Papers by Author: Kee Do Woo

Abstract: Turbochargers of automobilehave been widely used in commercial vehicles by diesel engine. Nowadays, turbochargeris being tried to use passenger car by gasoline engine such as deluxe and compact car. Therefore, fabrication method and conditions of turbine wheel which is core parts ofautomobile turbochargers,arereviewed by Lost wax method. To fabrication of turbine wheel using Inconel 713C alloy, vacuum induction melting (VIM) with alumina (Al2O3) crucible was used. Inas- cast Inconel 713C alloy, the strength at the elevated temperature is not satisfied to commercial standard. Therefore, heat treatment was introduced in Inconel 713C alloy to increase strength. The strength at the elevated temperature was increased by aging treatment.

Abstract: Ti-6Al-4V ELI (Extra Low Interstitial) alloy have been widely used as alternative bone due to its excellent biocompatibility, although it still has many problems such as high elastic modulus and toxic. Therefore, biomaterial with low elastic modulus and nontoxic has to be developed. In this study, the raw materials which are nontoxic elements such as Nb and Mo were mixed and milled in a mixing machine (24h) and a high energy mechanical ball milling machine (1h, 4h and 8h) respectively. Ti-Nb-Mo-CPP composites were fabricated by spark plasma sintering (SPS) at 1000°C under 70MPa using mixed and milled powders. The effects of CPP contents and milling time on biocompatibility and mechanical property have been investigated. By X-ray diffraction (XRD), chemical reaction during the sintering was occurred and revealed new phases, Ti2O, CaO, CaTiO₃, and Ti_xP_y. Vickers hardness of composites increases with increase milling time and addition of HA contents. Biocompatibility of HA added Ti-Nb-Si alloys were improved.

Abstract: A novel biocomposite Ti-35wt%Nb-2.5wt%Sn-15wt%HA powders was synthesized by high energy ball mill(HEBM) for various periods of time. The microstructure and characteristics of the milled powder particles were investigated. Results showed that in the composite powders milled for 4h, Ti was still exhibited primary α phase, with the increase of ball milling time up to 8h, Ti transformed into primary β phase and a little α phase, after ball milling for 12h, Ti transformed into β phase fully. the transform temperature was 380.06°C. And TEM and PSD results indicated that nanostructure was obtained after 12h milling..

Abstract: Nowadays, many economist and scientist worried for sharply increased to fuel consumption. Developed and developing countries investigation new energy system. It is base on the low emission gas, high energy efficiency, permanence and cogeneration. Specially, transportation system has restricted to system’s total weight. It is concern with light weight offer to increase to performance by itself. And give to other benefit that reduce to oil consumption, improved fuel economy, engine downsizing and increased to Market-value. Fuel cell is one of the new energy systems for next generation. Normally, fuel cell is consisting of bipolar plate, MEA (Membrane Electrode Assembly) and GDL (Gas Diffusion Layer). Conventional bipolar plate material was applied Graphite. Graphite has very weak for external shock, machining process is not easy, and the main problem is that the graphite material supplied by oxidizing and reducing agent composition of the gas leak comes. Thus, by this reason, the manufacturing cost is increased. In this study, to replace the graphite material was selected as a metal material. In this experiment, STS304 austenite base stainless steel is used. This study was based on the metal material’s corrosion behavior observed with 2 type’s surface pretreatment on the STS304 with similar PEMFC operating condition. Experimental results, As-Polished condition has good corrosion resistance better than As-Received condition. It was concerned with passivation layer.

Abstract: High frequency induction heated sintering (HFIHS) method is one of the rapid sintering methods. The advantage of rapid sintering method is that grain growth can be prevented during sintering at high temperature. Refinement of grains was known to increase the yield and flow stresses of crystals. The relation between the yield stress and the grain size is known as Hall-Petch relation. NbC-10vol.%Co, Ni and Fe composites were fabricated by HFIHS at 1060°C for 0 and 3 min as holding times under a pressure of 80MPa.The relative density of NbC-10vol.%Co, Ni and Fe composites which were sintered at 1060°C for 0min as holding time under 80MPa were 91.90%, 91.26% and 91.26%, respectively. These composites are difficult to use industrial parts due to low relative density. The longer sintering time was conducted for increasing relative density in this study. Nano-sized specimens, which were calculated grain size by full-width at half maximum (FWHM), can be obtained by HFIHS. The value of hardness and fracture toughness was investigated using 20kgf load Vickers indenter.

