Study On Non-Equilibrium Solidification Mechanism Of Al-Si Alloys

Studying of non-equilibrium solidification mechanism of alloy plays an important role in the solidification science field. Many studying have indicated that melt structure has a remarkable effect on non-equilibrium solidification process and structure of cast alloy and melt structure has to do with overheating state. So studying the influence of overheating state of melt to non-equilibrium solidification mechanism of cast alloy has a remarkable theoretical significance. Al-Si cast alloy takes an important part in all kinds of Al cast alloys, in this thesis, eutectic Al-Si alloy is studied as the main object and some other typical kinds of hypoeutectic and hypereutectic Al-Si alloys are also studied based on the predecessor's research results and combining DSC and metallographic method and the law of melt structure changing with overheating state is also discussed. Studying the evolution discipline of Al-Si alloy's melt structure with overheating state and solidification rate in non-equilibrium solidification process and the effects of overheating state and solidification rate on Al-Si alloy's non-equilibrium solidification structure and eutectic coupled zone. Now the method which improves solidification structure of alloy through changing overheating state of melt is seldom used in actual production process, so all studyings of this thesis also have certain practical significance to actual production Al-Si cast alloy.The DSC experimental results show that the precipitation effect of primary Si in the eutectic and hypereutectic Al-Si alloy is strengthened by elevation of overheating degree. Overheating degree has a little of effect on initial precipitation temperature of all kinds of phases in the Al-Si alloy, the solidification caloric value changes from big to small and then to big as the overheating degree rises. Holding time also has a little of effect on initial precipitation temperature of all kinds of phases in the Al-Si alloy, the solidification caloric value changes from small to big and then to small as the holding time extends. The solidification rate has a remarkable effect on the initial precipitation temperature of primary phases but it has little effect on initial precipitation temperature of eutectic phase, the solidification caloric value changes from small to big and then to small as the solidification rate rises. As solidification rate rises, the solidification process varys from equilibrium state to non-equilibrium state and the solidification mechanism will change.The studying of structure morphology shows that as overheating degree rises, morphology ofα(Al) in the Al-7%Si alloy becomes more and more regular, eutectic phase's distribution is discrete. There are some primary Si andα(Al) in the solidification structure of Al-12%Si alloy at low overheating degree and this two kinds of structure disappear at high overheating degree. Moreover, halos which are composed ofα(Al) and primary Si are found in Al-12%Si and Al-20%Si alloy. The morphology of primary Si in Al-20%Si alloy changes from small size regular and polygonal to big size five-petal star-shaped and then to small size regular and polygonal as overheating degree rises. The effect of holding time to non-equilibrium solidification organization of Al-Si alloy is weak and ruleless to some extent.The theoretical and experimental results show that Al-Si alloy melt becomes homogeneous as the overheating degree ascends. The structural parameter of Al-Si alloy changed abnormally at the eutectic point, reflecting the changing process of melt structural from hypoeutectic to hypereutectic alloy. There are structure transformation temperature intervals of Al and Si atom clusters above liquidus line of Al-Si alloy. The state of alloy melt affects the alloy's non-equilibrium solidification process and structure. The structure characteristics of hypoeutectic and eutectic Al-Si alloy are similar to that of pure Al and"contraction"of Al matrix is found at low overheating degree range. Si clusters have more remarkable effect on melt structure of hypereutectic Al-Si alloy than on hypoeutectic Al-Si alloy.The results of theoretical analyze show that overheating degree has a remarkable effect on neighboring atom coordination number but little effect on atomic separation. The ultimate reason why Al melt"contracts"at low overheating degree is that melt contracts spontaneously at initial stage of overheating which contra increasing of energy caused by augmentation of volume of system and it is found that this phenomenon is not related to hydrogen content varation in melt. The kinds and number of atom clusters in Al-Si alloy melt are changed by component. Component has a complicated and non-linear connection to do with atomic spacing and ligancy. As component rises, the connections between atomic spacing or arest neighbors ligancy and component are approximatively ruleless when component is in the transition zone of melt structure and are somewhat functionally when component is out of the transition zone of melt structure.The results of theoretical analyze show that overheating degree has some effects on eutectic coupled zone of alloy. The rising of overheating degree will broaden the eutectic coupled zone, reduce it conversely. As solidification rate changes, the non-equilibrium solidification structure Al-Si alloy will also be vary. Halos in whichα(Al) tend to grow and attach at primary Si are easily found in near eutectic and hypereutectic Al-Si alloy. The solidification mechanism of eutectic phase in the Al-Si alloy varys with solidification rate and overheating degree. When we study the law of alloy's structure changing from hypoeutectic to hypereutectic state, the structure of eutectic alloy will provide the key information.