발간논문

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This paper describes the empirical rule of strain rate modified linear accumulation of creep damage(SRM rule) for low-cycle fatigue(LCF) life prediction of Waspaloy in the creep-fatigue interaction regime and Chaboche type unified viscoplastic model predicting the stress-strain response in various cyclic loading conditions. The comparison of the experimental data and the predictions for strain controlled LCF tests carried out for various strain ranges at 600℃ and 650℃ was made. Chaboche type unified viscoplastic model described efficiently the inelastic deformation behavior during LCF tests. Crack-initiation lifing method to predict the material life was investigated with strain rate modification(SRM) rule. The application of SRM rule to LCF tests on Waspaloy indicated a good agreement between measured and predicted cycles to failure.

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In this study, effects of under bump metallurgy (UBM) structure and Cu content in solders on redeposition rate of Au-containing ternary intermetallics at solder/UBM interface were investigated. A UBM structure with Ni diffusion barrier, Au/Ni/Cu, appeared to promote the redeposition during solid state aging treatment, leading to an Au-embrittlement of solder interconnections. Addition of Cu into eutectic Sn-Pb and Sn-Ag solders, however, were observed to be very effective in retarding the redeposition and preventing the Au-embrittlement. These effects were discussed with the microstructures observed.

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UBM-coated Si-wafer was fluxlessly soldered with glass substrate in N₂ atmosphere using rolled solder sheet. The bulk Sn-37wt%Pb and Sn-3.5wt%Ag solders were rolled to the sheet of 100㎛ thickness in order to achieve a bonding to Si-wafer by fluxless reflow process. The oxide layer on the solder surface was analysed by AES(Auger Electron Spectroscopy). Through rolling, the oxide layer on the solder surface became thin. It was possible to bond a solder disc to the Si-wafer with fluxless process in N₂ gas. Wetting angle of the fluxless soldered disc on the Si-wafer was about 9∼10°. The Si-wafer with a rolled solder was bonded to glass without flux in N₂ atmosphere. The joint was sound, and the thicknesse of intermetallic compound along the interface of Si-wafer was less than 2㎛.

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Silver solubility technique was employed to deduce the relationship between quantitative basicity index and physicochemical properties of molten slags such as viscosity and refining abilities. Also, the theoretical assessment for the slag design concept on the basis of slag basicity was discussed. The relative basicity of basic oxides and CaF₂ with respect to CaO and that of acidic oxides with respect to B₂O₃ was estimated from the solubility difference of silver in MO-B₂O₃ (MO=CaO, BaO, Na₂O), CaO-RO (RO=B₂O₃, SiO₂, Al₂O₃), and CaO-Al₂O₃-CaF₂ slags. By using the relative basicity values of each component, a new basicity index for the complex slags was proposed. The viscosity and the refining ability of molten slags could be predicted as a function of basicity index within the experimental uncertainty. In addition, the estimation for the optimized composition of slags according to the goal of refining process and the basic concept for the slag design was discussed.

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The refractory samples from the steel ladle of 300 ton capacity, POSCO, have been recovered from a joint between MgO bricks and Al₂O₃-MgO castable, from the castable wall for slag removal and steel tapping and from the bottom of ladle. The samples were cut into thin sections and then observed under optical microscope, by means of X-ray diffraction and electron optics. The slag penetration into ladle wall, a portion of which serves as a trough for slag removal, is far deeper than that for steel tapping. This controls the ladle lining life. For balanced wear, the trough may be replaced by MgO bricks which is more slag resistance than the castable. This will increase ladle lining life. Al₂O₃ in the slag represented by tricalcium aluminate reacts with MgO in bricks to form MgAl₂O₄ and the reaction between the remaining CaO and MgO forms eutectic at 2300℃. But the reaction between CaO in slag and Al₂O₃ in castable makes low-melting calcium aluminates such as CaO·Al₂O₃ and CaO·2Al₂O₃. As a result, MgO bricks have higher slag resistance than the castable.

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A comprehensive methodology to elucidate the effect of the gate speed and the gate geometry on the flow characteristics of the molten Al alloy was presented in this study. A commercial Al alloy (ACD12) used for a general purpose diecasting was chosen as the model fluid to monitor the flow patterns during filling the die cavity. A high speed photography was employed to visualize the real time flow patterns of the molten alloy during die filling sequence. The effects of the gate speed of the incoming fluid and the gate geometry on the die filling characteristics were investigated. In general, the flow patterns of the molten Al alloy were observed to be a laminar type when the melt was injected at a gate speed less than 0.3m/s through a gate having a thickness larger than 10㎜.

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