Papers by Author: Sang Yong Lee

Abstract: In the present study, process developments for net-shape forming of aluminum roof joint node have been described. These joint node parts were designed for low speed electric vehicles of which operating condition is more moderate than general compact cars. Thixoforming of Al-7%Si-0.5%Mg (A357) alloy has been applied to the net-shape forming of a complex roof joint node to support aluminum space frame structure of low speed electric vehicle. Optimum heating temperature for the A357 billet was between 580 and 585°C corresponding to the semi-solid temperatures showing 20-30% of liquid fraction. An injection speed of around 100mm/s and preheating of die at temperatures of 200~250°C were also necessary conditions to obtain reasonable thixoformed parts.

Abstract: X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (TEM) with high annular dark-field (HAADF) microscopy have been used to characterize the nanoscale oxide film formed naturally on the surface of cold-rolled sheet of mild steel. Main aim of this research was to confirm that XPS could be used in determining the thickness of thin oxide film as a substitute for the laborious TEM analysis. The preparation and analysis technique for the XPS depth profiling has been investigated to characterize the oxide film. The selection of appropriate sputter rate and sputter area according to sheet material components and the exact estimation of sputter depth from sputter rate during depth profiling were essential for the determination of the nanoscale oxide thickness.

Abstract: The effects of thixoforming defects on the stress-strain curves of aluminum structural part were quantitatively investigated. The A357-MHD billet was thixoformed to manufacture a shock tower part of a small electric vehicle. All tensile specimens were machined from only a shock tower product to minimize the effects of different processing conditions. Tensile tests were performed under the almost same conditions. The fraction of solid and liquid phase, the sizes and area fractions of defects in the specimens were measured and analyzed by an image analyzing program using OM and SEM micrograph. The fracture strain values of the stress-strain curves were greatly affected by amount of defects. However, the analyses with defects existed in the area of gauge length did not show a clear explanation about the relation with the fracture strain values. The measurements of defects in the fractured surface represented more plausible results about the correlation of forming defects and fracture strains.

Abstract: Filling and solidification phenomena during thixoforming of copper rotor for small sized induction motors have been analyzed and characterized by experiment and computer simulation. Forming defects in slots of thixoformed rotor were examined by microstructural observation. Most filling and solidification failures were mainly related to complexity in slurry flows through slots in rotor core. Computer simulation with single slot model showed the effects of die temperature and ram speed on the filling characteristics clearly. Simulations with flow guide model showed that the control of filling velocity of slurry at slot and the consideration of possible differences in flow velocity in each slot are important.

Abstract: Cu-Ca alloys and the squirrel cage rotors for induction motors of small capacities were used for the development of thixoforming processes. Processing conditions, motor efficiency and forming defects in macro- and microscale for thixoforming of Cu-Ca rotors have been performed to investigate the microstructural features and the filling phenomena in slots of squirrel cage rotor. Inadequate filling due to the complexity of slot structure, separation of solidified metal from the slot, porosities and phase inhomogenitites were typical microstructural defects found in thixoformed Cu-Ca rotors. Exact flow control in terms of billet and die temperatures at slot gate especially, was necessary to prevent significant defects such as incomplete filling of slot.

Abstract: In order to investigate the effect of PVD coatings on the performance of proto-type high temperature mold and to identify the best candidate thin films for Cu semi-solid processing, TiAlN/CrN multilayer coatings with various superlattice periods was synthesized using a closed field unbalanced magnetron sputtering method with separate Cr and TiAl and characteristics of these films were compared with those from TiN/CrN and AlN/CrN films in terms of microstructure, nanoindentation hardness, and oxidation test at 900
in air. TiN/CrN film showed the best mechanical properties but their oxidation resistance, the most critical property for high temperature semi-solid processing was the poorest. Therefore TiAlN films with a superlattice period of 6.1nm which has intermediate plastic deformation resistance with the best oxidation resistance were considered to be the best candidate out of three different films studied in this work.

Abstract: TiAlN/CrN multilayer coatings with an superlattice period of 6.1nm was applied to a proto-type high temperature segment mold for Cu semi-solid processing so that it was investigated the possibilities of TiAlN/CrN multilayer coating as a candidate protective coatings to extend the lifetime of high temperature molds and dies. Much improved performance was obtained from the TiAlN/CrN coated molds, although different behaviors were observed depending upon the type of substrate mold materials. . Improvement of mold performance was observed by TiAlN/CrN coating on AISI H13 steel, but TZM alloy with TiAlN/CrN coating was found to be the best candidate as a mold for high temperature Cu semi-solid processing.