| Multi-principal-element alloys(MPEAs)include high-entropy alloys(HEAs),medium-entropy alloys(MEAs),and low-entropy alloys(LEAs).Among them,the concepts of MEAs,LEAs,and MPEAs are all derived from HEAs.HEAs affect the phase stability of solid solution by controlling the configuration entropy.Due to the combined effects of hysteresis diffusion effect,high entropy effect,cocktails effect,and lattice distortion effect,the presence of multiple principal components in HEAs helps to achieve better material mechanical properties.However,MEAs and HEAs have similar properties,which can produce materials that meet high-end production requirements,challenging current design practices.Laser cladding technology is the use of high-power laser beams to instantly melt alloy powder and the surface material of the substrate simultaneously and rapidly cool,allowing the cladding material to deposit on the surface of the substrate to form a coating with good metallurgical bonding.It is considered the most valuable green manufacturing and remanufacturing technology.However,due to the rapid cooling characteristics of laser cladding itself,the temperature gradient and coefficient of thermal expansion of the cladding layer and the base material are different,and the defects such as pores,cracks,residual stress,composition segregation and structure inhomogeneity in the cladding layer are difficult to eliminate.With the aim of improving the wear resistance of GCr15 bearing steel,GCr15 was selected as the matrix material,and three different MPEAs,Co Cr Ni,Fe Co Ni Cr,and Fe Co Ni Cr Mo,were selected as laser cladding powder materials.Three types of laser cladding coating samples were prepared,and ultrasonic burnishing(UB)was performed on the three samples after laser cladding.The surface characteristics and friction and wear performance of three coatings before and after ultrasonic burnishing were compared and analyzed using the matrix material as a comparison sample.The main research conclusions include:(1)The microstructure of Co Cr Ni coating prepared by laser cladding from top to bottom is equiaxed crystal,columnar crystal,and planar crystal,respectively.The microstructure of Fe Co Ni Cr and Fe Co Ni Cr Mo coatings prepared by laser cladding from top to bottom is equiaxed crystal,cellular dendritic crystal,and planar crystal,respectively.The element distribution in the three coatings is uniform,and there is a clear boundary between the substrate and the coating,showing metallurgical bonding.Due to the formation of a molten pool and the presence of stirring on the metal surface by the laser beam,the matrix has a certain dilution effect on the multi principal alloy,resulting in a significant increase in the Fe content of the coating.The element diffusion zones of Co Cr Ni,Fe Co Ni Cr and Fe Co Ni Cr Mo coatings are about 40μm,52μm and 38μm,respectively,and the dilution rates are 2.6%,3.3% and 2.5%,respectively.(2)The roughness,porosity,and grain size of Co Cr Ni,Fe Co Ni Cr,and Fe Co Ni Cr Mo laser cladding coatings were tested and compared before and after ultrasonic burnishing.The results showed that compared with the samples without ultrasonic burnishing,the coating roughness of ultrasonic burnishing decreased by87.6%,88.7%,and 87.1%,respectively,and the porosity decreased by 73.4%,63.8%,and 67.0%,respectively.The average thicknesses of the strengthened layers were92.50μm,41.25μm and 50.50μm,respectively.(3)The microhardness,nanohardness,and elastic modulus of three laser cladding coatings before and after ultrasonic burnishing were tested and compared.The results showed that the hardness of Co Cr Ni,Fe Co Ni Cr,and Fe Co Ni Cr Mo coatings without ultrasonic burnishing increased by 49.9%,33.6%,and 86.4% compared to the substrate,respectively.The surface hardness of the coatings after ultrasonic burnishing increased again by 32.7%,41.7%,and 18.4%,reaching 452.0HV,430.0HV,and 501.6HV,respectively.The cross-sectional hardness of the three types of ultrasonic burnishing specimens showed a stepwise decrease from the outer surface to the substrate.The nano hardness and creep resistance after ultrasonic burnishing also showed a significant improvement.In addition,although ultrasonic burnishing significantly improved the hardness of the coating,the elastic modulus of the three materials did not show significant changes before and after strengthening.(4)The friction and wear experiments showed that the average friction coefficients of Co Cr Ni,Fe Co Ni Cr,and Fe Co Ni Cr Mo coatings without ultrasonic burnishing decreased from 0.592 of the substrate to 0.456,0.503,and 0.418,respectively.After ultrasonic burnishing,the average friction coefficients of the three coatings decreased to 0.375,0.410,and 0.352,respectively.The wear rates of the three coatings without ultrasonic burnishing decreased by 59.3%,52.3%,and 69.2%compared to the substrate,respectively,and the wear resistance was improved.The wear rates of the coatings after ultrasonic burnishing decreased again by 72.0%,68.5%,and 85.2%,respectively,indicating that ultrasonic burnishing can significantly improve the wear resistance of MPEA coatings.The main forms of wear are adhesive wear,abrasive wear,layered wear,and oxidation wear. |
PDF Full Text Request