The fabrication processes and mechanical properties of advanced metallic syntactic foams

Research Doctorate - Doctor of Philosophy (PhD)In the last two decades a great deal of research has been focused on the development and characterisation of metallic foams for special purpose applications. Due to their high strength to weight ratios and highly porous structures, metallic foams have unique energy absorption, damping, and thermal properties. However, these materials have not yet been widely used in industry, simply because of their higher costs when compared to their polymeric competitors in the market. In recent years, researchers have shown considerable interest in metallic syntactic foams, which are produced by embedding hollow or porous low density heat resistant particles in a metallic matrix. Owing to their relatively simple manufacturing processes, metallic syntactic foams have lower costs when compared to other foams. However, the typical aluminium syntactic foams have significantly higher densities (reportedly more than 1.4 g/cm³). This is mainly due to the high densities of the filler particles (typically more than 0.6 g/cm³) and the failure of particles during the manufacturing process. In this thesis, the major limitations of the metallic syntactic foams, i.e., their high densities and relatively high costs, are addressed by introducing a novel light porous filler material, Expanded Perlite (EP). A large volume fraction of internal porosity (≥95%) reduces the density of this natural volcanic glass down to only 0.18 g/cm³. Being mined in large quantities, to the author’s knowledge EP has the lowest price when compared to its competitors. The large particle size range, from 300 μm to 6 mm, allows for the simple, cost efficient manufacture of foams with the desired properties. EP/A356 aluminium syntactic foams were successfully fabricated using a melt infiltration technique. Depending on the manufacturing parameters, the densities of the foams may vary between 0.7 and 1.05 g/cm³, which are the lowest among the typical syntactic foams. The produced foams were then subjected to a wide range of microstructural, structural, and mechanical testing for a comprehensive characterisation of the material. With a special focus on the energy absorption capabilities of the foams, attempts were made to improve the mechanical responses of the foams by adjusting their structures and microstructures. Heat treatment, a smaller EP particle size, and a higher sphericity of the particles were shown to be effective parameters which increase the mechanical strength and energy absorption capacities of the foams. The positive strain rate sensitivity of the compressive properties makes this foam attractive for crash cushioning applications. The foams also showed outstanding performances under cyclic compressive loading conditions. Following the major characterisations, an application of EP/aluminium syntactic foam, as the core of hollow steel tubes, was investigated. The compressive and bending properties of the foam filled tubes improved considerably when compared to empty tubes. A second novel filler material, with a higher density and crushing strength than those of expanded perlite, was employed for the manufacture of high strength syntactic foams, while maintaining a low price. Syntactic foams with a density of 1.5 g/cm³ were made by the infiltration of packed beds of pumice, a natural porous volcanic glass with a particle density of 0.75 g/cm³, with molten aluminium. The pumice/aluminium syntactic foams showed a 35% increase in their energy absorption capacities when compared to the EP/aluminium syntactic foams