Aluminum Foams: On the Road to Real Applications
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Aluminum Foams:
On the Road to Real Applications John Banhart
Foaming Liquid Metals Metallic melts can be foamed by creating gas bubbles in the molten liquid, provided that the melt is prepared in such a manner that the emerging foam is fairly stable during processing. This can be done by adding fine ceramic powders or alloying elements to the melt to form stabilizing particles, for example. The exact stabilization mechanism is still undetermined, but most likely involves the generation of repulsive forces that prevent the film from thinning. Currently, there are three known ways of foaming metallic melts: first, by injecting gas into the liquid metal; second, by causing an in situ gas release in the liquid
Abstract Metallic foams have become an attractive research field both from a scientific viewpoint and the prospect of industrial applications. Various methods for making such foams are available. Some techniques start from specially prepared molten metals with adjusted viscosities. Such melts can be foamed by injecting gases or by adding gas-releasing blowing agents that decompose in situ, causing the formation of bubbles. Another method is to prepare supersaturated metal–gas systems under high pressure and initiate bubble formation by pressure and temperature control.Yet a further class of techniques begins with solid precursors containing a blowing agent. These can be prepared by mixing metal powders with a blowing agent, compacting the mix, and then foaming the compacted mix by melting. Alternatively, casting routes can be used to make such precursors. The unique properties of foams offer promise in a variety of applications ranging from lightweight construction and impact-energy absorption to various types of acoustic damping and thermal insulation. Keywords: alloys, casting, cellular solids, industrial applications, metals, mechanical properties, powder metallurgy, structural materials.
Introduction Solid metallic foams exhibit many unusual combinations of physical and mechanical properties that make them attractive in a number of engineering applications. For instance, when used as cores in structural sandwich panels, they offer high stiffness in conjunction with low weight. Their use in energy-absorption devices exploits their capacity to undergo large deformation under almost constant stresses. In the literature and in practical use, there is some confusion concerning the term “metallic foam,” which is often used for any kind of non-dense metallic material. In this article, these materials are defined as follows: Cellular metals are materials with a high volume fraction of voids, usually more than 70%, composed of an interconnected network of struts and plates. Porous metals have isolated, roughly spherical pores. Mechanically, pores do not interact if the porosity is less than about 20%.
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Metal foams have polyhedral cells that may be either closed with membranes separating the adjoining cells, or open with no membranes across the faces of the cells so that the voids are interconnected. So
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