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The rare earth element yttrium was added into 7055 aluminum alloys in the form of Al-Y intermediate alloy. The thermodynamic mechanism during melting and effect mechanism during solidification of yttrium were analyzed based on the classical theories of physical chemistry and crystal growth. Results show that there are strong binding between yttrium and impurity elements, such as oxygen, hydrogen, nitrogen, sulfur, iron and so on, which can purify the aluminum melt. The grain size of as-cast alloys is decreased from 60-70 μm to 40-50 μm after yttrium addition. It is due to the small solubility of yttrium element in molten aluminum. Most of yttrium segregates at the solid-liquid boundary, and prevent the Zn, Mg and Cu atoms from diffusing resulting in the constitutional supercooling at the frontier of solidification, which accelerate the growth of cellular dendrite. Furthermore, the amount of eutectic increases, the size of which is smaller than that without yttrium.