Reduced-Pressure Foaming of Aluminum Alloys
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INTRODUCTION
THE process of making closed cell aluminum alloy foams directly from liquid metal alloys involves gas that is either obtained by the decomposition of blowing agent powders such as TiH2[1–4] or CaCO3,[5,6] which are admixed to the liquid metal, or is based on directly injected gases, usually air, nitrogen, or argon.[7,8] TiH2 is the most frequently used blowing agent for foaming aluminum alloys and decomposes inside the melt while releasing hydrogen gas that forms the bubbles. Gasar[9] or lotus[10] materials are made by foaming molten metals without using blowing agents or injecting gas but rather by melting and subsequently solidifying metals under pressurized (usually, a few hundred bars) hydrogen or nitrogen gas. Sometimes inert gas such as argon is also mixed with the hydrogen/nitrogen.[9–11] Both the Gasar and lotus methods are based on directional solidification. During solidification, dissolved gas is partially rejected by the melt and forms directional or radial pores in the metallic matrix. Every metal and alloy that dissolves gas is amenable to this method. Aluminum responds rather poorly to this process due to its low hydrogen solubility in the molten state in comparison to other metals.[10] Still, aluminum and its alloys can pick up from the ambient atmosphere and dissolve in the melt enough hydrogen gas to create some porosity during solidification, during which part of this hydrogen is rejected from the solid-liquid interfaces due to the pronounced jump of gas solubility. Gas volume fractions are low in this case and depend on the liquid solubility as a function of temperature[12] and alloying elements.[13,14] Foundries study porosities by applying G.S. VINOD KUMAR, Postdoctoral Researcher, is with the Technische Universita¨t Berlin, 10623 Berlin, Germany. Contact e-mail: [email protected] M. MUKHERJEE, formerly Postdoctoral Researcher, with the Technische Universita¨t Berlin, is now Assistant Professor, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, India. F. GARCIA-MORENO, Scientist, and J. BANHART, Professor and Head, are with the Institute of Applied Materials (Fl1), HelmholtzZentrum Berlin fu¨r Materialien und Energie, 14109 Berlin, Germany. Manuscript submitted April 4, 2012. Article published online October 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
the reduced-pressure test (RPT), also known as the Straube–Pfeiffer technique, to quantify the dissolved hydrogen.[15] Molten aluminum alloy is poured into a pressure-tight chamber under normal pressure and is then allowed to expand under reduced pressure, usually in the range of 60 to 130 mbar, after which the alloy is finally solidified.[15] The growth of porosity is promoted by the pressure drop and the amount of porosity that is created by a given amount of hydrogen gas and is much larger compared with what is obtained under isobaric casting conditions. The gas porosity is governed by the gas law; i.e., the volume of the gas is inversely related to the pressure appli
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