Investigation on in-situ laser cladding coating of the 304 stainless steel in water environment

The 304 stainless steel coating was prepared successfully in water environment directly through an in-situ underwater laser cladding technique utilizing an underwater gas-shielding laser cladded nozzle, and the interaction mechanism of water on cladding coating formation, grain morphology and size, microstructure and corrosion performance was investigated. For underwater laser cladding, the high-density aerosol particles decreased the laser power density and reduced the stability of cladding process, causing the conduction mode of molten pool and decreasing the wettability of molten metal. In underwater laser cladding coating, columnar dendrites nucleated near the fusion line and track band and grew up along the opposite direction of maximum temperature gradient whilst equiaxed grains formed in the center region; besides, the grain size decreased overall and the number of equiaxed grain dropped owing to the water cooling effect compared with in-air coating. For in-air and underwater cladding coatings, the solidification mode of molten metal was ferrite-austenite mode and the ferrite mainly distributed in the austenite matrix with skeleton shape. Attributed to the higher cooling rate, a small amount of lathy ferrite was observed in underwater cladding coating, and the ferrite content increased within the overlap region of tracks, causing the corrosion resistance of coating reduced.