Microstructure And Properties Of CoCrFeNi Based High Entropy Alloy Coating Prepared By Laser Processing

45 steel is a kind of high quality carbon structural steel,which is widely used in practical production because of its low price,wide source,excellent cold and hot processing properties.However,the poor corrosion resistance and wear resistance limit the further application of 45 steel in complex and severe conditions such as wear and corrosion.In recent years,Co CrFeNibased high entropy alloys have attracted much attention due to their excellent corrosion resistance.In order to improve the corrosion resistance and wear resistance of 45 steel,Co CrFeNihigh entropy alloy coating was laser cladding on the surface of 45 steel.Through the control of molding process,phase and microstructure analysis and performance test,the following conclusions are obtained as followed:（1）By calculating the thermodynamic parameters and analyzing the formation law of high entropy alloy phase,the composition design was carried out,and the alloy forming process was adjusted based on single factor experiment and orthogonal experiment.The results of single factor experiment show that with the increase of laser power,the width,depth and height of cladding layer increase,and the dilution ratio increases gradually.With the increase of laser scanning speed,the width,depth and height of cladding layer decrease,and the dilution first increases and then decreases;On the basis of single factor experiment,orthogonal experiment was designed to optimize the process parameters.The results show that the primary and secondary relationship of each factor on the forming quality of cladding layer is scanning speed>laser power>powder feeding speed.When the parameters are 1000W,4mm/s,1.2r/min,the macro forming quality of high entropy alloy cladding layer is good,and there are no pores,cracks and other defects.（2）CoCrFeNiMo_0.2high entropy alloy coating is composed of single-phase fcc structure.The microstructure of the alloy coating from the bottom to the top is planar crystal columnar dendrite with obvious directivity dendrite with secondary dendrite arm equiaxed crystal.After adding 5%,10%,15%and 20%WC particles,the WC/CoCrFeNiMo_0.2high entropy alloy coating does not form complex compound phase,and is still composed of single-phase FCC.The addition of WC particles hinders the grain growth of high entropy alloy,resulting in grain refinement.（3）The average hardness of CoCrFeNiMo_0.2high entropy alloy coating is about206HV,and the hardness of heat affected zone is higher than that of alloy coating,about560HV.The hardness of the alloy coating increases to 298HV,316HV,327HV and347HV after adding 5%,10%,15%and 20%WC,respectively.Although the introduction of ceramic reinforcement does not show its advantage in improving the hardness of traditional alloys,the friction and wear tests show that the wear resistance of high entropy alloy is better than that of 45 steel.When the WC content reaches 15%,the friction loss rate is 3.48×10^-6.Compared with 45 steel substrate and coating without WC particles,the wear mechanism of high entropy alloy is different from that of traditional ceramic composite alloy.In 5%nitric acid,0.5m sodium hydroxide and 3.5%sodium chloride solution,CoCrFeNiMo_0.2high entropy alloy coating is self passivated,showing good corrosion resistance.After adding 5%,10%,15%and 20%WC,the corrosion resistance of the alloy coating in the three solutions is improved.In 5%nitric acid solution,the corrosion resistance is improved obviously,and the self corrosion current is reduced by two orders of magnitude.