Research On Microstructure And Performances Of Laser Cladding Compound Coating By Fe-based Alloy And VC Powder

As the important nuclear equipment,the nuclear valve often works in high temperature and high pressure medium environment.The quality of the sealing surface affects the valves’service life directly.The material of nuclear valve sealing surface is generally cobalt alloy.But the element Co is a kind of rare metal and very expensive.The worst is,in radiation environment Co can be easily activated to be isotope Co-60 which can cause nuclear pollution for its long radioactive half-life.Laser cladding is an important surface modification technology,which uses the cladding material to clad a layer on the substrate surface with high-energy density laser beaming,to improve the quality of products,and extend the service time of mechanical products.Therefore,laser cladding has been widely used in the area of parts manufacturing and repairing.A new Fe-based alloy powder was designed in this article.Fe was set as the main element,and C,Cr,Si,Mo as added elements.Some ceramic powder was mixed to improve the comprehensive perforances.The Fe-based alloy and VC powder was cladded by diode laser on the surface of 316 substrate.The influences of different powder injection,laser power,laser scanning speed,powder feeding rate,VC content and powder size on the forming and performance of the cladding layer were investigated.The microstructure,component distribution and phase composition of cladding layer were analyzed in detail through optical microscopy（OM）,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）and X-ray diffraction（XRD）.The result shows that,a cladding layer with excellent surface formation with slag-free and crack-free can be obtained under 80° powder injection angles,1800W laser power,4mm/s scanning velocity,powder feeding rate of 45g/min,mass fraction of 30%VC and powder size of 320-400 Mesh.The phase composition of cladding layer with 0%VC is（Fe,Cr）₇C₃ and Cr₇C₃.Because of the addition of VC,the solidification characteristic of cladding layer changes from dendrites to equiax crystal finally.The phase composition of cladding layer with 30%VC is（Fe,Cr）₇C₃、Cr₇C₃、Fe₃C and VC.Analysis on the mechanical properties of the coating shows that the average micro-hardness of Fe based alloy cladding coating is 640HV_0.2,and the average micro-hardness of the cladding coating is 820HV_0.2.Dispersed carbides displays net structure,so that the high-temperature wear mechanism of cladding coating is dominated by abrasive wear,oxidation wear and adhesive wear.The cavitation resistance is 2.4 times of the Fe-based alloy cladding coating and 8.6 times of 316 substrate.The results show that Fe-based alloy and VC powder is a kind of laser cladding material with excellent performance and can be applied promisingly to improve the wear resistance of valve sealing surface.The study will provide the guidance for the design and application of metal matrix surface composites.