Effect Of Electrode Spacing And Thermal Stress On Aluminum Alloy Microarc Oxide Coatings

Micro-arc Oxidation（MAO）technology can be used to obtain an improved oxide film layer on the surface of aluminum,thus it is widely applied for surface modification of aluminum.In this paper,a finite element model of Micro-arc Oxidation of aluminum alloy is established based on COMSOL,the influence of pole spacing change on the growth of oxide film is studied,and then the model is solved and analyzed.The study investigated the effect of changes in inter-electrode distance on the thickness and distribution of the oxide film,and based on this,further explored the principle of how the electric field affects the film formation.In addition,a finite element model of micro-arc oxidation thermal stress was established to study the temperature change and thermal stress generation during film formation,and further explore their influence on the microscopic morphology of the film surface.The main research contents are as follows:（1）The change of pole spacing between 2～12mm was studied,and it is found that the increase of pole spacing played a negative role in the increase of film thickness,and the distribution law of film layer is related to the cathode position.The growth of the micro-arc oxide layer is affected by the surface current,when other external conditions remain unchanged,the pole spacing increases from 2mm to 12mm,the average thickness of the film layer is reduced by 0.117μm,and the average current density is reduced by488.77A/m².The current density is positively correlated with the electric field strength,so the increase of the electrode spacing not only reduces the current density but also reduces the electric field strength,fixes the electrode spacing,and controls the forward voltage change between 500V～620V to change the micro-arc oxidation electric field.It is found that the increase of forward voltage within the scope of this paper promotes the growth of the film layer.The total oxidation time was set to 20min,and when the voltage was increased from 500V to 620V,the final thickness of the film layer increased to 41.698μm;When the voltage is constant at 500V,the final thickness of the film layer increases to37.165μm;As the voltage increases,the final thickness of the film layer increases by4.533μm.In addition to the final thickness of the film,the growth rate of the film layer continues to increase at different time periods.In this paper,the principle of forward voltage increase to promote the growth of the film layer is to increase the electric field strength between the electrodes,thereby increasing the surface current density,which is the same reason as the decrease in the thickness of the film layer caused by the increase of the pole spacing.（2）The temperature and thermal stress of the film layer were studied,and it was found that the electrolyte played a key role within 50μs after the formation of the discharge channel,and after the formation of the discharge channel,the nearby stress could reach about 1200MPa within 2μs,far exceeding the tensile strength of Al₂O₃,and eventually resulting in crack defects on the surface of the film.The presence of cracks will reduce the resistance of the film layer near it,and the low resistance characteristics will cause more dense electrical breakdown in their vicinity,resulting in densely distributed micropores.（3）Based on numerical simulation conditions,the micro-arc oxide film layer is prepared and analyzed.The experimental results are compared with the simulation data,and the results show that the data errors of the two are within a reasonable range,the change trend is consistent,and the conclusions are consistent with the existing relevant data.Therefore,it is verified that the simulation is reliable and can provide certain theoretical guidance for the research of micro-arc oxidation.