Papers by Author: Ki Bae Kim

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Authors: Dock Young Lee, Suk Won Kang, Ki Bae Kim
Abstract: The mechanical properties of electromagnetically stirred billet of Al alloy during continuous casting such as a tensile, impact, and fatigue property was examined with respect to a size and roundness of primary α phase to examine the influence of the globularization and refining of primary α phase on the dynamic mechanical property. The billet was continuously cast in a casting speed of 100 to 600 mm/min during the electromagnetic stirring with a magnetic flux density of 700 Gauss and then was heat-treated. All tensile properties were enhanced with decreasing the size of primary α crystal and the ultimate tensile strength (UTS), yield strength, and elongation was obtained 320MPa, 235MPa, and 17.8%, respectively, at a size of primary α phase of 77 um. The fatigue life at fracture was largely improved by 37% from 1.25×105 to 1.7×105 cycles and the fracture toughness was obtained approximately 7.25 joules as a maximum value at a minimum size of primary α phase on the present experiment condition. Therefore, it indicates that all mechanical properties can be improved at the higher casting speed owing to the fine microstructure of a primary α phase and also eutectic phase due to the higher cooling rate.
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Authors: Dock Young Lee, Suk Won Kang, Ki Bae Kim
Abstract: Recently, a rheocasting process has been interested to produce a structural part for an automobile industry and so the slurry-on-demand process to make the semi-solid slurry having a fine and globular microstructure has been very important to produce a high quality and cost effective part in the rheocasting process. An electromagnetic (EM) stirrer employing for a slurry making process was designed and prepared to induce simultaneously both a circumferential and vertical fluid flow of a melt in order to control a rotation angle of EM stirring of a poured melt. In the present study, the semi-solid slurry of Al-15%Cu alloy was produced in the EM stirrer and its microstructure and the orientation distribution function calculated from the pole figure data was examined in accordance with a various rotation angle of EM stirring between 0˚ (circumferential flow) and 90˚ (vertical flow). The size and morphology of primary α phase was affected with a rotation angle of EM stirring and the finest and the most globular primary α phase could be obtained at a rotation angle of EM stirring of 45˚ and 60˚. Also, due to the EM stirring the tendency to random orientation was appeared. Also, the tendency of random orientation was the most at a rotation angle of 45˚ and 60˚. Therefore, it was considered that the rotation angle of EM stirring of 45˚ and 60˚ was the most effective to induce the non-dendritic growth of primary solid phase of EM stirred Al-Cu alloy.
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Authors: Jung Pyung Choi, Ki Bae Kim, Eui Pak Yoon
Abstract: In general, the element (Sr, Na, Sb, etc.) is used as a modifier of eutectic Si phase in the hypoeutectic Al-Si system. When these elements are added into the hypoeutectic Al-Si melt, the flake shaped Si phase transforms to fibrous shape and the size of Si phase is also decreased. In this study, the electromagnetic vibration is adopted for modifying eutectic Si phase and reducing its size. The higher the current density and frequency of electromagnetic vibration(EMV), the finer the size of eutectic Si phase. The tensile strength and elongation of EMVed alloy were highly improved. Measured twin probability of EMVed alloy at a frequency of 1000Hz was approximately six times as high as that of the normal alloy and a half of that of Sr modified alloy. The mechanism for the increase in twin density due to EMV during solidification could be supposed from the fact that the preferential growth along <112> in silicon was suppressed by preventing Si atom from attaching to the growing interface of Si phase and by changing the solid/liquid interfacial energy of silicon.
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Authors: Dock Young Lee, Suk Won Kang, Ki Bae Kim
Abstract: In this study, in order to develop the cast product of Al alloy having a globular microstructure by using the elctromagnetic (EM) stirrer, which was specially designed and manufactured to induce a various fluid flow type of melt during solidification, a morphlogy and size of primary solid phase of the solidifying slurry was investigated with respect to EM stirring condition such as an induced magnetic flux density (MFD) and a frequency of input current. The magnetic flux density of EM stirrer was measured by using a gaussmeter and its distribution and magnetic force within Al melt was simulated in ANSYS program. The induced MFD was increased with decreasing a frequency of input current at the same input voltage due to the increased penetrating depth of magnetic field. But, the magnetic force related directly with a stirring strength of melt was increased with the frequency. Both a roundness and size of primary α phase of Al alloy was decreased with increasing a frequency of input current and MFD within the experimental range. Therefore, the primary α phase was refined and globularized at the higher frequency and MFD.
