Process Optimization And Mechanism Investigation Of Micro-arc Oxidation On Aluminum Alloy

Micro-Arc Oxidation is a new surface treatment technique, developed from anodic oxidation. In this research, a silicate electrolyte system components and its corresponding process parameters for the Micro-Arc Oxidation techinque on aluminum alloy 6063 was experimented by the methods of check experiment, orthogonal experiment and single-factor experiment. Their effects on the cross-section and surface morphologies, composition, phase constitution and wear-resistant performance of the ceramic coating obtained were investigated by means of many modern analyses. Furthermore, their influences to the ceramic coating formation course and mechanics were discussed, with the aid of analyze on the electrolyte conductivity, positive current density and positive electrical energy.An optimized silicate electrolyte system compounds and its corresponding process parameteres, which could fast prepare a high quality ceramic coating on the aluminum alloy 6063, were obtained as follows:[Na2SiO3] 8.0g·L-1, [NaOH] 2.0g·L-1, [Na2WO4] 1.0g·L-1, [C10H14N2O8Na2·2H2O] 2.0g·L-1, positive voltage 440V, negative voltage 160V, impulse frequency 100Hz, oxidization time 30min.The experiment results have shown that, the electrolyte component affects its conductance,which causes variety of the coating's breakdown voltage, and causes the ceramic coating growth rate changing.The impulse frequency affects the ceramic coating surface quality. With the impulse frequency increase, the ceramic coating evenness degree was improved, but the ceramic coating growth rate decreased. The positive voltage affects coatings thickness and evenness linearly but not evidently, while the negative voltage promotes growth of coating obviously, but its action gradually becomes feeble when voltage exceeds certain value. The prolonging oxidtion time is propitious to increasing coatings thickness, improving the evenness simultaneitily.Experiment results show that the ceramic coating on the aluminum alloy is mainly compose ofα-Al2O3 phase,γ-Al2O3 phase and mullite phase, whose mass percents are influenced by the voltages significantly but individually. The positive and negative voltages all impetus the transition fromγ-Al2O3 toα-Al2O3, and increase the mullite phase mass percent. Theα-Al2O3 phase mass percent reached a maximum as the positive voltage rising in this research. As the calculate results have shown, there is a quite valuable linear correspondence between the phase composition and the electrical energy consummated on the anode region. 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.Based on the experiment results, according to the change of the positive current, the MAO ceramic coating formation process can be divided into two stages: the primary film formation stage and the ceramic coating growing stage. During the first one, the positive voltage increasing can expedite the formation of the primary film, through enhancing substrate electrolyzed, accelerating the anion migration to increase ion current density in the plasma, while the negative voltage increasing can promote the primary film formation, through accumulating the gas-film producing more plasma and increasing the electron current density in the plasma. Although the primary oxide film will be destroyed in the later stage, it can influence the thickness, evenness and phase constitution of the final ceramic coating seriously.However, during the second stage, the positive voltage increasing acts as an main factor to intensify the matter conveying from one side of ceramic coating to another, and to accelerate the ceramic coating forming process,while the negative voltage increasing acts as an main factor to enhance the action of positive voltage by stimulating the production of the plasma, and to provide more passages and microspores for the matter transportentation by breakdown the primary ceramic coating.Thus, the thickness, evenness and the phase constitution of ceramic coating of aluminum alloy are ultimately fixed up.