Effect of Microstructural Anisotropy of PM Precursors on the Characteristic Expansion of Aluminum Foams
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THE applicability of powder metallurgical (PM) aluminum foams in many sectors has been widely validated due to their excellent combination of properties and the facility to produce samples with complex shapes.[1–3] However, they still present a nonuniform and poorly reproducible cellular structure. As a direct consequence, they exhibit a high scattering in mechanical properties,[4–7] which implies the use of higher safety indexes to compensate for this uncertainty, as well as a deviation from the general foam models (properties below predicted values).[8] This fact makes them less competitive in comparison to other materials and limits the applicability of aluminum foams. The causes of this poor cellular structure quality are diverse. One of the already reported phenomena occurring in the materials foamed via the power metallurgical route is the presence of an initial expansion, while the precursor is still in the solid or semisolid state, called anisotropic early expansion,[9] which influences the later evolution of the liquid foam and the final cellular structure.[10] The effect of this expansion has been reported as detrimental for the cellular structure quality because more cracks and defects are created at this early stage.[10] In that way, any investigation focused on the characterization (including causes or related production parameters) of this early growing in semisolid state is of obvious interest for a better JAIME LA´ZARO and ESTER LAGUNA-GUTIE´RREZ, Ph.D. Students, EUSEBIO SOLO´RZANO, Associate Researcher, and MIGUEL ANGEL RODRI´GUEZ-PE´REZ, Professor, are with the Cellular Materials Laboratory (CellMat), Condensed Matter Physics Department, Faculty of Science, University of Valladolid, 47011 Valladolid, Spain. Contact e-mail: [email protected] Manuscript submitted September 11, 2012. Article published online April 26, 2013. 984—VOLUME 44B, AUGUST 2013
understanding of the foaming process and as a first step to optimize the cellular structure and the physical properties. This early expansion is known to be mainly associated with the blowing agent decomposition (typically TiH2) starting at a temperature below the melting point of most aluminum alloys.[11] In fact, the use of pretreated TiH2, to delay the H2 generation,[12] results in lower expansion below the melting point and improved cellular structures.[13] However, it is also known that the presence of gas at early solid or semisolid stages cannot be completely avoided due to the adsorbates present in the surface of the initial powders; it is possible to use them for foaming precursors free of a blowing agent under special pressure conditions.[14] Recent studies have attempted to eliminate these adsorbates by producing the precursors under vacuum conditions with interesting results,[15] but further investigation is still needed in this area. Finally, it has been reported that particular Mgrich low-melting-point alloys present sufficient hydrogen, dissolved in the solid state, to generate a highquality foam structure in normal conditions.[16] In any ca
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