Carbonate-Foaming Agents in Aluminum Foams: Advantages and Perspectives
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INTRODUCTION
LIGHTWEIGHT metallic foams have alluring potential for different sectors of industrial applications due to the unique combination of low density and novel physical and mechanical properties. In particular, the remarkable absorbing ability of aluminum foams offers significant performance gains for crash protection of vehicle and other applications where effective utilization of impact energy is required. Aluminum foams are also nonflammable, ecologically harmless, and easily recyclable. There are many possible applications for aluminum foams ranging from lightweight construction, sound insulation, and heat insulation to energy absorption applications and lightweight ballistic structures.[1 12] In general, two processes have been invariably used for Al foaming: (1) the liquid metal route where foaming is accomplished by direct foaming of melt with gas or some foaming agents and (2) the powder metallurgy (PM) route where foaming is affected by foaming a sintered compact.[1] Each production method gives its own characteristic range of densities, cell sizes and shapes. The principle of PM is simple and the process consists of three stages: (1) mixing the metallic powder with foaming agent powder, (2) compacting the powder mixture, and (3) sintering at temperatures slightly above the melting point of aluminum. At these temperatures, the blowing or foaming agents are expected to volatilize and the arising gas forms pores in the metal phase. All
ALI SOLOKI, Researcher, is with the Department of Metallurgy, Islamic Azad University, Saveh Branch, 1513634714 Saveh, Iran. Contact e-mail: [email protected] MOHAMMAD ESMAILIAN, Professor, is with the Iranian Research Organization for Science and Technology, 3353138646 Tehran, Iran Manuscript submitted February 23, 2014. Article published online December 19, 2014. 1052—VOLUME 46B, APRIL 2015
three steps are important for the quality of the final production and the properties of the aluminum-foam products.[2] Foaming agents for aluminum foams can be metal hydrides like (titanium hydride) TiH2 or magnesium hydride (MgH2), carbonates, hydrates, or other volatilizing substances. Among these, TiH2 was mainly applied as blowing agent for both the casting and powder metallurgical procedures of foaming of aluminum.[1] However, TiH2 is expensive and cost reduction can be achieved by replacing expensive TiH2 with alternative inexpensive blowing agents, particularly carbonates such as calcium carbonate (CaCO3), magnesium carbonate (MgCO3), and dolomite (CaMg)(CO3)2.[3] As detailed and discussed by Gergely et al., carbonates react with molten aluminum and creating the foaming gas (CO2) and various solid particles (such as CaO, Al2O3, Al4C3, and MgAl2O4), depending on the composition of the aluminum alloy.[4] In contrast to TiH2, in which decomposition leads to the formation of chemically inert hydrogen, the CO2-foaming gas obtained by the decomposition of CaCO3 reacts with melt and results in stabilizing the foam suspension.[3] The results of Gergely et al.[4] suggested that, as a r
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