발간논문

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A numerical technique has been developed to simulate the combined fluid flow, heat transfer and solidification during filling of castings. A modified SMAC method was applied to analyze fluid flow during mold filling, and a new concept of energy marker has been proposed not only to track the moving free surface of the melt, but also to calculate convective heat transfer during the mold filling process. The present code was applied to simulate solidification sequences and to predict shrinkage cavities in automobile crank shaft castings. The comparison of two types of solidification analyses, simulations with and without consideration of fluid flow during mold filling revealed that the first method is more effective in evaluating the formation of casting defects. The present model which is based on the coupled analysis of fluid flow and heat transfer during mold filling was successfully applied to predict shrinkage cavities in shaped castings.

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This study was conducted on the specific damping capacity(SDC) of rolled Zn-22Al alloy(Superplastic Zinc, SPZ). SDC is found to increase with increasing reduction ratio, because of the microstructure change from network to particle type. SDC is very sensitive to the change of frequency, so that the variation of frequency with reduction ratio was investigated. Frequency is almost constant with reduction ratio. But the frequency is decreased when the specimen is heat treated. SDC of both as-rolled and heat treated sheets are increased largely with increasing temperature. That is, SDC is about 11~14% at room temperature, but 50% at 130℃. This reason is considered as follows. Relaxation time τ subjects to thermal activation process and SDC becomes a maximum(has a peak) when ωτ is equals to 1 in SDC =△0ωτ/ (l +ω2τ2).

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The deformation mechanism of a strip-cast 304 stainless steel has been investigated by carrying out a series of load relaxation and tensile tests at the temperatures ranging from room temperature to 300℃. The experimental results were then analyzed according to the inelastic constitutive theory proposed recently. A thermomechanical treatment was performed before the test to remove the inhomogeneous cast structures providing comparable mechanical properties to those of conventionally processed 304 stainless steel. The inelastic deformation behavior of this steel is found to consist of two simultaneously occurring parallel processes, viz., viscous drag and long range interaction of dislocations. The total flow stress appears, in this regard, to consist of a friction stress to overcome lattice friction and an internal stress to overcome mutual interaction force of glide dislocations. The hardness parameter and the static friction stress were found to decrease as the temperature increased. The scaling parameter in the inelastic constitutive relation was found to be equal to the inverse of the power exponent(M). The value of M was obtained as M=0.17 at room temperature providing a unique scaling relation of flow curves generated by consecutive load relaxation tests. The activation energy for dislocations to overcome lattice friction was measured as 78.5 kJ/㏖ similar to that of carbon diffusion in iron.

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The movement behavior of fine sinter in hearth layer has been known to be influenced by suction pressure, particle size of sinter and charging condition of hearth layer i.e., the ratio of the height of upper hearth layer to that of lower hearth layer. In this study the behavior of fine sinter has been investigated using sintering pot simulator. From our experimental results, especially the charging condition among the factors has been found to have a great influence on the movement behavior of fine sinter. When the suction pressure in sinter bed was increased, the weight loss of fine sinter was increased. The weight loss of fine sinter was also decreased with the height of lower hearth layer. The optimum charging conditions are obtained as follows : (a) When the particle size of sinter in upper hearth layer is over 3 ㎜, the amount of fine sinter remains unchanged during the suction. (b) The weight loss of fine sinter in hearth layer is severe when the height of lower hearth layer is below 10 ㎜.

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Study in contrast between low and medium carbon Al-killed steels has been made in terms of morphology and composition of the oxide and sulfide inclusions resulting from calcium treatment. CaS activity calculated based on Lu`s model in complex sulfide (Ca,Mn)S inclusions in the medium carbon steel is 0.70±0.12, which is good accordance with the values of Fruehan et al. Inclusions in the low carbon steel was characterized as the calcium aluminates as cores which were enclosed with (Ca,Mn)S layer in comparison with those of the medium carbon steel whose cores were alumina alone surrounded by complex sulfide (Ca,Mn)S. Calcium in the medium carbon steel with the higher concentrations of S and Al reacted preferentially with MnS to Al₂O₃. Converting MnS into (Ca,Mn)S inclusions by the calcium treatment in the medium carbon Al-killed steel will be applied to the manufacturing process for the mold steel leading to the simultaneous improvements in free machinability and etching workability.

