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The laser-MAG hybrid welding in horizontal-vertical (PC) position was used to study the effect of different groove types on the quality of a root weld. Six combinations of bevel angles (50° + 0° (represents the groove angle of upper plate and lower plate respectively), 45° + 5°, 40° + 10°, 35° + 15°, 30° + 20°, and 25° + 25°) and root face sizes of 2 to 4 mm were designed. Hannover analyzer was used to collect electrical signals to analyze the stability of the horizontal-vertical position welding process. The results show that different combinations of groove angle have achieved one-sided welding with root formation. The groove angle has an obvious effect on the weld inclination and molten weld metal flowing down. The combination of bevel angles in which that of the lower plate is smaller than that of the upper plate was more conducive to avoid welding defects such as weld inclination and molten weld metal flowing down. The best groove angle combination was 30° + 20°, which allowed one-sided welding with root formation using a 4-mm root face.

Combined with the technologies of underwater local dry laser cladding (ULDLC) and underwater local dry laser remelting (ULDLR), a duplex stainless steel (DSS) coating has been made in an underwater environment. The phase composition, microstructure, chemical components and electrochemical corrosion resistance was studied. The results show that after underwater laser remelting, the phase composition of DSS coating remains unchanged and the phase transformation from Widmanstätten austenite + intragranular austenite + (211) ferrite to (110) ferrite occurred. The ULDLR process can improve the corrosion resistance of the underwater local dry laser cladded coating. The corrosion resistance of remelted coating at 3 kW is the best, the corrosion resistance of remelted coating at 1kW and 5kW is similar and the corrosion resistance of (110) ferrite phase is better than grain boundary austenite phase. The ULDLC + ULDLR process can meet the requirements of efficient underwater maintenance, forming quality control and corrosion resistance. It can also be used to repair the surface of S32101 duplex stainless steel in underwater environment.

Taking S32101 duplex stainless steel as the research object, underwater laser wire filling welding technology was used for U-groove filling welding. The influence of different shielding gas compositions on the ferrite content, microstructure, mechanical properties and pitting corrosion resistance was studied by simulating a water depth of 15 m in the hyperbaric chamber. The results show that, under the same process parameters, the size and proportion of austenite in the weld when using pure nitrogen as the shielding gas are larger than those protected by other shielding gases. In a mixed shielding gas, the increase in nitrogen content has little effect on the strength and toughness of the weld. Regardless of the shielding gas used, the base metal was the weakest part of the weld. At the same time, intermetallic inclusions have an adverse effect on the impact toughness of the weld. The pitting corrosion resistance of the welds depends on the Cr2N content in the heat-affected zone. The precipitation and enrichment of Cr2N causes local chromium deficiency, which is the main factor for the weak pitting corrosion ability of the heat-affected zone. Pure nitrogen protection has a better corrosion resistance than other gas protection.