Study On The Preparation And Heat Treatment Of Al-Cu-Li Alloys

Al-Li alloys, with low density, high elastic modulus, high specific strength and stiffness and many other excellent comprehensive properties, has been one of the most rapidly developing advanced lightweight construction materials in aerospace materials in recent years. The structural weight can be reduced about10%to20%and the stiffness can be increased by15%to20%, if the Al-Li alloy is used to replace the conventional high-strength aluminum alloy. Therefore, Al-Li alloys show a broad application prospect in the aerospace field and to study these alloys has theoretical and practical significance.In the present work, the ingots of Al-Cu-Li alloys were prepared and homogenization treatment, rolling, solid solution and aging treatment were carried out. The homogenization process, aging process of Al-Cu-Li alloys and the effect of the concentration of iron impurity on the microstructure and mechanical properties were systemically studied by means of differential scanning calorimetry (DSC), optical microanalysis (OM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) analysis, tensile mechanical properties test, hardness test and other methods. Significant results have been obtained.The differential scanning calorimetry (DSC) result shows that the overheat temperature of the Al-Cu-Li alloy is507℃. With the application of three-stage homogenization treatment (450℃x3h+470℃×12h+500℃×12h), the endothermic peak at lower temperature disappears and the endothermic peak at higher temperature becomes much smaller, indicating the dissolution of most of the low temperature eutectic structure. With the application of450℃×3h+470℃×12h+500℃x48h, endothermic peaks of both the lower and the higher temperatures disappear indicating the dissolution of all the dissoluble low temperature phases. With regard to the treating time and the efficiency, based on the results of the present work, three-stage homogenization treatment process,450℃×3h+470℃×12h+50℃×12h, was suggested. The concentration of iron impurity has a significant effect on the homogenization treatment process and the residual phases. With the increase of the concentration of Fe, the remelting temperature of the low temperature eutectic microstructure is improved. The main residual phase is Al2Cu at low content of Fe (0.09%). The main residual phases were transformed to Al2Cu, Al7Cu2Fe and Al-Fe when the content of Fe is higher than0.35%.After solid solution treatment at505℃and quenching, the Al-Cu-Li alloy sheet was held at180℃. The peak strength appears when the holding time is20h. After T6treatment, the tensile strength, yield strength and elongation of the alloy are613.2MPa,562.4MPa and3.4%, respectively. With the identical solid solution treatment and quenching process, a pre-deformation of12%was applied to the quenched Al-Cu-Li alloy sheet and then the sheet was held at160℃. The peak strength appears when the holding time is22h. After T8treatment, the tensile strength, yield strength and elongation of the alloy are621.3MPa,607.3MPa and4.7%, respectively. Therefore, the Al-Cu-Li alloy sheet with T8treatment has better mechanical properties than the sheet with T6treatment. The concentration of iron impurity has a very important effect on the mechanical properties of Al-Cu-Li alloy sheets with solid solution and aging treatment. The strength of the alloy significantly decreases with the increase of the concentration of iron impurity in the range of the present work.