Influence of Compaction Conditions on the Foamability of AlSi8Mg4 Alloy
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A successful technique to manufacture aluminumalloy foams is the powder metallurgical (PM) foaming route.[1–3] The foams manufactured in this way are already in small-scale industrial use.[4] The method is comprised of the mixing of metal powders and a blowing-agent powder (usually TiH2) compacting the mix, and then foaming by melting the compacted precursor.[3] There are many process parameters that have an influence on foaming, such as the heating rate and foaming temperature,[5,6] the type of blowing agent and its pretreatment before use,[7,8] the alloy composition,[9,10] and the atmospheric composition and pressure during foaming.[11,12] Among the most important parameters are the compaction conditions of the powder blend. It is surprising that the influence of these has not yet been studied systematically. Reference 5 contains information on the influence of the pressing temperature and pressing time, but not on the variation in pressure that came from a limitation to a maximum pressure of H.-M. HELWIG, PhD-Student, Group for Structure and Properties of Materials, Berlin Institute of Technology, is Researcher, Metallic Foam Group, Institute of Materials, Helmholtz-Centre Berlin for Materials and Energy, Berlin, Germany. Contact e-mail: h.helwig@ gmx.de S. HILLER, Student, is with the Berlin Institute of Technology, Group for Structure and Properties of Materials. F. GARCIA-MORENO, is Head, Metallic Foam Group of the Institute of Materials, Helmholtz-Centre Berlin for Materials and Energy, and the Metallic Foam Research, Group for Structure and Properties of Materials, Berlin Institute of Technology. J. BANHART, Professor, is Head, Institute of Materials, Helmholtz-Centre Berlin for Materials and Energy, and Group for Structure and Properties of Materials, Berlin Institute of Technology, Berlin, Germany. Manuscript submitted November 9, 2008. Article published online July 28, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
120 MPa of the hot press used. Reference 13 deals mainly with the influence of powder oxidation during compaction at different temperatures. Reference 14 focuses on the influence of the tool design on the precursor quality for fixed compaction parameters. In the additional literature, compaction ‘‘recipes’’ are presented that yield ‘‘good’’ foam, but parameter optimization is largely poorly documented. Most of the work done aims at densifying the powder to a threshold density, e.g., 98 pct,[5] in order to sufficiently encapsulate the hydride particles in the matrix and prevent the evolving blowing gas from escaping. Therefore, a density as high as possible is implicitly assumed to be desirable. The parameters that can be varied to achieve densification in a die are the compaction temperature, time, and pressure. The compaction temperature is limited by the decomposition temperature of the blowing agent. Moreover, the hotworking steel used for the tool limits both the temperature and pressure as its strength rapidly decreases upon heating. If uniaxial pressing is applied in mass production, the cost
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