| Al-Si alloy is more and more widely applied in industrial products,owing to its low density,high specific strength,as well as good casting and machining performance.Micro-arc oxidation(MAO)technology has been regarded as an effective and environmental-friendly method for depositing a ceramic coating on the substrate surface to enhance the properties of Al alloy.Efficiently forming ceramic coatings with good performance on the high silicon content Al alloys has important economic significance.However,the silicon in the substrate has an adverse impact on the micro-arc oxidation of high silicon content alloy,delaying the sparking initiation time and decreasing the growth rate of coatings significantly as well.Although many efforts have been devoted to solving the problems of the MAO of high silicon content aluminum alloy,few works have studied systematically the critical silicon content.Besides,the effect of sectionalized MAO mode of constant voltage oxidation followed by constant current oxidation on the MAO ceramic coating of high silicon content alloy is rarely investigated.In addition,the researches on MAO process mainly focused on single factor exploration.There is serious shortage of research on the synergistic effects of multiple factors,such as optimizing the electrolyte cmpositions and electrical parameters together.In this study,based on these problems,the main results are as follows:(1)The convetional MAO process developed for low silicon content alloy of 6061 was applied to high silicon content alloys and the effects of the silicon content on the coating properties were studied.The results showed that the maxium current density and the pore diameter of discharge channels was increased while the coating growth rate,corrosion resistance and wear resistance was reduced with the increase of silicon content in the Al substrate.By taking the effect of silicon content in the substrate on the properties of MAO coatings into comprehensively consideration,it was deemed that the critical silicon content of the traditional MAO process was 5%.Namely,the MAO process for 6061 aluminum alloy is applicable only when the silicon content in Al alloy is less than 5%.(2)By comprehensively optimizing the electrolyte compositions and electrical parameters,a new constant voltage MAO process was developed for the Al alloy with 7 wt% Si.Further study indicated that the developed process could significantly improve the anti-corrosion performance of the coatings while the anti-wear performance couldn't be effectively improved.(3)It was also found that,in the single constant voltage MAO mode,the current density suffered a continuous decrease and the arc energy became weaker and weaker,resulting in a significant reduction of the coating growth rate.Therefore,a sectionalized oxidation mode was introduced,namely a MAO process with the constant voltage oxidation followed by a constant current oxidation model.In this two-step model,the constant voltage oxidation mode at the early stage of MAO could significantly shorten the sparking initiation time while the followed constant current oxidation could make up for the decrease of current density at the later stage.The effects of the treatment time of constant voltge oxidation step and the current density of constant current oxidation stage in the sectionalized oxidation mode on the coatings performance were carefully investedgated.It was found that the ceramic coating on Al-7 wt.% Si alloy had the best comprehensive performance when the constant voltage processing time was 1 min and the current density at the stage of constant current oxidation was 13 A/dm2.Finally the sectionalized MAO mode was applied to aluminum alloys with the silicon content more than 7 wt.%.The result showed that the critical silicon content of the sectionalized MAO process was 12 wt.%.Namely,the aluminum alloys with silicon content less than 12 wt.% could be effectively oxidized by the sectionalized MAO process. |
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