Rare earth element is located in the third subgroup of the periodic table. The atomic radius is relatively larger, and has a unique 4f electronic structure, large electron magnetic moment, strong spin orbit coupling. Its chemical properties are very lively, and the chemical activity is only after the alkali metal and alkaline earth metal elements among the metal elements, so that it is able to form a stable chemical substances with almost all non-metallic elements (oxygen, sulfur, halogen, etc.), such as oxide, sulfide, Halides, etc. For the formed rare earth complexes with a variety of crystal structure, the balancing number changes in between 3 to 12. The Unique physical and chemical properties determine the rare earth elements play a variety of roles in the aluminum profile.
1. Metamorphism
Metamorphism refers to the process of changing the microstructure and properties of the profile by adding a small or little amount of a modifier to the metal and the profile to change the crystallization conditions of the profile. Studies have shown that rare earth in the profile has a good metamorphism, mainly in the refinement of grain and dendrites. The atomic radius of the rare earth element is larger than the radius of the aluminum atom, and the properties are livelier. It is easy to fill the surface defects of the profile in the molten aluminum solution, which reduces the interfacial tension of the old and the new phase and improves the growth rate of the nuclei. And a surface active film formed between grain and the aluminum solution can prevent the formed grain from growth, and refine the profile organization. Adding rare earth can also reduce spacing between the columnar grain and the secondary dendrite arm, improving the grain morphology, and to a certain extent, controlling the material grain size. Experiments show that there is a certain latent period of rare earth metamorphism, so only at high temperatures to maintain a certain time, rare earth will play the greatest metamorphism.
2. Refining purification
Rare earth has a good purifying effect on aluminum profile melt. First, the rare earth is prone to stable chemical reaction with O, S, halogen, etc, producing like RE2o3, RE2S3, RES2, RE3S4, REH2, REH3, REX3, and REX3 (X is a halogen element), and at a temperature of 250 to 300C, it can have chemical reaction with N to produce refractory REN. At high temperature, rare earth reacts with C, Si and B to produce REC2, RE2C3, REC, RE2C, RE3C, RE4C, RESi2, REB4, REB6 and so on. In additions, rare earth with a particularly large adsorption to hydrogen can adsorb and dissolved hydrogen in a large number, so that the hydrogen in the profile can be better to remove. For rare earth and hydrogen compounds have higher melting point and dispersed in the aluminum liquid, this hydrogen compounds does not form bubbles, greatly reducing the hydrogen content of aluminum and pinhole rate. Second, rare earths can form high-melting, low-density binary or multi component compounds with low melting point elements in aluminum profiles, such as Sn, Bi, Pb, Zn, etc. When the metal smelting temperature is lower than their melting point, these compounds slag thereby purifying the aluminum liquid, and their tiny particles become the heterogeneous nuclei on the aluminum crystallization process so as to refine the grains. Finally, the addition of rare earth can improve the surface tension, fluidity, viscosity and other physical and chemical properties of aluminum profile melt and slag, which is beneficial to the non-metallic inclusions and promote the floatation, which can effectively remove non-metallic inclusions
3. Alloying effect
There are three kinds of existence form of rare earths in the of aluminum profile: solid solution in the matrix α (Al); segregation in the phase boundary, grain boundary and branching boundary; solid solution in the compound or in the form of compounds. The existence forms are mainly related to the addition amount. When the rare earth content is low (less than 0.1%), it exist in the front two forms enhanced by the limited solid solution and increasing the deformation resistance to promote the dislocationmultiplication; When the rare earth content is more than 0.3%, it is mainly in the form of the third existence, with other elements in the profile to form a number of the new phase containing rare earth, distributed in the grain or on grain boundaries. At the same time, the shape, size of the second phase (The majority of the rare earth containing the second phase has the characteristics of the particleization, spheroidization and refinement) changes with a large number of dislocations, to a certain extent, strengthening the aluminum profile.
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