| Due to their easily machining,lightweight cast Al-Si alloys are widely used in aerospace,automotive and other industry.However,poor wear and corrosion resistance often decrease the lifetime of cast Al-Si alloy components.So,it has great economic and functional significance to inprove its performance of cast Al-Si alloy by surface treatment.Plasma electrolytic oxidation?PEO?,which has the advantages of simple technological process and environmental electrolytes,can be used to improve the surface properties of aluminum alloy.Al-Si alloy contains a large number of silicon,which may influence the plasma electrolytic oxidation behavior.The electrolytes used for PEO technique are usually dilute alkaline solution of silicate,phosphate on the Al-Si alloy in the previous research,but less researchers use aluminate as the electrolytes to form the PEO coatings.This paper is based on two aspects to research the behavior of PEO on cast Al-Si alloy.On the one hand,exploring the different concentration of sodium aluminate on Al-Si alloy by plasma electrolytic oxidation,and also testing the related properties of the coatings.On the other hand,in order to prevent the decomposition of sodium aluminate,adding sodium hydroxide into the electrolytes,and studying the properties of the coatings by different conditions.Utilizing sodium aluminate electrolyte with different concentration(2 gl-1,16 gl-1,24 gl-1)and silicate electrolyte(8 gl-1,for comparison with sodium aluminate)of automobile hub with A356 aluminum silicon alloy by plasma electrolytic oxidation process.The coatings' wear and corrosion performances are evaluated with tungsten carbide ball dry friction experiment and electrochemical experiments,and the coatings' hardness is tested by microhardness tester.The results showed that the coating had the best wear resistance and corrosion resistance in 24 gl-1 sodium aluminate electrolyte with processing time of 480 s,the film could continue 30 min sliding friction under 30 N,the wear rate is only 4.5×10-7 mm3 N-1 m-1;Meanwhile,the average value of coating micro-hardness in 24 gl-1 sodium aluminate with 480 s reached 1352±251 HV,equivalent to 15 GPa,similar to the hardness of ?-Al2O3,meaning good wear performance;The electrochemical tests showed that A3 56 Aluminum Alloy by PEO membrane treatment,significantly improve the corrosion resistance,the breakdown voltage with highest value of-0.236 V,corrosion current density of 8.81×10-9 Acm-2 in 24 gl-1 electrolyte had the best corrosion resistance compared with the substrate,nearly 3 orders of magnitude higher.Although the content of ?-Al2O3 phase in the film is low,the film's compactness and monolayer structure guarantee the friction and corrosion resistance.In order to improve the stability of sodium aluminate solution,different concentrations of NaOH were added to 32 gl-1 in sodium aluminate solution to investigate the effects of NaOH concentration on the stability of solution and the corresponding properties of the film,The coatings' wear and corrosion performances are evaluated with tungsten carbide ball dry friction experiment and electrochemical experiments.The results show that the electrolyte can be preserved for 35-40 days or longer with higher concentration when the concentration of sodium hydroxide is 5 gl-1.But NaOH concentration in sodium aluminate solution is too high that would cause significant field assisted dissolution phenomenon in film formation process.The stability would improve and had good film performance with the NaOH addition of 5 gl-1.The coating with reaction time of 30 s could withstand 30N load,and could not been worn down with wear rate of 4.1×10-7 mm3 N-1 m-1.The electrochemical test shows that the corrosion current density of the film is 8.52×10-9 A cm-2,which shows excellent corrosion resistance. |
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