【作者】 李科；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 工程热物理， 2009， 博士

【摘要】 闭孔泡沫金属是一种内部结构含有大量孔隙的新型功能材料,以其独特的结构和优异的物理性能、机械性能、声学性能和热性能,以及可回收利用性等,成为一种极具开发前途的工程材料。在众多制备泡沫金属的方法中,吹气发泡法因为设备简单、成本低、可以连续生产等特点,更适用于规模化生产。生产过程中,如何控制气泡的尺寸大小及与分布、以及其拓扑结构是该项工艺的核心问题。从发泡到凝固成型,从湿泡沫到干泡沫再到泡沫金属成品过程中,影响泡沫尺寸及其分布的因素众多,单纯通过传统的以小规模实验为主的经验方法,不仅工作量大,周期长,成本高,而且很难全面了解各种参数的影响,更无法掌握其机理与规律。本文以相关实验研究为依据,采用数值模拟方法对金属泡沫从液态到固态的演化过程动力学行为进行系统分析,从深层次上揭示液态金属演化过程的动力学机制,为吹气法制备泡沫金属提供准确而可靠的科学依据和理论预测模型。主要研究内容和成果包括以下几个方面:对泡沫析液现象从二个方面进行了研究。一是宏观方面,即研究泡沫群的整体行为;二是微观方面,即对单条Plateau边界、节点、液膜等的研究。宏观方面,初步尝试了泡沫金属析液过程的二维数值计算,计算结果与实验结果进行了定性比较,研究了在不同工况参数下金属泡沫铝的孔隙率的变化规律。结果表明:泡沫最终的孔隙率受到泡沫初始孔隙率、泡沫层的高度、泡沫的直径、重力加速度和熔体粘度等因素的影响。其中泡沫的孔径对最终的孔隙率有较大的影响,而粘度对最终的孔隙率影响较小,只是对析液速率有较大的影响。微观方面,研究了单条Plateau边界及节点处的析液现象。对单条泡沫体Plateau边界内部的速度场的研究表明:泡沫体内部Plateau边界内的速度要小于同等条件下的容器壁面处Plateau边界内的速度,间接解释了宏观析液过程中析液速率与不同容器条件的关系。分析了Plateau边界中界面流变特性对析液过程的影响,进而将单条Plateau边界内液体流动的分析结果应用于泡沫体析液过程的宏观研究,建立了一个析液模型,计算结果与实验结果的比较显示:在泡沫层上部、中部吻合较好,在底部存在一定误差。建立了描述了吹气法制备泡沫铝过程中泡沫孔隙率随空间的变化的数学模型。计算结果表明,在不考虑气泡的合并的情况下,熔液粘度、表面张力、重力加速度、气体的流量对泡沫孔隙率,即Plateau边界的横截面积,有显著的影响。重力加速度和表面张力在析液过程中起着关键的作用。通过理论建模和数值计算预测了发泡层高度随时间,气体流速等的关系,模型能较好的解释泡沫层高度的变化。利用相场(Phase field)方法对界面演化过程进行了数值模拟,较为直观合理的解释了液膜破裂现象。通过析液方程和由于气体扩散造成的气泡长大的方程的耦合,建立了铝合金熔体泡沫中气泡尺寸分布发展的数学模型。通过数值计算,得出了不同工况下泡沫尺寸分布的发展过程,讨论了表面张力、亨利常数、扩散率等因素对泡沫尺寸分布发展的影响。计算结果表明:表面张力越大、亨利常数越大,泡沫尺寸演化过程越剧烈;含液率较小的泡沫体析液量很小,所以,泡沫尺寸分布变化主要由气体扩散引起。利用Surface Evolver软件及MacPherson等人的最新理论成果,对金属泡沫多面体气泡的演化过程中的泡沫尺寸和拓扑结构进行了数值计算研究,数值模拟结果与试验研究结果进行了定性的比较,一些特征非常相似。利用Potts模型对液态金属泡沫胞元结构进行了随机模拟,得到了二维泡沫的胞元尺寸、拓扑参数以及分布等定量特征信息。计算结果与实验结果完全吻合,表明Potts模型应用于熔体泡沫演化过程研究的可行性、可靠性。建立了一个简化的泡沫铝合金凝固过程数学模型,通过对模型的求解,获得不同时刻泡沫铝合金的温度场、凝固界面的位置以及形状。通过求解液体体积分数分布,间接得到孔隙率的分布。在凝固过程中,考虑了粘度、比热容等物性参数随温度的变化现象。整个凝固过程与液态泡沫析液现象有着密切的关系;重力和表面张力在析液过程中起着关键作用,在微重力条件下凝固的泡沫铝产品比较理想,即孔隙率分布比较均匀。更多还原

【Abstract】 As a representative of metal foam material,foamed aluminum,which has been used widely in the fields of spaceflight,architectural structure and automobile,is a promising functional and structural material.Because of its outstanding thermal,acoustical and mechanical performances,foamed aluminum has become a hotspot of research and development in the material science and technology.There are many approaches to manufacture cellular metallic materials.From which the gas injection method has special advantages in the respect that metallic foams can be produced continuously and their size is little limited.In this technique,a major issue is how to control the size and uniformity of the cells during the foaming process of molten aluminum.In order to explore approaches through which one can effectively control the manufacture process and the performance of aluminum foams,it is necessary to investigate and understand deeply factors affecting the foaming process.In this thesis,hydrodynamic behaviors of metallic foam flow and bubble moving and distribution characteristics in the melt were systematically studied by numerical simulation, and on this basis some insights into effects of relevant parameters on the foam structure have been gained.The main works and conclusions are summarized as follows:Firstly,microscopical and macroscopical numerical studies are performed on the drainage process in fabricating foamed aluminum.The former studied the liquid flowing in a single Plateau border(PB) of aluminum foam during drainage process and a structural model of a single node with different liquid holdup is presented.Then the CFD software FLUENT is used to compute the velocity field in a single node.The latter proposes,based on the results from the microscopical model,a new macroscopical drainage model for aluminum foams. Furthermore,the liquid/gas interface mobility is taken into account,which is characterized by the Newtonian surface viscosity.Computational results indicate that at the