Composite Modification Preparation Of (large) Hypereutectic Al-si Alloys

The current theory and application of Al-Si alloy mainly focus on the study of hypoeutectic and eutectic, therefore we systematically analyzed the modification process of hypereutectic Al-Si alloy based on the methods and results of predecessors. Several factors including the modification temperature, the content of modifier, the available holding time, the times of repeat melting , the casting temperature and the speed for cooling have been studied in order to understand its effects to the composition, function and performances of primary silicon and eutectic silicon. The results of the correlations of composition, function and performances will lay a solid foundation for the fabrication of new model hypereutectic Al-Si alloy, provide evidences for theory and show the direction for industry development.The experiments results show that, for Al-20%Si alloy with working temperature about 820癈, the coarse primary silicon turns to be obtuse, and that the average diameter of the primary silicon decreased from 150 m to 40 m, resulting in ideal primary silicon. After the modification with RE at temperature around 800 C, we have got granular and stubby eutectic silicon. The Al-20%Si and Al-25%Si alloys by modification with P and RE produced good effects. The average size of both primary silicon and eutectic silicon is less than 50um, and the crystal particle is very small after melted for 5 times .The average size of primary silicon is still less than 50u,m even after holding the temperature at 840 C for 4 hours. However by way of using this kind of modifier we can not get what we expected with the contents of silicon reaching 30%. Mechanical properties of the Al-20%Si and Al-25%Si alloys by modification with P and RE increased evidently. Mechanical properties of tensile test sample in normal atmospheric temperature increased from 92.7Mpa and 82.6Mpa to 125.4Mpa and 102.5Mpa. Extendability increased from 0.62% and 0.57% to 2.3% and 1.4%. Linearity expansion have not evidencechange. The abrasive wear and tear decrease with the increase of the contents of silicon, and then increase after reaching certain points under the same wear condition. After the modification, the abrasive wear and tear rate of the alloy with the same component reduced 14-39 %.The characteristic of this essay is that we systematically analyzed the modification process of hypereutectic Al-Si alloy with high contents of Si on a pioneer stage. The results will be applied to fields as structure material with high abrasive wear and tear, which is a big boost of economics and its development.