Collapse of Aluminum Foam in Two Different Atmospheres
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Metal foams can be manufactured by various methods, among which the powder metallurgical route is one of the most important ones because it allows for processing a wide range of materials and component geometries. In this route, ambient pressure and atmosphere influence foam evolution.[1–3] The effect of ambient atmosphere (oxidizing or inert) on foam expansion has been attributed to the mechanical resistance of the outer surface oxide layer that is created during foaming.[1,2,4] In aqueous foams, coarsening is governed by intercellular diffusion of gas; large bubbles grow at the expense of small ones.[5] In contrast, coarsening of aluminum foams occur solely through coalescence[3,6] as intercellular diffusion is negligible because of the low pressure difference between adjacent bubbles and a thick cell wall.[6] However, at the outer surface that is in contact with the surrounding atmosphere, the conditions for diffusion can be quite different when hydrogen is allowed to be removed continuously, which is the case in industrial foaming practice where open furnaces are used. Then, the driving force for diffusion is high, and because hydrogen has a significant diffusivity in liquid aluminum at 933 K (660 C), it can diffuse up to 103 m in only 10 seconds.[7] This should imply that when the usual metal hydrides are used for foaming, a significant M. MUKHERJEE, Postdoctoral Researcher, F. GARCIAMORENO, Head of Metallic Foam Group, and J. BANHART, Professor, Head of Institute of Applied Materials, are with the Institute of Applied Materials, Helmholtz-Centre Berlin for Materials and Energy, Hahn-Meitner-Platz, 14109 Berlin, Germany and also with Structural Properties of Materials, Berlin Institute of Technology, Hardenbergstrasse 36, 10623 Berlin, Germany. Contact e-mail: manas. [email protected] Manuscript submitted November 10, 2009. Article published online April 14, 2010. 500—VOLUME 41B, JUNE 2010
loss of gas (hydrogen) to the surrounding area takes place, resulting in a low expansion of the foam. The associated fast collapse of foams actually has never been reported; foams appear very stable in most cases, suggesting that diffusion is hindered. In the context of a general study of foam collapse,[8] the question was formulated of whether out-diffusion can contribute to instability. In this article, we show experimentally that the foaming atmosphere does play an important role for the evolution of foam by influencing the gas loss rate, provided that foaming is carried out under a very pure argon atmosphere. Foam expansion was monitored in situ using X-ray radioscopy to detect even small changes in volume and internal structure. A calculation is carried out to check the results obtained. The collapsed structure of foams was also studied by X-ray tomography. Aluminum (Al) powders (Alpoco, London, UK; purity 99.7 pct) and titanium hydride (TiH2) powders (Chemetall, Frankfurt, Germany; grade N, purity 98.8 pct), heat-treated at 753 K (480 C) for 180 minutes in air, were used to prepare foamable precursors. Thirty grams of Al powder w
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