Hee Woong Lee

Cold Rolling Technique for Eliminating Cutting Process in Manufacturing Precise Product Using Non-Heat-Treated Micro Alloys

In general micro alloy steel have the higher strength relative to conventional steels, which limits the utilization of conventional plastic forming processes. Incremental forming processes are more suitable for cold forming of such a high strength material. In particular, cold cross wedge rolling (CWR) can be a potential tool to fabricate axi-symmetric components with multi steps using high strength micro alloy steel. Obviously, optimization of die shape design is a crucial factor to apply cold cross wedge rolling to micro alloy steels. In this regards, a simulation-based process design using an elasto-plastic FEM has been carried out in order to obtain an optimum die shape for cold cross wedge rolling in this paper. Analysis results provided that the stretching angle and the shoulder angle at knifing and guiding zones were significant parameters for the stable forming process. It was demonstrated that proper stretching and shoulder angles reduced an excessive slip between a work piece and die in CWR process despite the condition of the low friction coefficient.

Forming Technology for Cold Forging Processes of Ball Stud Using Non-Heat-Treated Cold Forging Materials

A microalloyed (MA) forging steel is non-heat-treated materials that have been replacing for conventional quenched and tempered (Q/T) structural steels since the MA forging steels are very cost-effective compared with Q/T steels for the production of automotive parts. However, due to a high strength and low elongation, it has been difficult to apply the MA cold forging steel to the ball stud for automobile. In this study, finite element analyses were carried out to investigate the forming loads, the stress and strain distributions of the workpiece in the cold forging processes of the ball stud using the MA cold forging steel. Compression test at room temperature and fatigue test were also performed to obtain the flow stress and fatigue life, respectively. From these results, it was found that the fatigue life was greatly affected by the strength in the neck region of the ball stud and the cold forging processes should be designed to improve the fatigue life of the ball stud used the MA cold forging steel.

Effect of carbon on the coefficient of thermal expansion of as-cast Fe−30wt.%Ni−12.5wt.%Co−×C invar alloys

The segregation (distribution) of nickel and the composition of its constituents influence the low thermal expansion characteristics (Invar effect) in Fe−30 wt.% Ni−12.5wt.% Co−×C Invar alloy. The change of coefficient of the thermal expansion and magnetic properties were studied as an aspect of carbon addition causing the segregation of Ni in primary austenite of as-cast Fe−30wt.% Ni−12.5wt.% Co Invar alloy. The coefficient of thermal expansion of Fe−30 wt.% Ni−12.5 wt.% Co−×C Invar alloy showed its lowest value at 0.08 wt.% carbon, increased with increasing carbon content in the range of 0.08–1.0 wt.%C, kept constant at 1.0–2.0 wt.%C and decreased at carbon higher than 2.0 wt.%. The effective distribution of the coefficient of nickel in as-cast Fe−30 wt.% Ni−12.5 wt.% Co−×C Invar alloy increased with increasing carbon content. The volume fraction of the γ phase of Fe−30 wt.% Ni−12.5 wt.% Co−×C alloy increased with increasing carbon content. The microstructure of Fe−30 wt.% Ni−12.5 wt.% Co−×C alloy changed with the carbon content was independent of the coefficient of thermal expansion. The Curie temperature changed linearly with the carbon content and was similar to the change of the coefficient of thermal expansion. Moreover, the coefficient of thermal expansion decreased when the ratio of saturation magnetization to Curie temperature (σs/Tc) increased, decreasing the Curie temperature and showed a specific relationship with the magnetic properties of the Fe−30 wt.% Ni−12.5 wt.% Co−×C Invar alloy.

Effects of Fillers on Electrical Properties of PTFE Composites for Circuit Breaker

This paper presents the effects of fillers on electrical properties of PTFE composites for nozzle of circuit breaker. PTFE has been used widely as a material for circuit breaker nozzle. Adding some filler into PTFE material is expected to be efficient for improving the endurance against arc radiation. In this experiment, effects of fillers on electrical properties such as dielectric constant, dissipation factor, electrical resistivity, dielectric strength and corona resistance of PTFE composites were investigated.

