Experimental Study on the Pore Structure of Directionally Solidified Porous Cu-Mn Alloy
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NTRODUCTION
BASED on the solubility difference of gas between liquid and solid metals, regular porous metals with oriented pores could be fabricated by directional solidification of molten metals saturated with gas (such as hydrogen), which is called directional solidification of metal-gas eutectic method,[1] or the GASAR process.[2,3] Compared with other metal foaming techniques, the porosity, pore size, and distribution of oriented pore structure produced by this process could be well controlled by adjusting processing parameters. A porous metal with oriented pore structure shows promising mechanical and functional properties. Many experiments show that various kinds of porous metals with oriented pore structure could be produced successfully by the GASAR process, such as copper,[4] magnesium,[5,6] iron,[7,8] etc. However, porous alloys with similar pore structure, which have more important potentials than porous pure metals in industry, could not be made by this process.[9] Really, during the fabrication process of porous alloys by the GASAR process, three critical factors should be considered. (1) The solubility variation data of gas between liquid and solid for various alloys are not abundantly documented. The author summarized the hydrogen solubility in some pure metals in the liquid and solid states[12] and indicated[13] that the gas solubility variation between liquid and solid state is one of the controlling factors for producing oriented pore structure. In early research by the author,[14] a theoretical model for calculating hydrogen solubility in molten multicomponent single-phase GUANGRUI JIANG, Postdoctoral Candidate, and YANXIANG LI, Professor, and YUAN LIU, Associate Professor, Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, are with the Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P.R. China. Contact e-mail: yanxiang@ tsinghua.edu.cn Manuscript submitted January 21, 2010. Article published online September 14, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys had also been developed based on thermodynamic analysis, which could well predict the hydrogen solubility in many molten alloys. (2) Velocity of directional solidification. In fabrication of porous copper by the continuous casting process, it was found by Park et al.[15] that, if the solidification velocity is smaller than a critical value, the pore structure will be irregular. Considering that alloying usually decreases thermal conductivity of metallic materials, it is hard to get high solidification velocity with normal directional solidification processes. (3) Morphology of the solid-liquid interface. The morphology of the solid-liquid interface is regarded as planar in the directional solidification of the metal-gas eutectic;[16] it is hard to keep planar in the GASAR process for alloys.[18] As pores often appear on the solid-liquid interface, the morphology of the solidliquid interface should be one of the most important influencing factors on the growth of pores. In the researc
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