Microstructure evolution and mechanical properties of 308L stainless steel coatings fabricated by laser hot wire cladding

Abstract Laser hot wire cladding (LHWC) technology has been proved to be a very efficient laser cladding manufacturing process due to its advantages of fast deposition rate, high material utilization rate, and less environmental pollution. In this study, LHWC technology was used to prepare multiple layers of 308L stainless steel coatings on ordinary mild carbon steel substrates. The microstructure of the weldment section was evaluated by optical microscopy (OM). The results showed that the heat-affected zone (HAZ) of the sample was small, with a height of 1.58 mm and an area of 14.28 mm2. The metallurgical bonding between the substrate and coating was good. Because of the temperature gradient and grain growth rate, the grain morphologies in the different coating regions were different. From the bottom to the top, the grain morphologies were planar crystals, columnar dendrites, and equiaxed grains. Electron backscatter diffraction (EBSD) was used to obtain crystallographic information on the entire section of the sample. The interior of the cladding layer was mainly composed of columnar grains growing along the structural direction, most of which were coarse grains. The matrix and the HAZ consisted of fine equiaxed grains, and the crystal orientations were randomly distributed without obviously preferred orientations. Hardness analysis performed that the hardness of the HAZ was the highest, and the matrix had the lowest hardness. The hardness was higher in the layer-to-layer fusion zone and then gradually decreased with an increase in the cladding height inside each layer. The tensile test results illustrated that the elongation of the cladding layer (44.8 %) was higher than that of the HAZ (32.1 %) and the matrix (20.4 %) because the tensile direction of the tensile sample in the cladding layer was parallel to the laser scanning direction, which improved the tensile performance. The average grain size of the HAZ was the smallest, owing to the effect of fine grain strengthening, and the tensile strength was the highest(585 MPa). According to the tensile fracture profile, the sample matrix, HAZ, and the cladding layer exhibited ductile fractures.