| In order to meet the needs of aluminum alloy structural materials of lightweight, high hardness and wear-resistant in modern industrial, micro-arc oxidation(MAO) technology was an effective measure to solve the industrial application,which could improve the hardness and wear resistance of aluminum alloy.7075aluminum alloy was widely used in aerospace and military industry,but micro-arc oxidation on which was not enough in-depth and comprehensive studied at present.In order to improve the properties of micro-arc oxide coatings on7075aluminum alloy,this paper studied mainly through the electrolyte system and characteristics of electrical parameters.The characteristic electrical parameters studied in the paper mainly were power supply modes,reverse duty ratio and AC asymmetric sectional regulation oxidation method.The loose and dense layer of microstructure,surface morphology,element composition of the best oxide coatings were tested by using various modern testing methods.In order to verify the best process of the research which was carried out experiments on different grades of aluminum alloy.Through orthogonal and single factor optimization experiments, an optimized silicate electrolyte compounds which could fast prepare a high quality micro-arc oxide coatings on the aluminum alloy7075,were obtained as follows:[Na2SiO3]8.0～10.0g.L-1;[Additive A]7.0～9.0g.L-1;[Na2WO4]1.5～2.5g.L-1;[pH]10～12.In the paper,the characteristic electrical parameters were optimized and studied on the basis of the best silicate electrolyte compounds.The results show that the properties of MAO coatings were better under AC asymmetric compared with constant DC and DC pulse,because the negative current has leveling and modification effect on MAO coatings.The reverse duty ratio was a way of introducing negative current which taken an important role in improving the properties of MAO coatings,but there were few research into it.Therefor, in this paper the reverse duty ratio was system researched separately. The research shows that the reverse duty ratio have effects on the MAO coatings could divided into three stages.Firstly,the reverse duty ratio was increased from10%to40%,because of the introduction time of the negative current and it has dissolution effects on MAO coatings,the total thickness of the coatings decreased,but the ratio of dense layer increased. Secondly,the reverse duty ratio was40%～50%,the properties of MAO coatings achieve the best state.Lastly,the reverse duty ratio keep increasing,the whole properties dropped sharply due to the dissolution ratio of negative current was higher than the forming of forward current.Negative current was introduced through several ways for it taken a great effect on MAO coatings.The experiments found that the properties of MAO coatings was best under using AC asymmetric sectional regulation oxidation method. The oxidation method was at different MAO stages used sectional reverse duty ratio,which could obtain a thickness more than100μm,a hardness above1500HV,a resistant of salt spray test more than1500H,the were-resistant increased6times more than the substract,and the coatings adhesion close to hard oxidation coatings.Ultimately determined the MAO process as follows:[Power supply mode] AC asymmetric sectional regulation oxidation method;[Ja=Jc]4～6A·dm-2;[Frequence]100Hz;[Temperature]10～70℃;[Positive duty ratio]20%;[Reverse duty ratio for oxide time]50%for60min,40%for35min,30%for25min.Analysis of the best MAO coatings by means of the modern testing methods,the results show that the loose layer of MAO coatings surface hole was relatively larger and more than the dense layer of MAO coatings.The loose layer was mainly composed by metastable γ phase aluminum oxidation and the compounds of solutions.On the contrary,the dense layer of the MAO coatings was composed of stable a aluminum oxidation and metastable γ aluminum oxidation which was the key to improve the comprehensive properties of MAO coatings. |
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