Study On Fabrication And Properties Of Micro-arc Oxidation Coating On Al-Si Alloy

In this paper,the micro-arc oxidation?MAO?coatings on the surface of Al-Si?11¹4%silicon content?aluminum alloy were prepared and researched to meet the special requirements of surface protection of Al alloy piston of High Power Density Engine?HPDE?.The MAO process parameters were optimized by the orthogonal test method.The microstructure,elements and phases composition and growth mechanism of MAO coating were analyzed by means of X-ray Diffraction?XRD?,Scanning Electron Microscopy?SEM?and Energy Dispersive Spectrometer?EDS?etc..The corrosion behavior in different corrosion medium,the friction and wear properties,heat insulation and thermal fatigue resistance of MAO coating were evaluated,aiming at elucidating the corresponding mechanisms.Bench test was carried out on evaluating the performance of MAO coating on the top of the aluminum alloy piston.The MAO coating prepared by optimized processing consisted of mullite?3Al₂O₃.2SiO₂?,?-Al₂O₃ and?-Al₂O₃.The thickness of MAO coating increased with the increase of oxidation current density and oxidation duration time.The content of mullite and?-Al₂O₃ in the coating increased with the increase of the thickness of MAO coating as well as the thickness of the dense layer.The growth rate of MAO coating increased firstly and decreased then.During oxidation process,the primary silicon in Al-Si alloy migrated to the surface of MAO coating or electrolyte,resulting in the existence of larger pores in MAO coating.In 3.5%NaCl solution,Oxygen ion(O^2-)in Al₂O₃ was replaced by Chloride ion?Cl^-?which stayed on the MAO coating to produce Al-O-Cl which was a soluble compound,leading the MAO coating to be damaged.At the same time,The insoluble corrosion products which the the products of anodic reaction(Al³⁺)reacted with the products of cathodic reaction?OH^-?gathered in the pores of MAO coating,and the pores were sealed to prevent Cl^-from permeating through the MAO coating.To a certain extent,the corrosion reaction was suppressed in during the anodic polarization process and the suppression became more obvious with the thickness of MAO coating increase.Meanwhile,In 1%Na2S solution,the hydrolysis of sulfur ions(S^2-)formed the high concentration of hydroxylthe?OH^-?that permeated through the MAO coating and reacted with the substrate of Al-Si alloy and it caused the substrate of Al-Si alloy corroded;moreover,it made MAO coating dissolved and damaged.So the effect of corrosion resistance of MAO coating and the substrate of Al-Si alloy in 1%Na₂S solution was worse than that in 3.5%NaCl.The corrosion products gathered in the pores of the MAO coating made the MAO coating passivated during later stage.The passivation was more obvious with the thickness of MAO coating increasing.The corrosion resistance of the MAO coating prepared by optimized process increases with the thickness of MAO coating increasing and decreased over a certain thickness.Under the dry friction test condition,the friction coefficient of MAO coating is higher than that of Al-Si alloy substrate,but its wear performance was better than that of the Al-Si alloy substrate.The wear resistance of MAO coating increased with the increase of the thickness of MAO coating.The mass loss rate of Al-Si alloy substrate and MAO coating increased with the increase of friction load and sliding velocity.The friction coefficient of MAO coating decreased with the increase of load,and the friction coefficient of the top layer of MAO coating was lower than that of the bottom layer of MAO coating.The friction coefficient of MAO coating under oil lubrication was 1/8¹/5 of that under dry friction test condition,and the friction coefficient of Al-Si alloy substrate under oil lubrication was 1/7¹/5 of that under dry friction test condition.The friction coefficient of MAO coatings and Al-Si alloy was greatly improved under oil lubricating medium.Under the high temperature condition,because of the mechanical properties of the Al-Si alloy decreases,the plastic deformation of Al-Si alloy increased under the friction load,which resulted in the increase of the friction coefficient.At the same time,the MAO coating like the egg shell was easily cracked and peeled off under the cyclic friction load.The surface of the substrate was reinforced by the wear particles of the MAO coating,so its friction coefficient was significantly lower than that of the Al-Si alloy.The mass loss rate of the Al-Si alloy and MAO coating increased with the rise of temperature.The mass loss rate of the MAO coating was lower than that of the Al-Si alloy under the same condition.The thermal diffusivity,specific heat capacity and thermal conductivity of MAO coating of the Al-Si alloy increased with the increase of the thickness of MAO coating,and decreased with the rise of test temperature.The thermal conductivity of MAO coating was decreased by2³ orders of magnitude contrasted to that of the Al-Si alloy substrate.The heat-insulation performance of MAO coatings with different thickness were simulated by ANSYS software,the results showed that the heat insulation of the 55?m MAO coating was the most favorable under the steady-state conditions.The heat resistant test showed that the heat-insulation of the55?m MAO coating was about 20?,the heat-resistance of the Al-Si alloy with MAO coating was improved by 67?,and the service life of heat resistant test was improved by one time.The MAO coating on the surface of the Al-Si alloy was with favorable thermal shock resistance and adhsion.After 60 times of air-cooled and water-cooled thermal shock tests,the MAO with 55?m thickness was free of visible cracks and coating peeling.The top surface of the MAO aluminum alloy piston was with favorable thermal fatigue performance:after 1000cycles of thermal shock test,the aluminum alloy with MAO coating piston surface was free of coating peeling and cracking failure.It passed the 50-hours bench test.