A Study Of Ceramic-Metal Composite Coating Technique Prepared By Laser-Induction Hybrid Cladding

Laser cladding ceramic-metal composite coating (LCCC) has high hardness, excellent wear resistance and corrosion resistance, and thus has a wide application ranges in the surface strengthening and repairing of the key components. However, the low cladding efficiency, high volume of porosities and cracks in LCCC has restricted its applications in many fields. In order to enhance the cladding efficiency and reduce the metallurgical defects, induction heating, another heat source is introduced into the laser cladding process, which is named as laser-induction hybrid cladding (LIHC) in the dissertation. The processing characteristics, the heat damage mechanisms of WC particles and the wear resistance of the composite coating by LIHC were studied systemically. The main results were as following.A system of LIHC was developed, which was composed of a CO₂ laser, a high frequency induction heater, an auto-feeding powder apparatus and CNC controller, etc, based on which the processing parameters of the ceramic-metal composite coatings produced by LIHC were investigated. The results showed that the minimum laser specific energy, the maximum cladding height and the contact angle had a linear relationship with the maximum powder density per area. The maximum cladding efficiency increased with the increasing of the preheated average temperature of the substrate by induction heating. Moreover, the efficiency of LIHC can be increased to 1-4 times higher than that of the individual laser cladding.The effects of the laser processing parameters on the quality of the composite coatings by LIHC were investigated systemically. The results demonstrated that the choice range of laser processing parameters and the cladding efficiency using the parallel spot were wider and higher than those using the perpendicular spot during LIHC, respectively. By increasing laser scanning speed and powder feeding rate, the dilution of the composite coating by LIHC was decreased, the homogeneity of WC particles distribution was improved. As a result, the free-crack ceramic-metal composite coatings with a low dilution and high microhardness were obtained, with the weight fraction of WC particles higher than 35%wt and the metallurgical bonding between the composite coatings and the substrate. The heat damage fashions and mechanisms of WC particle in Ni-based WC composite coatings by LIHC were studied in detail. The results proved that the formulas of half quantity on the heat damage level of WC particles introduced before can be adopted to evaluate the heat damage extent of WC particles in multi-pass overlapping Ni-based WC composite coatings if the high laser scanning speed was adopted during LIHC, which demonstrated that the formulas not only had an extensive application range and strong practicality, but also may provide an important theoretical basis to evaluate the microstructure and properties of laser-induction hybrid cladding Ni-based WC composite coatings.When Ni-based alloy was used as the matrix alloy in ceramic-metal composite coatings by LIHC, the heat damage mechanisms of WC particles usually took the following models:(1) The dissolution-diffusion heat damage fashion: WC particles were continually dissolved by the molten Ni-based alloy at high temperature and exchanged their atoms each other, which resulted in WC particles dissolveng directly into Ni-based alloy solution and then precipitated in the format of M₆C, M₁₂C and other carbides during the rapid solidification processing. In the meantime, the alloying elements such as Ni, Cr and Fe diffused into WC particles rapidly, thus forming an alloyed reaction layer with limited thickness.(2) The dispersion-dissolution diffusion hybrid heat damage fashion: a part of WC particles with small size were dispersed into several small pieces of WC particles when they were irradiated by laser, which was heat damaged as the dissolution-diffusion heat damage fashion above mentioned. However, these dispersed WC particles with relatively smaller size may change into the alloyed carbides with high W concentration after they interacted with Ni-based alloy solution at high temperature.(3) WC particles with relatively smaller size may dissolve into Ni-based alloy solution completely, and then precipitated into the composite coating in the format of carbides such as M₆C, M₁₂C, M₂₃C₆ and M₇C₃ with different morphology (i.e. blocky, bar-like and fish-bone shape).The compositions of the matrix alloys have a big effect on the heat damage of the WC particles. It was found that WC particles had a complete dissolution in Fe-based WC composite coatings produced by LIHC. The coatings was composed of M₆C, M₂₃C₆ martensite and the retained austenite and the microhardness of the binder metal was lower than that in Ni-based WC composite coating under the same processing condtions. This result has not reported before. This difference was mainly caused by the difference of the dissolution level and rates of WC particles in the binder metal, melting point and latent heat between Ni-based and Fe-based alloy. The above results were highly helpful to choose a reasonable binder alloy.The sliding wear resistance and mechanisms of Ni-based WC composite coatings and Fe-based composite coatings prepared by LIHC were studied systematically. The adhesion wear, abrasive wear, fatigue wear and oxidation wear were simultaneously found in the samples by dry sliding wear process of the ceramic-metal composite coatings, the debris with lamellar structure were sheared from the composite coatings by the delamination pattern. Especially, the heat damage extent of the WC particles had a big effect on the sliding wear resistance of the composite coatings. By increasing laser scanning speed, the wear resistance of Ni-based WC composite coatings by LIHC can be 1.4 times as much as that of Ni-based WC composite coatings by the laser cladding with the same composition of the alloying powders. The main reason was that increasing laser scanning speed can decrease the heat damage extent, which can increase the microhardness of the composite coatings, improve the homogeneous distribution of WC particles, decrease the average distance of WC particles and limit the plastic transformation of the binder.In summary, the above results demonstrated that laser-induction hybrid cladding ceramic-metal composite coatings had more excellent propeties than the laser cladding ceramic-metal composite coatings with the same composition of alloy powders, which may find a wide application field in industrial area.