Study On The Variation Pattern And Properties Of The Melting And Solidification Organization Of Scanning Electron Beam Surface Alloying

30Cr Mn Si steel has the characteristics of high toughness,high strength and good heat hardenability,and is widely used in the manufacture of shaft and gear parts.Scanning electron beam surface alloying technology has the advantages of high energy utilization rate,no pollution to the workpiece,small workpiece deformation,etc.The use of scanning electron beam technology to alloy the surface of 30 Cr Mn Si steel to obtain an excellent surface strengthening layer is a proven method.In this study,30 Cr Mn Si steel was used as the object of research,and different ratios of WC-12Co/Ni mixed powder and different thicknesses of coating were prepared on its surface by plasma thermal spraying technology,and then the surface alloying treatment was carried out using the scanning electron beam method;a finite element model for the temperature field analysis of scanning electron beam alloying of WC/Ni was established based on the transient heat transfer equation,and the temperature distribution law of the surface alloying process was investigated.The effect of recoil pressure,surface tension,metal vaporization,gravity and metal liquid kinetic pressure on the fluid flow in the melt pool is investigated.The fusion process between the coating powder particles and the substrate by the electron beam was analysed from a microscopic point of view;the effect of different process parameters on the mechanical properties of the specimens was investigated by optimising the coating powder parameters and the electron beam process parameters through an orthogonal test.The surface and cross-sectional microstructure of the specimens,the composition of the reinforced layer,the hardness and wear resistance were analysed and tested using a scanning electron microscope,an X-ray diffractometer,a microhardness tester and a friction and wear testing machine.The reasons for the formation of the composite gradient coating in the alloy layer and its effect on the properties were discussed,and the reasons for the formation of nano grains in the alloy layer were investigated.The results of the study show that: the scanning electron beam can make the surface of the specimen instantly warm up to the melting point,so that the coating layer of powder and the base steel fusion together,scanning electron beam energy ring can have the function of secondary heating,scanning ring along the electron gun moving direction of the first half of the ring first heating specimens,followed by the second half of the ring to reach the previous moment of the first half of the ring action area,the second heating the action area,and in the scanning electron beam closing phase,the A sudden rise in temperature occurs.After the scanning electron beam has formed a molten pool,the surface of the material solidifies rapidly and the workpiece is not deformed to any great extent.After the scanning electron beam alloying process,the specimen cross-section is divided into three parts: the alloying layer,the heat affected zone and the matrix.The alloying layer is mainly composed of martensite and lysite carbide,and its microhardness can reach 1121.3 HV0.2.The microstructure of the alloy layer is a composite gradient,with fine grains on the upper surface as fine equiaxed crystals,and columnar crystals in the lower region,accompanied by the precipitation of nanoscale intermetallic compounds,which are uniformly distributed in all parts of the alloy layer and greatly improve the properties of the alloy layer.The microhardness of the upper part of the heat-affected zone is composed of fine martensite grains up to 900 HV0.2,while the microhardness of the lower part of the heat-affected zone is composed of coarse mixed martensite,with a microhardness of around 600 HV0.2.The microhardness of each part of the reinforced layer is higher than that of the matrix by 267.1HV0.2.With the increase of the thickness of the coating layer,the surface microhardness first rises and then falls;with the increase of the electron beam current,the surface microhardness first falls and then rises;with the increase of the electron gun movement speed,the surface microhardness first rises and then falls.The optimised solution is: the thickness of the coating layer is 50?m,the ratio of WC-12 Co to Ni is 20:80,the electron beam current is 16 m A,and the electron gun travel speed is 300mm/min.The surface microhardness of the alloy layer is 1177.8HV0.2,which is nearly four times higher than that of the matrix,and the friction coefficient of the surface layer is stable at 0.532.