Micro-arc Oxidation Process And Uniformity Of High-speed Train Car Body Underframe Large Beam

The large beam of the high-speed train'car body underframe is made of A7N01aluminum alloy.Due to the narrow space and difficult spraying of the large beam chute of the car body underframe,the existing coating system can not effectively guarantee the formation of the overall coating of the large beam of the underframe.Especially when the high-speed train'car is in a poor environment,the corrosion problem will seriously affect the service life of the large beam.Micro-arc oxidation?MAO?technology can form a ceramic layer mainly composed of alumina on the surface of aluminum alloy.The ceramic layer has the advantages of good wear resistance,good corrosion resistance,good electrical insulation performance,small thermal conductivity and high bonding strength with the substrate.The technology has the characteristics of high voltage,large current,and large power consumption per unit area,so MAO treatment of large-sized workpieces cannot be realized.This paper proposes the idea of using the mobile cathode nozzle to treat the A7N01 aluminum alloy beam.The mobile cathode nozzle MAO treatment system is composed of an immersed micro-arc oxidation treatment system,a cathode nozzle control system and an electrolyte circulation system.The micro-arc discharge electric field characteristics and coating uniformity of the mobile scanning cathode structure have been studied,and the micro-arc oxidation treatment process parameters are optimized,including the grid cathode embedded depth,electrical parameters,electrode spacing and scanning parameters.The thickness and roughness of the film were measured by eddy current thickness gauge and roughness meter.The structure and organization of the film were analyzed by scanning electron microscopy?SEM?,X-ray energy spectroscopy?EDS?and X-ray diffraction?XRD?.The mechanical properties of the samples were tested by universal experimental tensile machine and microhardness tester.The corrosion performance of the samples was tested by drop test,salt spray test,slow strain rate stress corrosion test and electrochemical method.The micro-arc discharge electric field characteristics of mobile cathode structure and the coating uniformity study show that,with the depth of the electrode is increased,the surface voltage and film thickness of the sample are significantly reduced,and the surface electric field and film thickness uniformity of the workpiece are decreased.With the voltage and duty cycle increase,or the frequency decreases,the surface voltage and film thickness,and the voltage distribution and thickness distribution uniformity decreases.With the distance of the electrode increases,the surface voltage and film thickness decrease,and the uniformity of the voltage distribution and thickness distribution decreases.With the increase of the moving speed of the cathode nozzle,the thickness,roughness and corrosion resistance of the micro-arc oxidation film of the sample are reduced.The uniformity of film thickness distribution is best when the processing speed is 0.16mm/s and 0.32mm/s.When the processing speed is 0.16mm/s,the uniformity of film roughness distribution is the worst.The uniformity of overall corrosion resistance distribution first increases and then decreases.With the increase of treatment times,the overall thickness,roughness and corrosion resistance of the film increase,the uniformity of the film thickness increases first and then decreases,the uniformity of the film roughness decreases gradually,and the uniformity of the film corrosion resistance increases first and then decreases.With the increase of moving overlap rate,the overall thickness,roughness and corrosion resistance of MAO film increase.With the increase of treatment times,the uniformity of overall film thickness increases,roughness uniformity first increases and then decreases,and corrosion resistance uniformity first increases and then decreases.The optimum composition of electrolyte by orthogonal test is as follows:20g/L Na₂SiO₃·9H₂O,5g/L of?NaPO₃?₆,3g/L KOH and 1.5g/L NH₄VO₃.The surface of A7N01aluminum alloy film deposited by mobile cathode micro-arc oxidation has a large number of pores and particle deposits.The film layer is divided into loose layer and dense layer.The film layer is mainly composed of?-Al₂O₃ and?-Al₂O₃.Salt spray corrosion and electrochemical test results show that the film layer effectively slows the erosion of Cl^-on the aluminum alloy matrix and improves the corrosion resistance of the matrix.The weld zone film hole of A7N01 aluminum alloy MIG welding joint is larger than the other two zones,the defects of the base metal are the least,and the defects of the weld zone are the most.The weld zone has the largest film layer and the heat affected zone is the thinnest.Each region of the joint film layer is mainly composed of?-Al₂O₃ and?-Al₂O₃,and the content of?-Al₂O₃ in the weld zone is relatively small.The strength and elongation of the joint sample decreased after MAO treatment,and the surface hardness and uniformity of the joint increased significantly.MAO treatment improves the stress corrosion resistance of the joint.Electrochemical test results show that the film layer effectively improves the corrosion resistance of the joint,and the corrosion resistance of the film in each zone of the joint is different.The thickness and defects of the dense layer are the key factors affecting the corrosion resistance of the welded joint of A7N01 aluminum alloy.The micro-arc oxidation treatment system and the corresponding process of the self-made mobile cathode nozzle successfully realized the overall preparation of A7N01aluminum alloy underframe large beam film,and the whole film is complete.It provides a feasibility test for the industrial treatment of micro-arc oxidation of the underframe large beam.The overall uniformity of the film thickness was 68.2%;the overall roughness uniformity of the film layer was 72.8%;the overall corrosion resistance uniformity of the film layer was 76.2%.