Papers by Author: Hyoung Wook Kim

Abstract: The sheet of ZK60 alloy with a thickness of 1mm was prepared from a casting ingot followed by homogenization and warm-rolling. Variations in microstructure and mechanical properties of ZK60 alloy sheets were investigated during T6 treatment. Especially artificial aging after solution heat treatment affected both precipitates distribution and mechanical properties with aging treatment. Variations of mechanical properties were related to precipitates, i.e. rod-shaped ( 1 β ′ ) or disc shaped ( 2 β ′ ) particles. Around the peak of hardness values, regularly distributed rod-shaped ( 1 β ′ ) precipitates were found. The rod-shaped ( 1 β ′ ) precipitates were oriented with a growth direction of [0001]. When over-aged, rod-shaped ( 1 β ′ ) precipitates were expected to decrease and the density of disc-shaped ( 2 β ′ ) precipitates to change. The rod-shaped ( 1 β ′ ) precipitates mainly consist of {Mg, Zn}, while disc-shaped ( 2 β ′ ) precipitates, {Mg, Zn, Zr} or {Mg, Zn}. In this study the optimum T6 treatment was determined as solution treatment at 430 °C for 6 hours and subsequently aging treatment at 175 °C for 18 hours. At this T6 condition the tensile strength, yield strength and elongation are 321MPa, 280MPa and 16%, respectively.

Abstract: Al-Mg alloy sheets with high Mg contents (3~10wt%Mg) were fabricated by twin roll strip casting. The optimum process conditions to get a good surface quality of Al-Mg strip have been suggested in this experiment. Controlling the cooling rate of cast roll was important to improve the surface quality of the strip and a compositional homogeneity through the thickness. The size of intermetallic particle like Al-Fe compound was reduced down to 1~2μm due to a high cooling rate of Al melt during strip casting. In addition, the dendrite structure was fine and the segregation of Al8Mg5 phase between grains was remarkably reduced. Therefore, the strips with a thickness of 3mm have good workability during additional hot/warm rolling processes. The hot/warm rolled Al-5wt%Mg sheets show high strength and elongation. When the rolled sheets were annealed at 300 oC for 1hr., the tensile strength and total elongation of the sheets reached at 290 MPa and 30%, respectively.

Abstract: Variations in microstructure and mechanical properties of ZK60 alloy sheets were investigated with aging time. ZK60 alloy sheets with a thickness of 1mm were prepared from a casting ingot followed by homogenization and warm-rolling. Artificial aging process after solution heat treatment (T6) affected both hardness variations and precipitates distributions with aging time. Hardness variations were related to precipitates, i.e. rod-shaped ( 1 β ′ ) or disc shaped ( 2 β ′ ) particles. Rod-shaped ( 1 β ′ ) precipitates mainly consist of Mg and Zn without Zr.

Abstract: The evolution of hot rolling texture in FCC materials has been simulated numerically using a visco-plastic self-consistent (VPSC) polycrystal model. A finite element (FE) analysis with ABAQUS/StandardTM was conducted to evaluate the deformation gradients during hot rolling deformation. In order to capture crystallographic rotation during hot rolling deformation, an octahedral slip system was considered in a microscopic hardening model. The FE analysis with the VPSC polycrystal simulations successfully predicted the inhomogeneous texture development through the thickness direction in the hot-rolled Al-5wt%Mg alloy sheets.

Abstract: Evolution of crystallographic texture by hot rolling deformation at the temperature of 200°C was investigated by hot rolling tests on as-cast Al-5wt%Mg alloy fabricated by a new strip cast technology. Texture variation through the thickness direction in the Al-5wt%Mg alloy was examined experimentally. Macrotexture and microtexture measurements were conducted using X-ray diffractometer and electron backscatter diffraction (EBSD), respectively. Experimental investigation reveals that the evolution of texture and microstructure is strongly dependent on a distance from center of the Al-5wt%Mg alloy sheet. It was found that the shear texture components tend to be increased at the surface region of the hot-rolled specimen.

Abstract: Recently the method for obtaining ultra-fine grained metallic materials has developed using severe plastic deformation (SPD), such as equal channel angular pressing (ECAP), accumulative roll bonding (ARB), torsion straining, and warm multiple deformation (WMD) etc. In order to enhance thermal stability of ultra-fine grained aluminum alloys manufactured by SPD process, the addition of Sc and Zr elements has been considered to devise fine Al3Sc, Al3Zr and Al3(Scx Zr1-x) precipitates for inhibiting the grain growth. In this study, the microstructure evolution has been investigated in Al-Mg alloys with and without Sc and Zr addition during the warm multiple deformation process. In addition Al-Mg alloys were compressed at a strain rate of 10-1 sec-1 by two different routes, that is, route A and route B. Route A is to rotate the specimen throughout 90o around the vertical axis of loading direction at every pass. Route B is to rotate the specimen throughout 90o around the parallel axis of loading direction and then rotate it again as route A. The specimen deformed by route B had finer grain size and more uniform distribution of grains than those deformed by route A. When the warm multiple deformation process repeated up to 8 passes at 673 K, the specimen consisted of ultra-fine grained structure with the average grain size less than 3 μm. The superplastic behavior can also be observed at the high strain rate and low temperature regime.

Abstract: The effects of the damage induced during ion implantation on the surface roughening and oxide growth rate were investigated. Using several scheme of doses and acceleration energies, it is found that the amount of the dose predominantly produce damage rather than the acceleration energy, especially near the surface region. It was also found that the damage affects not only the oxide growth rate but also the surface roughening during high temperature annealing. The edge of highly implanted area may have higher doping concentration due to the vicinal side wall effect of the thick oxide mask for ion implantation. It was confirmed by the trench formation after thermal oxide remove.