| H13steel is currently widely used as stir welding head in friction stir welding technology due to its excellent mechanical properties. However, the stir welding head is easily worn and get deformation during stir welding process. This, in turn, restricts the usage and development of the technology. Hence, it is very essential to improve the surface properties of H13steel to prolong its lifetime, and broaden its usage in welding such materials that posess high melting point. It is well known that the initial stage of all failure is from the surface. Hence, gradient coatings were prepared by laser cladding technology on the surface of H13steel. The main purpose of this thesis is to improve the surface properties of H13steel, further, this technology is expected to provide important foundation for reparing and forming of stir welding head.Ni based and Co based alloy powders were used as cladding material substituted by WC and TiCN reinforced particles. Proper parameters were found by orthogonalization experiments as following:defocus distance of20mm, pulse frequency of30Hz, laser scanning speed of100mm/s, pulse duration of2.0ms, single pulse energy of6.46J, and overlap rate of30%. Gradient coatings were prepared using the above parameters, and the microstructure, microhardness distribution, wear resistance and high temperature oxidation property were investigated, and draw the following conclusions.The mocrostructure of WC and TiCN reinforced Co and Ni alloy coatings presented the same characteristics. The higer content of WC and TiCN is benefit for getting fine microstructure. Different reinforce particle has no obvious influence on microstructure distribution of Co based alloy. However, different reinforce particles influence the microstructure of Ni based alloy. The TiCN reinforced Ni based alloy coating presented better-structural homogeneity than WC reinforced Ni based alloy coating. Furthermore, compared to single coating, gradient coating presented better structural homogeneity.The microhardness of H13steel was significantly improved by laser cladding WC and TiCN reinforced Co and Ni based alloy coating. Not only had the value of microhardness increased by gradient coating design, but also the gradient distribution improved. The triple gradient coating presented a smaller grad of microhardness, which in turn will prevent the sudden change of mechanical properties of the coating. Comparing all results, it can be concluded that the coatings reinforced by WC presented higher microhardness value than that of the TiCN reinforced coatings. The microhardness values of all coatings can be arranged from high to low as following:WC reinforced Co coating, WC reinforced Ni coating, TiCN reinforced Co coating, and then TiCN reinforced Ni coating.The laser cladded coating presented better abrasive resistance than H13steel substrate. The weight loss of WC and TiCN reinforced double and triple coatings was far lower than that of the single coating and the substrate, indicating that the gradient design of the coating is favor of improving the wear resistace of H13steel. The wear mechanism of H13steel can be characterized as adhesive wear, and that of Co based and Ni based single coating is adhesive and abrasive wear and adhesive wear, respectively. In contrast to the single coating, the double gradient coating and the triple gradient coating presented the characters of typical abrasive wear. WC reinforced coatings presented more excerllent wear resistance than TiCN reinforced coatings.High temperature oxidation property of H13steel was improved after laser cladding gradient coatings. A compact oxide of Co, Cr, and Ni coating was formed after oxidation, which in turn prevented the substrate from being oxided. The gradient coating presented better high temperature oxidation property, Co based coating also presented better antioxidation property than Ni based coating. WC is a better candidate than TiCN in respect of improving antioxidation property of H13steel. |
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