• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.10a
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• pp.240-243
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• 2001

Recently, many automobile parts produced by semi-solid forming(SSF) process has been applied for improved weight reduction, better environmental protection and energy savings. SSF process was well developed for high volume production of light weight aluminum components. In this paper, knuckle has been manufactured with A357 by SSF and thor investigated for microstructures and mechanical properties followed by various heat-treatment conditions. It was found that the examined microstructure was equiaxed at the whole cross-section area and as a result, the mechanical properties were satisfied by 100MPa YS, 260MPa UTS and $14\%$ elongation.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10a
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• pp.281-284
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• 2003

The microstructural changes of Al-Zn-Mg-Cu alloy containing Sc during hot extrusion and post heat treatment is investigated. Two kinds of Al-Sc alloys with different alloying elements (B1, B2) are hot extruded to make I-shape bars at 380$^{\circ}C$, then the bars are solution treated at 480$^{\circ}C$ for 2hrs followed by artificial aging at 120$^{\circ}C$ for 24hrs. The interior microstructure of as extruded bar consists of elongated grains, however, fine equiaxed grains are mainly observed around surface. The microstructural gradient suggests that different restoration process can proceed during the hot extrusion. For B1 and B2, different grain growth behaviors are found around the surface during the post heat treatment. Rapid grain growth behavior around the surface is discussed related with the crystallographic orientation of the grain.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.10a
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• pp.163-167
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• 2003

The large deformation at cryogenic temperature would be one of the effective methods to produce large bulk UFG materials. The effects of annealing temperature 150∼300$^{\circ}C$, on microstructure and mechanical properties of the sheets received 85% reduction at cryogenic temperature were investigated. In comparison with those at room temperature. Annealing of 5083 Al alloy deformed 85%, at 200$^{\circ}C$ for an hour, results in the considerable increase of tensile elongation without the great loss of strength and the occurrence of equiaxed grains less than 300nm in diameter.

• Journal of the Korean institute of surface engineering
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• v.34 no.6
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• pp.537-544
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• 2001

In the present study, tin-zinc alloys were coated on a cold-rolled steel sheet with variations of electrolyte concentration, additives quantity and current density employing the Hull cell and circulation cell simulator. With an addition of additives of 2m1/L, tin-zinc deposits containing 10 to 40 percent Zn revealed a good surface appearance with weak acidic electrolytes. The organic additives suppressed the Sn deposition rate and thus increased the zinc contents in tin-zinc coating layers. The zinc contents in the tin-zinc coating layers depended almost linearly on the concentrations of metal ions of tin and zinc. Temperature of the electrolyte affected the composition tin-zinc coating layer. However, the concentration of complexants revealed little effectiveness. The surface morphology of tin-zinc coating showed dense tin and zinc phases with fine equiaxed grains with the high current density.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.03a
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• pp.149-152
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• 1999

The effect of initial orientation on the microstructure and texture evolution of two ferritic stainless steels was investigated. the columnar and equiaxed crystal specimens which were prepared from continuous casting slab were hot rolled annealed cold rolled and annealed respectively. The rolling and recrystallization textures at each process stage were examined by orientation distribution function (ODF) and electron back-scattered diffraction (EBSD); The observation showed that the orientation density of the $\alpha$-fibre of hot rolled band of columnar crystal specimen was more pronounced than that of the equaxed one at the center layer. Nevertheless the cold rolled textures of Type 430 steel have demonstrated a rather similar development . Compared to Type 430 steel the development of the $\alpha$-fibre in the center layer of Type 409L steel was much more pronounced. The relation between texture evolution and ridging behaviour has been discussed.

• Proceedings of the Korean Nuclear Society Conference
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• 1998.05b
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• pp.136-141
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• 1998

재결정된 Zircaloy-4 합금을 35$0^{\circ}C$ 의 여러가지 알칼리 금속 수산화물 수용액에서 부식시켜 동일한 무게증가량을 갖도록 조절한 뒤, 1M H$_2$SO$_4$ 용액에서 그 시편에 대한 임피던스 특성을 분석하였다. LiOH, NaOH, KOH 순서로 알칼리 금속의 이온반경이 클수록 수소이온의 이동에 대한 임피던스가 증가함에 따라 산화막을 통한 수소이온의 흡수가 점차로 어려워졌다. 이것은 Zr$^4$$^{+}$와 이온반경이 비슷한 알칼리 금속 수용액 조건에서 얻은 부식시편은 open grain boundary 와 equiaxed microstructure의 산화막을 갖기 때문이었고, 반면에 Zr$^4$$^{+}$와 이온반경의 차이가 큰 알칼리 금속 수용액 조건에서 얻은 부식시편은 compact 한 columnar microstructure의 산화막을 갖기 때문인 것으로 생각된다.

• Journal of the Korea Institute of Military Science and Technology
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• v.5 no.3
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• pp.89-97
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• 2002

In this study, the effect of $\alpha$-phase morphology on the dynamic deformation behavior at ultra high strain rate was investigated by EBW(Explosive Bridge Wire) test. All of tests and analyses were conducted on three typical microstructures of Ti-6Al-4V alloy, i.e. equiaxed, widmanstatten and bimodal microstructures. The spall strength and HEL(Hugoniot Elastic Limit) of the specimens that have the thickness of 2mm and 4mm were highest with the bimodal microstructure. These results were similar with previous study which was performed by dynamic torsion test(Kolsky torsion test).

• Journal of Powder Materials
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• v.7 no.4
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• pp.228-236
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• 2000

An investigation was carried out on the possibility whether the ball-milling process of low energy could successfully improve the packing density and flowability for MIM application in W-20wt%Cu system. In this study, W-20wt%Cu powder mixture was prepared by ball-milling. W powder was not fractured by low mechanical impact energy used in the present work during the critical ball-milling time, but the ductile Cu powder was easily deformed to the 3 dimensional equiaxed shape, having the particle size similar to that of W powder. The ball-milled mixture of W-20wt%Cu powder had the more homogeneous distribution of each component and the higher amount of powder loading for molding than the simple mixture of W-Cu powder with an irregular shape and a different size. Accordingly, the MIM W(1.75)-20wt%Cu powder compacts were able to be sintered to the relative density of 99% by sintering at $1400^{\circ}C$ for one hour.

• Journal of Powder Materials
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• v.6 no.4
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• pp.277-285
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• 1999

Mechanical properties of nanocrystalline Al-5at.%Ti alloy were investigated through high temperature compression test. Al-5at.%Ti nanocrystalline metal powders, which had finer and more equiaxed shape than those produced at room temperature, were produced by mechanical alloying at low temperature. The powders were successfully consolidated to 99fo of theoretical density by vacuum hot pressing. XRD and TEM analysis revealed that $Al_3Ti$ intermetallic compounds formed inside powders and pure Al region with coarse grains formed between powders, especially at triple junction. Mechanical properties in terms of hardness and strength were improved by grain size refinement, but ductility decreased presumably due to the formation of the weak interfaces between Al pool and powders.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.3
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• pp.392-398
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• 1996

$Si_{3}N_{4}/SiC$ nanocomposites reinforced with tow different mean particle size were fabricated by hot press. Grain growth of matrix gran was inhibited by adding of SiC particles, and then number of equiaxed and fine grains were increased. The effect of grain growth inhibition was higher in the nanocomposites dispersed small size SiC. herefore fracture strength and hardness were increased, but fracture toughness was decreased in small size SiC dispersed samples.