Papers by Author: Dae Cheol Ko

Abstract: Common joining methods used in automotive industry are welding, adhesive bonding, friction stir welding, mechanical fastening, self-piercing rivets, mechanical clinching and so on, for multi-material designed automotive bodies. Among different joining methods, mechanical clinching which achieves geometrical interlocking by plastic deformation has several advantages such as no need of additional joining elements and fast joining. But mechanical clinching is difficult to join a ductile material with a high-strength or low ductility material. Therefore the hole clinching as a new mechanical clinching process has been proposed to join these material combinations. In the hole clinching process, as punch force is applied to a upper sheet (a ductile material), it is indented into hole of lower sheet (low ductile material) on die and then interlocked by plastic deformation. It is very important for a successful design of hole clinching to predict the failure mode such as neck fracture and button separation and the strength of hole-clinched joint. For this an analytical approach was carried out for the hole clinching process of Al6061 and DP980. Tool geometry used in hole clinching was designed by the predicted failure mode. Preliminary finite element simulation was performed to validate the geometrical interlocking and joinability. The predicted failure mode and strength were verified by the results of cross tension test.

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: 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: In this paper, the adhesive properties of hard coatings(TiN, TiCN) made by plasma chemical vapor depositions on non-nitrided and nitrided substrates was estimated using the scratch test, where adhesion was 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. Results of the test showed that harder substrates and coatings give higher values of critical loads.

Abstract: The aim of this study is to design flash geometry of bearing hub to apply the automatic transfer system in hot forging process. The flash geometry is very important in hot forging process because the flash geometry effects on the metal flow, material losses, forging load, die pressure and so on. In this study, the problem of designing the flash geometry is studied with flash thickness and width considering the maximum die pressure to apply an automatic transfer system in hot forging process for bearing hub. The numerical analysis was conducted by means of the commercial S/W DEFORM. On the basis of numerical analysis the flash geometry of hot forging die was redesigned, and experiment was conducted. From the experimental results, it was possible to produce bearing hub with an automatic transfer system without any deterioration of die lifetime.