Study On Combined-warm Extrusion And Micro-arc Oxidation Process Of High Strength Aluminum Alloy

7A04aluminum alloy has been widely used in aerospace and automotive fields due to itshigh specific strength and high yield strength （σ0.2>500MPa）. Recently, there is increasingdemand of7A04aluminum alloy in shell and hub preparation. However, the application of7A04alloy is limited in extreme environment such as heating etching, drastic change intemperature or corrosion, for its relative low melting point. And since7A04aluminum alloy isone of difficult-to-deformation materials at room temperature, the processing of7A04aluminum alloy is complicated and costs high. Therefore, in order to expand the applicationof high strength aluminum alloys, it is necessary to improve their thermal resistance anddevelop new forming processes. In this study, a7A04aluminum alloy work-piece wasproduced through combined-warm extrusion, heat treatment and micro arc oxidation （MAO）process. And processing parameters in each step were investigated.Based on the characteristics of the work-piece structure and preparation demand,combined-warm extrusion was employed to produce the work-piece. Billet preheatingtreatment, friction and lubrication, billet temperature, mould temperature and ram speed werestudied to improve the quality and processing efficiency of work-pieces. It was found that thepeak load of the extrusion decreased with the billet preheating temperature elevating, while itwasn’t sensitive to the mould preheating temperature and ram speed. The surface quality ofwork-piece can be improved by pre-oxidizing the billet, reducing the billet and mouldtemperature. And the parameters of preparation for the flawless work-piece are determined asfollows: the billet is annealed at360℃for120min and then pre-oxidized for20min in3.5g/L NaOH solution of70℃; graphite and oil （cylinder oil） with a volume ratio of2/3areutilized to lubricate the mould and billet during extrusion; the appropriate preheatingtemperature of billet and mould is200²50℃, and the ram speed is1.81³.51mm/s.T6treatment was employed to strengthen7A04aluminum alloy work-pieces afterextrusion. Microstructure and mechanical properties of work-pieces prepared under differentextrusion temperature and heat treatment were examined. It was found that recrystallizationappeared in solution treatment temperature. And the proportion of recrystallization increasedwith the increasing of solution treatment. In addition, compareing with samples processed bywarm extrusion, the mechanical properties of samples obtained through T6treatment wasimproved obviousely. T6treatment meets the application is485℃/80min+140℃/11h, andthe ultimate tensile strength and elongation of the specimen is616MPa and11.2%,respectively. A layer of Al₂O₃ceramic coating can be prepared by MAO treatment and the coating isin situ formed on the surface of aluminum alloys. In addition, since ceramic has excellentthermal resistance and anti-oxidation property, the thermal resistance of7A04aluminum alloycan be greatly improved by MAO treatment. In the MAO processing, the effects ofelectrolytes, MAO time, current ratio of anode and cathode and anode material on MAOcoating were analysed. The results show that the coating thickness increased and thendecreased with the increase of MAO time. And the coating thickness increased with the raiseof current ratio of anode and cathode. Moreover, the electrolytic temperature should be under50℃and the electrolytic pH should be above11.0in MAO processing, otherwisedetrimental effects on the MAO coating would generated. The coating is mainly composed ofγ-Al₂O₃. Compactness, thickness, energy efficiency, and thermal resistance of7A04coatingare superior to those of7075aluminum alloy. Cellulose can decreases anode peak voltageduring MAO processing. Also, an effecient MAO electrolytical compositions are determined:10g/L Na2SiO3,0.7g/L NaOH,1.0g/L Na2B4O7and0.2g/L Na2EDTA. And when the MAOcoating thickness exceeds12.0μm, the7A04aluminum alloy has excellent thermal shockresistance and ablation resistance.