Fabrication of Lotus-Type Porous Al-Si Alloys Using the Continuous Casting Technique
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INTRODUCTION
POROUS metals have unique characteristics that are different from those of bulk metals, such as an inherently low density and a large surface area. Thus, these metals are expected to be utilized as lightweight materials, catalysts, electrodes, vibration- and acousticenergy-damping materials, impact-energy-absorbing materials, etc.[1,2] In particular, porous Al alloys are expected to be used for lightweight structural applications because of their low specific weight, high stiffness, and high energy-absorbing capacity. Recently, lotus-type porous metals, one class of the porous metals, have been investigated by many researchers;[3–9] these materials possess long cylindrical gas pores aligned in one direction. Lotus-type porous metals have superior mechanical properties originating from unidirectional pores, compared with conventional porous metals, the pores of which are almost isotropic and spherical.[10] As a result, lotus-type metals are attracting attention as new lightweight structural materials. Lotus-type porous metals have been developed based on the unidirectional solidification method, using the gas solubility gap between the liquid and the solid.[4–9,11] Recently, our group successfully fabricated a long-sized lotus-type porous copper slab using the continuous casting technique,[9,12,13] and we reported that both the J.S. PARK, Specially Appointed Assistant Professor, S. SUZUKI, Associate Professor, and H. NAKAJIMA, Professor, are with The Institute of Scientific and Industrial Research, Osaka University, Osaka 567-0047, Japan. Contact e-mail: [email protected] S.K. HYUN, formerly Associate Professor, The Institute of Scientific and Industrial Research, Osaka University, is Assistant Professor with the School of Materials Science and Engineering, Inha University, Inchon 402-751, South Korea. Manuscript submitted August 21, 2007. Article published online January 6, 2009 406—VOLUME 40A, FEBRUARY 2009
porosity and pore size can be easily controlled by the solidification velocity and the hydrogen pressure in the continuous casting technique. Lotus-type porous aluminum is also expected to be utilized as a lightweight structural material. For aluminum and its alloys, hydrogen is available for pore formation and growth.[9] However, it is difficult to obtain high porosity due to the small hydrogen gas solubility gap between the liquid and the solid.[14] Furthermore, in the case of casting in nonporous aluminum and its alloy, it is also reported that both the porosity and pore diameter are reduced at a high solidification rate, because there is no time for the hydrogen to diffuse from the aluminum into the pores.[14–16] Therefore, both an increase in the hydrogen gas solubility gap and control of the solidification rate are required in order to increase the porosity of the material. It is thought that the addition of an alloying element is an effective method of changing pore formation. Silicon is one of the important alloying elements in Al alloys, and Al-Si alloys have been used as commercial cast all
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