Underwater laser welding of tube-plate structure of aluminum alloy

Underwater local dry laser welding of the tube-plate structure was performed first. The effects of laser power and welding speed on welding quality were studied. With the increase in the laser power or decrease in the welding speed, the melting zone (MZ) area and depth-width ratio of the welded joint increased, and metallurgical porosity also increased. When the laser power was 2 kW and the welding speed was 12 mm/s, a better metallurgical bond could be formed between the tube and plate, and there were fewer metallurgical pores in the welded joint. Due to a large amount of evaporation and ionization of water in the gap between the tube and plate, metallurgical pores formed in the welded joint. By adding a waterproof layer on the back of the substrate, the metallurgical porosity decreased from 2.1% to 0%. In order to investigate the influence of a water environment on welding quality, the in-air laser welding was performed. Compared to in-air welded joint, the MZ area, depth to width ratio, and grain size were smaller. The average microhardness of underwater welded joint was higher than that of in-air welded joint due to the refined crystalline strengthening. In addition, because the magnesium burning loss in an underwater environment was less than that in an in-air environment, the microhardness values at the top of the underwater MZ were similar to those at the bottom, while the microhardness values at the top and bottom of the in-air MZ were much different.