RETRACTED ARTICLE: Fabrication of Lotus-Type Porous Al-Si Alloys Using Thermal Decomposition Method Combined with Mold C
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TRODUCTION
ALUMINUM alloy foams have attracted much attention, because they exhibit superior light-weight properties originating from the intrinsic light-weight properties of aluminum matrix. However, most aluminum alloy foams have a disadvantage that their mechanical properties are low because of the large stress concentration around the spherical or irregular pores. Therefore, to improve the pore morphology, fabrication processes have been studied intensively. In general, the porous, cellular, and foamed metals are characterized by pore growth direction, pore size, and porosity. As mentioned earlier, while the pore direction, pore size, and porosity of the cellular and foamed metals are not easy to control by the fabrication methods, those of the lotus and gasar metals are relatively easier to control by those fabrication methods because the direction of the solidification, gas pressure, gas content, and the solidification velocity can be controlled.[1–8] Recently, porous metals cylindrical pores of which are oriented in one direction have attracted much attention.[1–8] Such porous metals are known as lotus-type porous metals (lotus metals) or gasar metals. Shapovalov[9] first fabricated the anisotropic porous metals from the viewpoint of applicability to functional materials.
TAE BUM KIM and TAEK KYUN JUNG, Researchers, YONG HWAN KIM and TAEK SOO KIM, Principal Researchers, are with the Korea Institute of Industrial Technology, Incheon, Korea. Contact e-mail: [email protected] SOONG KEUN HYUN, Professor, is with the Inha University, Incheon, Korea. Manuscript submitted January 20, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
On the other hand, Hyun et al. revealed that the mechanical properties along the orientation direction of pores of lotus metals are much superior to those of metal foams with spherical or isotropic pores, because stress concentration hardly occurs, and crack formation is difficult under loadings along the orientation direction.[10,11] Therefore, lotus metals are expected to be used not only as functional materials but also as light-weight structural materials; in particular, lotus Al alloys are expected to be used as light-weight structural materials. Lotus Al and Al alloys can be fabricated by the unidirectional solidification in pressurized hydrogen gas atmospheres. In this fabrication method, a hydrogen solubility gap between the solid and liquid phases is utilized for the pore formation. When a metal is melted and solidified in a hydrogen atmosphere, hydrogen insoluble in the solid is rejected at the solid/liquid interface. The rejected hydrogen forms pores which grow along the solidification direction. As a result, a lotus metal with directional pores is fabricated. This fabrication method is known as a pressurized gas method (PGM).[12–14] First, the PGM method was adapted to a mold casting and continuous zone melting methods which cannot produce a large ingot. Next, it was adapted to a continuous casting method which can produce a large slab suitable for a mass production. Lotus Al and
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