Expansion Mechanisms in Foaming Aluminum Melts
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TRODUCTION
CELLULAR or porous materials are commonly found in nature and include familiar examples such as wood, bone, sponges, and some forms of lava.[1] Synthetic cellular materials are also common, and include plastic foams for cushions, packing materials, and protective gear. Metallic foam, though less common, is a synthetic cellular material in which discrete gas cells are separated by thin membranes or walls, and the volume fraction of the solid phase is typically small (10 to 30 pct). Metallic foams exhibit engineering properties that offer potential advantages in applications in special service environments and at temperatures where polymer foams and woods cannot be used. Like most cellular materials, these characteristics include high specific stiffness, ultra-low density, thermal insulation, and impact energy absorption. However, unlike most polymers and woods, foam metals provide electromagnetic shielding and are nonflammable. Furthermore, when exposed to fire, foam metals do not release toxic gases, an important consideration for materials used for building construction, personnel quarters on ships, and on subway cars. Metallic foams can be produced by solid-state or liquid-state processing. Solid-state foaming is normally achieved by a powder metallurgical process, and several kinds of metallic foams have been fabricated by this process route.[2–4] This method affords good process control (compared with liquid-state foaming), although there are substantial cost increments associated with the use of powder materials and the powder consolidation equipment. In contrast, liquid-state foaming is more amenable to production of large volumes, although process control can be challenging. Foam expansion from the melt can be achieved by (a) blowing gas into ZHENLUN SONG, Doctor, is with the Materials Science Department, University of Southern California, is with the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China. Contact e-mail: [email protected] STEVEN NUTT, Doctor and Professor, is with the Materials Science Department, University of Southern California, Los Angeles, CA 90089-0241. Manuscript submitted June 19, 2007. Article published online June 19, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
the melt, (b) creation of gas bubbles by a nucleation and growth reaction, or (c) agitation of the melt by external force. For industrial production of metallic foams, all three methods are used. However, the second method affords superior structural control compared with the other two. Metallic foams can be obtained by means of method (b) mentioned previously, i.e., by injecting a blowing agent directly to the molten metal. The idea of direct metal foaming using a blowing agent was first demonstrated in 1947 using vaporized mercury.[5] Subsequently, Elliott pioneered the use of solid blowing agents, including hydride titanium or hydride zirconium.[6] Improved processes for foaming liquid metals by this method appeared after 1960.[7,8] In 1987, Unno and Akiyama repo
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