Thermal Analysis On Modification Effect Of Aluminum-silicon Alloys With Rare Earth

Al-Si alloys, which boasts lower density, high specific strength, well processabilityand casting performances, is widely used in transportation and aviation industry. Inrecent years, a lot of research of Al-Si alloys has been focused on its refinement,modification of primary Si, eutectic Si and primary phase α-Al etc.. In this sense, myresearch will focus on the Rare Earth modification mechanism since it is considered anideal modification additive for Al-Si alloys.This paper, employing diversified developed test method, such as thermal analysisinstrument, scanning electron microscope(SEM) and X-ray diffraction(XRD), etc.,studies how different quantity of RE added influence the microstructure and properties ofAl-Si alloys and the corresponding relationship between modification effect and thermalcharacteristic values. The study results of RE modification effect in hypoeutectic Al-Sialloys show that, with the addition of RE contents(0~0.6%), the primary phase α-Al inZL101converts from plate into regular polygon, the average grain size decreases, and theprimary crystal growth temperature on the cooling curve raises. When the difference ingrowth temperature of primary crystal ΔTGranges between0.4℃~0.9℃, the ZL101alloyhas the best modification effect. However with the increasing addition of REcontents(0.8%~1.0%), ΔTG＞2.2℃, part of primary phrase α-Al grain size decreases.Since ΔTGand α-Al enjoy excellent corresponding relationship in refinement and equalaxle, can be regarded as the evidence to judge for the modification effect of hypoeutecticAl-Si alloys. Comparing the cooling curve of ZL101with RE and without RE, we foundthat the primary crystal growth temperature raises is mainly due to weakening thecombine of the Si-Si and Si-Al atoms, strengthening the Al-Al atoms with the addition ofRE, and promoting α-Al first to be formed. When the RE content ranges between0~0.6%,the eutectic Si has no modification effect, and the difference in growth temperature ofeutectic crystal ΔTEGis small on corresponding cooling curve. The main reason is thatwith the precipitation of primary phase α-Al, RE elements are left, the rest REconcentration in the melt becomes low. With the increasing addition of RE(0.8~1.0%), the primary Si growth temperature decreases, ΔTEGincreases, the eutectic Si transfersfrom needle shape into small plate, the average grain size decreases, and the modificationphenomenon starts to appear. Its modification mechanism is because of the distributionof elements, the constitutional undercooling caused by the gathering effect of REelements in the solidification front, the decrease of eutectic crystal growth temperature,the refinement and modification of eutectic Si.To the RE modification effect of hypereutectic Al-Si alloy, RE is able to modify theprimary Si with RE added alone. With the increasing addition of RE contents（0~1.0%），the primary crystal growth temperature decreases on the Al-20Si-2Cu-1Ni-0.4Mg alloycooling curve. When the absolute value of ΔTGranges between2.9℃～3.9℃, eutectic Siwith small average grain size presents round shape, the alloy has the best modificationeffect. Therefore ΔTGhas an excellent corresponding relationship with the modificationeffect of primary silicon with RE. However the average grain size and shape of eutecticSi stays the same, ΔTEGhas no regulation, eutectic Si has no modification effect. Whenthe RE addition content ranges between0.8%~1.0%, the grain size of part primary Sikeeps decreasing, but the other part increases, the shape becomes worse.When P and RE modify the hypereutectic Al-20Si-2Cu-1Ni-0.4Mg alloy together, theprimary growth temperature raises, the eutectic growth temperature decreases. When theRE addition content ranges between0~0.6%, ΔTEGcontinually increases, and eutectic Sihas excellent modification effect when ΔTEGranges between6.5℃~7.6℃. WhenΔTEG<4.3℃, alloys have no modification effect, therefore ΔTEGhas an excellentcorresponding relationship with the modification effect of eutectic silicon with RE. Whenthe RE addition contents are0.8%and1.0%, the eutectic grain size is huge, grain shapebecomes worse, the number of RE-containing intermetallic compounds increases. Theeutectic Si growth temperature decreases when the hypereutectic Al-Si alloys aremodified with RE. This is because the RE elements gathering in the solidification frontleads to constitutional undercooling, easily dendrites of precipitation phrase andpromotes refinement.