| Al-Si alloys are widely used in industrial products, but their properties, such as wear resistance and corrosion resistance are not satisfactory. Their surface performance could be greatly improved by preparing a ceramic coating on the surface by micro arc oxidation(MAO) technology. Nevertheless, the deposition efficiency of MAO layer on high-silicon Al alloy is low, and the performance of the layer is not as good as that of Al alloys with lower Si content(Si<1%). The reason is related to the Si in their substrate. In this thesis, pure Al and binary Al-Si alloys with Si content of 5%, 9%, 12% and 15%, were treated by MAO. The mechanism of Si and the influence of two preprocessing techniques on the MAO of Al-Si alloys were investigated.Firstly, by a common MAO process of deformation Al alloys, when the Si content in Al alloy was increased to 12%, the layer deposition efficiency became much lower and its corrosion resistance became much worse than layers on Al-Si alloy with less Si. Therefore, processing parameters were optimized by orthogonal experiment, in which the electrolyte was composed of Na2SiO3(12 g/L), NaOH(2 g/L) and C6H12N4(5 g/L), the positive and negative current densities were 10 A/dm2 and 4 A/dm2 respectively, the frequency was 400 Hz and the duty ratio was 25%. With the optimal MAO process, the pure Al and Al-Si alloys were treated for 30 min. Results show that, as the Si content in the substrate increasing, their anodic oxidation time was increased from 48 s to 276 s, the dielectric breakdown voltage of the layer was gradually decreased. The gap of their voltage was diminished with time, the final average deposition rate of the layers was decreased significantly, while the corresponding specific energy consumption was increased.Then, by constant voltage regime, the positive and negative voltages were 450 V and 50 V respectively, while other parameters were the same as that used in constant current oxidation, all substrates were also MAO treated for 30 min. The Si phase in all substrates was mostly oxidized under the drastic discharge occurred at peak current density at the earlier stage, the negative effect of Si on the layer formation became much weaker, the gap of growth rate of all layers became much small. As the Si content increasing, both the forward and negative current density of the substrates were increased gradually. The specific energy consumption of the layer was increased. In addition, the layers on all substrates by the two oxidation regimes were both composed of γ-Al2O3, α-Al2O3, mullite and SiO2.The optimizing of process parameters is unable to significantly eliminate the bad impact of Si content on the MAO of Al-Si alloys, which may be associated with morphology of Si phase. Hence, the influence of the size of Si in the Al-Si alloy matrix on its MAO was studied. The eutectic Al-Si phase in the matrix was obviously refined with 0.05% or 0.1% Sr modification, and the Si in the substrate with 0.1% Sr was smaller and more uniform than that with 0.05% Sr. Over 30 min constant current MAO treatment, as eutectic Si phase become smaller, it was easier to be covered by the anodized alumina formed on the Al nearby for volume expansion. Hence, the impact of Si on the layer growth became much weaker. The anodic oxidation time was also decreased. But the effect of Sr modification became feebler and feebler with the MAO time. The voltages and characteristics of the two Sr modified substrates were close to each other all the time, which means that when the eutectic Si was refined to some certain size(10~15 μm), its influence would be almost steady even with further refinement. All layers were also composed of γ-Al2O3, α-Al2O3, mullite and SiO2. The MAO layers on the Al-12% Si alloy with Sr modification became more uniform and compact. And their micro hardness, wear resistance and corrosion resistance were all greatly improved. The characteristics of layers on the two Sr modified Al-12% Si alloys were similar to each other all the time, but the property of the layer with 0.1% Sr modification was a bit better, in that its matrix structure is more uniform.The refinement of eutectic Si could promote the performance of MAO layer on Al-Si alloy, but it was useless to improve the deposition efficiency of the layer. In fact, the deposition efficiency and properties of MAO layers on deformation Al alloy with low Si content is much better than the layers on high-silicon Al alloy. Thus, the Si element in the skin layer of Al-Si alloy was eliminated by an etching technology, and the effects of such pretreatment on its MAO was investigated. The results indicate that the mixture of concentrated HNO3(65 wt%) and HF(40 wt%) with a volume ratio of 4 to 1 was most suitable for the etching processing of Al-12% Si alloy. By 45 sec etching treatment, the Si in the skin layer of the alloy became little(Si<0.56%), and just decreased slightly with prolonging the etching time. Such etching treatment made the Si about 1~1.5 μm deep in the skin layer of the substrate was dissolved, while the Al was remained. Over 30 min constant current MAO of none, 15, 30 and 60 sec etched Al-12% Si alloy, the etching treatment significantly accelerated the forward voltage, leading to a shorter anodic oxidation stage and a higher layer growth rate. But the impact of etching time on the voltage of the etched samples was tiny. The influence of etching preprocessing on the layer growth also became weaker and weaker, on that, the Si in the deeper layer of the etched substrate would also be oxidized with MAO continued. All layers were composed of γ-Al2O3, α-Al2O3, Al6Si2O13, mullite and SiO2. With 30 min MAO treatment, the micro hardness, wear resistance and corrosion resistance of layers on the etched substrates were all significantly enhanced. The difference of Si content on the matrix by three kinds of etching time were small, the characteristics of their layers were similar, and the performance of their layers was just improved a bit with extending the etching time. In addition, the etching pretreatment can also remove most of the Si in the skin layer of Al-9% Si and Al-15% Si alloys. Thus, it could also improve the MAO layer deposition rate, and reduce the specific energy consumption. |
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