Study On The Regulation Mechanism Of Safe Service Performance Of Laser Micro Melting Cr Coating Fabricated By Multi Arc Ion Plating On Zirconium Alloy

As the fuel cladding tube of nuclear power water-cooled reactor,zirconium alloy is the first safety barrier of nuclear reactor.Therefore,its safe service is the key to ensure the safe and efficient operation of nuclear reactor.Chromium,which has good corrosion resistance,low neutron scattering cross section and good thermal matching with zirconium alloy substrate,has been adopted as the first full-scale coating material.However,the Cr coating fabricated by multi arc ion plating technology is easy to fall off and fail under the harsh conditions of creep,fatigue and thermal shock due to the disadvantages of mechanical bonding,large particles and holes on the surface of coating.In this paper,a new composite technology of laser micro melting and multi arc ion plating is adopted,which can not only improve the bonding performance of the coating,but also realize the accurate deposition of micron coating thickness.The regulation mechanism of performance of laser micro melting the Cr coating on zirconium alloy fabricated via multi arc ion plating is studied,with the purpose of marked improvement of adhesion,corrosion resistance and high temperature oxidation resistance of Cr coating by controlling the diffusion degree of the film-based interface via adjusting energy density of laser.Such technique may be important for improving the safety and reliability of zirconium alloy clad tube in accident environment.(1)The quality evaluation index of zirconium alloy surface coating for safety service is analyzed.The coupling design of Cr coating thickness was carried out based on the wear resistance and corrosion resistance of the coating under normal operation and transient conditions,and the multi factor conditions of high temperature steam oxidation resistance of the coating under design basis accident conditions.Moreover,the surface micro morphology and composition,surface porosity and surface roughness of Cr coating prepared by multi arc ion plating were evaluated,and the performance of coatings was also tested.(2)In view of the small technological window conditions of laser micro melting micro Cr coating,the simulation model of laser micro melting temperature field oriented to interface diffusion was established.Through the orthogonal experiment of temperature field,the laser absorptivity of Cr coating prepared by multi arc ion plating was 40%.Under this condition,the laser micro melting process parameters were optimized.The optimization results were as follows:spot diameter is0.1mm and scanning speed is 1600 mm·min^-1,When the energy density is in the range of 9J/mm²to 18J/mm²,the whole stage of micro fusion diffusion can be displayed.(3)The laser micro-melting technology of multi-arc ion plating Cr coating was researched.The effects of laser micro melting on phase composition,micro density,roughness and microhardness of Cr coating were investigated.It is found that the diffusion of coatings and substrate is induced by laser microfusion and Zr Cr₂phase formed in the coating.When the energy density is in the range of 9J/mm²to 18J/mm²,with the increase of laser power,the micro density and microhardness are improved,the support length ratio RMR(c)is enhanced,which indicates that the surface wear resistance is improved.while the surface roughness of the coating increases.The area ratio of micro holes on the surface of the coating decreased from 4.6%to 0.3%,the micro particles decreased from7%to 0.4%,and the microhardness increased by 116%from 302.2HV_0.2to 652.7HV_0.2.(4)The influence of energy density on the interface of Zr coating/Cr substrate was studied.According to the diffusion degree,it can be divided into four stages:interface micro fusion diffusion,surface micro fusion densification,film substrate mutual fusion and laser remelting.When the energy density is 9 J/mm²,only micro fusion diffusion occurs at the interface,due to the existence of Zr/Cr eutectic region at the interface between the film and the substrate,and the lowest melting point(1332°C)of the eutectic alloy is much lower than that of Cr(1863°C).With the increase of energy density,the interface diffusion intensifies,and the surface of Cr coating begins to produce micro melting.As the energy density continues to increase,the mutual fusion occurs between the substrate and the coating,and the Zr/Cr mutual fusion zone is formed in the whole coating and part of the substrate.When the energy density is increased to 15.75 J/mm ²,the high energy is conducive to the full diffusion of Zr and Cr,and the stable Cr₂Zr coating is formed by remelting.(5)The influence of laser energy density on the bonding property,high temperature oxidation property and corrosion property of Cr coating was studied.The results show that the coating prepared by multi arc ion plating with brittle property and mechanical bonding between film and substrate is transformed into plastic property and metallurgical bonding by laser melting treatment,which indicates that the bonding property is improved.At the same time,the corrosion resistance is improved.The results of high-temperature oxidation experiments show that the high-temperature oxidation resistance of Cr coating prepared by laser micro melting is slightly reduced due to the dilution effect of Cr₂Zr region generated by laser micro melting diffusion on the high-temperature oxidation resistance of Cr coating.The results show that the Cr coating with spot diameter of 0.1 mm,scanning speed of 1600 mm·min^-1and energy density of 11.25 J/mm²has excellent comprehensive service performance.The reason is that under the condition of this process parameters,the interface of laser micro melting coating is micro melting diffusion,the film substrate bonding form is changed from mechanical bonding to metallurgical bonding,and the fracture morphology is changed from brittleness to toughness,which shows that the bonding performance is improved;the coating surface is micro melting,the local surface is densified,and the grain is refined,which improves the corrosion resistance.The non diffused Cr coating is retained in the coating,which makes it have good high temperature oxidation resistance.