The Effect of In Situ Intermetallic Formation on Al-Sn Foaming Behavior
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ODUCTION
THE metallic foams often exhibit excellent stiffness to weight ratios, high mechanical energy absorption, and good acoustic and thermal insulating properties.[1] Metallic foams with closed cell porosity can be obtained by the powder metallurgy technique where a large variety of metals are employed. Nevertheless, foam reproducibility is one of the processing issues because foaming happens at elevated temperatures involving sudden foam expansion and rapid collapse with problems of drainage and pore coalescence. Foam reproducibility can be directly related to foam stability.[1] Foam stabilization using powder metallurgical route has been addressed mainly by the existence of solid oxide networks.[2,3] Experiments performed under micro-gravity demonstrated that small amount of solid components (Al2O3 from oxidized powder surface) can prevent films (cell walls) from coalescing but with very little influence on viscosity.[4] Other experiments[5] using reduced pressure test (RPT) technique indicate that surface tension and viscosity of the melt (without adding stabilizing particles) play an important role in pore nucleation and stabilization at different stages of the foaming process. Surface tension would influence pore nucleation while viscosity determines if there is pore coalescence during solidification. Although extensive research[6] has been carried out to understand the mechanisms governing the stabilization of metallic foams, the mechanism is not well known yet.
Recently, we proposed a Al-Sn foam with stability which was significantly improved compared with that of other Al foams.[7,8] The foam stabilization in the Al-Sn alloy results mainly from the gradual decomposition of TiH2 and the decrease in surface tension of the cell wall by the addition of small amounts of Sn, which can prevent the coalescence of pores. By optimizing the process parameters, the Al-3 wt pct Sn alloy was found to have the best expansion and foam stability. The purpose of the current study is to further investigate the foam stabilization and expansion of Al3 wt pct Sn alloy with addition of alloying elements which can generate solid intermetallic compounds during the foaming process. These alloying elements can modify the Al-Sn foaming behavior by (1) enhancing foam expansion (reaching higher maximum expansion) and/or (2) stabilizing the foam (regularizing pore size and distribution) by means of intermetallic formation. As a consequence, such alloying elements can homogenize the mechanical properties of Al foam. In order to design the alloying elements, thermodynamic calculations were extensively used and thermal analysis employing DSC was carried out. Five different alloying elements were selected to change the foaming behavior, type of intermetallics, and foam macrostructure.
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ALLOY DESIGN CONCEPT
A. Foaming Behavior of Al-Sn Alloy LYDIA Y. AGUIRRE-PERALES, Ph.D. Candidate, and IN-HO JUNG, Professor, are with the Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, H3A 2B2, Canad
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