Fabrication of lotus-type porous stainless steel by continuous zone melting technique and mechanical property
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I. INTRODUCTION
POROUS and foamed metals and alloys have recently attracted much attention since they exhibit some superiour physical properties for application to lightweight materials, damping materials, and so on. So far, 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 deposit in polyurethane form, a powder sintering metallurgy method, and so on.[1,2,3] Usually, the pores in such porous metals are spherical and randomly distributed. On the other hand, several investigators[4–10] have studied the formation of elongated gas pores during solidification of metals and alloys. Recently, Nakajima and co-workers[10–21] have investigated the fabrication of various porous metals and alloys with elongated pores under pressurized hydrogen, nitrogen, or oxygen by a unidirectional solidification method. They designate the porous metals lotus-type porous metals in order to distinguish the porous metals with elongated directional pores from sintered and foamed metals, because they look like lotus roots.* So far, lotus-type *Shapovalov et al. named GASAR, which is a Ukrainian acronym for gas-reinforced composite metals (For example, Ref. 22). However, it turns out from our systematic investigation that gas pores do not reinforce porous metals. Therefore, a term based on their shape is considered to be more suitable for the name of the porous metals.
porous metals such as copper,[17] iron,[19] nickel,[23] and magnesium[24] have been fabricated. It has been found that the lotus-type porous metals exhibit superior mechanical properties,[17,19,21,25] internal friction,[26] and sound absorption characteristics.[27] Thus, industrial applications are expected to expand to various fields. T. IKEDA, Research Associate, and H. NAKAJIMA, Professor, are with The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: nakajima@sanken. osaka-u.ac.jp T. AOKI, Research Graduate Student, is now with Toyota Industries Corporation, Kariya, Aichi 448-8671, Japan. Manuscript submitted May 5, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Recently, the present authors[28] fabricated lotus-type porous stainless steel, which is a practically important material in various applications, by unidirectional solidification under hydrogen atmosphere using the casting technique. However, there is a shortcoming that the pore size is coarsened in the part farther from the chill plate in the porous stainless steel ingots. In order to overcome such a shortcoming, in the present study, a new technique, the continuous zone melting technique,[29] has been applied to fabricate lotus-type porous stainless steel (SUS304L) with uniform pore size and porosity distribution. The tensile tests on porous stainless steel thus fabricated have also been performed. II. EXPERIMENTAL PROCEDURE The chamber of the apparatus for fabricating the lotustype porous metal rods can be evacuated to a pressure less t
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