Cell Structure Evolution of Aluminum Foams Under Reduced Pressure Foaming
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oams with closed cell structure show a combination of light weight and unique physical and mechanical properties.[1] Foaming of metallic melts or powder compacts are two common methods for obtaining closed cell metal foams, in which gas is either directly injected or introduced as foaming agents.[2] In the past decade, efforts are being made to understand foam stability, improve production reproducibility, and develop new manufacture techniques.[3–8] A number of novel foaming techniques based on reduced pressure foaming principle were reported, with
ZHUOKUN CAO, Associate Professor, YANG YU and MIN LI, Ph.D. Students, and HONGJIE LUO, Professor, are with the School of Materials and Metallurgy, Northeastern University, Box 117, Shenyang 110004, China. Contact e-mail: [email protected] Manuscript submitted November 20, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
the aim of processing net-shaped foam parts from cost-efficient raw materials. Renger and Kaufmann developed a vacuum foaming technique (VFT) to make magnesium foams from molten foundry scraps that contained a large amount of entrapped air and dissolved gas.[9] No blowing agent was used and the bubble expanded under reduced pressure. Vinod Kumar et al. also designed a reduced pressure foaming process (RPF) to apparently produce aluminum-based foams without a blowing agent.[10] Due to low hydrogen solubility in molten aluminum, a sharp reduction of both pressure and temperature was performed simultaneously to obtain sufficient expansion. A different process called Alporas admixes TiH2 into an AlCa melt to cause foaming.[11] Because the thermodecomposition of TiH2 at foaming temperature is very fast in this process, the melt starts expanding immediately when stirring is stopped and the fluidity of the liquid melt is greatly weakened when massive bubbles become foam. The advantage of our method reported here is the potential to produce net-shaped foam parts directly from molten aluminum. In our process, the melt only starts to expand when the pressure is reduced, so it is possible to move the melt to a mold first, and then perform the reduced pressure foaming. Our experiments showed that the gas content of the molten aluminum was of crucial importance to avoid foam collapse at low expansion rates. Despite the convenience of RPF processes in foam part production, experimental results indicate that reducing ambient pressure has negative effects on foam stability. Foaming of aluminum powder compact precursors under reduced pressure shows strong bubble coalescence or extreme instability.[12,13] The RPF process used by Vinod Kumar et al. shows Al alloys can be foamed by using the intrinsic gas in the melt and applying reduced pressure of higher magnitude combined with pressure quenching (rapid application of pressure) and that calescence is avoided by very fast solidification which freezes the foamed structure before the film ruptures. Nevertheless, RPF foam products exhibit wide distribution in cell size and cell shape. The mechanism of foam instability at reduced pres
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