Research on interface characteristics of 308L stainless steel coatings manufactured by laser hot wire cladding

Laser hot wire cladding is an efficient and energy-saving additive manufacturing process, which is used in coating, repair, and 3D printing. In this study, 308L stainless steel wire was deposited on the low carbon steel substrate using Laser hot wire cladding technology. The microstructure evolution and microhardness changes in the heat-affected zone of the substrate and the cladding layer near the laser cladding interface were studied in detail by using optical microscopy, electron backscatter diffraction and microhardness tests. The phase transformations and the relationship between microstructure and mechanical properties of the cladding layer were discussed. Owing to the dual effect of temperature gradient and cooling rate, the grain morphology in the central region of the cladding layer is equiaxed, while the edge region is mostly columnar dendritic morphology along the structural direction. There is a dilution zone composed of single austenite near the interface of the cladding layer. In the cladding layer, the ferrite-austenite solidification mode is dominant, and the austenite-ferrite solidification mode is also found in local areas. The carbon migration process from the substrate to the cladding layer promotes the austenite-ferrite mode. The heat-affected zone microstructure consists mainly of ferrite and tempered bainite, and slatted martensite can be observed near the interface. The electron backscatter diffraction results demonstrate the existence of martensite in the heat-affected zone. The texture is more apparent in the cladding layer, while the heat-affected zone contains more residual strain and higher dislocation density. The average grain size in the heat-affected zone region is about 4 mu m, while the average grain size in the cladding layer is above 14 mu m. The hardness of the heat-affected zone near the interface is the highest due to numerous factors, such as grain size, microstructure transformation and dislocation density. The yield strength increases with the decrease of grain size and texture strength, demonstrating the correlation between microstructure and mechanical properties.