발간논문

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Effects of mechanical properties of Cu/Cr metal films in Cu/Cr/polyimide structures on the peel strength of Cr/polyimide interfaces have been investigated. With increasing the yield strengths of Cu/Cr metal films from 156 MPa to 325 MPa, the peel strengths of Cr/PMDA-ODA and Cr/BPDA-PDA interfaces were lowered from 75 g/㎜ to 57 g/㎜ and from 69 g/㎜ 20 g/㎜, respectively. Compared to the peel strength of Cr/PMDA-ODA interface, the low value was obtained on Cr/BPDA-PDA interface with identical Cu/Cr metal films, indicating that the interfacical fracture toughness of Cr/BPDA-PDA is lower than the value of Cr/PMDA-ODA.

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A numerical analysis of solidification process of hypereutectic Al-Si alloy was performed by microscopic modeling of solidification which incorporates nucleation and growth kinetics. Examining the solidification characteristics of hypereutectic Al-Si alloy, growth kinetics of equiaxed primary Si phase and eutectic phase was considered by linear growth kinetics and modified Jackson-Hunt theory respectively. Conventional macroscopic soldification model (enthalpy method) in which unique relationship of temperature and solid fraction assumed was compared with present model in the situation of conventional mold casting and gas atomization of Al-20wt%Si alloy. Latent heat difference of primary phase and eutectic phase and nucleation-growth kinetics of present model resulted in more realistic cooling curves. Some useful information about thermal history was presented from the inspection of the cooling curve. It is also demonstrated that calculated results and growth kinetics can be related with microstructural characteristic scales of the solidified alloys in good agreement.

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A two-dimensional numerical model for the vertical twin-roll strip casting process has been developed. The boundary fitted coordinate (BFC) is employed to handle the complicate boundary geometry and the resultant transformed governing equations for the melt pool and roll are solved separately using the finite volume method based on control volume method. The effects of casting conditions of roll speed and superheat on the heat transfer and flow pattern were studied. As roll speed increased, the positions of solidification end point and stagnation point were lower and the temperature of the strip at the roll nip increased. The effects of superheat on the heat transfer and flow pattern were smaller than that of roll speed. The surface temperature of strip out of the roll nip increased due to reheating of strip and the temperature differences between surface and center of strip were in the range of 60∼80℃.

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Using Cu, Al and Ti powder as model materials, milling experiments were carried out to investigate the particle coagulation and coating on balls surface and or mill container wall during mechanical alloying of ductile metal powders. The change in particle number per unit weight during milling was analyzed as a function of particle coagulation and coating by Gillespie-Langstroth coagulation ratio equation which is based on probability theory. In the initial milling stage, the decrease in particle number was found to be due mainly to the particle coagulation. As the milling time increase, however, the contribution of the particle crating dominated the particle number decrease. It has been demonstrated that the particle coagulation and coating during milling occurred easily with increasing the ductility of metal power.

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This work was carried out in order to join dissimilar stainless steels(STS304 and STS430) by laser beam welding which had many advantages over conventional welding processes. In this work, laser welded specimens were compared with TIG welded specimens in microstructure analysis, mechanical properties and corrosion resistance tests. Laser welding resulted small heat-affected zone and low distortion due to low heat input, and thus weld defects like hot cracking, Cr-carbide precipitation and grain boundary coarcening at HAZ which usually occurred in TIG welding were prevented. Although microstructure of weldment in TIG welding was ferrite/martensite duplex structure, only ferrite structure was obtained in laser welding due to suppression of δ→γ transformation by rapid cooling. It could be known that laser welded specimens resulted better ductility, toughness, pitting and SCC resistance.

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Oxidation resistant ultra fine iron powder has been prepared by silica coating and reduction of magnetite powder. A uniform coating of silica on the surface of ultra fine (75 A˚) magnetite powder was possible in a water-glass solution at pH 10-11. The silica-coated magnetite powder was reduced in H₂ at 500-650℃. The size of reduced iron powder decreased as the reduction temperature decreased ; the mean size of the particles could be reduced down to 110 A˚. When the treatment was done at 550℃ for 5 hrs, the silica-coated iron particles with SiO₂/Fe=3.7 wt% showed the good oxidation resistance under 150℃ and magnetization value of 139.6 emu/g (at the magnetic field 10 kOe).

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