Abstract: A planetary ball milling (PBM) technique was employed to fabricate mechanically alloyed (MA processed) Al-Nb2O5 composite powder. Nano or sub-micron sized Nb2O5 particles were homogeneously embedded in the Al particles after milling for various periods. None of cracks, by-products and pores were observed in the areas between embedded Nb2O5 particulates and Al matrix powder after milling. The sequence of the in-situ reaction was confirmed by DSC, XRD measurements, optical microscopy and EPMA. The specific temperature of the in-situ reaction was between 650 and 700°C. Al-based metal matrix composites (MMC) reinforced with the sub-sieve sized θ-Al2O3 particulates and Al3Nb intermetallic compound were successfully fabricated by the in-situ reaction process. The substituted Nb by the in-situ reaction was fully reacted with Al to form the Al3Nb intermetallic compound during sintering. A number of sub-sieve sized θ-Al2O3 particulates and Al3Nb intermetallic compound formed by the in-situ reaction between Al and Nb2O5 were homogeneously distributed in the Al matrix during sintering. Nano sized θ-Al2O3 particulates are preferentially distributed near the Al3Nb intermetallic compound and no by-products are formed in the interfaces with the Al matrix.

Abstract: Texture distribution through thickness in 6xxx aluminum alloy cross-roll rolled sheet has been investigated using the electron back-scattered diffraction (EBSD) technique. Analyses of texture in three layers from the surface to the center of the sheet were performed. Near the surface, shear type textures including {012}<142> orientation and {112}<110> orientation were found and β-fiber texture were dominating. In the center layer, very weak β-fiber texture {112}<111> orientation, {123}<634> and {011}<211> were found with Cube, Cube-ND and Goss orientation plus others.

Abstract: A near-α high temperature titanium alloy, Ti-6Al-2.5Sn-4Zr-0.7Mo-0.30Si, was produced with various Y contents from 0 to 0.7wt% by arc-melting technique to study the influence of Y on its microstructure and mechanical properties. It was found that small amount of Y obviously refines the grain size of Ti-6Al-2.5Sn-4Zr-0.7Mo-0.30Si alloy. SEM and TEM observations revealed that Y-containing phase precipitated at the β grain boundary in the form of Y2O3 particles. Hardness and compression tests performed at room temperature revealed the strengthening effect of Y.

Abstract: In the present study, we tried to make Ti matrix/hydroxyapatite biocomposites using powder metallurgy and chemical treatments. Three composites were produced: Ti-30vol%HA, Ti15Mo-30vol%HA and Ti13Nb13Zr-44vol%HA. After mechanical milling and spark plasma sintering, Ti/HA composite compacts were produced. Microstructure observations and mechanical property tests showed that composites with two components (Ti-HA) have high hardness, moderate compress strength and lower density. The produced Ti/HA bulk composites were subjected to chemical treatment of immersion into phosphoric acid solution and porous structure was obtained. The obtained porous structure was characterized.

Abstract: The displacement reaction (in situ reaction) is an effective process to solve the interfacial problem between Al matrix and ceramic particles in Al alloy matrix composites. In this study, an Al alloy matrix composite powder containing nanometer was produced by using high energy ball milling, and a bulk Al alloy matrix composite reinforced with nanometer and submicrometer sized Al2O3 particles was obtained by sintering the ball milled composite powder at 950 °C for 2h. It was found that the Al2O3 particles were homogeneously distributed in the Al matrix after sintering. Some of the Si particles have lots of fine Al2O3 particles distributed homogeneously in the Al matrix. The hardness of the sintered specimen produced using as-milled composite powder is higher than that of the sintered specimen produced using as-mixed powder.