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A duplex surface treatment process of calorizing process followed by plasma nitriding was design to develop a process which improves wear resistance as well as oxidation resistance at high temperature. The surface properties of the duplex treated mild steel and STD61 steel were characterized and compared with those of both steels treated by single surface process of calorizing or plasma nitriding, in terms of microstructure, microhardness, wear resistance at room temperature and 500℃, and the oxdation behaviours at high temperatures. Duplex treated specimens were calorized at 1,050℃ for 5 hours and then were subsequently plasma nitrided at 500℃ for 5 hours. The pack cementization process on mild steel had created FeAl compound layer of approximately 120 ㎛ thickness on the surface. Much improved surface microhardness above 1200Hv, (0.1 ㎏) was measured after subsequent plasma nitriding and the nitriding depth was of the order of 80 ㎛. Although STD61 steel showed an decrease in microhardness in the Al-diffused layer upon calorizing, the subsequent plasma nitriding increased the hardness up to 1280Hv and the nitriding depth of over 80 ㎛ was obtained. The room temperature wear resistance of the duplex treated steels was slightly better than that of the nitrided one, but there was considerable improvement of the high temperature wear resistance at 500℃ in the duplex treated steels when both wear volume and weight change due to oxidation were considered. In addition the duplex treated steels showed an improved oxidation resistance than the plasma nitrided steels.

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To improve the high temperature oxidation resistance of STD61 steel used as hot dies or cutting tools, CrN films of single phase with the (111) preferred orientation which were deposited on STD61 steel substrates by arc ion plating. The oxidation characteristics of CrN films were studied at the temperatures, ranging from 800 to 950 ℃ in air(70sccm) by employing thermogravimetry. XRD, SEM and EPMA The high temperature oxidation resistance of STD61 steel was improved by the CrN films. The oxidation of CrN films follows nearly parabolic rate laws and the activation energy of oxidation was 206 kJ/㏖ at the temperatures ranging from 800 to 950℃. According to the marker test using Al₂O₃, particles, the conclusion of investigation for the interface migration of oxide was that the growth of oxide(Cr₂O₃) is controlled by external diffusion of cation(Cr ion)

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The directed metal oxidation process has been used for the fabrication of Al-Mg/Al₂O₃ functionally gradient reaction layers. It was found that the reaction of the Al-Mg-Al₂O₃ powder compact at 900℃ under air atmosphere led to the formation of reaction layers with varying ceramic phase contents. Two different types of reaction layers were observed depending on the Mg contents : ceramic/channel/composite or channel/ceramic/channel/composite arrangements from the sample surface. The microstructural development of the reaction layers at various reaction stages was investigated and the mechanism of the characteristic microstructure formation was discussed on the basis of MgO phase formation on the surface of the reaction layer.

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Interfacial reactions in squeeze cast AC2B Al alloy reinforced with Saffil and Carbon fibers were investigated using scanning and transmission electron microscopy, and secondary ion mass spectrometry. In the case of the as-cast composite, MgO crystals were observed in the vicinity of Saffil fibers. It indicates that the reaction occurred between SiO₂, which was used as a binder to fabricate the preform, and Mg present in the Al alloy during squeeze casting. In the case of T6 treated composite, however, very small MgAl₂O₄ crystals as well as MgO were observed at the surface of the Saffil fibers. Based on SEM and TEM study on the T6 treated composite, the interfacial reactions were observed to occur directly between Al alloy and SiO₂ binder to form both MgO and MgAl₂O₄ crystals. However, a direct reaction between the Al alloy and Saffil fiber were not observed to be significant in T6 treated composite.

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The deformation mechanism of a strip-cast 304 stainless steel has been investigated by carrying out a series of load relaxation and tensile tests at the temperatures ranging from room temperature to 300℃. The experimental results were then analyzed according to the inelastic constitutive theory proposed recently. A thermomechanical treatment was performed before the test to remove the inhomogeneous cast structures providing comparable mechanical properties to those of conventionally processed 304 stainless steel. The inelastic deformation behavior of this steel is found to consist of two simultaneously occurring parallel processes, viz., viscous drag and long range interaction of dislocations. The total flow stress appears, in this regard, to consist of a friction stress to overcome lattice friction and an internal stress to overcome mutual interaction farce of glide dislocations. The hardness parameter and the static friction stress were found to decrease as the temperature increased. The scaling parameter in the inelastic constitutive relation was found to be equal to the inverse of the power exponent(M). The value of M was obtained as M=0.17 at room temperature providing a unique scaling relation of flow curves generated by consecutive load relaxation tests. The activation energy for dislocations to overcome lattice friction was measured as 78.5 kJ/㏖ similar to that of carbon diffusion in iron.

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