Replicated aluminium foam:processing and properties

This thesis explores the processing and properties of microcellular aluminium-based materials produced by replication. It includes the development of a pressure infiltration process for net-shape component production, structural characterization of the foams, measurement of their fluid flow permeability and their uniaxial deformation characteristics, together with contributions towards explaining and modeling these properties. Sodium chloride particles are classified and compacted to form an open-pore pattern that is infiltrated with molten pure aluminium under gas pressure. After solidification of the metal, the salt is dissolved in water, yielding an open-cell aluminium foam, the structure of which replicates in negative that of the preform. Resulting highly porous aluminium foams (or "sponges"), regular and homogeneous in pore size, pore shape and density, are produced with an average pore size ranging from 5 to 500 μm and a relative density between 10 and 35 % (corresponding to porosity between 65 and 90 %). The structure of these replicated foams is characterized by optical and scanning electron microscopy. A collaboration with Ms A. Marmottant and Dr. Luc Salvo, GPM2, INP Grenoble, France, allowed three-dimensional images to be produced by X-ray tomography using synchrotron radiation, at the ESRF in Grenoble, France. The Darcian fluid flow permeability of replicated foams is measured varying the average cell size (75 and 400 μm) and the relative density (from 12 to 32 %). Data show the expected dependence on the square of the pore size and agree with other experimental data in the literature for coarser aluminium foams produced by a different casting process. A predictive model for the permeability of open-pore microcellular materials is derived based on a quantitative description of the foam structure, leading to a simple analytical expression. Predictions agree well with present and published experimental data. Mechanical tests are performed in both compression and tension to evaluate the response of pure aluminium replicated foams and its dependence on principal microstructural parameters. Present and published experimental data for the stiffness and the flow stress are compared with the usual Gibson-Ashby equations and with predictions of mean-field models for the deformation of composite materials adapted to metal foams. The best predictor of the behaviour of replicated foams is found with the differential effective medium calculation of composite elastic moduli. This, coupled with the modified secant modulus model of composite non-linear deformation by Suquet is also shown to provide a good predictor of the plastic flow characteristics of these foams. Damage accumulation during foam deformation is evidenced, lowering the foam stress in both tension and compression and causing an evolution in foam stiffness and electrical conductance. In-situ compression and tension tests are performed to observe by X-ray microtomography the mechanisms of deformation and fracture at the foam cell scale. The formation of plastic hinges in compression and strut necking and fracture in tension are evidenced. Damage is shown to govern the tensile failure of the foams, in accordance with theory. The influence of microstructural features on the mechanical behaviour of replicated aluminium foams is investigated. It is shown in particular that the average pore size of the foams influences their plastic flow stress and work hardening rate, both increasing with decreasing pore size. Variations in the heat treatment of the foams are used to show that this plasticity size effect is dislocational in nature, and that it has its origin in dislocation emission during thermal excursions of the Al/NaCl precursor of the foam, before dissolution of the salt. The infiltration pressure and the salt grain shape are also shown to influence the foam structure and the foam uniaxial compressive mechanical response