Investigation On Technology Of Micro-arc Oxidation On The Surface Of Aluminum Alloy

Aluminum and its alloy have the advantages of lightweight, high strength and easy processing in shape, good solderability and polishing.It has been widely used in industrial production. But their application is limited by their low surface hardness , Poor wear-resistance and poor anti-corrosion to the solution of halide and inorganic acid. By means of the micro-arc oxidation(MAO)technique , ceramic coatings with high micro-hardness,strong adhesion,high strength and high wear resistance can be fabricated.In this research, a silicate electrolyte system components and its corresponding process parameters for the micro-arc oxidation technique on aluminum alloy 6060 was experimented. And because the oxide layers often have a porous surface structure, it exhibits relatively high friction coefficients. In this work, in order to enhance the surface and mechanical properties of the films produced by micro-arc oxidation, Al2O3 coatings embedded with SiC nano-powders of different thicknesses were produced on aluminum alloys by adding SiC nano-powders into the electrolyte during MAO. Compared to the Al2O3 coatings without SiC nano-powders, the MAO Al2O3 coatings with SiC nano-powders are much denser and harder, and the wear resistance is also improved significantly. The enhancement can be attributed to the enhancement of the surface structure and morphology of the MAO Al2O3 coatings with embedded SiC nano-powders. In the way of the characterization of the MAO coating, the microstructure and composition of the coating are analyzed by modern surface analysis methods such as CLSM ,SEM, FESEM, Metallurgical Microscopy and XRD. The micro-tribology properties under dry sliding were measured on a Universal Micro-Tribometer.And the corrosion resistance properties were measured by electrochemical methods and EIS.An optimized silicate electrolyte system compounds and its corresponding process parameteres, which could fast prepare a high quality ceramic coating on the aluminum alloy 6060,were obtained as follows: [Na2SiO3]12.0g·L-1, [NaOH]3.0g·L-1, [H3BO3]1.0g·L-1, [C6H5Na3O7·2H2O]1.0g·L-1; current density 5A/dm2; impulse frequency 700Hz,oxidization time 30~45min. The results are as followed:1. The electrolyte concentration affects its conductance, which causes variety of the coating's breakdown voltage, and causes the ceramic coating growth rate changing. The growth rate first increases and then decreases with increasing the electrolyte concentration .While the PH value is in the range of 10~11, the coating surface is good.2. The surface roughness and the growth rate of the MAO coating increase with increasing current density. And the proportion of the denser layer decrease with increasing current density.3. In the beginning of MAO, the thickness of MAO coating increases fast. At last,the thickness increases very slowly. And the surface roughness increases.4. The main frequency has no significant effect on the surface roughness. The growth rate first increases and then decreases with increasing frequency. And frequency effect on the content ratio (P) ofα-Al2O3 toγ-Al2O3 under the condition of constant current density, which P of the coating prepared in Na2SiO3 solution gradually increases with the increased frequency.5. Experiment results show that the ceramic coating on the aluminum alloy is mainly composed ofα-Al2O3 phase andγ-Al2O3 phase. And the ceramic coating can be approximately divided to the dense layer and the loose layer. The boundaries between the dense layer and the loose layer are not clear. The ceramic coating, possesses good compactness, adhesion, integrity and protective ability, can enhance wear-resistance of aluminum alloy, especially for heavier load. And micro-hardness is also better than the matrix, the biggest value come to 1540HV in this work.6. The friction coefficient of the MAO coating is obviously less than the aluminium alloy's.The friction coefficient of the aluminium alloy is about 0.75,but the MAO coating's is about 0.68.The friction coefficient is less,the wear resistance is better.So the wear resistance of MAO coating is better than the aluminium alloy.7. Compared to the Al2O3 coatings without SiC nano-powders, the MAO Al2O3 coatings with SiC nano-powders are much denser and harder, and the wear resistance is also improved significantly. The enhancement can be attributed to the enhancement of the surface structure and morphology of the MAO Al2O3 coatings with embedded SiC nano-powders.