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Hypereutectic silicon aluminum has low expansibility、high wear resistance、smaller specific gravity and good heat stability. However,the coarse primary crystal Si phase existed in hypereutectic Al-Si alloy seriously fragmented the matrix, deteriorated the mechanical properties and limited its wide application in industry. An effective way to solve such problems is to refine the primary crystal Si phase. At present, there are many ways to refine the primary crystal Si phase, for example: Modification of primary crystal Si phase processing, extrusion, using rapid solidification technology. Except for rapid solidification technology is an ideal effect, other methods to improve performance are not obvious, but the rapid solidification technology, there are high costs and process complexity and other issues. So this paper has studied the technique to improve the phase shape for increasing the mechanical property of hypereutectic Al-Si alloy. According to Al-25Si alloy, through the modification, electromagnetic casting, semi-solid extruding formation technologies, thus across primary silicon thinning or control in size, to achieve the purpose of improving mechanical properties of alloys.It can be observed in this research: the shape and sizes of primary silicon from coarse and irregular became finer and more even after rare earth modification; The average size of the primary silicon can be reduced to 35μm after modification; Moreover, the RE modification has a suitable scope of time , 30Min～40Min is the best time for RE modification. Through the semi-solid formation, needle-plate eutectic silicon is disappeared, the size of the primary silicon reduced slightly and the shape of the primary silicon change into spherical, their distribution more homogeneous in the alloy.The ball hardness of Al-25Si increased from 99.1 to 99.8 by modification with RE; Mechanical Properties of tensile test sample in normal atmospheric temperature increased from 134.4 MPa to 178.4 MPa, improved by 32.7%; After semi-solid forming, The tensile strength of Al -25% Si alloy increased 78.9%, reached 240.5MPa, extension rate increased from 0.2% to 4.3%, Brinell hardness is 120; the mechanical properties increase with the increasing of extrusion rati o.
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