발간논문

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In this study Al-(20,30wt%)Si-(3,5,8wt%)Fe alloy powders were manufactured by gas atomization and the characteristics of alloy powders were investigated. In rapidly solidified Al-Si-Fe alloy powders, primary δ phase (Al₄FeSi₂) with 20∼50 ㎛ in size is observed with primary Si. And the matrix of the powders consists of ternary eutectic of α-Al, Si and intermetallic compound. This intermetallic compound is found in the intercellular regions of α-Al in very small alloy powders, too. When Al-Si-Fe alloy powders are heat treated, the metastable δ phase disappears and equilibrium β phase(A1_5FeSi) is formed and residual Si precipitates around intermetallic compound. It is expected that the materials formed of Al-Si-Fe alloy powders will show better wear resistance, heat resistance and mechanical properties than Al-Si binary alloys due to finely distributed primary and eutectic intermetallic compounds.

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Unidirectionally reinforced Al 2024 and Al 7075 composites with SiC whiskers were fabricated by high pressure infiltration technique and subsequent hot extrusion. The microstructure, age hardening behavior and mechanical properties of the composites were examined. Hot extrusion enabled the unidirectional alignment of whiskers, but substantial amount of whiskers was broken during extrusion. The whisker reinforcement tends to accelerate the age hardening, and decrease the amount of aging, due to the inhomogeneous precipitation of stable phases at the lattice defects in the matrix. The beneficial effect of whisker reinforcement was observed up to 473K. Above 573K, however, the mechanical properties of the composite were comparable to those of the unreinforced alloys, attributable to the enhanced grain boundary sliding due to the fine grain structure formed during hot extrusion.

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Dynamic recrystallization controlled rolling(DRCR) process was simulated by multistage torsion tests in the range of 1000∼900℃, 0.1/sec∼3.0/sec with decreasing temperature and increasing strain rate(ε). The critical strain(ε_c) for the initiation of dynamic recrystallization(DRX) was determined by plots of the work hardening rate(θ) against stress(σ) and the relationship ε_c/ε_p was found to be around 0.7. Grain refinement was accelerated by the penultimate pass in the DRCR. After the final deformation at 900℃, grain growth was restricted from controlling the cooling rate. When the cooling rate was 7℃/sec, refined ferrite, pearlite and bainite were observed and grains were refined down to 20∼30 ㎛ which gave tensile strength of 780MPa. The precipitates of V(C, N) distributed homogeneously at higher cooling rate and improvement of tensile strength confirmed precipitation hardening.

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The vacuum brazing of Silicon Nitride(β-Si₃N₄) and Ni-Cr Steel with Ag-Cu-Zr active braze alloy has been conducted to study the brazability o the ceramics and the interfacial reaction. The reaction of Si₃N₄ with the braze alloy resulted in the formation of ZrN produced by only redox reaction between Si₃N₄ and Zr, however the presence of Zr-silicide(Zr_5Si₃) wasn`t identified obviously. Therefore it was suggested that the interfacial reaction of Si₃N₄ and Ag-Cu-Zr could be controlled by only brazing variables such as temperature and holding time because the reaction product didn`t comprise any elements that could be diffused from brazing alloy or bonded metal. The maximum fracture shear strength was obtained at the brazing condition of 950℃, 30min, as average value of 202MPa. The fracture strength has influenced by the thickness of the reaction product and, also the morphology of intermetallic compounds formed in the brazement. It was found that the brazability of Ag-Cu-Zr braze alloy on S₃N₄ was admirable in the aspect of the joint strength and the interfacial reaction.

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Spot welding, a kind of electric resistance welding, is a welding method widely used for automobile, aircraft, and appliance industry. By measuring and observing tensile shear strength of spot welding specimens, alloying degree of electrode tips, and nugget cross section under various welding conditions, we have the following results; 1) The spot weldability of the galvanized steel sheet increases with the welding current within the range of 8-11 KA. 2) For a high current density spot welding, the Al₂O₃-Cu electrode tip was better than the Cr-Cu electrode tip on the spot weldability. 3) Electrode tip pressure did not affected the spot weldability, in the tested pressure range (200∼300Kgf).

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Complex tungsten carbides of fine grain sire and Ni as a binding metal were developed with TiC and SiC as additives. The process to produce cemented carbides based on (W-Si-Ti)C-10wt.%Ni consisted of mixing W and TiC powders with liquid glass(SiO₂) and carbon powder and carburizing at 1500∼1550℃ to form SiC coating, adding 10wt.% of Ni powders and sintering. As a result, we obtained a fine grain size in the range of 0.4∼1.0 ㎛, a high micro-Vickers hardness of 1830 ㎏/㎟ and a relative density higher than 97%. TiC appeared to retard grain growth and enhance hardness by forming (W, Ti)C and Ni₃Ti. This was evidenced by the morphological features of (W, Ti)C and WC phases as well as the distribution characteristics of the elements observed by electron microscopy.

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Since the pin point rust is the major quality problem of cold rolled steel sheets, the effects of atmospheric conditions and chloride ions on the formation of pin point rust were investigated. To know rust susceptibility, annealed specimens were prepared and placed in the cold rolling plant by everymonth in 1994. Temperature and relative humidity around annealed specimens were continuously measured by using thermohygrograph(model R-704). From the experimental results, the propensity of rust formation was remarkably influenced by absolute humidity which was calculated from the temperature. It was considered that relative humidity was not related to the rust formation When impurities containing water or moisture were deposited on the surface of steel sheets, rust formed preferentially at those sites and then propagated as the form of thin long tubes due to the Cl ions.

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The effects of cooling rate of heater, melt weight, seed direction, base materials of crucible and purity of silicon were investigated in the experiment of the silicon single-crystal(diameter 7㎝) growth by Heat Exchanger Method in this paper. Silicon single-crystal which purity was electronic-grade could be grown at the cooling rate lower than 0.8℃/min when the seed with $lt;100$gt; direction was used and twins in parallel with the radial direction were formed at the cooling rate higher than 0.2℃/min when the seed with $lt;111$gt; direction was used. But, in any case, some polycrystal formation at the low edge part of the crystal could not been avoided. When the alumina with low thermal conductivity instead of graphite with high thermal conductivity was used as the base material of crucible, the trend of the poly-crystal formation at the low edge part of the crystal could be diminished. Especially, Silicon single-crystal which purity was metallurgical-grade could be grown at the cooling rate lower than 0.2℃/min when the seed with $lt;100$gt; direction was used.

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