Papers by Author: Ha Guk Jeong

Abstract: In Mg-Al-Zn alloy sheets, the grains were refined from 30 mm to 2 mm during the process of differential speed rolling at 473 K, having approximately the same size dominate at a high thickness reduction rate of 60 %. In addition to these microstructural changes, the rolled sheets showed a weaker texture characterized by pole than those of commercial sheets. The DSR sheets have equal value of elongation at 0, 45, and 90 degrees from the rolling direction. With the mechanical properties, the results of Erichsen test indicate that DSR process remarkably enhanced ductility of the present Mg alloy sheets at room temperature.

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: 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: This study deals with the dimensional accuracy of outer diameter and geometrical workability in rotary swaged product for various process parameters such as percent reduction in outer diameter and the ratio of thickness to outer diameter of a tube. It is generally known that greater cold strengthening is achieved by rotary swaging process rather than by conventional process such as rolling with respect to the same reduction of cross-sectional area. Percent reduction in the diameter and the ratio are considered and selected as important process parameters because of playing a key role during rotary swaging process. In case of tube under rotary swaging process the ratios have influence on geometrically proper workability without defect for different percent reductions in the diameter. In addition the change of metal flow of workpiece under the swaging process is microscopically and globally observed to analyze the reason why dimensional accuracy of the outer diameter of final product is improved after the rotary swaging process. This work might provide available information for the optimum rotary swaging process.

Abstract: The microstructure and mechanical properties of the Mg97Zn1Y2 alloy prepared by spark plasma sintering of gas atomized powders have been investigated. After consolidation, precipitates were observed to form in the α-Mg solid solution matrix of the Mg97Zn1Y2 alloy. These precipitates consisted of Mg12YZn and Mg24Y5 phases. The density of the consolidated bulk Mg-Zn-Y alloy was 1.86 g/cm3. The ultimate tensile strength and elongation were dependent on the consolidation temperature, which were in the ranges of 280 to 293 MPa and 8.5 to 20.8 %, respectively.

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: In automotive industries, components have replaced steel with aluminum alloys since the automotive parts that used aluminum alloys have the ability to improve the environment by lower weights and recycling of waste materials. In this study, experiments were carried out to investigate the characteristics of the developed aluminum alloys according to the forming temperature and speed. The microstructure of forged product in hot forging process was also investigate. The results revealed that the grain size of aluminum alloys was grown according to the forming temperature and speed. Three-dimensional finite element analyses were also carried out to investigate the forming load, deformed shape, and the stress distribution of the workpiece in the hot forging process. The deformed shape of the workpiece was consistent with the trend of experimental results.