same liquid/gas interfacial mobility(M) and same radius of curvature,the max velocity inside an exterior Plateau border is about 6～8 times as large as that inside an interior Plateau border.It is indicated that drier forms have smaller drainage rate and show rapider coarsening,implying that gas diffusion between bubbles is the predominate factor for coarsening of foams.Besides, surface tension,and fluid properties(Henry constant,diffusion rate etc) have also remarkable effects on the evolution of bubble size distribution.The holdup of foams is gained by a mathematic model based on the gas injection method.The results suggested that gravity, viscosity,surface tension and the velocity of gas injection affected the holdup of foams greatly.The theory of phase field is applied to studying the evolution of the liquicuid/gas interface.The results explain the coalescence of neighboring bubbles.For any foams,gas diffusion through the film between bubbles in foams is inevitable.A mathematical model for predicting the evolution of bubble size distribution in aluminum foams is presented,which takes into account effects of both the coarsening due to gas diffusion between bubbles and the liquid drainage.A bubble size distribution equation and a one-dimensional drainage equation are solved coupled by a finite difference approach. Comparison with experimental results from the literature shows a reasonable agreement.The model predictions indicate that the bubble size increases exponentially with time that is in good agreement with MacPherson’s theory.Furthermore,computational results reveal that bubble size distributions are dependent strongly on the drainage behavior,the Henry constant, gas diffusivity and surface tension of the aluminum foam in liquid state.Furthermore,a method for geometrical and topological modeling the evolution of close-cell metallic foams based on the Voronoi tessellation in three-dimensional space is presented.Numerical computations were carried out to examine the evolution of bubble size distribution and topological and geometric properties of aluminum foams in liquid state,which were implemented by using McPherson’s new theory on coarsening of microstructures as well as the topological transition rules(T1 and T2 processes) in 3D foams,accounting for remarkable effects of both the gas diffusion and surface tension.Computational results show that the bubble size distributions of metallic foams are strongly coupled to the evolution of the cellular structure and dependent on the gas diffusivity and surface tension.Gas diffusion between bubbles dominates the evolution of bubble sizes and foam structures.Additionally,the cell structure of foamed aluminum is predicted by using a Monte Carlo Potts model,which takes into account effects of surface tension between gas and liquid.The statistical results of numerical simulation and experiment were compared,which indicate that the Potts model can be used in predicting the cell structure of foamed aluminum in liquid.The results show that the cell size distribution of foamed aluminum can be fitted by the Weibull function approximately.Finally,a mathematical model for the coupling process between drainage and solidification of aluminum foams is presented based on the coupling of the foam drainage equation and the energy equation.The time evolution and spatial distribution of the aluminum volume fraction and temperature during the solidification process are numerically predicted. Effects of relevant parameters e.g.gravity,viscosity and surface tension are discussed. Computational results show that foam drainage and solidification are two closely coupled and interactive processes and that the melt properties have significant influences on the solidification time and foam porosity.更多还原