Papers by Author: Sang Mok Lee

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 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.

Abstract: In order to investigate the effects of physical melt treatment on microstructural feature and anodizing characteristics of Al-Si system casting alloys, A380 and A356 aluminum alloys were chosen and a twin-screw melt-shearing process was 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.

Abstract: Carbide precipitation behavior in Si-Mo ferritic ductile cast iron was investigated as functions of vanadium and chromium contents. Vanadium addition promoted the precipitation of carbides both in ferrite grains and at its grain boundaries. Pearlite was found to form near the cell boundary next to the eutectic carbides in the as-cast chromium alloyed specimens, and was fully decomposed by an annealing heat treatment. Vanadium addition led to the formation of fine precipitates, prohibiting the ferrite growth, resulting in smaller ferrite grains. The precipitate phase at grain boundaries in vanadium alloyed specimens was identified as vanadium carbide (VC1-x) and the stoichiometry of the eutectic carbide was almost same as that in Si-Mo ferritic ductile cast iron except for a higher vanadium content. However, in the case of chromium alloyed specimen, the ratio of iron in the composition of eutectic carbide was higher than those of Si-Mo and Si-Mo-0.5V cast irons.

Abstract: The effects of copper addition on the microstructure and the elevated temperature properties of ferritic heat resistant cast iron were investigated. The as-cast pearlite formed due to the addition of Cu was fully eliminated by full annealing. As the content of Cu increased, the grain size of ferrite decreased. The grain refinement due to the addition of Cu enhanced the mechanical properties at room temperature, however, those at elevated temperature deteriorated. The addition of Cu diminished the volume change during α→γ transformation. The starting point of α→γ transformation increased with Cu contents under 1.15wt% Cu but this tendency was reversed above this point. This trend can be found also in the case of lattice parameter of ferrite matrix. It is inferred from Fe-Cu phase diagram that the addition of Cu enlarged the coexistence zone of α and γ, so it diminished the volume change during α→γ transformation.

Abstract: Effects of Cr addition on morphology of β-Al5FeSi phase formed in recycled aluminum alloy were studied as a function of cooling rate by technology of time-resolved radiography. The Cr addition to iron-contaminated recycled alloys modified the needle-like β-Al5FeSi into faceted dendritic α-AlFeCrSi phase in condition of conventional casting. Time-resolved radiography of the formation of α-AlFeCrSi revealed the transition of its morphology from polyhedron at about 10K/min of cooling rate to equiaxed dendrite of faceted arms at about 50K/min. of cooling rate.

Abstract: High temperature oxidation behavior of Si-Mo ferritic ductile cast iron was investigated in the point of the effect of chromium and vanadium addition. The addition of Cr promoted the formation of as-cast pearlite around carbide which exists in cell boundary, which was eliminated during annealing process. The addition of vanadium promoted the precipitation of tiny carbide and reduced the grain size of ferrite. As the test temperature increased, the change of volume increased, on the other hand, the change of weight decreased above 1173K. In the case of Cr added specimen, the change of weight decreased with the increase of test temperature because of the presence of Cr oxide layer. The vanadium added specimens showed higher increase in the weight and volume change. The oxide layer of vanadium added specimen had very porous structure and showed severe internal oxidation. It is due to the catastrophic oxidation characteristic of vanadium alloyed ferrous alloy.

Abstract: A Mo-free high-Cr cast iron with superior impact-wear resistance was tried to develop for mill balls by alloy design and subsequent heat treatment. The Cr/C ratio was varied up to 7.7 in order to minimize and eventually eliminate Mo addition, still ensuring hardenability. For the proper balance between abrasion resistance and toughness, the combined effects of each heat treatment parameters on the mechanical properties were systematically investigated using Taguchi method and response surface method (RSM), well recognized as powerful tools of the designs of experiments (DOE). It was found that hardness and impact energy were mainly affected by tempering and destabilization conditions, respectively. It is noted that high impact energy of 5.2 J/cm2 was obtained without deteriorating hardness (54 HRc) during the verification experiments under the heat treatment condition optimized by the DOE analyses.