Papers by Author: Kyong Whoan Lee

Abstract: The effect of silicon (Si) on the stability of heat-resistant ductile iron having ferrite matrix in high temperature was investigated by dilatometric test. Microstructure of heat-resistant ferritic ductile iron consists of ferrite, eutectic carbide at eutectic cell boundaries, precipitated carbide in grain and graphite. Pearlite was found around eutectic carbide in some specimens, however, all pearlite was decomposed by the annealing treatment. As Si content was increased, the number and size of carbide decreased and full ferrite matrix were obtained. The starting temperature of ferrite-austenite transformation and the coefficient of thermal expansion increased with the increase of Si content. The growth of Si 6.0wt% specimen during held at 1,000 ^oC is lower than other specimens, and it is considered that in the case of Si 6.0wt% specimen, the carbon movement is restrained due to the low solubility of carbon in matrix. In the case of annealed specimens, the contraction during ferrite-austenite transformation was not found when heating. This is because the re-distribution of the graphite was happened through the movement of carbon during annealing treatment.

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: 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: Ti-Al intermetallic compounds are regarded as promising materials for the hightemperature structural and coating applications. We focused on the joining of Al casting alloy with Ti-Al intermetallic compounds by in-situ combustion synthesis to improve the surface properties of Al casting components. Microstructures and phase formation behavior of Ti-Al based intermetallic compounds synthesized by combustion reaction were analyzed using scanning electron microscope(SEM) equipped with energy dispersive x-ray spectroscopy (EDS) and x-ray diffractometer(XRD) in Ti-Al intermetallic compounds. Three kinds of titanium aluminides of Ti3Al, TiAl and TiAl3 were synthesized by the heat from the Al molten metal and a coating layer of intermetallic phase were formed simultaneously on solidifed Al alloy surface. The shapes and microstructures of reacted compacts were varied by mixing ratio of elemental powders. The TiAl3 intermetallic compound was observed in the compacts regardless of the mixing ratio of elemental powders. And the unreacted Ti powders were remained in the reacted compacts due to the big size of Ti powder and low exothermic heat of reaction between Ti and Al powders. The zone that poured Al alloy diffused into the reacted Ti-25at.%Al compact of about 200 μm thickness was formed at the interface by the reaction between liquid molten Al alloy and solid Ti powders in green compact.

Abstract: We focused on the surface reinforcement of ligth weight casting alloys with Ni3Al intermetallic compounds by in-situ combustion reaction to improve the surface properties of nonferrous casting components. In the present work, by setting the mixture of elemental Ni and Al powders in a casting mold, the powder mixture reacted to form Ni3Al intermetallic compound by SHS reaction ignited by the heat of molten AZ91D Mg alloy and simultaneously bonded with the Mg casting alloy. The AZ91D Mg alloy bonded with the Ni3Al intermetallic compound was sectioned and observed by optical microscopy and scanning electron microscopy(SEM). The chemical composition of intermetallic compounds and diffusion layer formed around bonding interface were identified by energy dispersive spectroscopy(EDS), X-ray diffraction analysis(XRD) and electron probe micro analyzer(EPMA). The main intermetallic compound was Ni3Al phase and a little Ni2Al3 intermetallic compound was formed from the Ni and Al powder mixtures. Residual pores were observed in the synthesized intermetallic compound. The AZ91D Mg alloy and Ni3Al intermetallic compound were bonded very soundly by the interdiffusion of Mg, Ni and Al elements, but some cracks were observed around the bonded interface on the interdiffusion layer. The diffusion length formed between AZ91D Mg alloy and Ni3Al was different depending on the diffusivity of Ni and Al elements into the molten Mg alloy. Ni was more deeply diffused into the Mg alloy than Al. The diffusion layer was about 200
m thickness and various phases were formed by the interdiffusion of Mg, Ni and Al. From this challenge we have successfully produced a coating layer based on nickel aluminide on ligth weight Mg alloy using molten metal heat without any additional process. On the basis of the results obtained, it is concluded that near-net shaped nickel aluminide coating layer can be formed using this unique process.

Abstract: The name “high strength brass” is given to the wrought and cast alloys indicating their particular virtue of high strength, which can be achieved by additions of Al, Fe, Mn and Sn. Forgings made from copper base alloys offer a number of advantages over products made by other processes. However, because for forging more heat must be applied to the ingot which was solidified once, there are some disadvantages in the economy of energy and time. In this study, we investigated the microstructures and mechanical properties of high strength brass made by semi solid forging and compared them with those of conventionally forged product and gravity die casting. No shrinkage or gas hole was found in semi solid forgings. Fine equiaxed crystals developed at the center of semi solid forgings, while grains in the corner of semi solid forgings were elongated perpendicular to the pressure direction. The grains of semi solid forgings were smaller than those of conventional forgings and gravity die castings. It is suggested that a rapid heat transfer condition due to applied pressure is responsible for grain refinement. Tensile and yield strengths of semi solid forgings were as high as those of hot forgings but elongation was positioned between that of conventional forgings and gravity die castings.

Abstract: We focused on the surface reinforcement of Al casting alloys with Ni-Al intermetallic compounds by in-situ combustion reaction to improve the surface properties of Al casting components. Microstructure and phase formation behavior of Ni-Al based intermetallic compounds synthesized by combustion reaction were investigated in terms of thermal and phase analysis using scanning electron microscope(SEM) equipped with energy dispersive x-ray spectrometer (EDS) and x-ray diffractometer(XRD) in Ni-Al intermetallic compounds. Three kinds of nickel aluminides, NiAl3, NiAl and Ni3Al, were synthesized by emission heat from the Al molten metal in order to form a coating layer of intermetallic phase simultaneously on the solidifed Al alloy surface. The synthesized shapes and microstructures of nickel aluminides were varied by casting temperature, Si contents, and the mixing ratio of elemental powders. The synthesized reaction products formed in nickel aluminides were observed to be different depending on the mixing ratio of elemental powders. The reaction layer of about 25
m thickness was formed at the interface, and it mainly consisted of NiAl3 phase by the reaction between liquid molten Al alloy and solid Ni powders in green compact. With this information, we successfully produced a coating layer of Ni3Al intermetallic compound onto the casting Al alloy surface using molten metal heat without any additional process. These findings led us to conclude that a near-net shaped nickel aluminide coating layer can be formed using this unique process.

Abstract: Mold filling characteristics in the Mg Expendable Pattern Casting(EPC) process were investigated in terms of casting conditions such as reduced pressure, pouring temperature and casting modulus including foam materials. With increasing pouring temperature up to 775oC the filling velocity increased. However, the filling velocity decreased at temperatures above 775oC. This is likely due to the increase of back pressure. Concerning the effect of reduced pressure on filling velocity, it increased sharply at lower reduced pressure while became stable at higher reduced pressure. In thick pattern, high reduced pressure would be needed to obtain high filling velocity. In expanded polystyrene(EPS) patterns, mold filling was found to be faster in the thick pattern than thin pattern at temperatures below 750oC. This propensity was observed to be reverse at pouring temperatures above 750oC. In polymethyl methacrylate(PMMA) patterns, the filling velocity almost leveled off without showing a dependence of pouring temperature. This result is attributed to the difference in gas pressure between EPS and PMMA patterns during the EPC process.