Research On Preparation Of MAO Coating On Al-Cu Alloy And Its Corrosion Resistance

Al-Cu alloys are the earliest cast aluminum alloys used in industry.Their main properties are high mechanical properties at room temperature and high temperature,simple casting process,good cutting performance and excellent heat resistance,which are the basis for the development of high strength aluminum alloys containing Cu and various heat-resistant alloys The disadvantage is that the potential difference between ? phase(CuAl2)phase and ?(Al)matrix is large resulting in poor corrosion resistance.In order to improve the corrosion resistance of Al-Cu alloys,the "green" and effective surface treatment method of microarc oxidation(MAO)is adopted to protect the Al-Cu alloys.The content of Cu in the matrix affects the mechanical properties of the alloys,but the effect of Cu on the microstructure and corrosion resistance of MAO coating is seldom studied.The porosity and microstructure of MAO coating become the decisive factor of corrosion resistance.This thesis elaborated the effect of the evolution of plasma spark discharge state on MAO coating formation,and then systematically studied the effect of Cu content and microstructure of Al-Cu alloys on the microstructure and corrosion resistance of MAO coating.From the point of application,the effect of electrolyte composition on the compactness and corrosion resistance of MAO coating was studied.The change of plasma spark discharge state in the whole MAO process was studied and the whole process was divided into five stages according to the voltage-time curve.The effect of plasma spark discharge on the microstructure and corrosion resistance of MAO coating in various stages is as follows:The substrate is strongly dissolved by micro-arc discharge and only one layer of MAO coating is formed after oxidation.The dense layer and barrier layer gradually thicken after the appearance of "soft" spark.The interfacial defects of barrier layer/dense inner layer decrease obviously after "soft" spark transformation and the thickness of dense layer increases,which shows a certain corrosion resistance.However,the coating which has not been transformed is still very thin.The appearance of "chain sparks" distngctly reduces the defects in the dense layer and barrier layer.Meanwhile,the proportion of the external loose layer to the whole coating decreases.The uniform and dense MAO coating provides excellent barrier properties for the substrate.MAO coatings were prepared on aluminum alloys with different copper contents.The effects of copper content on plasma spark discharge,microstructure and corrosion resistance of the coatings were investigated.With the increase of copper content,the content and the size of? phase increases.The results show that ?phase increases the intensity and promotes the transition process of plasma sparks discharge in MAO process.Amount of oxygen would be generated due to the preferential oxidation of ? phase.Oxygen release brings a lot of pores and cracks leading to the increase of porosity in the coating.As a result,the corrosion resistancedecreases because corrosion medium penetrates through defects and arrives at the substrate.On the basis of the above study,the effect of the size and distribution of the ? phase on the microstructure and corrosion resistance of the MAO coating on Al6Cu alloys were studied.When the ? phase exists in a continuous network form,the volume of oxygen evolution originated from the oxidation process is large and concentrated.Therby,the size of pores and cracks in the prepared MAO coating increases and the corrosion resistance of the coating decreases.If a large amount of ? phases is dissolved into the matrix,the scattered Cu was oxidized accompanyng with a lot of oxygen.Which leads to the increase of porosity in the whole coating and the decrease of corrosion resistance.With small short rods precipitated in the matrix,only a small amount of oxygen can be produced accompanying with oxidation of the small size and discontinuous ? phase.The discontinuous oxygen quickly releases from the coating leading to a few defects in MAO coating,which improves the corrosion resistance obviously.In this part,the relationship between microstructure of matrix and corrosion resistance of coating is discussed,which provides an important reference method for improvingthe corrosion resistance of the MAO coating by alloy design in the future.The effect of NaOH,NaH2PO4 and Na2C2O4 on microstructure and corrosion resistance of MAO coatings on 2A12 aluminum alloys were studied by adjusting the composition of the electrolyte.The results show that a large number of OH' in electrolyte has a strong dissolution effect on the newly formed coating,which leads to the increase of the roughness.The side effect of oxygen precipitation increases the pore size in the MAO coating,leading to a significant decrease in the corrosion resistance.The NaH2PO4 has "sealing" effect on MAO coating,which greatly reduces the pores and cracks in the coating.The Na2C2O4 can react with metal cations in electrolyte,which not only increases the utilization rate of metal ions,but also changes the interfacial tension to accelerate the release of gases.These processes further reduce the defects of the coating and greatly improve the corrosion resistance.