Fabrication,Microstructure And Properties Of WC Reinforced FeCoNiCr High Entropy Alloy Composite Coatings By Laser Cladding

High entropy alloy has excellent mechanical properties,wear resistance,corrosion resistance and high temperature resistance.It has great application potential in extreme working conditions.Laser cladding has become one of the main methods to fabricate high entropy alloys with the characteristics of high energy density,low dilution ratio,rapid heating and cooling,interfacial metallurgical bonding and so on.However,single-phase high entropy alloys with face centered cubic(FCC)or body centered cubic(BCC)have poor comprehensive properties due to strength and plasticity mismatch,which limits their engineering application.The high entropy alloy with FCC structure reinforced by hard particles is an important way to solve the strength-plastic mismatching.It has become one of the research hotspots at home and abroad.In this thesis,the effects of laser power and scanning speed on on the microstructure,phase composition,microhardness,wear resistance and wear resistance of Fe Co Ni Cr high entropy alloy coatings by laser cladding were investigated systematically.The effects of different WC contents on the microstructure,phase composition,microhardness,wear resistance and corrosion resistance of laser cladding Fe Co Ni Cr high entropy alloy composite coatings were investigated.The structures of WC reinforced Fe Co Ni Cr high entropy alloy composite gradient/alternating multilayer coating by laser cladding were designed,and the effects of different structures on the mechanical properties,corrosion resistance and impact resistance of high thickness coatings were explored.The research results show that:the laser power and scanning speed have great influence on the molding quality and macroscopic morphology of Fe Co Ni Cr high entropy alloy coating.The height of single track increases first and then decreases with the increase of laser power,and gradually decreases with the increase of scanning speed.The width increases with the increase of laser power and decreases with the increase of scanning speed.The coating is composed of FCC single phase.The grain morphology at the top of the coating is fine equiaxed crystal,and the grain morphology at the bottom of the coating is coarse columnar crystal,which shows a double layer structure.The microhardness of the coating is low,and the average of microhardness is 170 HV_0.2.The wear resistance of the coating decreases first and then increases with the increase of laser power,and increases with the increase of scanning speed within limits.The corrosion resistance decreases with the increase of laser power and increases with the increase of scanning speed.It was found that after the addition of WC particles,the phase composition of WC reinforced Fe Co Ni Cr high entropy alloy coatings changed from FCC single phase to FCC phase,WC phase,W₂C phase and Co₄W₂C phase.The microstructure morphology developed from top equiaxed crystal and bottom columnar crystal to dendrite morphology,and the massive and fishbone carbide precipitates were found.The contents of the two carbide precipitates increased with the increase of WC content.The microhardness of the composite coatings was significantly higher than that without WC.After adding 60%WC,the microhardness of the composite coating reached 501 HV_0.2,which was increased by about 186%.The friction and wear test results show that the coefficient of friction,wear volume and wear rate of the composite coating decrease greatly with the increase of WC content,and the wear resistance of the composite coating was improved.After the addition of WC particles,the corrosion resistance of the coating decreased,but it was still superior to other laser cladding coatings like Ni60,Stellite6 and so on.The WC reinforced Fe Co Ni Cr high entropy alloy composite coatings of gradient and alternating structures were designed and fabricated.Gradient coating has good molding quality and no defects such as cracks;alternating coating continuously generates sound of cracking during laser cladding,and the molding quality is poor.The microstructure of the gradient coating shows a gradient distribution.With the increase of WC content,the carbide precipitates in the coating increased gradually.The microstructure of alternating coating is alternating distribution.The interface between the layers of the gradient coating is clearly visible and the bonding is good.The microhardness of the gradient coating also presents a gradient distribution,and the microhardness increases continuously from the substrate to the coating surface.The wear resistance of gradient coating is better than that of single layer60%WC reinforced composite coating.Corrosion resistance is slightly worse than single layer 60%WC reinforced composite coating,both surface have pitting corrosion.Impact toughness of gradient coating is 75.8 J/cm~2,which has better impact resistance than other laser cladding materials like H13,42Cr Mo and so on.Through the analysis of impact fracture morphology,the impact fracture mode of gradient coating includes ductile fracture and brittle fracture.