| The substantial life of the human has been improved step by step with the development of the society and manufacturing, but meantime the resource and power has being lacking and the environment has being damaged. Therefore, some permanent materials are imperative to be made and applied. On the other hand, the material is developed to be lighter, leading to be more efficient and decreasing the pollution. The cast hypereutectic Al-Si alloy we are researching is one of these materials, whose properties are as follows: the lower expanding and density behavior, the higher resistance to wear and corrosion and excellent conduct. And therefore, it has being widely applied to automobile and aviation industry. It is a vital subject for the automobile industry that some auto-materials with more excellent properties are explored and put in use, especially to some materials of the key parts such as some engine plunger and rotor etc. However, for a long time, the hypereutectic Al-Si alloy can't been widely applied because of it's the disadvantages: the bigger brittleness and the worse machining property, which can be attributed to the lath primary Si and coarse eutectic Si in the microstructure. The coarse primary Si, as hard particles, can improve the resistance to wear, but at the same time reduce the mechanical and machining properties due to its brittleness and the dissevering effect to the matrix. It is o great interest for the research development of the hypereutectic Al-Si alloy to improve the mechanical properties by fining the primary Si. Aiming at the modification of the cast hypereutectic Al-Si alloy, in this present study, refining modification of hypereutectic Al-Si alloy has been studied by different modifiers. A kind of better complex modifier(Al-Ti-TiC-P-Y) chosen among these ones is used for modifying hypereutectic Al-Si alloy with different Si content and furthermore some good parameters are optimized. Furthermore, the properties of hypereutectic Al-Si alloy modified by the modifier and treated by T6 are tested. 1,The chemical composition of the composite modifierIn order to research the effect of different modifiers on hypereutectic Al-Si alloy, some modifiers are prepared in the present study, in clouding 6 modifiers as follows:(1) Al-Ti; (2) Al-TiC; (3) Al-Ti-TiC; (4) Al-TiC-P; (5) Al-Ti-TiC-P; (6) Al-Ti-TiC-P-Y. The key modifier is the Al-Ti-TiC-P-Y, which SEM,EDS and XRD are shown the Fig.1-1, 1-2:Fig.1-1 SEM and EDS of composite modifier Al-Ti-TiC-PFig.1-2 XRD analysis of composite modifier Al-Ti-TiC-P2,Effect of the complex modifier on the structure By comparison of the 6 modifiers in the hypereutectic Al-20wt%Si, it is found that the modifier Al-Ti-TiC-P-Y is the best one.Fig2 Microstructure of hypereutectic Al-20wt%Si alloy r (a) unmodified as-cast (b) modified by addition of Al-Ti-TiC-P-Y(RE) composite modifieIt can be seen from Figs. 2 (a) and (b) that the size and morphology of the primary Si improve significantly after composite modification. The largest size of it is 20?m, and the morphology changes over from polygon and strip to regular block and its distribution is uniform. The processing parameter of hypereutectic Al-Si alloys modified by Al-Ti-TiC-P-Y (RE) composite modifier is shown as follows,the content of the composite modifier addition :0.5%;the modified time :5 minutes;the modified temperature :850 ℃.The amount of composite modifier addition increases accordingly with increasing amount of Si in hypereutectic Al-Si alloys.3. Effect of composite modification on the propertiesAfter composite modification of hypereutectic Al-Si alloys with various Si content, they was heat treat by T6 and then, properties measurement containing hardness (HB), wear resistance and tensile strength was conducted. The main results o are shown as follows. (1) After T6 heat treat, the microstructure of hypereutectic Al-Si alloys is improved greatly, and eutectic Si becomes grainy, But it is found that the primary Si starlike blunt only at the...
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