| As a kind of alloy with excellent properties such as low density,high specific strength and good biocompatibility,titanium alloys are widely used in various fields,but their poor wear property and low hardness limit their developments.In order to overcome these difficulties,domestic and foreign researchers have tried to modify the surface properties of titanium alloys by ion implantation,chemical heat treatment,plasma spraying,vapor deposition and other methods,improving the surface properties of titanium alloy to some extent.Among them,laser surface alloying technology has attracted extensive attention from scholars due to its high processing efficiency,small heat-affected zone and good bonding with substrates.By designing a reasonable alloying materials system and optimizing the process parameters,alloying coatings with good quality and excellent properties can be prepared on the surface of the titanium alloys.In this paper,nickel coated graphite(G@Ni)and nickel coated hexagonal boron nirtide(h-BN@Ni)composite materials systems were utilized for laser alloying on TC4 titanium alloy.During the alloying process,complex chemical reactions occurred in the molten pool,for example,Ti reacted with B,C and N,forming TiC,TiB and TiN;Ni,Ti and Al were able to form Ni-Ti,Ni-Al and Ti-Al intermetallics.The various phases acted as a composite strengthening effect on alloying coatings,which significantly improved the hardness and wear resistance of the TC4 substrate.In this thesis,the effects of process parameters(scanning velocity,laser power)on the macroscopic morphologies,microstructure and wear resistance of the alloying coatings were investigated.The influence of adding nano scale rare earth oxide n-Nd2O3 on phase constituent,microstructure and wear resistance was studied.Besides,the formation mechanism of the phase in the alloying coatings was analyzed,and the strengthening mechanism and wear mechanism were discussed.The G@Ni alloying coatings were mainly composed of TiC,?-Ni,Ni3Al,NiTi,NiTi2 and graphite.Appropriate process parameters could ensure the presence of graphite in the alloying coatings,and graphite could play a certain role in friction reduction in the subsequent wear process.When the laser power was 3.5kW and the scanning velocity increased from 150mm/min to 450mm/min,the content of the hard phase formed in the alloying coatings decreased and the microstructure became finer.When the scanning velocity was 300mm/min,the average microhardness was 1319.25HV0.2,which was the highest among the three.When the laser scanning velocity was 300mm/min and the laser power increased from 3.0kW to 4.0kW,the hard phase content in the alloying coatings increased,as well as the size and density of the coatings increased.When the laser power was 4.0kW,the average microhardness was 1423.76HV0.2,which was the highest among the three.The nano scale rare earth oxide n-Nd2O3 mainly played a role in refining the microstructure,but the addition amount should not be too much,and generally there was an optimum value.When the addition amount of n-Nd2O3 was too much,the dilution ratio of the alloying coatings would increase,and there would be no graphite existing in the coatings,and the microstructure would be coarse,thereby too much n-Nd2O3 addition was detrimental to the properties of the alloying coatings.In this study,when the addition amount of n-Nd2O3 was 0.5wt.%,there was graphite remaining in the alloying layer,which played a certain lubricating role in the wear process.The friction coefficient was 0.45,which was 10%lower than the TC4 substrate,and the wear mass loss was only 0.0017g,which was 52.8%lower than the alloying coating(0.0036g)without adding n-Nd2O3 under the same wear conditions,and its wear resistance was increased to 18 times that of the TC4 substrate.The alloying coatings fabricated with h-BN@Ni were mainly comprised of ?-Ni,TiN,NiTi,NiTi2,TiB,h-BN,?-Ti,Ni3Al and TiO2.The microstructure of TiN was mainly granular or dendritic,and TiB was a kind of rod-like structure.A small amount of residual h-BN could provide some lubrication.The average hardness of the alloying coating reached 1188.72HV0.2,which was about three times the TC4 substrate.Under room temperature wear conditions,the wear mass loss of h-BN@Ni alloying coating was only 0.0017g,and the wear resistance was increased to 10.65 times of the substrate.Under the wear condition at 800?,the friction coefficient of h-BN@Ni alloying layer was only 0.3,which was 40%lower than the substrate,the wear volume is 5.46 827?×10-9m3,and the wear resistance is increased to 4.25 times of TC4 matrix. |
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