A Study On Al-Li Alloy Cast Under Near-rapid Cooling

Al-Li alloy has the characteristics of light weight and high strength. It has become one of the most competitive materials and been widely used in the aerospace industry. The research on the low density and high performance Al-Li alloy was rich and mainly focused on the heat treatment. Little research was conducted on the preparation process, the optimal chemical composition and the homogenization system. In this paper, the Al-Li alloys with different contents of Cu and Li were cast under near-rapid cooling. The effect of the contents of Cu and Li and heat treatment on the microstructures and mechanical properties of the Al-Li alloys were evaluated by means of density testing, mechanical property testing and analyses with optical microscope(OM), scanning electron microscope(SEM), electron diffraction and energy spectrum(EDS), X-ray diffraction analysis(XRD) and different scanning calorimetric(DSC). The results were as following.1. The tensile strength of the near-rapid solidified alloy firstly increased and then decreased with Cu content increasing. The fracture mode is characterized by brittle fracture.When the Cu content reached 4.5%(wt), the best performance was obtained. The effect of the same homogenization treatment to the Al-Li alloy decreased with the increasing of Cu content.When the Cu content is low, it can be obtained the precipitation of fine by the homogenization treatment, at the same time, the microstructure of the alloy is uniform. When the Cu content is high, the segregation of Cu appeared in the alloy microstructure which can not be completely dissolved through the homogenization treatment making an influence to alloy mechanical properties. When the Cu content between 3.5%(wt) to 7.5%(wt), the optimal homogenization process for Al-Li alloy is conducting heat preservation for 24 h under 530 degrees Celsius. In this process, the low Cu alloy can obtain better uniformity effect, but if the Cu content is too high, there are still some coarse second phase which can not dissolved.2. With the increase of the content of Li element in near-rapid solidified Al-Li alloy, the performance of the alloy decreases gradually in the same homogenization treatment system.The reason is that the effect of the same homogenization treatment to the Al-Li alloy decreased with the increasing of Li content. When the Li content is low, the second phase of the alloy is dissolved in the matrix, while, when the Li content is higher, and the homogenization can not completely dissolve the coarse second phase which will aggregate toform a larger phase and reduce the performance of alloy.3. At low Li content, the microstructure of Al-Li alloy is mainly based on the dendritic structure. With the increasing of Li content, the Afterbirth-like crystal structure is beginning to appear, and the larger grain size is refined into a number of smaller grains in this process.4. When the Li content is low, it can be obtained that the better grain structure in the Al-Li alloy by homogenization treatment, and there are few and small second phase residues in the grain boundary. When the Li content is high, the effect is not satisfactory. The eutectic structure Al2 CuMg phase in the alloy can be dissolved at first, and the remaining block-phase will aggregate to form a bulky phase to reduce the performance of alloy.