| 5XXX series aluminum alloys are widely used in the area of aerospace, traffic, national defense and packing material because of its small specific gravity, high specific strength(rigidity), great corrosion resistance, well weld ability, easy shaping and recycling etc. It is the second metal structure materials after the amount of iron and steel, and it is also thought to be one of the ideal materials to achieve automotive lightweight. As the rapid development of industry, the demang for aluminum sheets is growing. However, the property of the aluminum alloy sheets made in our country is often unsatisfied, and it is owing to the homogeneity of microstructure.5XXX series aluminum alloys belong to unheat-treated strengthening alloy, and it is decided that they have only two strengthening methods: deformation and micro-alloying. Nowadays, there are many reports on the deformation strengthening of 5XXX series alloys at home and abroad, but short of researches about the micro-alloying theory. In fact, micro-alloying is one of the most effective means to optimize the microstructure and property. So it is necessary to research on the influence of alloying element on the microstructure and property of 5XXX series alloys in different states.5052 aluminum alloy is one of the most typical and commonly used alloys in 5XXX series aluminum alloys. In this paper, the test alloys are 5052 and 5052 micro-alloyed with Cu, Sr, Er and Zr respectively. The microstructure and properties of 5052, 5052+Cu, 5052+Sr, 5052+Er, 5052+Zr and 5052+Er+Zr in states of as-cast, homogeneous, the first cold rolled and the second cold rolled are researched by OM, SEM, EDS and electromechanical universal testing machine etc, expecting to improve the uniform structures and properties of 5052 aluminum alloy. The results show that:(1) the microstructure of as-cast 5052 aluminum alloy consists of α-Al and iron-rich phases which is acicular or skeleton shaped, and the size of iron-rich phases is more than 50 μm. The surface grain present equiaxial, the distribution of their sizes is not uniform, and about 100 μm. The values of tensile strength, yield strength and elongation of as-cast 5052 aluminum alloy are 197 MPa, 117 MPa and 22.5%, respectively. After adding Cu, a large amount of fine net skeleton phase,which is tested to be Al2 CuMg phase by EDS, is inclined to precipitate in the grain boundary. The strength of the alloy is remarkably improved, the plasticity is significantly reduced, and the corrosion resistance becomes worse. The effect of Sr on the grain is very small, but it has a slight refinement effect on the skeleton iron-rich phase. The strength and plasticity of the alloy adding Sr decreases slightly. The influence of Er on the microstructure of the alloy is related to the content of the alloy. When the Er content is low(below 0.4%), the morphology of the second phase is mainly acicular, and the grain has a coarsening tendency. With the growing of Er content, the skeleton iron-rich phase increases, the grain size first becomes smaller and then increases, and when the Er content reaches 0.4%, the grain is the finest. The strength and plasticity of the alloy first decrease, then increase and finally decrease. According to the EDS analysis, it is found that Er is mainly concentrated in the head of the skeleton of iron-rich phase. The effect of Zr on the alloy is mainly reflected in the grain refinement. With the increase of Zr content, the change regulation of grain size is similar to that of Er. The alloy strength increases gradually, and the plasticity decreases incrementally. When the Zr content is 0.29%, the grain refinement effect is the strongest, and the size is about 100 μm. When it is more than0.29%, it is obvious to see an aggregation phenomenon of Zr, which generates a coarse Al3 Zr.(2) The homogeneous microstructure of 5052 aluminum alloy and 5052 micro-alloyed with Cu, Sr, Er and Zr has little change, compared with that of as-cast microstructure, but it can be found that the corner of the second phase has a passivation trend for all the alloys after homogenization, and the grain size distribution is more uniform. The tensile strength, yield strength and elongation of the 5052 aluminum alloy after homogenization are 183 MPa, 111 MPa and 18.0%, respectively, which decreased compared with that of as-cast alloy. After homogenization treatment: the grain size of aluminum alloy with Cu has little change, but it will release a large amount of fine Al2 CuMg phase in the grain, resulting in precipitation strengthening, then increasing the strength slightly and the plasticity remarkably, and finally improving the corrosion resistance of the alloy. The grain size of aluminum alloy adding Sr becomes smaller, but the strength is improved slightly, and plasticity is reduced. With the growing of Er content, the phase of each kind of aluminum alloy with Er increases significantly, the strength and the plasticity first decreases, then increases, and finally decreases. When the content of Er is 0.2%, the second phase grows evidently, decreasing the strength and the plasticity. When the Er content is 0.4%, the second phase has a remarkable refinement phenomenon, and the strength reaches the maximum. With the increase of Zr, the iron-rich phase of each alloy has a “dissolve†phenomenon, while the aggregation of Zr become worse. Moreover, the strength increases gradually, and the plasticity first decreases and then increases. However, When the Zr content is 0.15%, the grain suddenly becomes coarsen, and the strength and plasticity decreases.(3) After hot rolling, annealing and the first cold rolling, the second phase of 5052 aluminum alloy and 5052 micro-alloyed with Cu, Sr, Er and Zr is elongated along the rolling direction, and the grain presents fibrous, after the first cold rolling: the tensile strength, yield strength and elongation of 5052 aluminum alloy are 313 MPa, 300 MPa and 5.0%. The second phase of alloy with Cu is crushed more dispersed, the strength increases, and the plasticity decreases. Sr can refine the skeleton iron-rich phase, inhibit the recrystallization of alloys during the hot rolling, and stop the growth of recrystallized grain in the annealing time, which is good for reducing the stress concentration caused by the deformation, and then promote the deformation and refine grain, improving the strength. Rare earth element Er can significantly improve the recrystallization temperature, keep the deformation structure and inhibit the nucleation and growth of the recrystallized grain in the hot rolling and annealing process, achieving the purpose of strengthening alloy. With the increase of Zr, the grain deformation degree of alloy increases. At this moment, the alloy strengthening mainly depends on the deformation, but not the grain refinement effect caused by Zr any longer.(4) The second cold rolling makes the second phase of 5052 aluminum alloy and 5052 micro-alloyed with Cu, Sr, Er and Zr broken completely, and uniformly distributed in the matrix. The microstructure of alloys is further elongated along the rolling direction, and the fibrous tissue is more obvious, which further improing the alloy strength and reducing the plasticity. |
