Research On Microstructure And Properties Of Co Based Alloy/MoSi₂ Composite Coating By Laser Cladding

Co based alloy and MoSi₂ have excellent high temperature property.However,laser cladding pure MoSi₂ or(Mo+2Si) powder is bad because of crak,poor moldability and so on flaw.Co based alloy has better wetting property,the compact and smooth coating be fabricated.So the better coating can be got by laser cladding Co based alloy/MoSi₂ powder, but the coating has excellent resistant oxidation and corrosion resistant.The Co based alloy/MoSi₂ composite coating was fabricated on the surface of 304 strainless steel because of its extensive application.It not only establishes the link between 304 strainless steel and MoSi₂,but also enhances the resistant oxidation and corrosion resistant on the surface of 304 strainless steel.So based on CO₂ laser,the Co based alloy/MoSi₂ composite coating was fabricated on the surface of 304 strainless steel by laser cladding.The composite coating was investigated by optical microscopy(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),electron probe microanalysis(EPMA), microhardness(HV),wearing instrument,electrochemiistry analysis instrumentation.The main research results are as follows:Firstly,the processing parameters of laser cladding were optimized.The optimal processing parameters of laser cladding were as follows:laser output power 2.2～2.6kW, laser scanning speed 8mm/s,and laser beam diameter 5mm.Based on optimal processing parameters,the composite coating was metallurgically bonded to the strainless steel substrate by the action of laser.The macroscopical pattern of cladding coating was crescent.The microstructure was coarse in the heat affection zone.The microstructure of coating became fine with the distance increasing from molten pool.The microstructure of surface coating was the finest.Secondly,the content of MoSi₂ had significant affection on the microstructure of composite coating in laser cladding with Co based alloy/MoSi₂ alloy powders.The microstructure of Co based alloy coating was homogeneous,but coare with none MoSi₂;the microstructure of coating was obviously finner by adding MoSi₂.It indicated that MoSi₂ could refine grain.Additionally,the morphological of precipitated MoSi₂ was affected by feezing rate and density of MoSi₂.When the amount of MoSi₂ was in the range of 0-20wt%,it completely dissolved and solided in the eutectic structure.None MoSi₂ precipitated.With increasing the amount of MoSi₂(above 30wt%),some MoSi₂ again precipitated.The morphological of MoSi₂ was various such as petal-like,star-shaped,caterpillar,edgewise and so on.The content of MoSi₂ had significant affection on the average microhardness of composite coating in laser cladding with Co based alloy/MoSi₂ alloy powders.With the different amount of MoSi₂,the microhardness of Co based alloy/MoSi₂ composite coating appeared graded distribution.When the amount of MoSi₂ reached the 40wt%,the average microhardness of composite coating was the highest(1455HV).Thirdly,under the room temperature,the dry sliding wearing experiment indicated that the composite coating had better resistant wear compare to the substrate.In wear processe,the hard phase partical effectively carried the loading weight.The hard phase was enclased by generated solid solution,it could depressed the plastic yield of coatact surface,stopped the expansion of crack and decreased the wearing capacity.Additionally,the fine grain existed in the cladding coating also had affection on enhanced the resistant wear.Under the room temperature,the resistant pitting of substrate and composite coating were tested in 3.5%NaCl solution by electrochemistry corrosion.It indicated that the(E_b—E_p) of Co based alloy/MoSi₂ composite coating appeared graded distribution.When the amount of MoSi₂ was in rang of 10-30wt%,the(E_b—E_p)of Co based alloy/MoSi₂ composite coating was less than strainless steel subatrate's.The resisitant pitting of composite coating was better than substrate.So the amount of MoSi₂ shoud be control in rang of 10-30wt%in order to improve the resistant pitting of 304 strinless steel surface.