The Melt Temperature Treatment Of Al-Si Alloy And Its Solidification Process

Al-Si alloy is one of the most important casting aluminum alloys, which has been widely used. In order to overcome its disadvantage in traditional refining method, the research on melt temperature treatment as a refining method has caused wide researchers. However, the present researches are focused on certain commercial hypoeutectic aluminum alloys and hypereutectic Al-Si alloy and do not form a complete research system. According to a lot of DSC experiments, the melt temperature treatment of Al-Si alloy and its effect to microstructure are investigated systematically in this paper and the research results may be of significance in both theory and practice.The experiment results show that the effect of superheating temperature on secondary dendrite spacing is unnoticeable when Al-7%Si alloy is superheated in the temperature range of 750-1 100C. Dendrite morphologies become well distributed with the increase of superheating temperature. The high or low temperature has different effect on solidification process. The influences of hysteresis and relaxation caused by melt superheating treatment will affect the morphological stability of interface in solidification. When superheating temperature is low, the stability of interface increases with increasing of temperature. When superheating temperature is higher than the critical superheating temperature, and with temperature increasing the concentration gradient in liquid increases significantly, which decreases the interface stability. The prolong of the holding time from short to long causes the reduction of the nucleating center in the melt, and then the concentration gradient increases. As a result, the stability of interface decreases, which leads to the volume proportion of dendrite a(Al) increases at 800 C.The solidification microstructure is refined and well distributed when Al-12%Si alloy is superheated in the temperature range of 750-1 100C. Dendrite a(AI) and primary silicon are coexisted in the same field of the microstructure under most melt temperature treatment condition. Dendrite a(Al) is disappeared in the microstructure after the alloy was holding at 1 000C. At 800C, the morphology of dendrite a(Al) experiences the state from broken to integrated with the increase of the holding time.The size of primary silicon changes from small to big and then to small when Al-20%Si alloy is superheated in the temperature range of 750-1 100C. By the 1 100C melt treatment, the precipitation of primary silicon is restrained, and its grain size isreduced, tending to become spherical. At 800 C, the grain size of primary silicon grows from small to big with the increase of superheating time. Therefore, effect of prolonging superheating time is the same as that of increasing superheating temperature on the same extent. Five-petal star-shaped primary silicon is also founded in the microstructure.A good many of DSC experiments on Al-Si alloys with different ingredients discover that there is extra heat effect in the melt of Al-Si alloy, which indicates the change of the alloy structure. Extra heat effect is founded in all the Al-Si alloys' DSC curve during heating process and the analysis results show it is caused by the structure change of master alloy Al. The melt structure is changed during the cooling process of hypereutectic Al-Si alloy and the analysis found it is caused by the continuous emergence and decomposition of Si-Si clusters.The research shows that there are three regions upon the liquidus according to Al-Si alloy's melt structure: the homogeneous region of ingredient and structure above high critical temperature; the structure-changing region between high critical temperature and low critical temperature; the heterogeneous region below low critical temperature. The melt structure of alloy will make a genetic effect to the microstructure, and the atom clusters in melt are genetic carrier from liquid state to solid state.