Investigation On Microstructure And Properties Of Wide-Band Laser Clad Nickel/Ceramic Composite Coatings On High Strength Steel

High strength steels made by TMCP?Thermo Mechanical Control Process?had the excellent properties such as high specific strength and good weld ability which made them widely applied in the fields of bridge,building auto industries,and engineering machineries.The weight percentage of high strength steel utilized in engineering machineries such as hydraulic support could be up to 70%.Laser cladding has the advantages of tight metallurgical bonding,designable coating properties.The service life of work pieces could be largely extended if composite coatings with high hardness and good wear properties were fabricated on surfaces of high strength steels utilizing laser cladding.Besides,it also has wide application prospects in the area of machinery remanufacturing.In this study,composite coatings with good properties were fabricated on surfaces of Q550 high strength steel.Firstly.Ni60 and WC were designed as the alloy systems of cladding materials.Comparative studies were carried out in coatings made by wide-band laser and round beam laser.By process design of wide-band laser cladding and addition of modification materials such as Cr,Ti and Y₂O₃,both the microstructure and coating properties were modified.Systematic investigations of micro and macro interfaces in wide-band laser clad coatings were carried out.The growth control methods of the in-situ reinforcement phases were explored.In addition?the bonding strength of coatings was quantitative measured by shear tests and the fracture behavior of macro interfaces was also studied.Ni60 and Co 12 self-fluxing alloy powers reinforced with WC particles were clad on surfaces of Q550 high strength steel using round beam laser with diameter of 5mm.Results indicated the microstructure of Ni60/WC composite coating was composed of nickel dendrites and interdendritic precipitates.Phase constitution of Ni60/WC coating consisted of ?-Ni?Fe??Fe-Cr?Cr₇C₃?Cr₂₃C₆ and W₂B₅.The WC particles showed good strengthen effects on Ni60 powders due to the higher hardness.Thus Ni60/WC mixed powders were chosen as cladding materials utilized in surface modification of Q550 steel.Besides,laser cladding was also carried out using wide-band laser spot with dimensions of 17mm ×1.5mm.Procedures of wide-band laser cladding process involving laser power and laser scanning aped were optimized.Contrastive study of Ni60/WC coatings fabricated by wide-band laser and round beam laser was carried out.Results indicated special cored and non-cored eutectics were synthesized in wide-band laser clad Ni60/WC coatings.The cored eutectics consists of inside core precipitates which was star-like Cr₅B₃ phase and eutectic lamellar of?Cr,W?₂₃C₆ and y-Ni which grew around the kernel.Microhardness of wide-band laser clad coatings was increased to 600?800HV0.1 and wear properties were enhanced by more than 30%.In coatings with Cr addition,the main in-situ synthesized phases were also?Cr,W?₅B₃ and?Cr,W?₂₃C₆,but parts of Cr₇C₃ phases were generated.When Cr addition reached wt.8%,morphology of in-situ precipitates varied from star-like patterns and blocks to long strips.According to microhardness and wear properties,the optimum content of additional Cr was wt.4%.The addition of Y₂O₃ exhibits obvious grain refining effects on the in-situ precipitates,since they increased the nucleation sites in the molten pool.Growth control of the in-situ particles in Ni60/WC coatings was realized by adjusting of wide-band laser power.Three kinds of strengthening phases were formed.The in-situ synthesized particles by laser power 3.6kW consist of hard boride cores,nickel dendrites and carbides eutectic lamellar,which showed the best wear resistance.It was found that the Ti element had a significant homogenization effect on the microstructure of Ni60/WC cladding layer.The net structure TiC was formed by in situ reaction of Ti element and C in the molten pool.Thus the growth behavior of?Cr,W?₅B₃ ceramic particles varied from free growth to restricted growth.And the formation of?Cr,W?₂₃C₆ eutectics in original coatings was suppressed.The microstructure of the cladding layer is significantly homogenized and the wear resistance is also improved.The effect of wide-band laser cladding and round beam laser remelting on microstructure homogenization of cladding layer was investigated.It was found that laser remelting could eliminate the deposition of WC particles at the interface and both the morphology and distribution of the secondary nucleated in-situ reinforcements are more uniform.Microstructure and evolution of the interfaces in wide-band laser clad Ni60/WC coatings were studied.Precise control of the microstructure at the micro-interface between in-situ phase and nickel matrix is realized by adjustment of laser energy density.The thresholds value of laser energy density which caused variation of the three micro-interface structures are near 50J/mm² and 67J/mm².Microstructure at the coating/matrix interface zone is composed of nickel planar crystals,dendrites and deposited WC particles.The high laser energy density increases the melting of Q550 base metal,and promotes the development of plane crystal and dendrites at the interface zone.The bonding strength of coating/matrix macro-interface was quantitatively determined by shear tests.It was found that a positive correlation was established between laser energy density and the shear strength of coating interface.When the laser energy density exceeds 55J/mm²,the interfacial shear strength of the cladding layer is more than 200MPa and a good metallurgical bonding is formed.The fracture mechanism of coating/substrate interface is brittle fracture.With the decrease of wide-band laser energy density,the radio of radial region on fracture surface increased.The fracture mechanism varied from cleavage fracture to quasi-dissociation fracture.It can be noted the fracture behavior of coating/matrix interface results from the interaction between nickel dendrites and WC particles.