Melt Treatment Structure Of Al-Si Alloys

In this paper, Al-7%Si,Al-12%Si and Al-20%Si alloys were investigated with their solidification structure and differential thermal analysis curve attained in different melt temperature treatment process. Based on the analysis of the differential thermal analysis curve and corresponding solidification structure, the effect of different melt temperature treatment process on degree of undercooling and solidification structure was discussed. The results will have important theoretical and practical significance.The quantitative metallographic analysis results indicated that dendritic of hypoeutectic Al-7%Si alloy got a certain degree refinement by melt superheat treatment at 750-1100℃ and the degree of refinement followed the relation of exponential function. The theoretical minimum dendritic size of hypoeutectic Al-7%Si alloy refined by melt superheat treatment is about 20μm.The effect of degree of superheat on degree of undercooling was studied. The result shows that superheat affected the size of solidification structure by the way of effecting degree of undercooling. When holding time and cooling rate remained constant, eutectic Al-12%Si alloy was superheat-treated at the range of 750℃-1100℃ temperature, Generally the solidification structure exhibited the trend of refinement and uniformity with the lifting of superheat temperature, and their degree of undercooling also lifted. The crystal grain of hypereutectic Al-20%Si alloy increased at first and then decreased, and its degree of undercooling decreased at first and then increased.As holding time and superheat temperature kept constant, solidification change point of hypoeutectic Al-7%Si, eutectic Al-12%Si and hypereutectic Al-20%Si alloys in the DSC curve was decreaseed with the increase of cooling rate, it indicated that the degree of undercooling of these alloys were lifted with the increase of cooling rate and the change trend gradually is weakened.When superheat temperature was kept at 800℃ and holding time was changed from 0min to 105min,the degree of undercooling of remained basically invariant. It indicated that declared that the elongation of holding time at 800℃ had little effect on degree of undercooling, but the melt structure would become more uniform as holding time was elongated. While the temperature was kept at 900℃, the degree of undercooling of hypereutectic Al-20%Si alloy would be enlarged with the elongation of holding time. So when the alloys were holded at a suitable temperature and prolonged...