Papers by Author: Tae Kwon Ha

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Authors: Hwan Jin Sung, Tae Kwon Ha, Sang Ho Ahn, Young Won Chang
Abstract: In the present study, powder injection molding (PIM) process, in which the porosity of sintered parts can be easily controlled, has been employed to produce specimens with the various porosities from the initial stage of sintering and to the stage of nearly full density. A series of tensile tests has been conducted on these specimens at room temperature to elucidate the effect of a wide range of porosity. The material used in this study was 17-4 PH stainless steel, which is the precipitation hardenable stainless steel containing 4% of Cu and well known to show high strength and the high corrosion resistance at the same time. The 17-4 PH stainless steel powders used in this study were produced by the high-pressure water atomization method. Based on the results of tensile tests on the specimens with the various porosity, a new approach to predict the elongation of sintered materials has been carried out and a new framework combining neck growth model and ideal pore model has been established.
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Authors: Hwan Jin Sung, Tae Kwon Ha, Sang Ho Ahn, Young Won Chang
Abstract: In the present study, PIM process has been employed to produce sintered specimens with the various porosities from the initial stage of sintering and to the stage of nearly full density. A series of tensile tests has been conducted on these specimens at room temperature to elucidate the effect of a wide range of porosity. The material used in this study was 17-4 PH stainless steel. Based on the results of tensile tests on PIMed specimens with the various porosity, a new approach to predict the strength and the elongation of sintered materials has been carried out and a new framework combining neck growth model and ideal pore model has been established. By applying this new model to not only the results obtained in this study but also to previously published data, its generality and validity were sufficiently verified.
785
Authors: Hong Chul Shin, Tae Kwon Ha, Woo Jin Park, Young Won Chang
667
Authors: Tae Kwon Ha, Kyu Dong Lee, Jin Hwa Song, Hyo Tae Jeong
Abstract: Strengthening method for the Fe-36Ni based Invar alloy for power transmission wire was investigated in this study. High strength of 1300 MPa could be obtained in this alloy through solution hardening, precipitation hardening and strain hardening by cold working. Phase equilibrium of the Invar alloy was calculated using FactSage®, revealing that thermodynamically stable phases are Mo2C, MoC, M23C6-type FeCrMo carbide, and M6C-type FeMo carbides. Aging treatments were carried out at temperatures ranging from 400 to 900oC for time intervals from 3 min to 30 hrs. Peak aging condition was obtained as 400oC and 1 hr. With temperature increased, peak hardness was decreased abruptly. Microstructure observation was conducted by optical microscopy, scanning electron microscopy, and transmission electron microscopy. By using the result from aging treatment, high strength above 1300MPa was obtained in the cold rolled Invar alloy plate.
109
Authors: Tae Kwon Ha, Jae Young Jung
Abstract: Ti-45.5Al-2Cr-4Nb-0.4B alloy was cast by vacuum arc melting at high purity Ar atmosphere using high purity sponge Ti, granular Al (99.99%), flake Nb (99.9%), lump Cr (99.9%) and TiB2 (99.5%) and subsequently heat-treated to obtain a couple of microstructures, i.e. lamellar and near γ. The heat treatment consisted of annealing at a high temperature (1200 ~ 1330oC) of different phase fields for 24 hrs and stabilizing at 900oC for 4 hrs followed by air cooling. Fracture toughness was measured on the specimens with different microstructures at room temperature. The value of KQ of specimen with fully lamella structure was obtained as 18.68 MPa √m, much higher than that of specimen with near γ structure (11.84 MPa √m). It was also revealed that the KQ value was decreased as the annealing temperature decreased.
189
Authors: Tae Kwon Ha, Hwan Jin Sung, Woo Jin Park, Sang Ho Ahn
Abstract: The effect of warm rolling under various conditions on the microstructure and mechanical property was investigated using an AZ31 Mg alloy sheet. Several processing parameters such as initial thickness, thickness reduction by a single pass rolling, rolling temperature, roll speed, and roll temperature were varied to elicit an optimum condition for the warm rolling process of AZ31 Mg alloy. Microstructure and mechanical properties were measured for specimens subjected to rolling experiments of various conditions. Warm rolling of 30% thickness reduction per pass was possible without any side-crack at temperatures as low as 200oC under the roll speed of 30 m/min. The initial microstructure before rolling was the mixed one consisting of partially recrystallized and cast structures. Grain refinement was found to occur actively during the warm rolling, producing a very fine grain size of 7 µm after 50% reduction in single pass rolling at 200oC. Yield strength of 204MPa, tensile strength of 330MPa and uniform elongation of 32% have been obtained in warm rolled sheets.
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Authors: Tae Kwon Ha, Hyo Tae Jeong, Young Won Chang
Abstract: Texture evolution and superplastic deformation behavior of a quasi-single phase Zn-0.3wt%Al have been investigated. It was attempted to produce a stable and fine-grained microstructure in a dilute Zn-Al alloy through a proper thermomechanical treatment process (TMTP). The grain size of about 1 µm was obtained in the Zn-0.3 wt.% Al alloy and a relatively coarse grain size of 10 µm was also obtained through a subsequent aging treatment. The fine-grained material showed typical rolling texture with basal poles tilted about 30 degrees away from the ND toward RD, while the coarse-grained material showed a typical recrystallization texture with basal poles parallel to ND. A series of load relaxation and tensile tests were conducted at room temperature. According to the internal variable theory of structural superplasticity, the grain boundary characters of fine and coarse-grained materials were different from each other. A large elongation of about 1400% was obtained in fine-grained material at room temperature.
889
Authors: Tae Kwon Ha, Yong Nam Kwon, Young Won Chang
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