Microstructure Control And Properties Of Microarc Oxidation Coatings On Aluminum Alloy

Aluminum alloy, is widely used. But its hardness, corrosion resistance and wear resistance is poor, limiting the further industrial application. Microarc oxidation is used to prepare ceramic oxide film on the surface of aluminum alloy. The ceramic film with the characteristics of high hardness, high corrosion resistance and high wear resistance, can effectively improve surface properties of aluminum alloy. The ceramic film contains double-layer structure: the inner dense layer and external loose layer. The inner dense layer with less defects is mainly composed of α-Al2O3, while the loose layer with more defects is mainly composed of γ-Al2O3. The properties of the dense layer is better than the loose layer, the performance of α-Al2O3 is better than γ-Al2O3, so we are committed to obtain ceramic film with high dense layer ratio and alpha phase content. In recent years, the research of microarc oxidation on aluminum alloy mainly focuses on the process, we can see relatively smaller efforts towards the study of the structure with some controversial results. The following questions are seldom reported:(1) the electrical conditions to obtain α-Al2O3;(2) the electrical conditions to obtain dense layer;(3) the effects of different electrical parameters on alpha phase content and dense layer ratio;(4) the effects of alpha phase content, dense layer ratio, thickness, roughness on corrosion resistance and wear resistance.In this thesis, SEM and XRD were used to analyze the microstructure and morphology of ceramic film. Eddy current thickness gauge and 3D shape gauge were used to measure the thickness and roughness of the films. The corrosion resistance and wear resistance of the coatings were characterized by electrochemical test and friction and wear test. Orthogonal test and single factor test were done to study the effects of different electrical parameters, such as frequency, positive and negative current density, positive and negative duty ratio, on alpha phase content, the dense layer ratio, thickness and roughness. The effects of alpha phase content, the dense layer ratio, thickness and roughness on corrosion resistance and wear resistance of the coatings is also studied. Draw the following conclusions:(1) positive and negative current density cannot be both less than 10 A/dm2 to form α-Al2O3;(2) the positive and negative current density cannot be both less than 4A/dm2 to obtain the dense layer. The dense layer can be obtained with the minimum thickness about 10 μm;(3) the effects of different electrical parameters on the structure is different;(4) the larger alpha phase content, the greater dense layer ratio, the bigger thickness of the film, the lower roughness of the film, makes the better corrosion resistance and wear resistance.