Papers by Author: Shae K. Kim

Abstract: The thermal conductivities of binary Mg-CaO and Mg-Zn, and ternary Mg-Zn-CaO alloys have been investigated by evaluating the effect of CaO on pure Mg and Mg-Zn alloys, with an emphasis to develop a new Mg alloy by compromising thermal conductivity, process-ability and mechanical property. The Mg alloys specimens were prepared by casting into a steel mold and then by machining. The thermal conductivities of the alloys were determined by evaluating the thermal properties of specific heat and diffusivity, from room temperature to 200 °C. OM, SEM, and EDS were used to analyze the microstructures and phases. The fluidity was also investigated by using a spiral fluidity mold for improved process-ability during actual die casting.

Abstract: In this study, the effect of CaO as alloying element on Mg-Zn-Y and Mg-Y-Zn alloys were investigated to develop wrought magnesium alloys available for aeronautic components. The evaluation for the effect of CaO addition in these alloy systems was performed by the processing map development, the tensile test, and the ignition temperature measurement. The addition of CaO changed the hot workability and improved the ignition resistance of both Mg-Zn-Y and Mg-Y-Zn alloys. Although the mechanical properties of CaO added Mg-Zn-Y alloy were lower than those of Mg-Zn-Y alloy, the CaO addition in Mg-Y-Zn alloy enhanced the ductility of the alloy. The effect of metallic Ca addition in these alloy systems were investigated to be compared with the CaO addition.

Abstract: In this study, the effect of plane strain on the microstructure and texture of AZ31 sheet was investigated with various reduction ratios. To achieve plane strain, that is, to prevent shear deformation during warm-rolling, AZ31 sheet was capsuled by pure Cu tube. The large strain induced by simple one-pass warm-rolling led to the significant grain refinement and texture randomization. The two-dimensional finite-element method showed that the warm-rolling of AZ31 sheet capsuled by Cu tube imposed a uniform plane strain in the whole sheet thickness. The high reduction ratio caused the dynamic recrystallization during annealing of the rolled sheet.

Abstract: The main task of this study was to investigate the effect of Mg content on hardness of extruded Al-Si-Mg-Mn alloy after solution and artificial ageing heat-treatments. The Al-Si-Mg-Mn alloys with the variation of Mg content were fabricated through casting, homogenization and extrusion processes. The extruded samples were heat-treated – T6 temper. Thermal analysis with metallurgical examination was performed in order to optimize the condition of solution heat-treatment. Then, microstructure change was discussed by using optical microscopy and SEM-EDS. And the corresponding Vickers hardness was measured as a function of ageing time. It was found that the Vickers hardness slightly increased due to improved ageing efficiency and increased hard phase as the Mg content increased.

Abstract: In this study, the microstructures and mechanical properties of the recently developed Eco-2024-T3 alloy were examined. Eco-2024 is made using Eco-Mg (Mg-Al₂Ca) in place of element Mg during the manufacture of alloy 2024-T3. This is an alloy that has economic advantage and excellent properties. Alloy Eco-2024 showed smaller crystal grains that were distributed more evenly compared to the existing alloy 2024-T3. It consisted of Al matrices containing minute amounts of Al₂CuMg, Al₂Cu, and Ca phases and showed microstructures with reduced amounts of Fe phases or oxide. As a result of tensile tests, this alloy exhibited yield strength of 413 MPa, tensile strength of 527 MPa, and elongation of 15.4%. In other words, it showed higher strength than the existing alloy 2024 but was similar to the existing alloy 2024 in terms of elongation. In fatigue tests, alloy Eco-2024-T3 recorded fatigue limit of 330 MPa or around 80% of its yield strength; this is a much more excellent property compared to the existing alloy 2024-T3, which has fatigue limit of 250 MPa. Based on the aforementioned results, the correlation between the excellent mechanical properties of alloy Eco-2024-T3 and its microstructure was examined.

Abstract: The microstructure, hardness and tensile properties of Mg-3Al, Mg-3Al-1.2Ca(AX31), Mg-3Al-1.2CaO(AO31), Mg-9Al, Mg-9Al-1.2Ca(AX91) and Mg-9Al-1.2CaO(AO91) alloys were investigated to identify the effect of CaO addition compared with Ca addition. The results show that the alloy element CaO has a good effect on high temperature tensile properties and CaO added alloys have superior high temperature performance compared with Ca added alloys. Addition of CaO results in the precipitation of Al2Ca phase (C15 phase) on the grain boundaries. This phase precipitated by the reduction of CaO leads to the refinement of a microstructure and improvement of tensile properties at elevated temperature similar to the Ca added Mg alloys. When added into molten Mg Ca makes solid solution in the Mg matrix up to 0.8wt %. But in case of CaO addition the thermally stable compound is precipitated directly without making solid solution with Mg matrix. Therefore, the high temperature tensile properties of CaO added Mg casting alloys are better than Ca added one at the same adding contents (1.2wt.%).

Abstract: CaO addition to Mg forms the Al2Ca and Mg2Ca phases without solid solution even below solubility limit. In this study, thermodynamic considerations of CaO added Mg alloys in recycling process are investigated in comparison with Ca added Mg alloys. Thermodynamic considerations were done by using the army-navy-air force (JANAF) thermochemical tables between Ca or CaO added Mg alloys and fluxes such as MgCl2, KCl, NaCl and MnCl2. The mechanisms during recycling process were thermodynamically discussed. Actual recycling experiments were also carried out to compare the thermodynamic results.

Abstract: For non-SF6 process and melt cleanliness, CaO added AZ31 Mg alloy has been developed for strip casting. Mg alloys were usually ignited during conventional casting, the continuous casting and strip casting. Protective gases should be used in order to prevent the ignition of Mg alloys during melting and casting processes. However, protective gases have the disadvantages of global warming potential, high production cost and corrosion of steel based equipments. To solve ignition problem and overcome environmental issue, this paper discussed the effects of CaO addition and amount on strip casting process in terms of non-SF6 process and melt cleanliness.

Abstract: This study was carried out to characterize thixoextruded AZ31 Mg alloy in terms of its symmetric behavior through the optical microscope, electron back scattering diffraction and mechanical test. The elongated grains to the extrusion direction were generally observed during hot extrusion, while the thixoextruded microstructures were isotropic. Texture of hot extruded bar showed the strong fiber component, while that of thixoextruded bar was random distribution. The aspect ratio of hot extruded bar was irregular at each position. On the contrary, the aspect ratio of thixoextruded bar was almost uniform below 2 at each position.

Abstract: The aim of this study was to improve extrudability limit, eliminate welding line and obtain optimum thixoextrusion conditions for manufacturing tubes of 7075 Al wrought alloy. By thixoextrusion, it was possible to improve deformability, control isotropy with extrusion direction, eliminate welding line (seamless) and save cost due to low energy consumption compared with conventional extrusion processes. The welded part was not observed at the welding line area. The grains of thixoextruded tube were homogeneously distributed and equiaxed grains were observed. Therefore, thixoextrusion is the most effective variable for the control of the magnitude of the welding line.