Papers by Author: Bong Hwan Kim

Abstract: The microstructural evolution of AlMg4.5 and high-Mg containing AlMg10 alloys were investigated during homogenization by metallurgical analyses and micro-Vickers hardness test. It was concluded that the increased amount of Mg solute in AlMg10 alloy leads to formations of both super-saturated Al-Mg solid solution and increased eutectic Mg₂Al₃ phase in interdentrictic region at solidified state. The hardnesses of both AlMg4.5 and AlMg10 alloys increased after homogenization proportionally to the temperature. The resultant improved hardness of AlMg10 alloy after homogenization is higher than that of AlMg4.5 alloy and mainly contributed to additional solid-solution hardening by Mg solute dissolved from eutectic Mg₂Al₃ phase.

Abstract: The effects of Mg and Mn contents on hot-rolling temperature and resultant tensile properties of Al-Mg-Mn alloys were investigated. The Al-3.5Mg-0.3 alloy as a reference and Al-7.5Mg-0.3Mn, Al-3.5Mg-1.0Mn alloys were prepared by casting for hot-rolling experiment. The rolling temperatures of both Al-7.5Mg-0.3Mn and Al-3.5Mg-1.0Mn alloys had to be decreased due to surface cracking during hot-rolling, which is caused by increased fractions of Mg-containing phases like Mg₂Al₃. The tensile strength of the hot-rolled Al-7.5Mg-0.3Mn alloy was highly increased by the combined effects of enhanced solid-solution and work-hardening at lower rolling temperature. And the resultant tensile strengths of the hot-rolled Al-3.5Mg-1.0Mn alloy were also increased due to dispersoid hardening by increased Mn content and work hardening during hot-rolling.

Abstract: Grain growth behavior of AlMg5 alloy fabricated by using a new Mg mother alloy containing Al₂Ca (referred to as AlMg5-Al₂Ca hereinafter) was investigated during homogenization and subsequent hot compression test. Normal AlMg5 alloy using a commercial Mg mother alloy showed abnormally grown large grains in its microstructure after homogenization at 520 ̊C for 12hrs, while the grain growth in the AlMg5-Al₂Ca alloy was completely suppressed by formation of stable Al₄Ca during solidification on grain boundary. The compressive flow stress of normal AlMg5 alloy at 400 ̊C was significantly increased after homogenization because of lack of grains having proper slip directions to the applied load. But the flow stress of AlMg5-Al₂Ca alloy showing no grain growth during homogenization was slightly decreased implying lower energy needed for subsequent thermo-mechanical processing.

Abstract: The formation behavior of intermetallic layers on die material immersed in aluminum molten metal was investigated by using dipping test facility. And the metallurgical parameters affecting on the kinetics of intermetallic formation were discussed as functions of additional elements such as Fe, Mn and Cr in order to propose a practical guideline for control of molten metal in manufacturing processes. It was found that the intermetallic layers consist of α-AlFeSi, β-AlFeSi and θ-AlFe. The kinetic of intermetallic formation was highly dependent on not only the gradient of iron content through the interface between die material and molten alloy but also the relative formation temperature of primary intermetallic phase.

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 paper, it was aimed to improve understanding about the effects of physical melt treatment on the morphologies of eutectic silicon crystal size, and then the effects of these microstructural features on anodizing characteristics. A380 and A356 casting aluminum alloys were used in this experiment. A twin-screw melt-shearing process and an electro magnetic stirring process were utilized before high pressure die casting. In order to refine and homogenize the microstructure of the diecast Al-Si alloys, the melt-shearing process parameters were controlled and T6 heat treatment was carried out. A uniform microstructure over the whole thickness of the diecast specimens caused smaller difference in oxidation coating layer thickness between machined and non-machined surfaces. More uniform anodic coating layer was obtained by AC/DC coupled anodizing and PEO processes compared to conventional DC anodizing process.

Abstract: The mechanism of die erosion in Al-Si alloys was investigated based on the formation behavior of iron-containing intermetallics. And the metallurgical parameters affecting on the erosion tendency were discussed as functions of chemical composition for a guideline of melt control in Al-Si foundry. It was found that the intermetallic layers of α-AlFeSi, β-AlFeSi and θ-AlFe were formed through metallurgical reactions. The erosion tendency of die material was determined by not only the gradient of iron content through the interface between die material and molten alloy but also the relative formation temperature of primary intermetallics.

Abstract: This study aims to investigate the effects of mechanical melt-shearing treatment during casting process on the morphological change of iron-containing intermetallic phases formed in the high-iron bearing Al-Si casting alloys. The process parameters of the melt-shearing treatment were controlled and optimized in order to modify the needle-like shape of the iron-containing intermetallic into a less harmful form. Throughout this study, it was found that the melt-shearing treatment shortened effectively the length of the needle-shaped iron-containing intermetallic phase in the Al-Si alloys. It was also confirmed that these modified morphology and microstructures result in the improvement of mechanical properties.

Abstract: This study aims to investigate the influence of intermetallics morphological modification on mechanical properties of iron-containing Al-Si-Mg casting alloys. The modification of the intermetallics was performed by using melt-shearing treatment prior to commercial high pressure die casting. The process parameters of the melt-shearing treatment were controlled and optimized based on the theoretical consideration of the intermetallics formation behavior. It was found that the mechanical properties of low-iron containing Al-Si-Mg alloy was improved due to the modification of primary α-Al from dendritic to globular form by the melt-shearing. Regarding to high-iron containing Al-Si-Mg alloy, harmful morphology of intermetallics was successfully modified by the melt-shearing at semi-solid region, which results in the improvement of mechanical properties such as elongation.

Abstract: This study aims to investigate the formation behavior of intermetallic phases on casting mold die and its effect on erosion tendency in molten Al-Si alloy with the variation of Fe content. For these purposes, dipping tests of die material into molten Al-Si alloys with 0.98 and 1.33wt%Fe were carried out as a function of dipping time. Three kinds of intermetallic phases, α-AlFeSi, β-AlFeSi and θ-AlFe, were found to form on the interface between die and molten alloy in sequence, as iron and aluminum solutes diffuse into the molten alloy and die steel, respectively. It was discussed that the erosion process was decelerated in the case of the high-iron containing alloy, because of the decreased gradient of iron solute through the interface and enhanced diffusion barrier of intermetallic layer.