| Aluminum alloy is one of the most widely used materials among non-ferrous metals.Various aluminum products are employed throughout almost every sector of the national economy.The application of pure aluminum is restricted owing to the low strength.As a light alloy,aluminum alloy is a series of alloy fabricated by alloying the pure aluminum with different elements.The elements of magnesium,silicon,zinc,copper and manganese are the main alloying elements,and the auxiliary elements are chromium,iron,nickel,cobalt,titanium and zirconium,respectively.However,the corrosion and wear resistance of aluminum alloys is still poor,limiting their application scope in some areas.In order to enhance the corrosion and wear resistance of aluminum alloys,it is reasonable to prepare appropriate coatings on aluminum alloys with various surface modification techniques.Micro-arc oxidation is a new surface treatment technology to prepare ceramic coatings on surface,which is also low-cost and has no pollution.Currently,the PEO process is in a transition phase from research to commercial application,with a primary focus on the hardness,thickness,corrosion and wear protection of light alloys.In this thesis,micro-arc oxidation(MAO)was used to prepare MAO coatings on 2A12 aluminum alloy.Negative voltage,frequency and composition of electrolyte was changed to investigate the structure and performance by scanning electron microscope(SEM),x-ray diffraction(XRD),x-ray diffraction(XPS),high resolution transmission electron microscope(HRTEM),and the ceramic coating with excellent performance was got.The effects of different elements in the substrate were also studied by alloying the pure aluminum.The MAO coatings were prepared ont the aluminum alloys.The effects of alloying elements(Cu,Mg,Zn)in the substrate on the formation mechanism and properties of the MAO coatings were analyzed by studying the MAO process,as well as the microstructure,elemental composition,phase composition,corrosion resistance,wear resistance,and thermal shock resistance of the coatings.Results showed that the coatings were mainly comprised of Al,α-Al2O3,γ-Al2O3 and W with the addition La(NO3)3 in CH3COONa-Na2WO4 electrolyte system.La2O3 can be foung in the MAO coatings,which leads to the high cooling rate.With more La(NO3)3 addition,the wear and corrosion resistance of the coating gradually increased.With 0.7 g/L La(NO3)3 addition,the MAO coating showed best wear and corrosion resistance,as well as better binding force.The corrosion form of MAO coating is mainly pitting corrosion,the defect plays an important part during the corrosion process.However,the thermal shock resistance of the MAO coating was slightly diminished.When the negative voltage is-60 V,the corrosion resistance is much higher.Different elements show quite different effects on the MAO process.The properties of the MAO coating could be predicted by the investigation of single alloying element.Results showed that element could influence the change of current,the time of maximum current,and the maximum current value.Under constant voltage mode,with less alloying element in the substrate,the maximum current during the MAO process appeared at about 18 s.However,the maximum current appears at the maximum voltage when the addition amount of the alloying elements reached a certain value.The thermal shock and corrosion resistance decreases,and the wear resistance increase when Cu is added in the aluminum alloys。While the result is different,when different amouts of Mg are added in the alloys.When 1%Mg is added in the alloy,the wear resistance of the coating will increase.When 3%Mg is added in the alloys,the corrosion resistance of the coating will increase and exceed the coating on pure aluminum,but the wear resistance and the thermal shock resistance will decrease.The Mg which was added in the alloys,will react with the 0 only when the MAO process starts and the amount of γ-Al2O3 decreased.The corrosion resistance of the MAO coating prepared on Al-Mg alloys is higher than that of the coating prepared on Al-Cu alloys.When Zn is added in the alloys,the wear resistance of both the coating and substrate increase,but the corrosion resistance and thermal shock resistance will decrease.The results show that the Al-Mg alloys are better to be coated by MAO coatings.The alloying elements in the substrate will form plasma with A1 and react.When the chemical activity of alloying element is high,the alloying element will react with O first,and the amount of Al2O3 will change.During the MAO process,the alloying elements will go through the discharge channel,and part of the alloying elements and Al will stay in the coating because of the high cooling rate.The alloying element dissolves in the A1 resulting in solid solution strengthening.In the end the alloying elements will keep in the coatings as elementary substance or intermetallic compound. |
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