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

Abstract

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.

Introduction

Interest in closed cell aluminium foams for structural and energy absorption applications is currently high [1], [2]. There is a strong incentive to develop economically attractive processing routes [3] for such foams, creating material having a fine, uniform cell structure (preferably without coarse, embrittling constituents [4]). Routes based on the handling of a melt [5], [6] are potentially cheaper and more tolerant of impurities than those based on a powder compact [7], [8]. They can also be used as a ‘recycling’ procedure for certain type of scrap metal matrix composites (MMCs) by converting them into MMC-based foams [9]. By far the most commonly employed foaming agent for aluminium alloys is titanium hydride (TiH₂), owing mainly to its high specific hydrogen content, a reasonably good correspondence between its decomposition temperature and typical alloy melting temperatures and the rapid kinetics of the decomposition reaction.

The recently-developed FORMGRIP process [10], in which a protective oxide coating is created on the surface of TiH₂ powder particles, eliminates the problem of premature gas release during the hydride dispersion in the melt, facilitating production of a foamable precursor via a melt route. This is subsequently baked in order to produce the foam component. However, while this procedure works well, there is a need for a gas-generating agent with superior cell face stabilisation characteristics. There is also a strong commercial incentive to find an agent which is cheaper and easier to handle than hydrides such as TiH₂. A highly promising alternative foaming agent suitable for the melt-route production, calcium carbonate (CaCO₃), was reported in a preliminary study by the present authors [11]. A coating procedure facilitating easy dispersion of CaCO₃ in aluminium melts has been proposed [12] by researchers in Japan. Although this compound was listed as one of the potentially suitable foaming agents in several early patents [13], [14], its suitability for production of Al-based foams via a melt-route has not yet been systematically assessed. In the current paper, the performance of calcium carbonate as a foaming agent is studied in detail and the associated Al-melt foaming behaviour is discussed.