Preparation Process Of Micro-arc Oxidation Ceramic Coatings With Different Colors On Aluminum Alloy Surface

Aluminum is the most widely distributed metal element in the crust. Its physical, mechanical and processing performances are excellent. It has been widely used in all areas of daily life. But now aluminum and its alloy products we used have single color, only white, black and other relatively simple colors, which affect their application in the decorative and optical equipment and cannot meet people's needs. Therefore, it is a very promising subject to let aluminum and its alloys have more colors to get colorful products. The color coatings prepared by micro-arc oxidation technology not only have excellent wear resistance and corrosion resistance, but also have a variety of colors. They can be widely used in automobile, machinery, aerospace, decoration and other fields.In this paper, LY12 aluminum alloy was used as substrate material. Different colorants were added to the electrolyte to prepare the color coatings by micro arc oxidation technology. The influence of the colorants on the coating's color has been studied, and the technology parameters have been optimized. The thickness of the micro-arc oxidation coating was measured with TT240 Digital eddy current thickness gauge. The surface microstructure, 3D morphology, diameter and depth of the holes were measured by means of CLSM, SEM, EDS and XRD. The porosity of MAO coating was quantities analyzed by Matlab software. The roughness of the MAO coating was measured with the surface roughness-measuring instrument. The corrosion resistance of the MAO coating was characterized with immersion corrosion test and electrochemical corrosion test. The best content of the colorants and the energy parameters were determined, and some conclusions were drawn:(1) Best electrolyte formula and energy parameters of the MAO green ceramic coating are as follows: The colorant's content and the energy parameters are: 9g/L Na₂SiO₃ +4g/L NaOH+2g/L EDTA-Na₂+2.0g/L K₂Cr₂O₄, the current density: 7A/dm², the duty ratio: 30%, the time: 9min¹3min, and the frequency: 550Hz⁸50Hz.(2) Best electrolyte formula and energy parameters of the MAO brown ceramic coating are as follows:The colorant's content and the energy parameters are: 9g/L Na₂SiO₃ +4g/L NaOH+2g/L EDTA-Na₂+1.5g/L CuSO₄, the current density: 5A/dm², the duty ratio: 20%-30%, the time: 12min, and the frequency: 550Hz-850Hz.(3) Adding K₂Cr₂O₄ to the electrolyte to prepare MAO green ceramic coating. When there was no colorant, the color of coating was white. When K₂Cr₂O₄ was added to the electrolyte, the color of coating was green. With the raising of the concentrations of K₂Cr₂O₄, the color of coating was gradually become dark. The green coating was analyzed with EDS, it was confirmed that Cr element made the coating turn to green. With the increase of the colorant's concentration, the hole of the coating surface became larger; porosity increased; and the corrosion resistance of the coating was improved.(4) Studying the effect of the current density on the MAO green ceramic coating. With the increase of the current density, the coating became thick; the color became dark; the surface of coating became gradually coarse; the number of the holes on the coating decreased; the diameter and depth of the holes became larger, and the porosity increased. The corrosion resistance of samples in different current density was not well when the current density was too large or too small.(5) Studying the effect of the frequency on the MAO green ceramic coating. With the increase of the frequency, the coating became thin; the color became shallow; and the surface of coating became more and more smooth. When the frequency not high, the hole's volume of the film surface was smaller; the number of molten particles was big; the porosity was also high; and the surface of film became rough. Through increasing the frequency, the hole volume reduced, and the film became smooth.(6) Adding CuSO₄ to the electrolyte to prepare MAO brown ceramic coating. When there was no colorant, the color of coating was white. When CuSO₄ was added to the electrolyte, the color of coating was brown. If the concentration was too high, the coating would ablate. With the increase of the concentrations of CuSO₄, the color of coating was gradually become dark. The brown coating was analyzed by EDS, it was confirmed that the Cu element made the coating turn to brown. With the increase of colorant's concentration, the hole of the coating surface became larger; the porosity increased; the corrosion resistance of the coating was improved first, and then was reduced.(7) Studying the effect of the time on the MAO brown ceramic coating. When the oxidized time was extended, the coating became thick; the color became dark; the number of the holes on the coating decreased; the volume of the holes became larger, and the porosity increased; the surface of film became rough; the corrosion resistance of the coating was improved first, and then was reduced.(8) Studying the effect of the duty ratio on the MAO brown ceramic coating. The color of the film became shallow first and then turned to dark with the duty ratio increased; the film thickness became thin first and then turned to thick; the volume of hole became small first then turn to big. When the duty ratio too high or too low, the corrosion rate of the coating was very big, and the corrosion resistance was poor.