| Aluminum and aluminum alloys play an important role in various fields of the national economy and defense construction. ZL205 A is a high-strength cast aluminum alloy developed and manufactured by China, which is widely used in China’s aerospace industry. For aluminum and aluminum alloy products, hydrogen contents, inclusions (AJ2O3) and other metallurgical defects will directly or indirectly affect their mechanical properties or performance. Therefore, in order to obtain high-quality products, we must find a way to eliminate these metallurgical defects before casting, thereby improving the purity level of the melts. Using refining agents is an effective method to improve the purity level of the aluminum melt. For aluminum refining agents on the market, they would include the ingredients containing the sodium. In the enterprises manufacturing ZL205A alloys, they found that the properties of the alloy would be affected adversely after using refining agent with sodium. In order to study and solve this industry problem, we investigated the existing position and the form of sodium and the impacts on performance in aluminum-copper alloys in this paper, and we also developed a refining agent without sodium.In this paper, the sodium is introduced into the aluminum-copper alloy through using refining agent with sodium as medium on purpose. The scanning electron microscope (SEM), electron probe micro-analyzer (EPMA), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and other analytical tools are used to investigate the existing position and the form of sodium, the adverse impacts on performance and how these impacts happen in aluminum-copper alloys. At the same time, we also formulated a variety of sodium-free refining agents, and then the melting points of every formulation were tested by DSC experiments. The effect of the degassing was evaluated by the pressure-reducing solidification method, and finally the performance experiments were made to determine whether the formula is appropriate with the rotating impeller degassing equipment in the production conditions. Eventually we won a sodium-free refining agent, which is specially used in aluminum-copper alloy.The main research work is as follows:(1) The sodium is introduced into the Al-4.5 wt.% Cu alloy through using refining agent. The existence of the sodium in the EPMA analysis and the amorphous film in HRTEM prove that the elemental sodium segregates at grain boundaries. The existence of the sodium atom layer makes the bond between the matrix and CuAl2 -phase weaken so that the CuAl2-phase is easy to detach from the matrix.(2) After T6 heat treatment, cracks take place in the Al-4.5 wt.% Cu alloy containing sodium. A large number of liquid films appear in the process of solution treatment and form a net of thin liquid film. The crack forms because of thermal stress and propagates quickly. The high temperature in solution process of T6 treatment will make the sodium atom layer melt to liquid film at the grain boundaries. This film is poorly soluble in the matrix and CuAl2-phase and prevents the pass of both sides’ atoms. This will result in the residual needle-like CuAl2-phases. These residual CuAl2-phases are easily detached.(3) The magnified images of fracture surface of tensile tests shows that the sodium results in transition of the fracture mechanism from dimple fracture to intergranular fracture, which makes the mechanical performance of Al-4.5 wt.% Cu decrease dramatically.(4) The starting melting point of the refining agent D we chose is 703.9 ℃, and the porosity of the sectional views is less than 1%after the pressure-reducing solidification. Then the performance is improved after using the rotating impeller degassing equipment with this refining agent. |
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