발간논문

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This study is concerned with the analysis of microstructure, hardness, wear resistance, and fracture toughnes of two-layered VC/Ti-6Al-4V surface composites fabricated by high energy electron beam irradiation. The mixture of VC powders and CaF_2 flux was deposited on a Ti-6Al-4V substrate, and then electron beam was irradiated on these powder mixture to fabricate a one-layered surface composite. A two-layered surface composite was also fabricated by irradiating electron beam again onto the powder mixture deposited on the one-layered surface composite. The composite layers of 1.2∼1.5 ㎜. in thickness were homogeneously formed without defects, and contained a large amount (25∼40 vol.%) of (Ti, V)C precipitates in the martensitic or V-rich β-Ti matrix. This microstructural modification including the formation of hard carbides and hardened matrices in the surface composite layers improved hardness and wear resistance. Particularly in the two-layered surface composite containing more carbides, the wear resistance was greatly enhanced by seven times higher than that of the Ti-6Al-4V substrate. In-situ observation of fracture process showed that microcracks were initiated at carbides and propagated along these microcracked carbides, and that in the two-layer surface composite, more microcracks were easily formed at carbides than in the one-layed composite, thereby showing lower fracture toughness.

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Microstructural changes of Al-Si-(X) alloys during semi-solid state processing have been investigated in the present study. The main objective of this research is to investigate the effect of the electromagnetic stirring on the microstructure of Al-Si-(X) alloys. Firstly by continuous electromagnetic strirring during cooling, primary Si particles in hypereutectic Al-Si alloy tend to distribute themselves to both mold wall sides. The high concentration of primary Si at the surface increases the surface hardness and abrasive resistance of the stirred Al-Si alloy. As the rotation velocity of the magnet increases, the trends become remarkable. It has been suggested that each element having different electric conductivity has different electromagnetic forces. Secondly by adding Fe element to the Al-Si alloy, particle having a larger conductivity than Si particle is formed. During continuous electromagnetic stirring of Al-Si-Fe alloys, primary Si particles and Al_9Fe_2Si_2 particles are distributed uniformly, and the particles are fractured and then become round. It occurs due to the lots of collision between the two particles - primary Si and Al_9Fe_2Si_2 particles, since each has different velocity.

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The formation of copper plate crack in thin slab funnel mold was investigated using metallographic studies and mathematical models. X-ray analyses of crack surface revealed that relatively brittle compound was formed in the copper matrix at high temperature where crack initiated. The scale deposition at cooling slot of copper plate below the meniscus was found after casting, which hindered the heat transfer between solidified shell and copper plate. To calculate the heat transfer in the mold at the high casting speed, new heat flux equation was made using measured average heat fluxes at different casting speed. Two dimensional heat transfer and stress analysis were done to investigate the temperature and stress fields associated with cracks. The high temperature during casting, and high tensile stress of copper plate after casting were found in the funnel transition region just below the meniscus, due to geometrical effect and uneven distance between hot face and cooling slot tip of copper plate. The tension bolt was removed to verify the stress concentration relief at funnel transition region and crack was decreased after test.

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Thermomechanically treated Fe_3A1-(0, 2, 4, 6 at%)Cr alloys were oxidized at 1000℃ in air, and the oxide scales were characterized using XRD, SEM, EPMA and TEM. The oxide scales formed were thin, but non-adherent. The oxide scales were primarily composed of submicron-sized α-Al_2O_3 oxide grains, which contained small amounts of dissolved ions of Fe and Cr. Beneath the oxide scale, numerous voids particularly along the grain boundaries of the matrix alloy were observed, indicating that the outward diffusion of Al occurred to form the Al_2O_3 oxide layer.

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The burden flows by modification of rotating chute were studied using 1/16 scaled model of blast furnace. Adoption of repulsion plate on the marginal end of chute turned out to be very effective to control the falling width of ore and coke. Because falling trajectory of burden moved to central part, tilting angle of chute needed to be decreased to maintain same profile with that of conventional chute. Furthermore the mixed zone of ore to coke and piling of large particle size at the furnace center were increased, which was anticipated to show some positive effects for ensuring central gas flow.

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Solvent extraction experiments for the separation of impurities from Ni-rich solution were carried out for manufacturing of high purity Ni compounds from acid leaching solution of spent Ni-Cd secondary battery. Synthesised and actual leaching solution were used as aqueous phases and PC88A saponified by sodium in kerosene were used as organic phases. The extraction order is Zn>Mn>Co>Ni and extraction percentage of metal ions was increased with an increase of the concentration of extractant, initial pH of aqueous phase and ratio of O/A The separation of Ni and other impurities was effectively accomplished at the condition of extraction stage = 1, O/A = 1, saponified percent 50%, initial pH 5.0, concentration of extractant 1.0M.

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The formation of a striation pattern and the variation of kerf width in a thin stainless steel tube were investigated by numerical simulation during a pulsed Nd:YAG laser cutting process. The simulated results were compared with the experimental results, which were performed under the same conditions for simulation. The simulated results showed good agreement with the experimental results. Although the formation of the striation pattern was influenced by various laser parameters, the laser power density had come out the most important factor in the formation of striation patterns. Since a high power density resulted in clear regular striation patterns, relatively low energy density caused the formation of a hot spot, which hindered the formation of regular striation patterns and caused less striation. The kerf width is increased with decreasing the power density and the hot spot is generated in the bottom of tube section in the case of low energy density condition. The numerical simulation calculations can be used to predict the shape of striation patterns and to offer a way to provide a smooth cut wall.

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