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RECENTLY, lotus-type porous metals by Gasar process which possess long cylindrical pores aligned in one direction have attracted much attention.[1–6] Usually, lotus-type porous metals are fabricated by a directional solidification process at a pressurized condition (Gasar), utilizing the hydrogen solubility gap between the liquid and solid phases. Since they exhibit peculiar physical and mechanical properties such as light-weight, air and water permeability, and anisotropy of thermal and mechanical properties, lotus-type porous metals are suitable for applications in heat sinks, filters, and biomedical devices. Common metals such as Cu, Mg, and Fe have been fabricated into the unimodal porous structure using the Gasar technique. However, it is difficult to fabricate lotus-type porous aluminum with high porosity, since the hydrogen solubility gap between the liquid and solid aluminum is rather low compared with that in other metals such as copper and magnesium.[7] Owing to the widespread use of aluminum and its alloys, fabrication of lotus-type porous aluminum is of great interest. So far, several efforts have been made to fabricate lotus-type porous aluminum (hereafter lotus aluminum). Zhang et al.[8] fabricated porous aluminum by the Gasar process and found that the porosity was unlikely to be XIAOBANG LIU and YUN HE are with the Department of Materials Engineering, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China. YANXIANG LI is with the Department of Materials Engineering, School of Materials Science and Engineering, Tsinghua University, and also with the Key Laboratory for Advanced Materials Processing Technology (MOE), Beijing, 100084, China. Contact e-mail: [email protected] Manuscript submitted September 2, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

over 5 pct. In addition, the pores were mainly spherical rather than cylindrical. In consideration of the small hydrogen solubility in aluminum, it was thought that the addition of alloying elements would be effective for increasing porosity in lotus aluminum. Park et al.[9] fabricated lotus-type porous Al-Si alloys containing various silicon content levels under a hydrogen pressure of 0.1 MPa. Researches were also done on porous Al-Cu,[10] Al-Ti,[11] and Al-Mg-Si[12] alloys. The more the content of alloying elements, the higher the porosity of porous aluminum alloys. However, due to the mushy zone effect of alloys, the pore morphology becomes more and more irregular with the increase of alloying element content. Besides, by controlling the solidification velocity at as low as 0.5 mm min1, T.IDE et al.[13] had successfully fabricated lotus aluminum with a porosity of 30 pct in a mixture of hydrogen (0.25 MPa) and argon (0.25 MPa).Theoretically speaking, a porosity of 30 pct in lotus aluminum cannot be obtained in such a high pressure only by the initially dissolved hydrogen. This work also showed that the porosity of lotus aluminum would increase as the solidification velocity decreased, contradicting the result