Research On The Grain Refinement Mechanism And Wear Properties Of Laser Cladding NiCrBSi Coating Assisted By An In-Situ Laser Shock Wave

As an advanced surface modification technology,laser cladding has been widely used in the mining machinery industry.It is especially suitable for remanufacturing the middle plate of the scraper conveyor to improve the wear resistance of the surface of key parts.Based on the analysis of laser cladding assisted by compound energy fields at home and abroad,this thesis proposed a new manufacturing method,laser cladding assisted by an in-situ laser shock wave（LSW）,aiming at refining the grains and improving wear resistance of the coating.In this thesis,the preparation of laser cladding Ni Cr BSi coating assisted by an in-situ LSW was the main line,a variety of tests,analysis methods and theoretical deduction are used to determine the grain refinement mechanism during experiment.In addition,laser cladding coatings with different distances between the two laser spots were prepared.After that,the microstructure and wear resistance of each coating were studied to determine the relationship between the process parameters and properties of the coatings.The main contents of this article are as follows:Based on the Taguchi-grey correlation analysis,the process parameters of the laser cladding Ni Cr BSi coating were optimized,and the powder feeding speed,laser power and scanning speed were selected as the process parameters to be optimized,and the cladding height,cladding width and dilution rate were taken as targets.An orthogonal experiment with three factors and five levels was designed,the signal-to-noise ratio of the three response targets was analyzed by variance analysis,and also the influence of each factor on the target.The gray relational method is used to convert the three response targets into a single gray relational degree for analysis,so that the process parameters with the largest coating width,the smallest coating height and appropriate dilution rate can be found.Finally,the prediction of gray relational degree is verified by experiment.After preparing the laser cladding Ni Cr BSi coating with an in-situ LSW,theγ-Ni grains in the coating were significantly refined,and the length and width of theγ-Ni grains were about 50μm,which decreased by 50%compared to the coating without an in-situ LSW,and tended to be more equiaxed.And the secondary dendrite arm spacing of theγ-Ni grains is reduced from 1.98μm to 1.21μm,which was 38.89%lower than the coating without LSW.In addition,the microhardness of the coating with LSW was increased from 526 HV to 602 HV.This is because when the LSW propagated to the bottom of the molten pool,the pressure attenuation was still 70 MPa,much higher than the yield strength of dendritic 25 MPa,and the dendrites were interrupted.The detached dendrites increased the nucleation center,which refined the grains in the coating and promotes the diffusion of C and B atoms to the grain boundary,increasing the proportion of Cr B phase.The extended experiments showed that the grain refinement and the aspect ratio of the high-entropy alloy Al Co Cr Fe Ni coating were significantly reduced,indicating that this new technology has huge application.The study of laser cladding coatings with different distances between the two laser spots showed that only at an appropriate distance can the structure of the coating be sufficiently refined and the Cr B phase can be generated.Which increasing the hardness of the coating with LSW at 0.9 mm,compared with the coating without LSW and other distances.This was determined by the solidification behavior of the coating.When the distance between the two spots was small,only a very tiny fusion zone has been generated,so the growth of dendrites was insufficient.After the shock wave acted,it can only stir the molten pool.When the spacing is moderate,the dendrites continue to grow and the molten pool was deeper,and the dendrites shocked by the LSW can enter the molten pool and proliferate continuously as nucleation centers to refine the grains.When the distance was large,the dendrite grows fully,and the shock wave can only break the top of the dendrite,so the impact is weakened.The wear mechanism of the coating under a load of 50 N is dominated by abrasive wear.When the distance between the two light spots was 0.9 mm,the hard phase of the coating was not easy to fall off,and there were fewer hard phase particles in the abrasive debris,which only cause light damage to the worn surface.Under the load of 100 N,more frictional heat were generated on the friction surface,which promoted the formation of oxide film.Therefore,the friction coefficient of each coating were reduced,and the wear mechanism was transformed into oxidative wear.The volume wear rate of the coating with LSW at 0.9 mm was the smallest,0.34×10^-4 mm³/N·m.There are 63 pictures,17 tables and 105 references in this thesis.