Research On Strengthening Of Plasma Cladding Coating On W6Mo5Cr4V2 High Speed Stee

W6Mo5Cr4V2（M2）high-speed steel（HSS）is commonly used to manufacture complex cutting tools due to its excellent red hardness and wear resistance.The cutting speed and accuracy are gradually improved with the rapid development of machining technology,which proposes higher requirements for cutting tools.Currently,creating the coating with an excellent performance on the surface of HSS is considered as one of the most effective and economical ways to improve its properties.In order to further improve the red hardness of Ni Cr Cu coating prepared by plasma cladding on M2 HSS,different contents of Co were added into Ni Cr Cu alloy powder.The coating with 20 wt.%（Co20）Co addition was selected to analyze and compare with Ni Cr Cu（Co0）coating to study the effect of Co on the microstructure evolution and mechanical properties of the coating.It provides theoretical support for optimizing alloy powder composition design and developing high-performance HSS surface coating with excellent red hardness.The coating has no pores,no macroscopic cracks,while owning uniform element distribution and good interface metallurgical bonding,which is prepared by plasma cladding process parameters of output power 4.4 k W,scanning speed 400 mm/s,gas flow 1 L/min,nozzle-to-sample distance 4 mm and nozzle diameter 2 mm.The width,thickness,characteristic microstructure（equiaxed grain,columnar dendrite grain and cellular grain）,grain size and phase composition of the coating are almost unchanged after 20 wt.%Co addition.In addition,the addition of Co increased the matching between the coating material and the substrate,and optimized the surface quality of the coating.The residual stress decreased from 718.17 MPa to494.33 MPa,which reduced the cracking tendency of Co20 coating.Co element effectively increases the martensite finish temperature（M_f）,and decreases the retained austenite content of the coating.The microstructure of Co0 and Co20 coatings before and after tempering is similar.Coatings before tempering is mainly composed of martensite,retained austenite and a small quantity of carbides,and the retained austenite decomposes after tempering.Co reduces the nucleation barrier of carbides,increases the activity of C element and reduces the activation energy of secondary carbide precipitation,which is conducive to the precipitation of carbides during tempering.Besides,Co also reduces the diffusion coefficients of W and Mo inα-Fe of coating,push the precipitation process of secondary carbides to high temperature,and inhibit the growth of carbides.Therefore,the secondary carbides in Co20 coating are more dispersed and fined,and the strengthening effect is better.Co20 coating has fine grains and dispersed secondary carbides,so it has good toughness while maintaining higher hardness;The addition of Co promotes the precipitation of secondary carbides,hinders the growth of carbides,and improves the red hardness obviously.The hardness of Co0 and Co20 coatings after tempering at different tempering temperatures for 4 h increases first and then decreases.The secondary hardening peak appeared at 550℃,and the addition of Co increased the secondary hardening peak value of the coating from 768 HV_0.1 to 941 HV_0.1.The decreasing rate of hardness of Co-containing coating is significantly slower than that of Co0 coating in the process of multiple tempering at 600℃,and the secondary hardening phenomenon still exists in Co20 coating;The wear resistance of the coating was significantly improved after adding Co.The average friction coefficient of the coating decreased from 0.97 to 0.76,and the volume wear loss decreased from 0.051mm³ to 0.022 mm³ after adding Co.This is due to Co increase the overall diffusion coefficient of the coating alloy system,which makes it easy to produce oxide films during friction and wear.In addition,the Co20 coating has higher hardness and good toughness,hence the formed oxide film is closely integrated and continuous with the substrate.The oxide film acts as a lubricant to protect the substrate during the friction and wear process,which improves the wear resistance of Co20 coating.The Co0coating is controlled by a mixed wear mechanism mainly severe adhesive wear,abrasive wear and oxidative spalling,and the Co20 coating is controlled by a mixed wear mechanism of oxidation wear and abrasive wear.