Papers by Author: Ki Young Kim

Abstract: Various metallurgical refining processes of SOG (Solar Grade)-Si are being developed aiming its cost reduction for the PV (Photovoltaic) market. High purity silicon can also be obtained from Al-Si alloys using solidification theory. Since the solubility of impurity elements in silicon decreases rapidly with temperature decrease much above the Al-Si eutectic temperature, the removal of impurities from silicon at low temperature is expected to be effective. Most of impurity elements with small segregation coefficient can be removed during solidification. Key technology in this process is to separate the primary solid silicon from the liquid eutectic matrix during solidification. Some methods by an electromagnetic force or by combining flocculation and filtration techniques have been proposed to separate the solid silicon. The present study describes a new way to extract the primary silicon crystal from Al-Si alloys using centrifugal force during solidification for a silicon solar feedstock. Primary silicon was separated in the shape of foam during solidification, and pure Si flakes after acid leaching could be obtained.

Abstract: Cu-2wt.Ag-2wt.%Zr alloy was directionally solidified with different growth rates(V=10-200 um/s) at a constant temperature gradient(G=3.1 K/mm) in a modified Bridgman furnace. The influence of growth rate was investigated by observing the microstructure and measuring the solutes’ compositions within the Cu-matrix and dendrite boundaries. The experimental results show that increasing the growth rate, decreased both the primary and secondary arm spacing and increased micro-Vickers hardness. The solutes’ concentration also increased as a result of the back diffusion caused by a decreasing growth rate. The electrical conductivity depends on the solutes’ distribution.

Abstract: The consolidation process of ultra fine Si powders, generated as by-product during the decomposition process of silane gases, was systematically investigated for use as economical solar-grade feedstock. Si powder compacts were tried to fabricate by a consolidation process without a binding agent and then their density ratio and strength were evaluated. The Si powders in as-received state were not pure enough to be used alone as solar grade feedstock material. After the adequate chemical treatments, a sufficiently high purity above solar-grade was able to be achieved.

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.