Aluminium foam production using calcium carbonate as a foaming agent
The current state of the art with regards to the production of metallic foams is reviewed, with melt-based processes identified as the most promising for cost-effective large-scale production. The potential for metal carbonates as an alternative to currently-used titanium hydride foaming agents is explored, with calcium carbonate identified as the most suitable. The influence of a range of material and processing parameters on the stability of metallic foams in the molten state is discussed, and current methods of controlling melt viscosity and surface tension are reviewed. Characteristic features of the compressive deformation of metallic foams are described in the context of use as an impact-absorbing material, with a review of work in the literature linking the bulk mechanical properties to details of the cell structure. Calcium carbonate is found to be a highly effective foaming agent for aluminium. The foams obtained have notably finer cell structures than can be achieved in foams produced with titanium hydride, coupled with enhanced stability in the molten state. This is attributed to the presence of a thin continuous surface film of metallic oxide that counteracts the effect of surface tension. This film, combined with the finer cell structure of the calcium carbonate-based foams, is found to significantly reduce the rate of gravity drainage of the melt. The formation of the thin oxide film during foaming gives rise to a number of artefacts on the cell surface, including stretch marks and tear bands. A range of chemical and surface analysis techniques are used to identify the chemical composition and thickness of the oxide film. The distribution of refractory particles in the cell faces, which are commonly employed to stabilise molten foam structures, is found to be highly non-uniform in foams which undergo significant gravity drainage of liquid metal during the foaming process. Experiments in which the concentration of particles is varied demonstrate the importance of their effect on the melt viscosity in addition to their known role as a surface stabilising phase. The effect of alloy content and foaming gas on the stability of standing molten foams is also investigated in the context of other foaming processes. The formation of an oxide film on the surface of the cells is shown thermodynamically to be a necessary step in the production of low-density aluminium foams with a calcium carbonate foaming agent. A temperature-dependent upper limit on porosity is observed. It is established that this is the result of inhibition of the calcium carbonate decomposition reaction by its products as the thickness of the surface oxide film increases. The effect of varying cell size, porosity and chemical composition on the thickness of the surface oxide film is derived. The rate of thermal decomposition of calcium carbonate is found to be dominated by the partial pressure of carbon dioxide, with particle size and small impurity contents having only a small effect. Compressive mechanical properties of the foams produced are compared with those of foams produced with a titanium hydride foaming agent and theoretical predictions. A reduced cell size apparently minimises the influence of point defects on the properties of specimens of finite dimensions. A significant difference in the shape of the stress-strain curves of calcium carbonate- and titanium hydride-based foams is noted, with the latter marked by extensive serrations. This difference is demonstrated to be independent of differences in cell size. Microstructural analysis of foams in various stages of failure suggests that this is due to differences in the distribution of refractory particles in the two foams, which is in turn a consequence of the reduced extent of gravity drainage of liquid metal in the calcium carbonate-based foams.