Papers by Author: Byung Min Kim

Abstract: This paper aims to predict the hardness of hot formed part for tailor rolled blank (TRB) by the FE-simulation coupled with quenching factor analysis (QFA). Dilatometry test of boron steel is performed at various range of cooling rates from 0.2 to 100°C/s using the dilatometer with forced air cooling system. The dilatometry test provides a hardness data according to cooling curves which are used to determine the material constants (K₁~K₅) of QFA and the time‒temperature‒property (TTP) diagram of boron steel. Then, FE‒simulation of hot press forming is conducted to predict the cooling curves of hot formed TRB part with a thickness combination of thicker 1.6mm and thinner 1.2mm which is called as rear side member of automotive component. The cooling curves of FE-simulation are applied to predict the hardness of hot formed rear side member using the QFA. Also, experiment of hot press forming is performed to verify the predicted results and to examine the effect of cooling curves on the hardness.

Abstract: In recent years, in order to achieve weight reduction and crashworthiness, automotive part was manufactured using patchwork blanks. The smaller patches were welded on a main sheet to achieve a local reinforcement, which is called the patchwork. In this study, the effect of the spot-weld line orientation on the formability of patchwork blanks for high-strength low alloy (HSLA) steel was investigated in uni-axial tension mode by limiting dome height (LDH) test. A FE-analysis and experiment was performed to gain a better understanding of the formability for limiting dome height and strain distribution. As a result of the LDH test and FE-analysis, differences in the dome heights and forming limits were observed patchwork blanks with spot-weld line orientation of 0°, 45°, and 90°, respectively. For the 0° and 45° patchwork blanks, the dome heights was lower than those of 90° patchwork blanks because of the stress concentration in the pole side.

Abstract: Use of light materials such as aluminum, magnesium and carbon fiber reinforced plastic (CFRP) has been increased to achieve the light-weight car body in automotive industry. For successful multi-material design of automotive body, the joining method for dissimilar materials is required to assemble the automotive components produced by various materials. Especially, hole clinching process is effective to fasten dissimilar materials without any additional joining element. In this study, effect of tool shape on the hole clinching is investigated by FE-analysis and experiments. The parameters related to clinching tool shape are punch diameter, punch corner radius and die depth. The geometrical interlocking is evaluated by the neck-thickness and undercut. Joint strength using single lap shear test is evaluated also to verify the effectiveness of hole clinching as automotive joints.

Abstract: Because customers are requiring front-loaded washing machine to handle big capacity laundry and have faster rotation speed to increase drying ability, there are being a lot of studies for achievement of high speed dehydration, high-strength and lightweight of washing machine in the latest washing machine business. It is essential that strength of Drum Assembly which is composed of spin drum, shaft, flange is improved to attain that target. In term of spin drum, it is difficult to realize joint strength required at high speed operation because joint strength of mechanical press-joining method is low remarkably in comparison with welding. Also in case of shaft system, stress from bending and twisting are complexly loaded onto the shaft supporting the horizontal drum, causing problems in fracture strength and fatigue life. The results of this study show optimal joining condition for mechanical press-joining by performing lots of tensile joining strength test with various specimen under multi-change of important design factor such as seaming width, bead area and bead depth etc. and the optimal design of shafting system for big capacity, high-rotation drying through strength analysis, experiment and evaluation.

Abstract: High speed multi pass wet wire drawing has become very common for production of high carbon steel cord because of the increase in customer demand and production rates in real industrial fields. Although the wet wire drawing is preformed at a high speed usually above 1000 m/min, greater speed is required to improve productivity. However, in the high carbon steel wire drawing, the wire temperature rises greatly as the drawing speed increase. The excessive temperature rise makes the wire more brittle and finally leads to wire breaks. In this study, the variations in wire temperature during wet wire drawing process were investigated. A multi pass wet wire drawing process with 21 passes, which was used to produce steel cord, was redesigned by considering the increase in temperature. Through a wet wire drawing experiment, it was possible to increase the maximum final speed from 1000 m/min to 2000 m/min.

Abstract: Galvannealed steel sheets are being widely used in automotive application for better corrosion resistance. It is generally known that uncoated steel sheets have better mechanical properties than coated steel sheets due to presence of zinc coating. But frictional characteristics of coated steel sheets are very different from those of uncoated steel sheets. Therefore the study on mechanical and frictional characteristics of these steel sheets is needed. In this study, tensile test was performed to evaluate mechanical properties of coated and uncoated steel sheets. Cup drawing test was performed to measure friction-coefficient. And frictional characteristics were analyzed by using FE-analysis. The effect of mechanical and frictional properties on cup drawing was investigated. It was shown that the frictional properties more affected cup drawing.

Abstract: This paper was designed to assess the adhesive properties of hard coatings made by physical vapor depositions on various substrates (AISI D2, AISI H-13 and M2) with and without an intermediate nitrided layer. An estimation of adhesion was carried out using the scratch test, where adhesion is measured by the critical load (Lc). This value was determined as the normal force affecting the indenter and causing the coating detachment as well as the acoustic emission signal containing the information on the extent of coating damage. The scratch track after the scratch test was also examined with an optical microscope to observe the failure modes of each coating. Hard coatings TiN, CrN and TiAlN were chosen for this study. Results of the test showed that harder substrates and coatings give higher values of critical loads.

Abstract: This paper focuses on fabricated slurry by electromagnetic stirring (EMS) with A6061 and A7075 wrought aluminum alloys. For this EMS process, it is important to find the optimal electromagnetic stirring conditions such as pouring temperature, stirring current and stirring time. After electromagnetic stirring according to each condition, the billets fabricated by EMS were investigated, as was the microstructure and cooling temperature of molten alloy which was directly cooled from liquid state to the semi-solid state during EMS. In use of cooling time and temperature data, it is possible to determine the forming time and stirring time required, respectively.

Abstract: Semisolid processing is now becoming of greater interest for the production of various parts than pressure die casting. In the rheo die casting process, the important thing is to control the solid particles behavior during the rheo die casting process. So, in this paper, to find out the effect of stirring current and stirring time on the microstructure and mechanical properties, rheo die casting experiments were performed according to the stirring current such as 0, 20, 40 and 60 A and the stirring time such as 20, 40 and 60 sec. The results to the experimental conditions were analyzed.