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Computer simulation of ceramic sintering processes
Y.T.Keum,K.H.Auh 한양대학교 세라믹연구소 2002 Journal of Ceramic Processing Research Vol.3 No.3
In this research, to see the effects of temperature on the grain growth during sintering in the spark plasma sintering process, the grain growth associated with sintering temperatures are simulated by the Monte Carlo Method (MCM) and the Finite Element Method (FEM) for the ceramic material, Al2O3.The spark plasma sintering process produces ceramics that are sintered at high temperatures with high energy and cooled rapidly in a short time. The grain growth at the center of the sintered body is different from that in the boundary between the carbon die and sintered body. This phenomenon is explained by the fact that the grain growth during the sintering is mainly affected by heat conduction in the sintered body. For verifying the dependency on heat conduction, the temperature distribution in the sintered body was first found. Based on the temperature distribution, the grain growth in Al2O3 in the interior and at the exterior interface with the die at sintering temperatures of 1450ºC and 1650ºC are simulated and compared with those observed in SEM micrographs. The grain growth results numerically simulated by MCM seem to agree well with those experimentally observed by SEM.
An efficient numerical method for incorporating phase changes in ceramic drying process
Y.T.Keum,K.H.Auh 한양대학교 세라믹연구소 2002 Journal of Ceramic Processing Research Vol.3 No.3
Crystalline anorthite (CaO·Al2O3·2SiO2) powder was synthesized below 1000oC by a solution process employing a polymer carrier. Polyvinyl alcohol (PVA), having different molecular weights, was used as an organic carrier for preparation of the precursor ceramic gel. The PVA content, its degree of polymerization and type of silica sol had a significant influence on the specific surface area and morphology of the powder, and its crystallization behavior. The polymer content and its molecular length had an effect on the cation distribution in the solution and resulted in different crystallization behavior. Abnormally, omisteinbergite (CaO·Al2O3·2SiO2), which has the same composition as anorthite, was observed prior to crystallization of anorthite. A more porous morphology and higher specific surface area were obtained with a higher content of the PVA polymer. The milled, amorphous-type anorthite powder was densified to a relative density of 94% below 1000oC.
Springback of FCC sheet in warm forming
Y.T.Keum,B.Y.Han 한양대학교 세라믹연구소 2002 Journal of Ceramic Processing Research Vol.3 No.3
These days, automobile industry tends to use aluminum alloy sheets to reduce weight of auto-body. However, aluminum alloy sheets have the drawback that they have tremendous springback in press forming operations. Springback deteriorates accuracy of the product as well as causes difficulties in assembling the parts such that technical plans for springback reduction before pressing have to be prepared. In this paper, the warm forming method, which changes material properties, is proposed to decrease springback. In order to suggest the warm forming method, first, the material properties of aluminum alloy sheets, AL1050 and AL5052, in high temperatures such as yield stress, plastic strain ratio, Young's modulus, true stress-strain curve, etc. are obtained from tensile tests. Next, the bending and draw bending tests in various forming temperatures are performed to measure the springback in various forming temperatures. Two kinds of experiment show that the springback tremendously reduces in warm forming, specially in the forming temperatures of above 150oC.
Anisotropy at high temperatures
Y.T.Keum,B.Y.Ghoo' 한양대학교 세라믹연구소 2002 Journal of Ceramic Processing Research Vol.3 No.3
In this study, special attention is paid to the definition of planar anisotropy of non-ferrous sheets at high temperatures. Barlats strain potential is employed for depicting the yield behavior. A smoothing function with a third order polynomial is introduced for non-isothermal Barlats anisotropy coefficients. The predicted Barlats coefficients are compared with experimentally obtained coefficients. As a crucial result, the smoothing function provides a big advantage in evaluating non-isothermal Barlats anisotropy coefficients: there is no necessity to carry out tensile tests in high temperatures.