Underwater laser welding for 304 stainless steel with filler wire

Underwater laser welding for 304 stainless steel with filler wire was successfully performed with a self-designed double-side gas-shielding nozzle. The effects of the back gas flow rate, travel speed and wire feed speed on the welding appearance, geometry characteristics of cross section, defects and mechanical property of butt joints were investigated. While the bottom surface of joint touched the water directly, the surface appearance was uneven and some small pores formed. With increasing the back gas flow rate to 50 L/min, the stable local dry cavity around welding zone was generated and an underwater laser welded joint without defects was obtained. After the optimization of process parameters, the tensile strength and impact toughness of the underwater laser welded joint approached those of in-air joint basically, and the using of filler wire improved the tensile strength of butt joint significantly. Compared to the in-air joint, the water cooling effect decreased both the area and width of fusion zone of underwater joint. This accelerated cellular dendrites grew up from fusion line to the bead center, which reduced the number of equiaxial grain and thus decreased the tensile strength slightly. In addition, the high cooling rate decreased the grain size, but increased the lathy ferrite content in fusion zone of underwater joint, decreasing the toughness of weld metal.