Effect Of Lithium On Microstructure And Properties Of Al-Si Series Cast Alloys

Al-Si alloys are widely used in aircraft and automobile manufacturing industries because of its favorable casting properties, favorable machining properties, medium strength and hardness. Lithium is the lightest metallic element. With addition of lithium to aluminum alloys, the density of the alloys greatly reduce, the stiffness increase, and the elastic modulus improve, at the same time, the good corrosion resistance and fatigue resistance of the alloys keep stable. To combine the good mechanical properties of the Al-Si alloys with the characteristics of the Al-Li alloys'low density, three types of Li containing alloys were produced by addition of 1%,2%and 3%Li, which was based on Al-Si series cast alloys such as ZL102, ZL101, ZL110, respectively. Microstructure and properties of the Al-Si series cast alloys containing Li were analyzed and investigated by optical microscopy, Vickers hardness, differential scanning calorimetry (DSC), tensile testing machine, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). While the level of lithium staying the same, the effects of Si and Mg level on Al-Si-Mg-Li alloy were investigated by modifying the level, respectively. The results are shown as follows.Lithium plays a refinement on grains of the Al-12Si alloys. With the increment of Li level from 1% to 3%, the volume fraction of eutectic Si phase decreases, the primary Si thickens and its volume fraction increases, so that the tensile strength and the elongation of the alloys decrease. There appear new precipitates when Al-12Si-Li alloy ageing at 350℃, and the ageing curve of the alloy has two peaks. The corrosion resistance of the alloys increases as lithium is added into the alloys. The alloy containing 2%Li has the best corrosion resistance on in 3% NaCl solution.With the increment of Li level, the microstructure of the Al-Si-Mg-Li alloys changes. The a-Al matrix refines, the amount and the size of the primary Si increases, the shape of primary Si gradually becomes triangle from small particles, and the LiAISi phase appears. The precipitation of the Al-Si-Mg-Li alloys is inhibited, while the tensile strength and the elongation decreases. The tensile strength of the Al-7Si-0.5Mg-2Li alloy is 245MPa. The fracture of the Al-7Si-0.5Mg-Li alloys depends on the Si phase.With the increment of Li level, the shape and the amount of the primary Si are changed in early ageing of the Al-5.5Si-6.5Cu-0.4Mg-Li alloys. With the increment of the Si level, the size of the Si phases enlarges, and the meanwhile the LiAlSi phase, the LiAl phase and the AlCuMg phase appeare. The elongation of the alloys is lower, and lithium has little action on it. Alloy containing 2%Li has the highest tensile strength.As a result of Si changing, the microstructure of the Al-Si-0.3Mg-lLi alloys changes. With the increment of Si level, the primary Si of alloy appears in the microstructure, and it is coarse, irregular and angular; After metamorphism of Al-P Alloy, the microstructure of the alloys is refined, the primary Si reduces and the eutectic compounds increase. Microstructure of the modified Al-15Si-0.3Mg-lLi alloy is fine. The properties of the alloys are best, the strength and the yield strength of the alloys are 255MPa and 95.6MPa, respectively, and the extension is 1.5%.Addition of Mg, the capacity of the alloys in aging is improved, the tensile strength and the elongation of the Al-Si-Li alloys increases. With the increment of Mg level, the a-Al and the primary Si phase are refined, the amount of primary Si decreases, the eutectic microstructure increases, so that the mechanical properties of the alloys increases. Since excessive Mg is added, herringbone Mg2Si phase appears in the alloys, the mechanical properties of the alloys is poor. With the Mg level increases, the precipitates in the Al-Si-Mg-Li alloys change.