| The excellent thermal conductivity, outstanding electric capability, good elasticity of copper and its alloys make them interesting materials for applications involving electrical apparatus, machinery, metallurgical equipment and aerospace components, etc. However, due to the poor wear resistance, resistance to deformation and hot corrosion resistance, the service life of copper components such as chill mould for continuous casting is decreased seriously in extreme conditions without surface treatment. Laser surface cladding (LSC) can raise the reliability of working surface and increased service life of components.Though LSC can be easily achieved for many nonferrous metals it is difficult to prepare crack-free and nonporous coatings on copper, due to the good thermal conductivity, poor wettability with many other materials and reflectivity to laser beam during laser processing.In the present study, Wear-resistant coatings were fabricated onto a Cu-Cr-Zr alloy by LSC using a pulsed Nd:YAG laser with uniformly powder preplacement beds to increase the laser energy absorptivity of Cu surface. Surface and cross-section topography, microstructures, phase constitutions, interfacial structure, formation mechanism of reinforcement and wear properties of coatings wear investigated by means of scanning electronic microscopy (SEM) with X-ray energy dispersive microanalysis (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC), as well as dry sliding wear test.Based on Ni and Cu has complete miscibility with each other, which forms an ultimate mutual solution, four Ni-based non self-fluxing alloy and two Ni based self-fluxing alloy were deposited by LSC. It is found that pure Ni, Ni-30Cu, GH-01, Ni30 and Ni45 alloys have lower wettability on copper, which induces cracks or pores in coatings and interface zone. One reason of cracks is borides or silicides formation in coatings and interface zone. Even though an amount of Cu is in coating, it still does not improve the wettability with copper. The KF-6 alloy coating was generated metallurgical bonding with copper substrate due to chemical heat release between Ni and Al induced by laser. The coupling interaction between marangoni effect and non-equilibrium rapid solidification resulted layer structure in interface zone.Co-based alloy coatings have been fabricated on copper substrate to improve heat and wear resistance of copper components. The results showed that technological parameters influence the wettability of HG-Co01 alloy with copper. Cracks were formed in HG-Co02 alloy coating due to borides and silicides. Microstructure of HG-Co01 coating was composed ofα-Co solution and Cr23C6. The Ni-based solid solutions (a-Co, Ni) and (Ni, Cu) were formed at interface, which generate metallurgical bonding by diffusion between Co-based coating and copper substrate. To obtain better interface between HG-Co01 coating and copper, the KF-6 alloy as an interlauer was prepared by LSC on copper firt and then HG-Co 01 multilayer coatings were cladded. By this mean, Co/Ni gradient coating was prepared on copper which dramatically improve the compatibility between Co and Cu. The compact-grainstructure was obtained without defects in gradient coating.With KF-6 alloy as an interlauer and HG-Co01 alloy as metal matrix, metal matrix composite (MMC) coatings were successfully fabricated on copper by LSC using introducing Ni-coated WC reinforcement phase and in-situ synthesis of TiC, respectively. The results indicated that composite coatings and copper substrate were bonded metallurgically. WC particles were dispersively distributed in MMC coatings with slightly heat damage. WC particles kept their original multiangular shape. The alloying elements such as Co, Cr and Ni diffused into WC particles rapidly during laser processing, thus forming an alloyed layer with limited thickness. In situ synthesized TiC+Ni/WC reinforced Co-based coatings were fabricated on copper substrate by Nd:YAG LSC using preplaced powder. Reindorcements dispersed uniformly in the matrix, and TiC shows the morphology of dendritic and particle. The atomic ratio for formating reinforcements is of importance to the constituent of raw components. During LSC, the laser-materials interaction induces the heating effect and formating melt first, then Ti element diffused into WC particles forming TiWC2 alloyed layer. In the meantime, heating effect results the Ti atom and C atom in interdiffusion action and formation of TixCy. The coupling of lsaer and exothermic reaction with concentration compensation generated stable TiC finally. The nucleation and growth mechanism of TiC is diffusion and dissolution-reprecipitation mechanismThe hardness and wear resistance of copper were significantly improved by LSC Co-based alloy coatings. The hardness of interfacial zone in composite coatings decreased gradually. The results of wear test at room temperature showed that the friction coefficients of MMC coatings were less than 0.25. The copper adhesive wear severely while after LSC composite coatings, its wear resistance was improved significantly.In engineering application, chill mould plate for continuous casting was taken as typical practical application copper components to treat by LSC. The main failure forms of electroplating coating on chill mould plate are wearing and fatigue crack. The results of wear test under 400℃indicated that the wear resistance of Ni/Co composite coating was 3.15 times of that of electroplating coating. The Ni/Co composite coating on chill mould plate for continuous casting by LSC has successfully fabricated for practical application. The service life of LSC Ni/Co composite coating is more than two times of that of Ni-Co electroplating coating. |
PDF Full Text Request