Study On Preparation And High Temperature Oxidation Resistance Of MCrAlY Coatings By Electron Beam Cladding

Because of excellent high temperature mechanical properties,nickel-based single crystal superalloys have become the primary raw materials for the manufacture of aero-engine turbine blades.However,the high temperature oxidation and hot corrosion resistance of the superalloys are poor,therefore,an additional high temperature protective coating needs to be applied.MCrAlY（M=Ni,Co,Fe）coatings are widely used as the protective coatings due to their excellent high temperature oxidation resistance,thermal corrosion resistance and other properties.In this study,the influence of process parameters on the coating quality in the process of electron beam remelting was studied,and then a layer of MCrAlY coating was prepared on the surface of PWA1483 nickel-based single crystal superalloy by electron beam cladding using the optimal parameters.Moreover,the high temperature oxidation resistance of MCrAlY coating and the substrate was investigated.Optical microscope（OM）,scanning electron microscope（SEM/EDS）,X-ray diffraction（XRD）and electron backscattered diffraction（EBSD）were used to analyze the microstructure,phase and crystal growth direction of the coating;The microhardness,surface roughness and high temperature oxidation resistance of the coatings were analyzed by microhardness tester,nanoindentation tester,three-dimensional profile and muffle furnace,respectively.Firstly,the influence of electron beam remelting process parameters on the quality of the remelting layers（surface roughness,hardness and thickness of the remelting layer）were investigated through a three-level and three-factor orthogonal test.Then,taking the quality of the remelting layers as the evaluation index,the optimal remelting process parameters were obtained by the range analysis.The research results show that among the three process parameters that the influence sequence on the surface roughness,hardness and thickness of the remelting layer is:remelting current>accelerating voltage>remelting time;remelting current>remelting time>accelerating voltage;remelting time>accelerating voltage>remelting current,respectively.In addition,the best process parameters of electron beam remelting MCrAlY coating are:accelerating voltage of 65k V,remelting current of 15m A,remelting time of 4.5s.Then the MCrAlY coating was successfully prepared by using the optimal remelting process parameters,the microstructure and phase of the coating were analyzed.At the same time,the growth mechanism of the crystal was also analyzed though the growth of epitaxial growth structure in the coating.The research results show that the MCrAlY coating prepared by electron beam cladding technology not only realizes the epitaxial growth which growth consistent with the substrate[001]direction,but also does not appear columnar to equiaxed（CET）transition;the phase in the coating is consistent with the matrix and the mainl phase in the coating areγ-Ni Al andγ’-Ni₃Al phase;while the average primary dendrite spacing of columnar dendrites in the coating increased from 2.0μm to 2.7μm for bottom region and top region,respectively.On the cross section which perpendicular to the scanning direction of the electron beam,the microstructure can be divided into three regions:the[001]region,the[100]overlapping region and the transition region.In the overlapping area of the electron beam,the growth direction of dendrite changes from[001]to[100]because the dominant direction of temperature gradient vary from vertical to horizontal.Finally,the high-temperature oxidation resistance of MCrAlY coatings were studied,high-temperature cyclic oxidation experiments at 1000℃for 100 hours were carried out on the coating and the substrate,respectively,and the high-temperature oxidation behaviors of the coating and the substrate were analyzed.The research results show that the high temperature oxidation resistance of the MCrAlY coating is about 2 times higher than that of the substrate.Moreover,the oxide of the coating is mainly dense Al₂O₃without spalling,while the oxide film of the substrate which contains Ti O₂,Ni（Cr₂O₄）and Cr₂O₃phase peeled off seriously.In addition,as the oxidation time extend,Cr and Ti elements in the coating gradually pass through the Al₂O₃film,forming a Cr and Ti-rich oxide layer on the coating surface.