Numerical Study On Foam Evolution Dynamics In The Foaming Process Of Metallic Foams

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.