Effects of Ceramic Fillers on Light Reflectance and Arc Resistance of PTFE for Circuit Breaker

This paper presents the effects of ceramic fillers on light reflectance and arc resistance of PTFE(polytetrafluoroethylene) for nozzle of circuit breaker. In arc environment of a circuit breaker, radiation generated from arcing is considered to play a key role in transport mechanism of arc energy to the wall. Adding some ceramic fillers into PTFE material is expected to be efficient for improving the endurance against arc radiation. In this experiment, three kinds of ceramic fillers for achieving good endurance in the high temperature were used. Light reflectance and arc resistance of PTFE composites were investigated. The arc resistance of PTFE composites increased with the amount of ceramic fillers. The light reflectance of PTFE composites filled with ceramic fillers increased in the long wavelength region and decreased largely in the short wavelength region in comparison with that of pure PTFE.

Characteristics of thin film of nano-hybrid synthesized from acrylic resin and colloidal silca-silane sol

Organic-inorganic hybrid blends were prepared in variation with the ratio of sol solution to urethaneacrylic resin. Such sol solutions were synthesized from colloidal silica (CS)/ methyltrimethoxysilane(MTMS)/vinyltrimethoxysilane(VTMS) by sol-gel reaction through two step reactions. Thin films of organic-inorganic hybrid blends were prepared using spin coater on the glass and aluminum sheets. In order to understand surface properties of thin films, contact angle and roughness were measured. Surface contact angle and roughness of thin films increased as the amount of sol solution increased. Thermal dissociation temperature of thin films was observed using TGA. Thin films were stable until 270 oC. Thermal dissociation temperature of thin films improved with increasing sol solution. In order to observe insulation property of thin films, electrical resistance measurements were performed. Surface and volume resistance of thin films increased as the amount of sol solution increased.

Optimum Design of a Pilger Mill Process for Wire Forming Using CAD/CAE

The optimum design of a die shape for Pilger mill process was carried out using FEM analyses considering various processing factors. The important design parameters of the Pilger mill machine are feed rate and profile of the grooved die. Optimum design procedure was conducted in order to investigate effects on forming load and the deformed shape of a material depending on the die surface profiles. Profiles of the die surface for the optimum design were testified with linear, cosine and quadratic curves considering physical forming process. The results of the analyses provided that the model of the quadratic profile gave the lowest forming load and the proper deformed shape.

Changes of Fatigue Property with Heat Exposure in INVAR/AW Wire

Fatigue properties of INVAR/AW wires have been investigated under the heat exposure in order to ensure stable operation and to estimate life span of their power line. In the case of heat exposure for 1000hr, fatigue life and limit increased. For further heat exposure, fatigue limit decreased due to the decrease in yield strength. The variation fatigue of strand wire was strongly dependent on its amplitude. Also, cracks in wires of 7 strands were caused by stress concentration at the outer surface and fretting between each wire during vibration.

Effects of Deformation on Electric Conductivity in Al-Zr Alloys

Electric conducting behavior, sectional residual stress and texture in deformed Al-Zr alloy were investigated in order to investigate effect of deformation on electric conductance. {111} textures were developed in the wire of deformed Al-Zr alloy and most strong intensity of {111} texture was observed at the core in the cross section of the wire. Residual stress was mainly measured at the surface and , especially, measured at the core in the case of 4.5Φ wire, also, electric conductance was higher at the surface than at the core. This means that electric conductance might be affected by distribution of the residual stress.

Strain Change and Creep Behavior of STACIR/AW Power Line with Heat Exposure

As a way to expand electric capacity in power line with hovering of electric power demand, STACIR/AW (super thermal-resistant Al alloy conductors Al-clad Invar-reinforced) overhead conductor which cans double ampacity has been developed. The STACIR/AW power line is mechanically composite stranded wire composed of INVAR/AW stranded wire as core for sag control and heat-resistant aluminum alloy for delivering doubled electric current. Recently, in order to ensure stable line operation and to predict its span of life, the changes of thermal properties for STACIR/AW have been investigated. In the present work, the changes of strain with temperature and the creep behavior as important factors in sag control will be presented. The transition temperature of STACIR/AW 410sqmm was estimated approximately 130°C and the creep rates were decreased with temperatures.