Papers by Author: Sang Yul Lee

Abstract: CrN/AlSiN multilayer coatings with bilayer period (Λ) between 2.3 nm and 8.0 nm were synthesized from Cr and AlSi (Si=20 and 66 at.%) targets by using a closed-field unbalanced magnetron sputtering (CFUBMS) and their crystal structure, chemical composition and mechanical properties have been investigated by glow discharge optical emission spectroscopy (GDOES), Xray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and nano-indenter. In order to evaluate the thermal stability of the coatings annealing treatments for 30 min at temperature between 800 °C and 1000 °C were performed in air. The thermal stability of the CrN/AlSiN multilayer coatings was improved with increasing Si content. In case of the coating with Si content of 18.2 at.%, a superior thermal stability was exhibited even after annealed at 1000 °C and the relatively high hardness of 25 GPa was maintained without a significant decrease in hardness. The reduced oxidation rate of the coatings at high temperature could be attributed to the formation of the amorphous Si3N4 phase around the crystalline AlN phase.

Abstract: Underground PE double wall pipes have been supplying waste water to sewage disposal plant. Many processes have been introduced to join PE pipes, but most of these methods have lots of disadvantages such as costs, lack of reliability and difficulties in joining, etc. Recently butt welding has been paid much attention to joint PE pipes as this process has many advantages such as cost, safety and reliability. In this study, a newly developed heat plate, a patent-pending heat plate with a groove, was used to butt-weld PE double wall pipes with different misalignment gaps to simulate real underground conditions. The resulting joining characteristics of double wall pipes were compared with those from a conventional heat plate, in terms of stiffness, flattening and leakage tests. The results from the stiffness and flattening test showed that there were no big differences between the butt-welded joints made from these two plates. From the leakage test, although PE pipes welded with a conventional heat plate did leak below the required test conditions (10 min. at 0.75kgf/cm2), the pipes welded with a patent-pending grooved heat plate did not show any leakage even at a pressure 1.3 times higher than the required conditions. It was noted that by utilizing a grooved heat plate more complete fusion at the pipe joints could be obtained, which in turn induced a high quality joints.

Abstract: Due to the environmental problem, automotive companies are trying to reduce the weight of the car body. Therefore, TRIP (TRansformation Induced Plasticity) steels, which have high strength and ductility, have been developed. However, there are only a few publication reported on the characteristics of GMA weld on TRIP steel. In this study, we will evaluate which of the shielding gases, i.e. Ar 100% CO2 100% and Ar 85% + CO2 15%, and welding speed make a better quality GMA welded TRIP steel. Bead-on plate welding and butt welding were performed with various welding conditions. In order to identify mechanical properties of welded TRIP steel experimental methods including fractography, tensile test, and hardness test were performed.

Abstract: Binary transition metal nitride films have excellent tribological properties but these binary systems are still inadequate for high temperature applications due to their low oxidation temperature. Above 700°C, formation of porous oxides at the film surface deteriorates their mechanical properties rapidly by the. Especially impact and indentation resistance of these films deteriorate very quickly. In order to overcome these problems, Al based Al1-xCrxN films with X=0.29 and X=0.69 were synthesized by closed field unbalanced magnetron sputtering with vertical magnetron sources and their chemical composition, crystalline structure, morphology and mechanical properties including impact and indentation resistance were investigated. Synthesized Al1-xCrxN films formed solid solution showing FCC B1 type structure with strong (111) preferential orientation and films with X=0.29 showed a superhard hardness value of approximately 41GPa while films with X=0.69 did approximately 31GPa. While there was insignificant difference between Al1-xCrxN films with X=0.29 and X=0.69 in terms of Rockwell C indentation resistance, much improved impact resistance could be observed from the Al1-xCrxN films with X=0.29.

Abstract: In this study, CrZrN film was deposited on AISI 4130 cylinder barrel and valve plate using a unbalanced magnetron sputtering. The tribological properties between a coated steel cylinder barrel and coated steel valve plate were evaluated by a custom-built disk-to-disk tribometer and the results were compared with those between a coated steel cylinder barrel and a conventional bronze valve plate. The lowest friction coefficients were observed between a coated steel cylinder barrel and coated steel valve plate and they were approximately one half of those between a coated steel cylinder barrel and conventional bronze valve plate in the range of testing up to 1600rpm. Coated valve plate and bronze valve plate showed a steady state of friction coefficient at approximately 0.03 and 0.06, respectively after about 1000 rpm, but steel valve plate without coating showed a continuous decrease in friction coefficient, which suggested that active wear is continuously occurring. By applying CrZrN coating on the commercial cylinder barrel and steel valve plate, much improved tribological results could be obtained, by a factor of approximately 2 in terms of friction coefficient than those from bronze valve plate. Under a tap water environment CrZrN coating appeared to perform better than CrSiN coating. Although further work to reduce the friction coefficient below 0.13 in the steady state after initial drop is still needed, possibilities of applying CrZrN coating to realize a water hydraulic pump were demonstrated through the present work.

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.

Abstract: In Gas Metal Arc Welding (GMAW), electromagnetic force is one of the most important factors that effects the metal transfer, short-circuit rate, spatter generation rate and mechanical properties for welding metal. Shielding gas and welding current have a great influence on metal transfer mode in GMAW. In this paper different ways for external electromagnetic forces are applied by attaching a cylindrically rounded conducting wire solenoid on touch tip holding. It was conformed that the intensities of electromagnetic force and kinds of shielding gases influence on the droplet transfer mode. With the applied electromagnetic field, the arc transfer mode changes from normal mode to rotating mode. The rotating direction changes with the change of electromagnetic current direction. Applied electromagnetic field intensity varied by electromagnetic current influences on the spatter generation rate in CO2 welding. In MIG welding, the influences of electromagnetic force on the spatter generation showed different tendency as in the CO2 welding. This paper is for the purpose of discussing these factors.