surface at more than a certain ambient pressure.Finally, Process test research on the butt welding of X65plate at the water depth of80m is studied. With the same welding parameters, average welding current of DCEN is20-40A higher than DCEP. Therefore, the parameters of multi-layer welding process have been planned. DCEP method is employed for the fist layer, backing weld, and second layer, filling weld. DCEN method is employed for the third layer, filling weld, and fourth layer, cosmetic weld. The welding joint without defect is gained. The tensile testing results show that the tensile property is close to the base metal. Therefore, the butt welding of X65plate with dry hyperbaric GMAW process can be satisfy the requirements... |
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