Study On Micro-arc Oxidation Of Cast Al-Si Alloys

Cast Al-Si alloys are widely used in automotive, aerospace, marine and other industriesdue to their excellent casting properties, high specific strength and easily machining, etc.However, the poor surface corrosion resistance and low hardness restrict its applicationseriously. So, it has great economic significance to improve its surface performance of Al-Sialloy by surface treatment.As an eco-friendly and simple technology, micro-arc oxidation (MAO) which deprivedfrom anodic oxidation, can prepare a ceramic coating with good performance. Now, thistechnology has been widely applied in the surface modification of aluminum alloys. But it isdifficult to obtain a ceramic coating with good performance on cast Al-Si alloy with highsilicon content. With the increasing application of cast Al-Si alloys, it becomes more andmore necessary and urgency for deep investigation about the MAO technology on Al-Sialloys.This study is based on the two mainly problems existed in the MAO of cast Al-Si alloycontaining high silicon. The first one is the delay of the emergence of the first arc during theMAO processing. The other one is that, the silicon participates in the formation of the coatingand its reaction products will deteriorate the properties of MAO coating. In this paper, all thesamples were from the smelted Al-Si binary alloys, with silicon content (weight percentage)of1%,5%,9%,12%,15%, respectively. SEM (scanning electron microscope), EDS (EnergyDisperse Spectroscopy), XRD (X-Ray Diffraction) and electrochemical workstation wereused for the detection of the coatings’surface and section morphology, elements compositionand microstructure and corrosion resistance. At the beginning, the influence of silica contenton the experimental phenomena in the early stage of the MAO process and the morphology ofthe coating. Then, the arc starting time was significantly shorted and the performance ofcoating improved by the optimization of process parameters. Finally, the effect mechanism ofsilicon on the MAO of Al-Si alloy was discussed based of the analysis of the whole reactionprocess.The influence of the concentration of Na2SiO3and NaOH on the MAO coating on Al-Sialloy was studied. The characteristics of MAO processing under the constant voltage andconstant current power mode were also analyzed systematically. It was found that thedischarge breakdown preferentially to occur in areas rich of silicon according to the morphology observation. It was found that additives like fluoride has an significant influenceon the shorting of arcing time and decreasing of dielectric breakdown voltage during theMAO process. The effects of Fluoride A and Fluoride B on the discharge, thickness,roughness, corrosion resistance of the membrane, etc. were investigated in detail. Themorphology, thickness and roughness of the coatings varied with the oxidation time werestudied in the electrolyte composed of Na2SiO3, NaOH and Fluoride B. The impact of a twosections MAO process consisted of constant current mode followed by constanThe voltage mode on the MAO coating on Al-Si alloys was also studied, and thetwo-step oxidation process could improve the surface roughness, hydrophobicity andcorrosion resistance of the MAO coating.