Papers by Author: Kwang Seon Shin

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Authors: Hwa Chul Jung, Kwang Seon Shin
Abstract: Semi-solid processing is recognized as an attractive alternative method for the near net-shape production of engineering components. Although there has been a significant progress in semi-solid processing of aluminum alloys, very limited information is available on semi-solid processing of magnesium alloys, except for the thixomolding process. Continuous casting process has been utilized to produce the billets with the desirable cross-section at a reduced production cost for many metals, such as steel, copper and aluminum alloys. It has also been commercially utilized to produce the aluminum billets with non-dendritic microstructure for subsequent thixocasting process. However, continuous casting of magnesium billets for semi-solid processing has not yet been commercialized due to the difficulties involved in casting of magnesium alloys. In the present study, a continuous casting process has been developed for the production of the cylindrical billets of magnesium alloys for the subsequent thixocasting process. In order to obtain the desired non-dendritic microstructure with an excellent degree of homogeneity both in microstructure and composition, an electromagnetic stirring system has been utilized. A continuous casting process has been proven to be an efficient way to produce the high quality billets of magnesium alloys for semi-solid processing. A prototype air conditioner cover was produced using the continuously cast billets of AZ91 alloy.
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Authors: Young Gee Na, Dan Eliezer, Kwang Seon Shin
Abstract: The development of new components with magnesium alloys for the automotive industry has increased in recent years due to their high potential as structural materials for low density and high strength/weight ratio demands. However, the limited mechanical properties of the magnesium alloys have led to search new kind of magnesium alloys for better strength and ductility. The main objective of this research is to investigate the mechanical properties and the corrosion behavior of new wrought magnesium alloys; Mg-Zn-Ag (ZQ) and Mg-Zn-Si (ZS) alloys. The ZQ6X and ZS6X samples were fabricated using hot extrusion method. Tensile tests and immersion tests were carried out on the specimens from the extruded rods, which contained different amounts of silver or silicon, in order to evaluate the mechanical properties and corrosion behavior. The microstructure was examined using optical and electron microscopy (TEM and SEM) and EDS. The results showed that the addition of silver improved the mechanical properties but decreased the corrosion resistance. The addition of silicon improved both mechanical properties and corrosion resistance. These results can be explained by the effects of alloying elements on the microstructures of the Mg-Zn alloys such as grain size and precipitates caused by the change in precipitation and recrystallization behavior.
839
Authors: Jung Woo Choi, Kwang Seon Shin
Abstract: The mechanical properties of extruded ZA (Mg-Zn-Al) alloys with different Al contents were examined. The effects of Al on deformation behaviour were examined by tensile and compressive tests. The changes in texture due to plastic deformation were examined using the X-ray diffraction method. It was found that the basal poles of the extruded ZA alloy were parallel to the normal direction with a slight tendency to incline toward the extrusion direction. The degree of inclination increased with increasing Al content. The inclination degree of the basal poles was found to be closely related to the change in the deformation behaviour of the ZA alloy. Computer simulations were also carried out using the VPSC (visco-plastic self-consistent) theory in order to predict the contributions of various deformation modes to the plastic deformation behaviour of the ZA alloy.
249
Authors: Young Gee Na, Dong Yim Chang, Soon Chan Park, Kwang Seon Shin
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Authors: Young Jin Ko, Dong Yim Chang, Jong Dae Lim, Kwang Seon Shin
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Authors: Yeon Jun Chung, Jung Lae Park, Nack J. Kim, Kwang Seon Shin
Abstract: The effects of alloying elements on the microstructure and high temperature mechanical properties of Mg-Al alloys were investigated in this study. In order to improve the high temperature mechanical properties, Sr or Mm was added to the Mg-9Al alloy. The effect of Sn on the Mg-9Al alloy was also examined since Sn was expected to improve the high temperature mechanical properties by forming the thermally stable Mg2Sn phase. The specimens used in this study were produced on a 320 ton cold chamber high-pressure die casting machine. The microstructures of the specimens were examined by optical and scanning electron microscopy and tensile and creep tests were performed at elevated temperatures. Tensile tests were carried out at room temperature, 150oC and 200oC using an initial strain rate of 2×10-4/sec. In addition, tensile creep tests were conducted at the stress levels of 50 MPa and 70 MPa. From the microstructure analyses of the specimens after heat treatment at 400oC for 12 hours, it was found that most of the Mg17Al12 precipitate dissolved into the matrix, while the thermally stable phases continued to exist. The high temperature mechanical properties of the Mg-9Al alloys were found to improve significantly with the additions of Sr, Mm and Sn, due to the formation of the thermally stable precipitates.
845
Authors: Yeon Jun Chung, Kwang Seon Shin
Abstract: The effects of precipitates and alloying element on the high temperature tensile properties of Mg-Al alloys were investigated in this study. In order to modify the precipitation behavior and microstructure of Mg-Al alloys, Sr and/or Mm (Misch metal) were added to the Mg-9Al and AZ91 alloys. All test specimens used in this study were die-cast on a 320 ton cold chamber high-pressure die-casting machine and the microstructures of the specimens were examined by optical and scanning electron microscopy. Tensile tests were carried out at room temperature, 150oC and 200oC at a strain rate of 2×10-4/sec. The microstructure analyses revealed that thermally stable MgAlSr and AlMm compounds were precipitated in the Sr and/or Mm added alloys and these compounds suppressed the precipitation of the discontinuous Mg17Al12 phase at grain boundaries. The high temperature mechanical properties of the Mg-Al alloys were found to increase with the addition of Sr and/or Mm. It was concluded that the addition of Sr and/or Mm improved high temperature tensile properties of the Mg-Al alloys by the formation of thermally stable precipitates.
537
Authors: Seung Hoon Yu, Kwang Seon Shin
Abstract: Various reactions and the in-situ formation of new phases can occur during the mechanical alloying process. In the present study, Al powders were strengthened by AlN, using the in-situ processing technique during mechanical alloying. Differential thermal analysis and X-ray diffraction studies were carried out in order to examine the formation behavior of AlN. It was found that the precursors of AlN were formed in the Al powders and transformed to AlN at temperatures above 600oC. The hot extrusion process was utilized to consolidate the composite powders. The composite powders were canned in an Al can and then extruded at elevated temperatures. The microstructure of the extrusions was examined by SEM and TEM. In order to investigate the mechanical properties of the extrusions, compression tests and hardness measurements were carried out. It was found that the mechanical properties and the thermal stability of the Al/AlN composites were significantly greater than those of conventional Al matrix composites.
181
Authors: Young Pyo Kim, Cheol Man Kim, Woo Sik Kim, Kwang Seon Shin
Abstract: A clear understanding of fatigue properties for the pipeline steel and its weld is important to provide information for pipeline design during pipeline construction and predict pipeline fatigue life during pipeline operation. The materials used in this study are API 5L X65 pipeline steel generally used for natural gas transmission. This pipeline was welded by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) with V-groove configuration. The fatigue crack growth behaviors of pipeline steel and its girth weld according to crack growth directions and stress ratios were investigated over a wide range of stress intensities in laboratory air.
303
Authors: Hwa Chul Jung, Dong Yim Chang, Won Wook Park, Chong Soo Lee, Kwang Seon Shin
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