Foaming of blowing agent-free aluminium powder compacts

We present a novel foaming method based on the powder metallurgical (PM) foaming route but working without a blowing agent. A precursor is produced by uniaxial hot compaction of metallic powders without any further additions. In a first step the samples are heated up to above their melting temperature under overpressure. Foaming is triggered just by pressure release to ambient pressure. Precursors made of pure Al were melted under different overpressures from 1 to 9 bar and foamed. In situ X-ray radioscopy was used to study the foaming step and to provide quantitative information on foam expansion. 2D pore size distributions of the solidified foams were also analysed. Different possible gas sources such as entrapped gas, dissolved gas, adsorbates, water vapour, hydroxides, etc. are discussed. As a result adsorbates seem to be the most likely gas source for the observed foaming effect.