Papers by Author: Woo Jin Kim

Abstract: Currently, magnesium billets produced by ingot casting or direct chill casting process, result in low-quality surfaces and peer productivity. Continuous casting technology for high-quality surface billets with fine-grained and homogeneous microstructure can be a solution for the cost barrier breakthrough. The latent heat of fusion per weight (J/g) of magnesium is similar to that of other metals, however, considering the heat emitted to the mould surface during continuous casting in meniscus region and converting it to the latent heat of fusion per volume, magnesium will be rapidly solidified in the mould during continuous casting, which induces subsequent surface defect formation. In this study, electromagnetic casting and stirring (EMC and EMS) techniques are proposed to control solidification process conveniently by compensating the low latent heat of solidification by volume and to fabricate magnesium billets with a high quality surface.

Abstract: For the Magnesium alloy AZ31, hot rolling is usually carried out in the temperature range between 250 and 400°C but the processed sheets usually exhibit high anisotropy in mechanical properties. In the current study, DSR process was found to be effective in improving anisotropy of mechanical properties and ductility at room temperature. Full recrystallization takes place from 200°C and above. A large drop of UTS occurs above 200°C where full recrystallization starts. Tensile elongation increases with annealing temperature but anisotropy degrades from 200°C onwards. Texture change during recrystallization is believed to be responsible for this result.

Abstract: An asymmetrical rolling of AZ31 alloy sheets was carried out at 473 K with a high reduction ratio of 70% in thickness prior to tensile-tests. For the as-rolled sheet, tensile tests were performed in a temperature range of 473 ~ 673 K at a constant strain rate of 1 × 10-3 s-1. Tensile elongation increases with increasing reduction ratio in the as-rolled sheets. A large elongation of above 530% was obtained in the specimen deformed at 673 K for the sheet fabricated by 70% thickness reduction. Although the extent of grain growth was remarkable in a high temperature range, the tensile elongation increases with increasing the tensile temperature.

Abstract: Equal-channel angular pressing (ECAP) is an efficient method of improving strength of metallic alloys through (sub) grain refinement to, typically, the sub-micrometer level by introducing intensive plastic strain into materials through repetitive pressing. In the present study, the post-ECAP aging effect on strength of age hardenable aluminum alloys (6061, 2024, 7075 Al) was examined and it could be concluded that pre-ECAP solid solution treatment combined with post-ECAP low-temperature aging provided much more significant room-temperature strengthening effect than conventional ECAP processing routes without involving special heat treatments.

Abstract: By controlling texture and grain refinement using ECAP (equal channel angular pressing) process that induces severe shear deformation on materials without change their diameter, strength and ductility of the AZ31 alloy in the bulk form of rod could be enhanced. The original texture was completely replaced by a new texture rotated to have a high Schmid factor during ECAP. The decrease of yield stress after ECAP despite achievement of significant grain refinement was attributed to the strong effect of texture modification. The effect of differential speed rolling (DSR) on mechanical properties of the AZ31 alloy in the form of sheet was examined. Significant grain refinement took place during the rolling owing to introduction of large shear deformation by application of high speed ratio between the upper and lower rolls. Hall-Petch relations of the ECAPed and DSRed AZ31 alloys were compared and notable difference in strength between the two alloys at a given grain size was found and it was attributed to strong texture effect on strength of Mg alloys.

Abstract: Magnesium alloy AZ31, which processed by conventional rolling or extrusion, has high anisotropy of mechanical properties in its strength and elongation at room temperature. We compared the influence of differential speed rolling with conventional rolling process on microstructure and mechanical properties of commercial AZ31 sheet. Commercial AZ31 alloy sheets were processed with conventional and differential speed rolled with thickness reduction ratio of 30% at a various temperature. The elongation of AZ31 alloy, warm-rolled by differential speed rolling is larger than rolled by conventional rolling. Besides, grain size and distribution on microstructure of the conventional rolling were coarsely(~30μm) and inhomogeneously but, that those of the differential speed rolling were fine(~13μm) and homogeneously.

Abstract: Superplastic behavior of the Zr65Al10Ni10Cu15 glass metallic alloy produced by powdermetallurgy method was examined in the supercooled liquid region. Stress-strain and stress-strain rate relationships showed that Newtonian viscous flow governed the plastic flow until strain hardening took place. The large strain hardening was proved to a result of occurrence of crystallization during deformation.

Abstract: Bulk metallic glass Zr65Al10Ni10Cu15 was fabricated in a sheet form with thickness 1.5 mm by a squeeze casting method. The structure of the as-cast Zr65Al10Ni10Cu15 sheet was confirmed to be fully amorphous. The sheet was punched into a blank under high hydrostatic pressure at room temperature. A round hole was created with a possible evidence for plastic-like deformation along the edge of rim. No visible cracks were observed around the hole. This result indicates that bulk metallic glasses, which are known to be very brittle at room temperature, can be deformed in a ductile mode under hydrostatic pressure condition. Hydrostatic pressure may suppress the formation and development of micro defects leading to ductile fracture