Microstructure Of Plasma Cladding Ni Based Composite Coating On TC4 Titanium Alloy

TC4 titanium alloy has been widely used in modern aerospace industry because of its light weight,high specific strength and excellect corrosion resistance.However,its wear resistance is low,and a sea of research on laser cladding of titanium alloy has been done,and surface strengthening effect has been obtained.The coating of Ni60 and Ni/ceramic phase composite coating based on TC4 alloy,which was fabricated by plasma beam surface modification technology.The ceramic phase by in situ formation after the mixing of graphite powder and Ti powder in proportion.Considering the three indicators of pool depth,surface quality and surface hardness,4 factors and 3 level orthogonal experiments were designed,The optimum process parameters are as follows:40A the current,gas flow rate 2L/min,4mm the nozzle distance,scanning speed 450mm/min.With this process parameters,Ni60alloy powder was precoated,and treated by single-pass cladding and multi-channel lap cladding,respectively.The single channel cladding layer was used for the hardness gradient test of the coating,microstructure observation,EDS composition analysis,XRD phase analysis,tensile test.The multi channel-lap alloying was used for wear and corrosion resistance test and the hardness and structure test of the lap joint.The following rules are obtained:1.The structure of coating surface was titanium substrate and other hard ceramic phases,including TiN,TiC,TiB solid solution,white mesh NiTi/Ti eutectic solution,dark colored titanium matrix solid solution with alloy elements,?Fe,Cr?₇C₃solid solution in the dark matrix,white reticular tissue matrix and the dendrite tip of the branch in.The internal microstructure of the cladding layer is the fine?'acicular martensite with solid solution and alloying element,which distributes TiC,TiB hard ceramic phases.The HAZ was a larger?'acicular martensite.The substrate was widmanstatten microstructure.2.The hardness of coating surface to matrix decreased step by step,the surface hardness of coating was approximately between 650⁷00HV_0.2,the hardness of inner coating was approximately between 400⁵00HV_0.2.The hardness value of HAZ(300³50 HV_0.2)is higher than that of matrix(270²90 HV_0.2).The hardness of cladding layer is the result of solid solution strengthening,fine-grained strengthening and dispersion strengthening.In vacuum condition,the cladding sample is tempered at 600?for three times.The secondary hardening occurs in the cladding layer,and the hardness of the cladding layer and the heat-affected zone is increased again by the fine strengthening phase precipitated by dispersion,which has very good tempering stability.The experimental results of multi-channel lap show that the lap zone is due to the secondary remelting of the plasma beam,precipitated more reinforcement phase,which makes it have higher hardness.3.The results show that the wear resistance of the Ni60 coating is improved significantly.After tempering,the precipitation of TiN hard phase increases,and the wear resistance is better.The corrosion resistance shows that the surface produces a compact natural TiN layer,which significantly improved the corrosion resistance.The binding performance of beam cladding layer was tested by tensile test.It is found that the cladding layer is well bonded with the substrate without falling off.The microstructure of the cladding zone was refined to increase the tensile strength and plasticity simultaneously.4.Cladding treatment of alloy powder with five ratios+vacuum 600?/1h tempering.The results show that the average hardness of the cladding coating with ratio of Ni60+5%[Ti+C]?Ni60+15%[Ti+C]is higher than that of pure Ni60 coating,and the secondary hardening effect is similar to that of Ni60 coating after high temperature tempering.The wear resistance is as follows:Ni60+15%[Ti+C]>Ni60+5%[Ti+C]>Ni60+25%[Ti+C]>Ni60+35%[Ti+C]>TC4>[Ti+C],and basically consistent with the hardness result.The wear resistance of[Ti+C]coating is the worst,which is due to the difference of physical properties between TiC particles brittle phase and matrix,so the adhesion is poor and the wear process is easy to fall off.Because the crystal structure of TiC is face-centered cubic and the cooling rate of each part of the molten pool is different,in situ formation of TiC by[Ti+C]coating is achieved from the surface of the molten pool to the bottom of the molten pool is:cross dendrite,radiate flower and cluster,bar shaped and granular tissue.5.The surface melting treatment of TC4 titanium alloy shows that the melting layer is divided into two sublayers,the solid solution in the titanium matrix is TiN,TiO₂ hard phase fine fir-tree dendrites,and the fine dense?'phase in the inner layer.Compared with the literature data of laser melting,the hardness of the melting layer is greatly improved.After 1³ times high temperature tempering,the hardness of the melting layer and HAZ is improved.The wear resistance and corrosion resistance increased significantly.The strong electromagnetic stirring of plasma beam makes N and O elements into the molten pool,and the surface produces a compact natural TiN and TiO₂ layer,which plays an important role in improving the hardness of the melting surface,and improving the wear resistance and corrosion resistance.