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Kim Y.G.,Green Manufacturing Process Technology Center | Kim I.J.,Green Manufacturing Process Technology Center | Kim J.S.,Green Manufacturing Process Technology Center | Chung Y.I.,POSCO | Choi D.Y.,POSCO
Materials Transactions | Year: 2014

The development of the automotive industry is now focused not only on improving basic vehicle performance but also on reducing weight and enhancing safety and durability. Various automotive high-strength steels are being developed, and Zn-coated steels are being manufactured to prevent corrosion of the external white vehicle body. The most commonly used welding method in the car body assembly process is resistance spot welding (RSW), which has been extensively studied worldwide. In this process, the work piece is basically heated according to the contact resistivity of the interfacial between the electrode and the material as well as the bulk resistivity of the material itself. At this point, if the metal is Zn, which has a lower melting point than the Fe base metal on the surface, it is mainly melted in the temperature range of 400900° C. It becomes easy to penetrate the grain boundary of the HAZ during welding. Also, the tensile stress in such a state decreases the ductility of the grain boundary and causes liquid metal embrittlement (LME). Cu5Zn8, an intermetallic compound, can be formed from the reaction of the alloy with the Cu material electrode in the expulsion current range at a high temperature. Its formation is likely to be facilitated by LME or a surface crack. In this study, the fatigue characteristics of a tensile shear specimen during spot welding was investigated with the welding parameters that occur in the surface crack of welds on Zn-coated steel. Finally, a controlled spot welding condition was suggested to prevent surface cracks. © 2013 The Japan Institute of Metals and Materials.

Shim D.-S.,Green Manufacturing Process Technology Center | Kim K.-P.,Green Manufacturing Process Technology Center | Lee K.-Y.,Green Manufacturing Process Technology Center
Journal of Materials Processing Technology | Year: 2016

Bent pipes are widely used in many high-end industries as a structural component. Consequently, pipe roll bending has become an attractive manufacturing technology for forming lightweight products. For industrial applications, bent pipes should be accurately shaped into 2D or 3D shapes without defects. In this study, the behavior of pipes with rectangular cross-sections under roll bending was considered in order to improve product quality. When a pipe product is bent with a large curvature, the pipe cross-section shrinks under the bending moment. In order to minimize defects and improve product quality, this paper proposes double-stage forming to regulate the shrinking load, which causes the axial wrinkles and poor strength, as structural members. In double-stage forming, the bending radius at the pre-bending stage is assumed to be a key parameter to avoid cross-section distortion. In order to determine the pre-bending radius, material behavior was analyzed to figure out the reason for the cross-section shrinkage. The bending stresses on the cross-section of the pipe material subject to bending moments bring about the distribution of inward forces, which induce shrinkage moments on the cross-section. The main idea in this paper is that the cross-section distortion can be minimized by regulating the pre-bending radius at the first stage. The proposed process design was evaluated through nonlinear finite element simulation using a commercial program and experiments. © 2016 Elsevier B.V. All rights reserved.

Kim Y.-G.,Green Manufacturing Process Technology Center | Kim I.-J.,Green Manufacturing Process Technology Center | Kim J.-S.,Green Manufacturing Process Technology Center | Park J.-H.,DanDan Co.
Materials Transactions | Year: 2014

Early studies have been conducted on the friction stir welding of steel, tungsten (W), molybdenum (Mo), tungsten carbide, and the like for the production of tools, but the tool materials were significantly worn out and deformed while they were being joined. So far, the polycrystalline cubic boron nitride (PCBN) tool, which is very strong at high temperatures and is wear-resistant, has performed relatively well, but the improvement of its productivity is limited by its high manufacturing cost. In this study, 1.4 mm-thick DP590 steel was subjected to FSW using the ceramic material of a Si3N4 tool. The main process parameters of the butt joint at rotation speeds of 600-1,000 rpm and traveling speeds of 180-300 mm/min were determined through the bead-on-plate experiment. Also, the tensile strength, hardness, and Charpy impact properties of each FSW joint were evaluated, the full extent of the wear was measured and the tool life of the PCBN tool that was about four times as costly to produce was compared with the maximum joint length to determine the durability of the tool. ©2014 The Japan Institute of Metals and Materials

Lee E.-M.,Gwangju University | Shim D.-S.,Green Manufacturing Process Technology Center | Son J.-Y.,Gwangju University | Baek G.-Y.,Gwangju University | And 2 more authors.
Journal of Mechanical Science and Technology | Year: 2016

Advanced high-strength steel (AHSS) is widely used in automobile manufacturing to reduce the weight of vehicles, thereby improving fuel efficiency. However, the high yield and tensile strength of AHSS leads to a serious springback problem in the cold sheet metal forming process. This phenomenon has delayed the implementation of AHSS in vehicle parts due to the resulting negative impact on part accuracy. In this study, parameter optimization and multi-stage die compensation were conducted with Finite element (FE) analysis to develop a progressive forming process for automobile structural members using DP980. The FE simulation used the Yoshida-Uemori model to predict the springback phenomenon accurately. The key parameters that significantly influence the springback behavior were optimized using FE simulation and the Taguchi method. The simulation results were used to determine the die and mold compensation. After the parameter optimization and multi-stage die compensation, the final part was obtained with acceptable dimensional accuracy. © 2016, The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg.

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