Preferential Site Precipitation and Subcell Stability in AA6061 Sandwich Cores
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Preferential Site Precipitation and Subcell Stability in AA6061 Sandwich Cores B. A. Bouwhuis and G. D. Hibbard Department of Materials Science and Engineering, University of Toronto, Toronto, M5S3E4, Canada ABSTRACT Periodic cellular metal (PCM) sandwich cores can be considered hybrids of the solid and gas type. These can be designed at both the architectural and microstructural levels. PCM cores with 95% open porosity have been constructed from perforated 6061 aluminium alloy (AA6061) sheets using a perforation-stretching method. This method places planar, periodically-perforated sheet metal in an alternating-pin jig. The pins apply force out-of-plane, plastically deforming the sheet metal into a truss-like array of struts (i.e. metal supports) and nodal peaks (i.e. strut intersections). The result is a non-uniform work-hardened profile exhibiting large deformation at the nodes and small deformation at the struts. For identical PCM architectures, this study looks at the interaction of microstructural strengthening mechanisms and the resultant performance of PCM truss cores. Beginning with fabrication, work-hardening induced a subcell network of dislocation tangles within the AA6061 matrix. Following this stage, a variety of microstructures were created through recovery, recrystallization and precipitation mechanisms. Microhardness measurements and electron backscattered diffraction (EBSD) characterization were employed through truss core cross-sections in order to study the microstructural gradients of subcell size as well as interaction between subcells and precipitates in the truss cores. To determine the effect of microstructure on mechanical performance, PCM cores were compressed to study deformation and collapse mechanisms. The present data can be used to illustrate engineering at the architectural and microstructural levels to achieve a range of mechanical properties in a hybrid sandwich core.
INTRODUCTION Periodic cellular metals (PCMs) are hybrids of space (air) and metal, i.e. an effective material with its own set of properties [1]. A recent review of PCM architectures and fabrication methods is given by Wadley [2]. PCMs are attractive in applications requiring high weightspecific mechanical properties (e.g. compressive strength and modulus) and can exhibit superior mechanical properties when compared to their stochastic cellular metal foam counterparts [3]. In the present study, combined thermal and mechanical processing (TMP) is applied to a pyramidal aluminum alloy 6061 PCM in order to reach a broader range of microstructures than is possible through thermal treatment alone. For a constant PCM architecture, different deformation and annealing pathways are used to increase overall PCM material properties using microstructural strengthening methods accessible to heat-treatable aluminum alloys (e.g. preferential site precipitation and subcell stability).
EXPERIMENTAL DETAILS Pyramidal stand-alone PCMs were fabricated from a 0.80 mm thick (t) square punched aluminium 6061-T6 sheet, purcha
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