Papers by Author: Soong Keun Hyun

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Authors: Takuya Tsumura, Taichi Murakami, Soong Keun Hyun, Hideo Nakajima, Kazuhiro Nakata
Abstract: Effects of pore directions on the profile of fusion zone for lotus-type porous magnesium by laser welding has been investigated by comparing the experimental observations and the results of numerical simulation. The three-dimensional finite element calculations were performed, which takes into account equivalent thermal properties and anisotropy of thermal conductivity. There is the pore anisotropy in the profile of fusion zone by laser beam irradiation and the good weldability was obtained when the growth direction of the original pore equaled to the direction of the laser beam irradiation. A good agreement was obtained between the calculated profile of the weld fusion zone and the experimental results.
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Authors: Hironori Seki, S. Yamazaki, M. Otsuka, Masakazu Tane, Soong Keun Hyun, Hideo Nakajima
Abstract: Tension and compression fatigue property was investigated for lotus-type porous copper possessing cylindrical pores aligned in one direction. The cyclic stress was applied in the direction parallel and perpendicular to the longitudinal axis of the pores. It was found that the fatigue strength at finite life of lotus-type porous copper is lower than that of nonporous copper, and the strength in the direction parallel to the longitudinal axis of the pores is higher than that in the perpendicular direction.
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Authors: Masakazu Tane, Soong Keun Hyun, Hideo Nakajima
Abstract: We studied the effective electrical conductivity of lotus-type porous nickel with cylindrical elongated pores by applying the extended effective-mean-field (extended EMF) theory that is based on Hatta-Taya’s theory and Bruggeman’s effective-medium approximation. We modeled lotus-type porous nickel as the composite materials with ellipsoidal void inclusion, and examined the effect of shape and orientation of the pores and applied-electric-field direction on the effective electrical conductivity. The extended EMF theory revealed that well-known Archie’s power law is quite useful for expressing the porosity dependence of the electrical conductivity of lotus-type porous metals even though the deviation of porous morphology exists.
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Authors: Hideo Nakajima, Soong Keun Hyun, Masakazu Tane, T. Nakahata
Abstract: Lotus-type porous materials whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification from the melt dissolving gas. The pores were evolved by supersaturation of gas atoms when the liquid was solidified. Although such porous metals with high thermal conductivity were produced by casting process, the process could not produce porous metals with low thermal conductivity, which possess uniform pore size and porosity. In order to obtain uniform pore size and porosity in metals with low thermal conductivity, we invented a new “continuous zone melting technique”. Using this technique various metals, alloys, intermetallic compounds and semiconductors were fabricated. Mechanical properties of tensile strength on lotus-type porous metals are described; lotus-type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength. The lotus materials can be applied to various functional materials. Some examples are also shown in this paper.
295
Authors: Soong Keun Hyun, Teruyuki Ikeda, Masakazu Tane, Hideo Nakajima
Abstract: Lotus-type porous iron and stainless steel (SUS304L) whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized hydrogen or nitrogen gas atmosphere. Pores are formed as a result of precipitation from the supersaturated gases when the liquid metal dissolved with gas atoms is solidified. The ultimate tensile and yield strengths of the porous iron produced in nitrogen atmosphere are about two times higher than in a hydrogen atmosphere. Such superior strength is attributed to solid-solution hardening due to solute nitrogen atoms in iron matrix.
337
Authors: Soong Keun Hyun, Tsuyoshi Awadu, Teruyuki Ikeda, Hideo Nakajima
Abstract: Lotus-type porous Cu-5at.%Al alloy whose elongated pores are aligned in one direction was fabricated by unidirectional solidification in pressurized hydrogen gas atmosphere. The porosity of the Cu-Al alloy was higher than that of pure copper under the same fabrication conditions because of difference in hydrogen solubility. The pore structure was not round shape because the directional pore growth was interrupted with dendrite arms formed during the solidification; the pores grew to detour the obstacle of the dendrite arms.
1898
Authors: Hideo Nakajima, Soong Keun Hyun, J.S. Park, Masakazu Tane
Abstract: Lotus-type porous metals with low thermal conductivity are fabricated by continuous zone melting technique, which possess directional elongated pores. The porous metals have been able to be fabricated through the conventional casting method by utilizing the solubility gap between solid and liquid in pressurized gas atmosphere. However, there is a shortcoming that the pores are coarsened in the part farther from the chill plate in the ingot. In order to overcome such a shortcoming, we developed the continuous zone melting technique and successfully produced the lotus-type porous metals with even low thermal conductivity such as stainless steel and superalloys. Furthermore, from the viewpoint of mass production with low cost, we invented novel ”continuous casting technique”. The molten metals dissolving gas are solidified continuously by passing through the mold cooled with chiller and thus, lotus-type porous metal plate as long as one meter was produced for short time. Sufficient uniformity of the porosity and pore size was obtained in such long porous ingots. This technique is prospective method for commercial mass production.
187
Authors: Soong Keun Hyun, Teruyuki Ikeda, Hideo Nakajima
Abstract: Lotus-type porous Ni- (15, 28 and 31) at.% Al alloys whose long cylindrical pores are aligned in one direction were fabricated by continuous zone melting technique under high-pressure gas of hydrogen of 2.5 MPa. A part of 5-10 mm in length of the rod in the vicinity of the coil was melted by high frequency induction heating, and was moved downwards by electric motors at a constant velocity of 330 μms-1 to 500 μms-1 for unidirectional solidification. The pores are formed as a result of precipitation from the supersaturated hydrogen gas when the liquid metals dissolved with gas atoms is solidified. The porosity and the pore size decrease with increasing aluminum content. An increase of solidification velocity from 330 μms-1 to 500 μms-1 leads to a decrease of pore diameter and an increase of pore number in the porous Ni-28at%Al.
323
Authors: Toshihiko Kujime, Soong Keun Hyun, Hideo Nakajima
Abstract: Lotus-type porous carbon steel with cylindrical pores was fabricated by a continuous zone melting method under pressurized mixture gases of hydrogen and helium. The microstructure of lotus-type porous steel was analyzed, and the tensile test was carried out. In the case of tensile test of the specimen with the cylindrical pores parallel to the solidification direction, the ultimate tensile strength was lower than the estimated value, but the yield strength was higher than the estimated value. It was assumed that the strength was decreased in proportion to decreasing cross section area of the specimen. The increasing of yield strength was attributed to precipitation strengthening. The influence of heat treatment (Quenching and tempering, normalizing) on the tensile properties was investigated.
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Authors: Kelly Alvarez, Soong Keun Hyun, Hideo Nakajima
Abstract: Lotus-type porous Fe-25wt.%Cr and Fe-23wt.%Cr-2wt.%Mo alloys were fabricated by continuous zone melting technique in pressurized hydrogen gas. After applying a high temperature nitriding treatment, the fabricated Lotus-type porous nickel-free stainless steel absorbed larger amount of nitrogen compared with non-porous alloy of the same composition since the surface area exposed to the gas is larger in the porous samples. In the Lotus-type porous Fe-25wt.%Cr and Fe-23wt.%Cr-2wt.%Mo alloys the nitrogen concentration after the nitriding achieved was approximately 1.2 wt.%. Only austenite peaks were detected in the profile of both Fe-Cr-N alloys after the nitriding treatment. Neither CrN nor Cr2N were identified by XRD in any specimen after the nitriding.
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