Research On Microstructure And Properties Of High-entropy Alloy Coatings With Magnetically Conductive And Wear-resistant By Laser Cladding

With the rapid development of aerospace,ships,construction machinery and other fields,the requirements for sealing technology are increasing.As a new sealing technology,magnetic fluid sealing has the advantages of low leakage rate,simple structure,long life and high reliability,and has a broad application prospect in many fields.However,some commonly used traditional rotating shaft materials,such as40Cr Ni Mo A,34Cr Ni Mo6,304 stainless steel,have low magnetic permeability and poor wear resistance,and do not meet the use requirements of magnetic fluid sealing,which limits the popularization and application of magnetic fluid sealing technology.Based on the service condition and performance requirements of the rotating shaft in the magnetic fluid sealing device,304 stainless steel is used to prepare the coating of high magnetic wear alloy with high entropy resistance.The coating system of FeCoNiSi_xAl_x（x=0,0.2,0.4,0.6,0.8,1.0）was designed,and the laser cladding process parameters were optimized by studying the surface morphology,defects,microorganization and formability of high entropy alloy cladding layer under different laser cladding process parameters.Based on the existing laser cladding process parameters,the effects of Si,Al element content and heat treatment process on the microstructure,micro-Vionliers hardness,friction and wear characteristics and soft magnetic properties of FeCoNiSi_xAl_xcladding layer were studied.After several control variable tests,it is concluded that when the laser power is2100 W,the scanning speed is 500 mm/min,and the powder feeding rate is 7.2 g/min,the dilution rate of the cladding layer is moderate,no cracks and obvious pores,and has good forming performance,and when the lap rate is 50%,the cladding effect is good.According to the analysis of microscopic organization and element distribution,the interior of the cladding layer are planar crystal,columnar crystal and isoaxial crystal from bottom to top.The interior of the cladding layer is separated to a certain extent,and Siand Al are mainly concentrated at the grain boundary.According to XRD analysis,the crystal structure of the FeCoNiSi_xAl_xis single face-centered cubic structure at x=0 and 0.2.When x is 0.4,0.6 and 0.8,the crystal structure of the FeCoNiSi_xAl_xhigh entropy alloy is mixed by face-centered cubic and body-centered cubic.The crystal structure of the FeCoNiSiAl high entropy alloy is face-centered cubic at x=1.0.The phase composition is mainly the silicide-enhanced phase and the ferromagnetic phase such as Fe Co and Fe Ni.With the increase of Siand Al content in the FeCoNiSi_xAl_xcladding layer,the microhardness and wear resistance of the cladding layer have increased significantly.The Vickers hardness of the cladding was increased from 176.2 HV_0.2（x=0）to 490.5HV_0.2（x=1.0）,and the friction coefficient and wear rate decreased from 0.6587 and31.15×10^-6mm³/N·m（x=0）to 0.5350 and 9.79×10^-6mm³/N·m（x=1.0）,respectively.However,Siand Al,as non-ferromagnetic elements,improve the hardness and wear resistance of high-entropy alloy materials,and change the soft magnetic properties of Fe Co Ni,such as B_s,coercivity H_c,initial magnetic permeabilityμ_iand maximum magnetic permeabilityμ_m.The saturated magnetic induction intensity B_s,initial permeabilityμ_iand maximum permeabilityμ_mdecreased from 2.49 T,6.58 and 7.23 at x=0 to 0.82 T,3.18 and 3.33 at x=1.0,respectively,while the coercivity H_cincreased from 107.60 A/m at x=0 to 933.15 A/m.After the annealing heat treatment of the high entropy FeCoNiSi_xAl_xalloy cladding layer（850℃,20 min）,recrystallization and element diffusion occurred inside the cladding layer,which reduced the segregation between the grain boundary and the crystal,and the grain size was significantly reduced compared with that in the unheat treatment.In addition,the hardness and friction wear properties of FeCoNiSi_xAl_xhigh-entropy alloy were significantly improved after heat treatment.The Vickers hardness of the cladding layer increased from 490.5 HV_0.2to 536.28HV_0.2（x=1.0）,and the friction coefficient and wear rate were reduced from 0.5350and 9.79×10^-6mm³/N·m to 0.4606 and 4.63×10^-6mm³/N·m（x=1.0）,respectively.However,the annealing process reduces the grain size of the cladding layer,and the increased grain boundary limits the motion of the magnetic domain,which eventually reduces the soft magnetic performance of the cladding layer.