Papers by Author: Do Hyang Kim

Abstract: The heat-treatable, precipitation-hardening, aluminum alloys are of special interest for automobile where high strength and dent resistance are required, and for bumpers, where good strength and shock absorption are needed. In both cases, good formability is also an important requirement. The heat-treatable 6000-series Al-Mg-Si alloys have been the material of choice for automobile. Despite the high strengthening potential of Al-Mg-Si alloys, processing factors such as the artificial aging processes, as well as the duration of artificial aging, can seriously impede the strengthening process. The highest tensile strength of A6082 alloy was obtained when Mg/Si atomic ratio is set to 1.1 due to the biggest volume fraction of intermetallic compound containing Mg and Si. It was to determine how aging process affected the microstructure and mechanical properties of the A6082 alloy as different ratio between Mg and Si.

Abstract: Viscous flow behaviour of supercooled liquids and mechanical properties in Zr55+XCu30-XNi5Al10 (X=0, 5 and 10 at. %) and Zr65Cu18Ni7Al10 bulk metallic glasses (BMGs) have been examined. Viscosity has been measured by using a penetration viscometer at a high-speed heating rate of 400 K/min. With increasing Zr-content, Tg tended to decrease and Tx tended to increase, resulting Tx (=Tx-Tg) increaseing up to about 170 K under a heating rate of 400 K/min by the DSC analysis. The viscosity lowered about one order of magnitude, e.g., minimum value of the viscosity decreased from 3~4x106 Pa-s for the Zr55Cu30Ni5Al10 and Zr60Cu25Ni5Al10 BMGs down to 5~6x105 Pa-s for the Zr65Cu20Ni5Al10 and Zr65Cu18Ni7Al10 BMGs under the same heating condition. Compression tests were also carried out at a slow strain rate of about 1 x 10-4 s-1 at room temperature. With increasing Zr-content, the apparent yield stress was decreased but the total strain at fracture was increased significantly, suggesting that there is a strong relationship between viscosity and the mechanical properties in these BMGs.

Abstract: Rotating components used in the hot sections of land-based gas turbine are exposed to severe environment of several ten thousands operating hours above 1100
. To protect such components against high temperature oxidation an intermediate bond coat is applied, typical of a MCrAlY-type metal alloy. Various processing methods have been studied for bond coat deposition. This study is concerned with the cyclic oxidation behavior of CoNiCrAlY coatings. Coatings were deposited by a vacuum plasma spray and high-velocity oxygen fuel method on a nickel-based superalloy (GTD-111). Cyclic thermal oxidation test condition is at 1100
in ambient air for various periods of time. Tests were used to evaluate the oxidation resistance of the spray-coated specimens. The microstructure and morphology of as-sprayed and of tested specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The oxide phases formed on the coating surface are NiO, CoCr2O4, and Al2O3. The nickel oxide of them was to be dominant with increasing cycles. The differences in microstructure and phase composition in the interface with coating layer are reported. The influence of coating process methods on coating characteristics and degradation mechanisms is discussed. The HVOF coating with the splats was more resistant on the high temperature oxidation than the VPS coating.

Abstract: The rotating components in the hot sections of land-based gas turbine are exposed to severe environment during several tens thousand hours at above 1100則 operating temperature. To protect such components from high temperature oxidation, an intermediate bond coat is applied, typical of a MCrAlY-type metal alloy. Various processing methods have been studied for bond coat deposition. This study is concerned with the thermal cyclic behavior of thermal barrier coatings. The MCrAlY bond coatings are deposited by a vacuum plasma, low vacuum plasma and highvelocity oxygen fuel spray on a nickel-based superalloy (GTD-111). Thermal cyclic tests at 1100則 in still air for various periods of time were used to evaluate the thermal cyclic resistance of the TBC coating with the various processing sprayed bond coating layer. The microstructure and morphology of as-sprayed and of thermal cycled coatings were characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The order of thermal cyclic resistance was YSZ-VPS bond coat, YSZ-LVPS and YSZHVOF. The influence of bond coat spray methods on durability of TBCs is discussed.

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.

Abstract: The effects of reheating in solid-liquid region on the microstructure of electromagnetically stirred Al alloy have been investigated. The billet of Al alloy was produced at a various casting speed from 200 to 500 mm/min in a continuous casting machine with an electromagnetic stirring device. The microstructure of the billet, which was isothermally reheated in a solid-liquid region during holding for from 0.5 to 15 min, was examined. The roundness and size of primary α phase of electromagnetic stirred Al alloy was measured according to the reheating time. The roundness of primary α phase was obtained a minimum during holding for from 3 to 7 min at a holding temperature of 584°C and was deteriorated at a longer reheating time due to a dominant coalescence. Also the spheroidization of primary α phase during reheating in solid-liquid region was significantly dependent on initial microstructure of the billet.