Microstructure And Property Control Of High Entropy Alloy By Electron Beam Surface Modification

In this paper,the surface remelting of Al Cr Fe Co Ni_2.1eutectic high entropy alloy and the preparation of Al Cr Fe Co Ni Cu high entropy alloy coating on 304 stainless steel were carried out based on electron beam surface modification technology.The microstructure of the modified layers was analyzed by X-ray diffraction（XRD）,scanning electron microscope（SEM）,and transmission electron microscope（TEM）.The mechanical properties and friction and wear properties of the modified layers were tested by nano-indentation equipment and friction tester,and the effect of electron beam modification parameters on the microstructure and properties of the high entropy alloy modified layers was studied.The experimental results are as follows:The electron beam surface remelting of Al Cr Fe Co Ni_2.1eutectic high entropy alloy was designed based on an orthogonal experiment.The results showed that the microstructure of the remelted layer was obviously refined,the volume fraction of BCC phase was significantly decreased,and the segregation phenomenon was reduced.Due to solution strengthening and fine grain strengthening,the average microhardness of the remelted layer was improved to 383.4 HV,which was 1.5 times that of the substrate.When the wear test time is 0.5 h,the wear mechanism of the remelted layer under the bearing steel,zirconia and silicon nitride grinding ball was abrasive wear and oxidation wear.The wear resistance of the remelted layer under the Si₃N₄grinding ball was the highest,which was twice higher than that of the matrix.When the testing time is 1 h and 1.5 h,the wear rate decreases with time and remains stable after 1 h.The wear mechanism of the matrix under the silicon nitride ball is abrasive wear and oxidation wear,and the remelted layer was transformed into oxidation wear.AlCrFeCoNiCu high entropy alloy coating was prepared on the surface of 304stainless steel by electron beam surface cladding in-situ synthesis.The effect of electric current on the microstructure and properties of the coating was studied.The results show that coatings at the cladding current of 12-18m A are composed of BCC and FCC phases with a large number of cracks.With the cladding current increased,the number of cracks in the coating decreased,the volume fraction of FCC phase increased,and the BCC phase transformed from dendrite to needle and block.TEM results showed that there were nano-Cu-rich phase and desolated BCC phase in the dendrites of coating at cladding current of 12 m A.The average hardness and elastic modulus fluctuated greatly due to the macrosegregation of microstructure,and the average hardness of 12m A coating was the highest,reaching 548.2 HV,which was about 1.5 times higher than that of the substrate.The wear mechanisms of the three coatings were abrasive wear wear.The wear resistance of coating at cladding current of 12 m A is the highest,which is 2.42×10^-4mm³/N·m,which is 4.6 times higher than that of the substrate.Furthermore,AlCrFeCoNiCu high entropy alloy coating was prepared on the surface of 304 stainless steel by hybrid resistance seam welding and composite electron beam cladding technology（RSW-EBC）.That is,the powders are pre-alloyed by resistance seam welding before electron beam cladding.The effect of cladding current on the microstructure and properties of the coatings were studied.The results showed that the effect of the cladding current on the microstructure and mechanical property of coatintgs were consistent with those of the single electron beam cladding coating,but the number of crack was obviously reduced and the macro segregation phenomenon was reduced.The results of SEM and TEM showed that the Cu-rich phase and dissolving structure in the dendrite disappeared,but the Cu-rich phase and two Me₃Al L1₂phases were found between the dendrites.The homogenization of microstructure improved the overall properties of the coating,and the average hardness of at cladding current of 12 m A was the highest,reaching 642.3HV,which was 1.9 times higher than that of the substrate.The wear rates of coatings at cladding current of 12-18 m A were 2.09,3.78 and 5.36×10^-4mm³/N·m,respectively,which showed higher wear resistance than electron beam cladding coatings.