Papers by Author: Jae Ik Cho

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Authors: Chang Yeol Jeong, Chang Seog Kang, Jae Ik Cho
Abstract: A quantitative study of the interactions between microstructural features such as secondary dendrite arm spacing (DAS), eutectic structure and fatigue behavior of two Al-Si-Mg casting alloys with silicon contents of 7% and 10% respectively, has been conducted. In the condition of minimizing casting defects, the influence of microstructural features on the mechanical performance becomes more pronounced. Depending on the magnesium content affecting the strength of the matrix, the tensile properties were changed upon experimental conditions; the tensile strength was increased with magnesium content, whereas the elongation was increased in the reverse case. The increase of both of high cycle fatigue and low cycle fatigue lives with decreasing the secondary dendritic arm spacing is observed, mainly due to homogeneous deformation owing to the fine size of eutectic silicon and Fe intermetallic particles. The fatigue dynamometer of a diesel cylinder head shows the same tendency with the results of specimen fatigue tests with microstructures.
Authors: Hyun Rok Cha, Hyeon Taek Son, Cheol Ho Yun, Jae Ik Cho, Ik Hyun Oh, Jae Seol Lee, Chang Seog Kang, Hyoung Mo Kim
Abstract: Magnetic core components are often made from laminated sheet steel, but they are difficult to manufacture in near net shape, resulting in large core losses at higher frequencies. In this study, the pure iron powder was treated with aqueous phosphoric acid to produce phosphate insulating layer on the surface. After drying the powder, it was mixed with 0.5wt% Zn stearate and compacted in a mold with a diameter of 20mm at 800MPa. The powder compacts were then heat treated at 500°C for 1 hour. The results showed that insulated iron powder was obtained with uniform phosphate layer by chemical reaction. With increased amount of phosphate layer, the core loss and density of compacts were decreased. It was also found that the addition of ethyl alcohol during insulating reaction resulted in improved core loss value.
Authors: Chang Seog Kang, Ik Hyun Oh, Jae Ik Cho, Jae Seol Lee, Cheol Ho Yun, Hyeon Taek Son, H.L. Cha, Jung Chan Bae
Abstract: Anelastic behavior of a 9Al2O3·2B2O3 (AlBw) whisker reinforced aluminum composite has been examined through the measurements of the dynamic Young’s modulus and internal friction over a temperature range of 25 to 500°C at frequencies of 0.01, 0.05 and 0.1 Hz. A standard servo-hydraulic mechanical testing machine equipped with an infrared lamp heater was employed, but the dynamic measurement system therein was especially designed by assembling a scanning laser extensometer and a frequency response analyzer for detecting the amplitude and phase lag of strain in response to a sinusoidal time-varying stress. Two peaks of internal friction were observed over the ranges 100 to 250°C (LT peak) and 250 to 400°C (HT peak), together with marked decreases in the dynamic Young's modulus in the same temperature ranges. From a quantitative analysis of the experimental data, it is concluded that the HT peak phenomenon is due to grain-boundary relaxation, whereas the LT peak phenomenon is ascribable to the relaxation caused by stress-directed interfacial diffusion of Al atoms along the whisker-matrix interface.
Authors: Young H. Ko, Hyeon Taek Son, Jae Ik Cho, Chang Seog Kang, Ik Hyun Oh, Jae Seol Lee, Hyoung Mo Kim, J.C. Kim
Abstract: Although glass wool and polyurethane foam are the most widely used sound-absorber and insulating materials, there are some critical problems related to the environmental issues. In this reason, there is a lot of attention on the aluminum foam as a sound absorption material, therefore, in this research, sound absorption and transmission loss properties were studied. The results showed that low density aluminum foam performed better in both sound absorption and transmission loss and the properties were improved as increased air cavity and space.
Authors: Young H. Ko, Se Hun Chang, Ik Hyun Oh, Jae Ik Cho, Chang Seog Kang
Abstract: Aluminum foam is lightweight structures, energy absorption and thermal management. In this reason, there is a lot of attention on the aluminum foam as a structural material. The present papers showed various conventional joining techniques can be applied for foam-sheet structures, i.e., riveting, screwing, welding, gluing and soldering. This research presents new joining technique of Aluminum foam/Aluminum metal using spark plasma sintering (SPS) process. The Aluminum foam/Aluminum metals were fabricated by changing of various SPS holding temperature and holding time conditions. With increasing holding temperature and holding time, the tensile stress increased. The specimen sintered at 550°C for 20 min shows σts = 1.47 MPa. Also, it was found that the SPS holding time is dominant factor than the holding temperature for sound joining of two joint materials.
Authors: Chang Seog Kang, Ik Hyun Oh, Jae Ik Cho, Jae Seol Lee, Hyeon Taek Son, Jung Chan Bae, Young Chan Kim, Sung Kil Hong
Abstract: The silicon-based aluminum casting alloy and 2 different types of gate designs were used to investigate experimentally the fabrication feasibility of sound thin-wall case casting by diecasting process. While split type gate design had no defect, tangential type gate design resulted in cracks in the all obtained specimens. Electrolytic Oxide (PEO) coating was performed as a surface treatment process. Before the PEO coating treatments some die cast specimens were heat treated at the temperature of 323K and N2 atmosphere for 1hr to evaluate the effect of heat treatment. The result showed that hardness was improved after PEO coating, however, the effect of heat treatment was not significant.
Authors: Ik Hyun Oh, Hyeon Taek Son, Chang Seog Kang, Jae Seol Lee, Jae Ik Cho, Jung Chan Bae, Byong Taek Lee, Ho Yeon Song
Authors: Jae Ik Cho, Xin Yan, Chang Seog Kang, Ik Hyun Oh, Jae Seol Lee, Hyeon Taek Son, Jung Chan Bae
Abstract: In this study, the microsegregation effect of copper during solidification of A356.2 aluminum casting alloy was evaluated by using solidification simulation and a series of experimental works. Electron probe micro analysis (EPMA) and the computational simulation revealed clearly that copper as a solute element segregates during solidification and it becomes more significant as the solidification approaches its completion. This microsegregation effect of copper is due to accumulated copper solute ahead of the eutectic interface during solidification. Consequently, the presence of a higher content of copper in A356.2 aluminum casting alloy causes influence on the solidification characteristics of the alloy and forms copper enriched phases at the last region to solidify in the microstructures.
Authors: Jae Seol Lee, Hyeon Taek Son, Ik Hyun Oh, Jae Ik Cho, Chang Seog Kang, Jung Chan Bae
Abstract: The aims of this study ares to investigate the microstructure evolution of AZ31 Mg alloys with normal rolling and cross rolling during the large strain hot rolling affects microstructure, texture and mechanical properties of AZ31 Mg alloy. In the microstructures of as-rolled both samples, twins are clearly apparent, small and recrystallized grains are visible along some grain boundary and twinned regions. The lankford values of large strain cross rolled sample obviously demostrate the higher average r-value and lower planar anisotropy value. The press formaility of large strain rolled Mg alloy might be improved due to control of texture and grain size by change of the rolling direction.
Authors: Hyeon Taek Son, Ik Hyun Oh, Jae Seol Lee, Chang Seog Kang, Jae Ik Cho, Jung Chan Bae
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