Effect Of Sc And Zr Addition On Microstructure And Corrosion Of Al-Si-Mg Alloy

Cast Al-Si-Mg alloy has been widely used in the fields of aerospace and transportation due to its low density,high fluidity,low coefficient of thermal expansion and high specific strength.The microstructures of untreated cast Al-Si-Mg alloys mainly consist of coarseα-Al dendrite and plate-like eutectic Si phase.The plate-like eutectic Si phase easily split the matrix of the alloy and result in stress concentration and crack,thus damaging the mechanical properties of the alloy.At present,adding both grain-refiner and modifier to cast Al-Si-Mg alloy melts is a commonly used treating method in industry.However,this double addition not only demands very high technology,but also there are some defects.Sc is a rare earth element.It has refining and modifying effects on theα-Al and eutectic Si phases of cast Al-Si-Mg alloy.The combined addition of Sc and Zr can yet enhance the refining and modifying effects of Sc and reduce the addition content of Sc.However,relative researches have still been limited.Especially,there have been very few studies about the effect of Sc addition or the combined addition of Sc and Zr on the corrosion resistance of cast Al-Si-Mg alloy.In this paper,cast Al-6.5Si-0.45Mg alloy was taken as a research object,the effects of Sc addition and the combined addition of Sc and Zr on the microstructure and corrosion resistance of the cast Al-6.5Si-0.45Mg alloy were investigated by using scanning electron microscopy,X-ray diffraction,intergranular corrosion test,weight loss corrosion test and electrochemical measurement technology.The following conclusions can be drawn from the present study.1、Sc had refining and modifying actions on theα-Al and eutectic Si phases of the cast Al-6.5Si-0.45Mg alloy.When the Sc content was lower,the refining and modifying effects of Sc were relative weak.But with the increase of the Sc content,the refining and modifying effects of Sc were enhanced.When the Sc content reached0.58wt.%,the modifying effect of Sc on the eutectic Si phase was almost same as that of 0.04wt.%Sr,and the eutectic Si phases were transformed to fine short bar or granular shapes.Comparing to Sr modifier,Sc could improve the corrosion resistance of the cast Al-6.5Si-0.45Mg alloy.Although high Sc content could exhibit better refining and modifying effects on theα-Al and eutectic Si phases of the cast Al-6.5Si-0.45Mg,it could not further improve the corrosion resistance of the alloy.The reason for it was that the fine distribution of the eutectic Si phase in the alloy increased the interface betweenα-Al phase and eutectic Si phase and the amount of corrosion cell in the alloy,resulting in the increase of the corrosion current and the decrease of corrosion resistance.2、The addition of proper content of Zr could enhance the refining and modifying actions of Sc on theα-Al and eutectic Si phases of the cast Al-6.5Si-0.45Mg.For the Al-6.5Si-0.45Mg with addition of 0.45wt.%Sc,the addition of 0.12-0.20wt.%Zr could make Sc exhibit excellent refining and modifying effects,but the corrosion resistance of the alloy decreased.It was related to the formation of a large number of Al₃(Sc_x,Zr_1-x)phase particles in the alloy melt with addition of Zr.When the Zr content was further increased,the refining and modifying actions of Sc were weakened,but the corrosion resistance of the alloy was improved to some extent.The addition of excessive content of Zr would increases the number of Al₃(Sc_x,Zr_1-x)phase particles,which were densely distributed in the alloy melt.This increased the likelihood of the aggregation between Al₃(Sc_x,Zr_1-x)phase particles and settlement to the bottoms of the alloy melt.It not only decreased the number of the Al₃(Sc_x,Zr_1-x)phase particles in the alloys,but the content of Sc that was really effective refining and modifying element in the alloys.So,the addition of excessive content of Zr would weaken the refining and modifying effects of Sc,but be beneficial to the corrosion resistance of the alloy.3、The corrosion of the alloy mainly occurred in the eutectic region.Theα-Al phases were dissolved and the eutectic Si phases were almost not corroded during corrosion.It indicated that the corrosion cell could be formed between theα-Al phase and eutectic Si phase,and the electrode potential of theα-Al phase was lower than that of eutectic Si phase.