Microstructure and properties of spiral gradient coating prepared by laser cladding

Abstract In this study, laser additive manufacturing technology is utilized with the objective of controlling the internal stress of the cladding layer to maintain it in a compressive stress state. The cladding material was carefully designed and selected, and an Fe-Cr-Ni alloy spiral gradient multi-layer coating with significant differences in alloy element content was prepared on the surface of 45 steel. Using an optical microscope, scanning electron microscope (SEM), and x-ray diffractometer (XRD), the effects of the angle between cladding directions on the crystallization morphology, microhardness, and wear resistance of the cladding layer were investigated. The experimental results indicate that the interlayer angle significantly affects the crystallization morphology and grain size of the cladding layer. As the interlayer texture angle increases from 30° to 90°, the grain size within the cladding layer is significantly refined, accompanied by improved hardness uniformity and higher hardness values. This phenomenon occurs because, when the layer is rotated by a certain angle, the intersection area between the fusion line of the cladding layer and the dendritic crystals within the contour of the previous layer increases. This results in a greater number of favorable sites for liquid-phase nucleation, thereby promoting grain refinement. Additionally, since the columnar crystals at the solid–liquid interface continue to grow along the secondary dendritic growth direction of the previous cladding layer, the spatial angle between the columnar crystals at the interface of the upper and lower layers changes as the interlayer texture angle increases. Consequently, the angle in the cross-sectional direction also increases. When the interlayer texture angle reaches 90°, an ‘L’-shaped structure is formed. The microstructure of the cladding layer with higher Ni content is mainly composed of martensite and austenite, leading to lower hardness and a higher tendency for adhesive wear. In contrast, the microstructure of the cladding layer with lower Ni content primarily consists of martensite with a small amount of retained austenite, resulting in higher microhardness and better resistance to adhesive wear. The spiral gradient structure of the coating effectively refines the grains and enhances the hardness of the cladding layer, providing a new method for the preparation of high-quality cladding coatings.