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Authors: Dock Young Lee, Jun Hyun Han, Suk Won Kang, Jung-Hwa Mun, Ki Bae Kim
Abstract: In this study, the orientation distribution function (ODF), which can be calculated from the pole figure data, was examined and tried in order to characterize the bulky morphology of primary solid phase of semi-solid slurry instead of erroneous 2D observation for the characterization of microstructure. Al-15wt%Cu alloys were electromagnetically stirred during the continuous cooling in the specially designed electromagnetic (EM) stirrer, which generates a rotating magnetic field, and the samples were prepared by interrupt-quenching semi-solid slurry at a solid-liquid region. Owing to EM stirring the temperature distribution of melt inside a crucible became uniformed compared with the case of unstirred melt and the normal dendritic structure was transformed to one consisting of the spherical and rosette shaped primary solid phase due to the fragmentation of the dendrites. Also by the effect of EM stirring the <110>//ND texture were still strongly developed, but the advancement of the orientations to lead <110>//ND texture became weaker, and the orientations to lead <100>//ND and <111>//ND texture were more strongly advanced in comparison with the unstirred case. Therefore the advancement of the texture became weak and so the tendency to random orientation was appeared due to the EM stirring. Therefore it was thought that the extent of random orientation could be valued for the characterization of bulky morphology of primary solid phase of semi-solid slurry.
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Authors: Suk Won Kang, Dock Young Lee, Ki Bae Kim
Abstract: In order to produce a high quality and cost effective part in the rheocasting process for an automobile industry it has been important to develop a slurry-on-demand process, which can manufacture the semi-solid slurry having a fine and globular structure of primary solid phase. The morphology of primary solid phase of semi-solid slurry is coarsened and globularized during an isothermal holding process in a solid-liquid region. Accordingly, in this paper, the microstructural evolution of semi-solid slurry during the isothermal holding at a various temperature in solid-liquid region was investigated to examine a coarsening and globularization behavior of primary solid phase. The semi-solid slurry of Al alloy was produced in a slurry maker using a multiplex-type slurry cup that we developed recently. A size of primary solid phase was the finer at the higher holding temperature, but under a constant holding time a roundness degree of primary solid phase was the lower at the lower holding temperature. Also, a coarsening rate of primary solid phase was not considered to be affected significantly by a holding temperature even though a little lower coarsening rate was obtained at the higher holding temperature.
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Authors: Suk Won Kang, Dock Young Lee, Ki Bae Kim
Abstract: Semi-solid slurry for a rheocasting process requires a fine and globular structure of primary solid phase. In the present study, in order to find an optimum preheating temperature of a multiplex-type slurry cup in the slurry maker that we recently developed for the rheocasting process, the semi-solid slurry was held for a various time of up to 60 min. in a multiplex-type slurry cup preheated at a different temperature. A size and morphology of primary solid phase of the semisolid slurry was measured and analyzed with the difference of the recorded cooling curve during the solidification with respect to a preheating temperature of the slurry cup. Also, a microstructural evolution and globularization mechanism occurred during the isothermal holding were discussed. Finally, as considering the effect of a different casting condition on the morphology of primary solid phase the optimum manufacturing condition of the semi-solid slurry for the rheocasting process was proposed.
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Authors: Kwang Dong Kim, Ki Bae Kim, Yu Chan Kim, Dock Young Lee, Do Hyang Kim
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Authors: Dock Young Lee, Ki Bae Kim, Do Hyang Kim
Abstract: Forming of alloy and composite within a solid-liquid region, i.e. thixo-forming and rheo-forming, has been recognized as a technology offering several potential advantages over casting and forging such as a low forming pressure, reduction of macrosegregation, and reduction of porosity. In this paper, a globularization for primary solid phase by a forced fluid flow in melt due to electromagnetic stirring during solidification and isothermal stirring at solid-liquid region, was examined to produce Al alloy having a globular solid phase to get a thixotropic behavior. The electromagnetic stirrer was specially designed and manufactured to create all kind of fluid flow pattern such as a circumferential, vertical, helicoidally and contra-rotating flow pattern during the solidification of a melt. The size and roundness of primary solid phase of the quenched sample after isothermal stirring at solid-liquid region were measured with respect to a stirring time and frequency. The globularization of solid phase was enhanced with incresing the stirring time and electromagnetic flux density, i.e. stirring strength.
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