Research On Neutral Salt Bath Chromaluminizing On The Surface Of Nickel-base Superalloy And The Properties Of The Coatings

As the representative of the aviation material,nickel-based superalloys are often used in high temperature and corrosive environment for a long time and usually can not meet the use requirements by its own performance.Preparation of high temperature protective coating on the surface is an effective way to solve this problem,in which diffusion aluminizing is a commonly used method for improving the high temperature oxidation resistance.Chromium-modified aluminide coating can improve the high temperature oxidation resistance and hot corrosion resistance,which has a high value in use.In this paper,the diffusion aluminized coating and the chrome-aluminum co-permeation coating were prepared on the surface of nickel-based superalloy GH625 by neutral salt bath infiltration method and the formula and process of the impregnation agent were optimized by the orthogonal method.In addition,the above-mentioned two kinds of coatings modified by rare earth La₂O₃ were also studied and the performance of each coating was also tested.?1?Orthogonal method is used to optimize the formula and process of neutral salt bath aluminizing.The optimum aluminizing formula?the following percentages are all mass percentages?obtained by using the coating thickness and residual salt cleaning time as evaluation indicators is 6wt%Al,8wt%NaF,26wt%NaCl,60wt%BaCl2.The best aluminizing temperature and holding time is 950°C and 4h,respectively.Based on aluminizing,chrome-aluminum co-permeation only requires the addition of Cr₂O₃ in the impregnation agent.Therefore,adopting the single factor method and referring to the thickness and quality of the coating,the optimum formula of the chrome-aluminum co-permeation is 6wt%Al,8wt%NaF,23wt%NaCl,53wt%BaCl₂ and 10wt%Cr₂O₃.The best co-permeation temperature and holding time is950°C and 4h,respectively.?2?Experiments show that the two coating surfaces are composed of uniformly dense Al-Ni compounds and XRD detection indicates the phases are mainly?-NiAl.Moreover,chrome-aluminum co-permeation coating contains solid solution Cr phase.The cross-sectional morphology analysis shows that the average thickness of the aluminized coating is 50?m,while the average thickness of the chrome-aluminum coating is 46?m and there is a diffusion layer of Cr under the external Al-rich layer.The average surface hardness of the aluminized coating and the chrome-aluminum coating are 495.6 HV and 577.32 HV,respectively,which are 0.65and 0.93 times higher than the 299.72 HV of the matrix.The interface binding force of the aluminized coating and the chrome-aluminum co-permeation coating are 53N and63N,respectively.?3?The experimental results of the matrix,aluminized coating and chrome-aluminum co-permeation coating oxidized at 1000°C for 100 h show that the oxidation weight gain of the matrix material is significantly higher than that of the other two coatings,indicating that the coating has excellent oxidation resistance.This is because the Cr₂O₃ protective film formed during the oxidation of the matrix is not stable and can easily generate internal stress.The NiCr₂O₄ generated at the later period of the oxidation can easily peel off the oxide film,causing the failure of the material.While aluminized coating and chrome-aluminum co-permeation coating can quickly form?-Al₂O₃ in the early oxidation stage,so the oxidation gain weight is faster than matrix.Then the metastable?-Al₂O₃ is transformed into the stable?-Al₂O₃ and the coating's high temperature oxidation resistance is greatly improved.Comparing the two coatings,the chrome in the chrome-aluminum co-permeation coating can promote the transformation of?-Al₂O₃ to?-Al₂O₃ and form a Cr-rich layer in the diffusion zone to prevent the internal diffusion of Al,so its high-temperature oxidation resistance is better than that of aluminized coating.?4?Hot corrosion experiments are performed on the matrix and the two coatings in a mixed salt of 75 wt%Na₂SO₄+25 wt%NaCl at a temperature of 1000°C for100 h.Experiments show that the oxide film of the matrix peeled off at the initial stage of corrosion and then NiO and NiCr₂O₄ spinel generated and internal vulcanization occurred and severe corrosion occurred after about 60 hours.While aluminized coating and chrome-aluminum co-permeation coating can form a continuous Al₂O₃ protective film on the surface during the 100-hour hot corrosion process,which can slow down the hot corrosion rate.Therefore,both coatings show good thermal corrosion resistance.However,at the later stage of hot corrosion,the aluminized coating layer has a peeling phenomenon and the Al-rich?-NiAl phase was consumed conspicuously and converted into a part of the?phase,resulting the decrease of hot corrosion resistance of the coating.The chrome in the chrome-aluminum co-permeation coating can inhibit alkaline dissolution of Al₂O₃ and only part of the oxide film cracked and there are also many Al-rich?-NiAl.Thus,the chrome-aluminum co-permeation coating has excellent hot corrosion resistance.?5?Single factor method was used to determine the optimum content of La₂O₃ in the permeating which is 1wt%.The thickness of aluminizing coating and Al-Cr coating after adding La₂O₃ can reach 93.59 and 51.61?m respectively.And the main phase of the surface is?-NiAl,which is more compact and uniform,and the La element can be detected on the surface.?6?The oxidation test of aluminized coating and Al-Cr coating with the addition of La₂O₃ at 1000?for 100h were conducted.Compared to the coatings that do not add La₂O₃,the oxidation weight gain of two coatings added with La₂O₃ obviously slow down,and the surface oxides are still dominated by?-Al₂O₃,the transformation rate of?-Al₂O₃ to?-Al₂O₃ is accelerated especially the Al-Cr coating,which demonstrations that La₂O₃ has the effect of promoting phase transition and reducing the rate of oxidation.In summary,the high-temperature oxidation resistance of the two coatings with the addition of La₂O₃ have been greatly improved than that of without La₂O₃.And Al-Cr coating still shows the best high-temperature oxidation resistance.?7?The thermal etching testing of two coatings with the addition of La₂O₃ at1000?for100hwerecarriedoutonthemixedsalt75wt%Na₂SO₄+25wt%NaCl.Compared to the coatings that do not add La₂O₃,the hot corrosion resistance of the coatings added with La₂O₃ is increased,this is because La₂O₃ can improve the adhesion of oxide film and refine the coating grain.So after100h's hot corrosion,slight exfoliation appears on the surface of the aluminizing coating,while the oxide film of Al-Cr coating still kept a continuous state.In summary,Al-Cr coatings with high interfacial bonding force,high hardness and high temperature resistance on the surface of GH625 alloy were prepared successfully and creatively by neutral salt bath infiltration,which provides a theoretical basis for the surface modification of the superalloy.