The FOAMCARP process: foaming of aluminium MMCs by the chalk-aluminium reaction in precursors

A brief outline is presented of the factors involved in the search for gas-generating agents offering superior performance for foaming of liquid aluminium alloys. These include kinetic and thermodynamic characteristics of decomposition reactions, the ease of dispersion of the powdered foaming agent in the melt, the nature and likely effect of decomposition products on melt flow, potential reactions between the foaming gas and the melt and the availability, cost and ease of handling of the powder concerned. There is one very promising candidate material, calcium carbonate, which offers advantages compared to currently-employed hydride powders in virtually all aspects of their performance. It is shown that foams can be produced having appreciably finer cells (<1 mm diameter) and more uniform cell structures than currently-available melt route foams, a potentially lower ceramic content in the cell walls and dramatically reduced raw material costs. The presence of an oxidising foaming gas in the cells leads to reaction with the liquid cell surface, forming a continuous oxide film. The presence of this film has a significant effect on foam stabilisation, slowing down cell coalescence and melt drainage.