Foam production process: TiC and TiH2

Porous Metals and Metallic Foams: Current Status and Recent Developments

....Foam stabilisation can be obtained by adding ceramic particles into the metallic melt, which adhere to the gas/metal interfaces during foaming and prevent pore coalescence. One foam production process (sometimes referred to as the ‘Alcan process’) uses liquid metal matrix composites (MMCs) containing 10–20 vol.% particles (typically 10 lm silicon carbide or alumina particles) into which a blowing gas is injected. Very regular and highly porous metal foams can be produced with this method. However, the high particle content makes the solid foams very brittle and hard to machine. Replacement of the large particles in the MMCs by nanometric particles is a solution to overcome this problem. Using nanoparticles, melts can be foamed at much lower particle loading.

Indeed, 5% of 70 nm SiC particles dispersed ultrasonically in the melt were shown to be sufficient to stabilised aluminium foams. Particles formed in-situ by chemical reactions have also been used with success for the same purpose. As an example, 4 wt.% of TiC particles (200–1000 nm) formed in-situ in liquid aluminum resulted in stable aluminum alloy foams. Another area of investigation involves the development and improvement of the blowing agents. TiH2 has been in use since the 1950’s and is presently considered the most powerful blowing agent available for the production of aluminium and magnesium alloy foams. Efforts to replace or to improve this agent are important for two reasons. The first is the high cost of this hydride (<80 u /kg, 2008 price). Since 0.5 to 1.5 wt.% TiH2 are needed to produce an aluminium foam, the blowing agent contributes significantly to the final cost of the material. Taking into account the price of Al melt (1.50 u/kg) or powder (3 u/kg), the blowing agent contributes to up to 25% of total raw material costs. Replacing TiH2 by a less expensive blowing agent, namely CaCO3, is being investigated by different researchers.

A second reason for the replacement of TiH2 is associated with its decomposition behaviour that does not perfectly match the melting characteristics of most aluminium alloys used for foaming. In fact, TiH2 decomposition starts at temperatures lower than the melting temperature of aluminium or magnesium alloys. This negatively affects foaming by premature hydrogen release in the solid state and losses through the porosity network between the aluminium particles. Various strategies are currently under study to overcome this problem. One option is to modify the TiH2 powder in order to alter its decomposition characteristics.
This can be done by oxidation or by coating the particles with a thin layer of nickel. Another option is to work without blowing agent and rely on gas residues contained in powder or scrap that is added to the melt. In this case, pressure is used to control the foaming of the melt.

Another cost reduction strategy is to use chip waste, e.g. from machining, as replacement for the expensive aluminium alloy powders. Chips are mixed with ceramic additives and TiH2 and densified by compressive torsion processing or thixo casting after which the compacted materials are foamed in the usual manner. An open question remains, however, whether the need for more expensive compaction techniques contradicts the materials cost saving issue.


Advanced Engineering Materials, Wiley InterScience, 2008