| The micro-arc oxidation（MAO）technology is a new green and eco-friendly surface treatment technique that utilizes high voltage to induce spark discharge on the surface of light metals in an electrolytic solution,generating complex physical and chemical reactions between the substrate and elements in the solution to form a fused metal oxide coating under high temperature and voltage,which is tightly bonded to the substrate and possesses excellent properties such as wear resistance,corrosion resistance,and electrical insulation.In the study of MAO electrical parameters,frequency and duty ratio,as one of the influencing factors,determine the spark discharge time and oxide cooling time of MAO,which in turn affects the growth state and growth effect of the coatings.The rationality of their combination is crucial for the structure and performance of the coatings.At present,more research only focuses on frequency and duty ratio itself,and there is little research on the appropriate combination of the two.However,its appropriate combination is closely related to the adequacy of cooling time,and there is a lack of more comprehensive comparative research on the effects of different power supply modes on MAO.Therefore,in this study,the spark discharge time is referred to as arc duration,and the cooling time of is referred to as the no-arc duration.This study sets the arc duration as a constant,takes the no-arc duration as the independent variable,and introduces the concept of the ratio of no-arc duration to arc duration to quantitatively reflect their proportional relationship.Two sets of experiments were designed,including U1（1336 Hz,40%）,U2（667 Hz,20%）in unipolar mode and B1（1336 Hz,40%）,B2（667 Hz,20%）in bipolar mode,with a total of four plans.The effects of no-arc duration on the AZ91D MAO magnesium alloy coatings are compared and studied under different power supply modes in terms of the micro arc oxidation process,coatings thickness,micro morphology,phase composition and corrosion resistance.The effects of different frequency and duty ratio combinations on the coatings are comprehensively analyzed,which provide guidance for the directionality of frequency and duty ratio combinations.And compare the similarities and differences of power supply modes through equivalent experiments under different power supply modes.The results are as following:In unipolar mode,the U1 scheme with a ratio of no-arc duration to arc duration of1.5 produce a thicker coating with better corrosion resistance as measured by spotting testing and electrochemical testing;The U2 scheme with a ratio of no-arc duration to arc duration of 4.0 has a milder micro arc oxidation reaction,smaller surface micropores and roughness,and denser coatings.The corrosion resistance of electrochemical detection is not significantly different from that of the U1 scheme.If there is a higher requirement for coatings thickness,the U1 scheme with high frequency and duty ratio has the highest coating-forming efficiency and excellent comprehensive corrosion resistance performance;The U2 scheme with low frequency and duty ratio not only ensures high-quality microstructure of the coatings,but also has excellent electrochemical detection corrosion resistance.Compared to the U1 scheme,it has lower energy consumption and higher overall cost-effectiveness in practical application scenarios.In bipolar mode,the B1 scheme with a ratio of no-arc duration to arc duration of4.0 produce a thicker coating with better corrosion resistance as measured by spotting testing;The B2 scheme with a ratio of no-arc duration to arc duration of 9.0 has a milder micro-arc oxidation reaction,slightly better coatings flatness and compactness,and slightly improvement of electrochemical detection corrosion resistance compared to the B1 scheme;The B1 scheme with high frequency and duty ratio has a more balanced coatings structure and performance,and higher coating-forming efficiency.The scheme with a ratio of no-arc duration to arc duration of 9.0 has low coating-forming efficiency and slightly improvement of coatings structure and performance,which does not have advantages in practical application scenarios.The U2scheme with low frequency and duty ratio in unipolar mode and the B1scheme with high frequency and duty ratio in bipolar mode both have a ratio of no-arc duration to arc duration of 4.0,with only differences in power mode.The comparison shows that the difference in coatings thickness,microstructure,and corrosion resistance between the two modes is very small,which basically conforms to the equivalent characteristics.The unipolar mode has slightly higher energy consumption compared to the bipolar mode,but the reaction process is more stable and the reaction controllability is relatively better. |
