Investigation on solidification structure and temperature field with novel processing of synchronous powder-feeding underwater laser cladding

Synchronous powder-feeding underwater laser cladding(SULC) has a great application prospect in underwater in-situ manufacturing/remanufacturing due to its advantages of small heat input to the substrate and adjustable of additive materials. In this study, the iron-based planar coating with no phase transformation and dilution zone less than 8 μm was prepared on A32 deck steel by SULC using self-designed powder-feeding nozzle and optimizing process parameters. The powder-feeding gas flow rate directly determines the size and stability of the local dry region above the deposition point, and the laser power and powder-feeding rate are the key factors to compensate the laser energy dissipation. Due to the influence of local temperature gradient in SULC, the coating has obvious solidification structure distribution. Columnar grains are mainly distributed upon the trough of fusion line, while dendrites and equiaxed grains are distributed on both sides caused by the temperature gradient becomes smaller and the direction of heat flow changes due to the external environment. In the upper region of SULC coating the equiaxed grains tend to grow up with the increase of laser power. To gain better scientific understanding on differences between atmospheric laser cladding and SULC, the numerical simulation was used to reveal the temperature field changes and coating formation under SULC working parameters. The results show that under the same laser power, due to the forced cooling of larger gas flow and external water environment in SULC process, there is a larger temperature gradient at the deposition point, and the temperature drop rate of the coating and the substrate is greater. In the microstructure, it is shown as columnar grains has a greater directional growth trend. With the increase of laser power, this phenomenon becomes more obvious.