摘要
关于合金非平衡凝固机理的研究是凝固学研究领域的重要组成部分。大量研究表明,铸造合金的熔体结构对其非平衡凝固过程及组织都具有显著影响,而熔体结构又与过热状态有关。所以研究铸造合金熔体的过热状态对其非平衡凝固机理的影响具有重要理论意义。Al-Si系铸造合金在铝基铸造合金系列中占有非常重要的地位,本论文以共晶Al-Si合金为主要研究对象,结合亚共晶及过共晶Al-Si合金,在总结前人研究结果的基础上结合DSC及金相法分析了Al-Si合金的熔体结构随过热状态变化的规律。研究了不同过热状态及凝固速率条件下Al-Si合金在非平衡凝固过程中熔体结构的变化规律,讨论了过热状态及凝固速率对Al-Si合金非平衡凝固组织及共晶共生区的影响。目前由于在实际生产过程中很少采用改变熔体过热状态的方法来达到改善合金凝固组织的目的,所以本论文研究对Al-Si系铸造合金的实际生产也具有一定的实践意义。
DSC实验结果表明:共晶及过共晶Al-Si合金内初生硅的析出效应都随过热度的升高而逐渐增强。过热度对Al-Si合金内各相初始凝固温度的影响很小,而凝固焓变值随过热度的升高经历了由大变小再变大的过程。保温时间对Al-Si合金内各相初始凝固温度的影响也很小,而凝固焓变值随保温时间的延长经历了由小变大再变小的过程。凝固速率对Al-Si合金各初生相初始凝固温度的影响较为显著,而对共晶组织初始凝固温度的影响很小,凝固焓变值则随凝固速率的提高经历了先减小再增大的过程。随着凝固速率的提高,合金的凝固逐渐由平衡过程向非平衡过程转变,从而导致凝固机理方面的变化。
凝固组织形态研究表明:Al-7%Si合金随着过热度的升高,α(Al)枝晶形态逐渐规则化,共晶组织也逐渐呈完全离散状分布。Al-12%Si合金在过热度较低时凝固组织中出现了初生硅与α(Al)枝晶,在过热度较高时这两种组织逐渐消失。凝固组织中还出现了初生硅与α(Al)枝晶共存的晕圈现象。Al-20%Si合金凝固组织中的初生硅随着过热度的升高经历了由小尺寸多边形状转变为大尺寸五瓣星状再回归为小尺寸多边形状的变化。过热保温对非平衡凝固组织的影响程度较弱且规律性不强。
理论及实验分析结果表明:随着过热度的提高,Al-Si合金熔体逐渐均匀化。Al-Si合金熔体结构参数在近共晶点成分处会发生反常突变,反映了熔体结构由亚共晶向过共晶转变的过程。Al-Si合金在液相线以上存在铝、硅原子集团的结构转变温度区间。合金的熔体结构会影响其非平衡凝固过程及凝固组织。亚共晶及共晶Al-Si合金熔体的结构特征与纯铝类似,在低过热度范围内会发生基体铝“收缩”的现象。硅原子集团对过共晶Al-Si合金熔体结构的影响要大于其对亚共晶及共晶Al-Si合金熔体结构的影响。
理论分析表明:过热度对熔体内原子间距影响较弱,但对原子配位数的影响较为显著。铝熔体在低过热度范围内发生“收缩”的根本原因在于熔体在过热初期会自发收缩以抵消体系由于体积膨胀而导致能量的增加效应,与熔体内氢含量的变化无关。Al-Si合金熔体内原子团簇的种类及数量因成分而异。成分与熔体内原子间距及最近邻原子配位数之间均呈复杂的非线性关系。随着成分的逐渐增大,在熔体结构转变区内成分与熔体内原子间距及最近邻原子配位数之间呈近似无规律的状态,而在熔体结构转变区以外则呈现出一定的函数关系。
理论分析表明:过热度会对合金共晶共生区产生影响,提高过热度将使合金共晶共生区左右边界扩大,反之则缩小。随着凝固速率的变化,Al-Si合金的非平衡凝固组织会出现显著差异。共晶及过共晶Al-Si合金中易出现α(Al)依附于初生硅析出的晕圈组织。Al-Si合金共晶凝固机理将随凝固速率及过热度的变化而改变。共晶合金在结构上比较特殊,其中包含了合金的结构由亚共晶向过共晶过渡的关键信息。
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.
引文
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