Fabrication of lotus-type porous carbon steel by the continuous zone melting method and its mechanical properties
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I. INTRODUCTION
POROUS metals have lately attracted much attention because of their light weight, huge surface area, good sound absorption, high damping, etc.[1] Various fabrication methods of the porous and foamed metals and alloys have been developed; a foaming method with gas bubbling, an electroplating method for metal deposition in polyurethane form, a powder-sintering metallurgy method, etc.[1,2,3] Usually, the pores in such porous metals are spherical and randomly distributed. On the other hand, attempts have been made to fabricate porous metals with elongated gas pores through unidirectional solidification of metals in a gas atmosphere.[4–10] Among these previous investigations, most recently, Boiko et al.[7] developed the so-called GASAR method, adopting the casting method. By these methods, one can produce porous metals and alloys with high thermal conductivity and which possess an almost uniform pore size and porosity throughout the ingot. However, there is a shortcoming that the pore size is coarsened in the part farther from the chill plate in the porous metals and alloys with a low thermal conductivity; therefore, a uniform pore size and porosity cannot be obtained. In order to overcome such a shortcoming, a new method, the continuous zone melting method[11] has been applied to fabricate a lotus-type porous stainless steel (SUS304L) with a uniform pore size and porosity distribution.[12] In this method, the solidification velocity can be controlled by changing the transference of the specimen rod, regardless of the magnitude of thermal conductivity. The porosity and pore size are controllable when the solidification velocity is kept constant during solidification. The porous metals possess long cylindrical pores oriented in one direction. Hereafter, we designate these porous metals with elonTOSHIHIKO KUJIME, Doctoral Student, Graduate School of Engineering, and SOONG-KEUN HYUN, Assistant Professor, and HIDEO NAKAJIMA, Professor, Institute of Scientific and Industrial Research, are with Osaka University, Osaka 567-0047, Japan. Contact e-mail: kujime23@ sanken.osaka-u.ac.jp Manuscript submitted April 10, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
gated pores as lotus-type porous metals, because their shape looks like lotus roots. Carbon steel as well as the stainless steel is also a practically important material in various applications. If lotus-type porous carbon steel (hereafter, it is shortened to lotus steel) can be fabricated, it is expected to be used as lightweight structural-machinery parts and devices. In the present work, lotus steel was fabricated by the continuous zone melting method, and the gas-pressure dependence of porosity and pore size was investigated in order to clarify the mechanical properties of lotus steel. II. EXPERIMENTAL PROCEDURES The apparatus for fabrication of lotus steel consists of a high-frequency induction heating coil, a pair of upper and lower specimen holders which can be electrically transferred in a vertical direction, and blowers. These are installed in
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