Processing and Pore Growth Mechanisms in Aluminum Gasarites Produced by Thermal Decomposition
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TRODUCTION
A gasarite structure, also known as a lotus structure, is a unique form of metallic foam resulting from cooperative gas–metal eutectic growth during directional solidification of gas-saturated molten material.[1] This structure resembles a rod-eutectic with discrete rodshaped pores distributed throughout a metal matrix. This cylindrical pore structure is potentially beneficial when designing for both functional and structural material applications. For example, in unidirectionally loaded components, these structures exhibit higher mechanical strength along the pore axis than foams of the same material and relative density.[2] A primary drawback of gasarites is their high processing costs related to implementation of batch production methods.[1,3] In addition, hydrogen is a commonly used soluble gas for pore formation, and this requires special safety procedures. Hydrogen pressures up to several atmospheres are required for processing some materials[2,4–6] resulting in additional costs for specialized highpressure casting environments. These gasarite-processing challenges have led to development of alternative processing techniques to reduce costs and increase the usage of gasarites in engineering design.[7–14] The thermal decomposition method (TDM)[12] involves casting metal upon compounds that decompose and evolve gas upon heating, JOSEPH J. LICAVOLI, Ph.D. Candidate, and PAUL G. SANDERS, Assistant Professor, are with the Michigan Technological University, Houghton, MI. Contact e-mail: [email protected] Manuscript submitted January 14, 2013. Article published online October 17, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
such as metal hydrides. The use of hydrides as a foaming agent for isotropic metallic foams has been implemented in processes designed by Shinko Wire (Alporas) and Fraunhofer IFAM.[15] The key to producing gasarites via thermal decomposition is the coupling of directional solidification with gas evolution. In preliminary studies conducted by Nakajima and his coworkers, samples of aluminum gasarites were produced using a number of gas-evolving compounds.[10] The pore morphologies of foams created by thermal decomposition were found to vary significantly both as a function of the decomposing gas source and within sample groups created under nominally the same conditions. In particular, titanium hydride was shown to produce high variability in both pore size and porosity levels vs calcium hydroxide and other thermally decomposing materials. In order to achieve better control over the production of gasarites by thermal decomposition, it is necessary to understand how pores begin to form, and how, after formation, their behavior may be controlled to create a desirable pore structure. Based on their own preliminary study and the study of Makaya and Fredrickson,[16] Kim et al.[10] suggested that the pore-formation mechanism for aluminum gasarites produced by TDM is via a nonequilibrium gas–metal eutectic mechanism, but no experimental evidence in aluminum systems was identified to support this h
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