Papers by Author: Geun An Lee

Abstract: Laser forming is an advanced process in sheet metal forming in which a laser heat source is used to shape the metal sheet. In this paper, shape correction of stamped product using the laser forming technique is numerically invesitgaed. As a fundamental study, laser irradiated bending process has been analyzed through the thermo-mechanical FE analysis. Laser scanning experiments has been also conducted for varificaiton of numerical simulations. Finally, the laser forming technique is applied to the u-shaped channel member for springback compensation. It is observed that laser irradiation gives the desired bending angle to compensate the springback by controlling the process parameters such as the laser power, scanning speeds and the number of laser scanning.

Abstract: In this study, a Hydro Co-Extrusion Process of Al-Mn/Al-Si hybrid alloys prepared by electromagnetic duo-casting was investigated. Sleeve of the duo-casted billets is 3003 Al-Mn alloy and core is 4004 Al-Si alloy. It is expected hydrostatic stress during the HCE process enhances formability of materials and minimize any defects in extruded billets due to different material properties between two Al alloys. The duo-casted billet was extruded at temperature of 423K and with extrusion ratio of 5. Finally, the effect of the HCE on microstructure of the billet was investigated by an optical microscope.

Abstract: New manufacturing process capable of high density patterns has been required. The most popular technique used today to form such fine pitch conductors is Semi-Additive Process(SAP) or variations of semi-additive process. New manufacturing method focus on the technologies designed to produce a smooth interface between resin and copper conductors for advantages in the high frequency application and fine conductor pitch capabilities. In this study, laser microprocessing was investigated for resin materials with various filler size, and was selected by suitable materials with processability and physical characteristics.

Abstract: In this study, the variation of workability of semi-continuously casted and extruded ZK60A magnesium alloy was investigated. To determine the deformation capability of two different billets, uniaxial compression tests were conducted at elevated temperatures and two different strain rates. In addition, the microstructural evolution was investigated using electron backscatter diffraction (EBSD) to compare the microstructure before and after the extrusion. The formability of ZK60A depending on the microstructure is discussed based on the experimental results obtained in this study, and is compared with earlier research in the literature.

Abstract: Laser forming involves heating sheet metal workpiece along a certain path with a defocused laser beam directed irradiate to the surface. During laser forming, a transient temperature fields is caused by the irradiation and travelling of a laser beam. Consequently, thermal expansion and contraction take place, and allows the thermal-mechanical forming of complex shapes. This is a new manufacturing technique that forming metal sheet only by thermal stress. Therefore, the analysis of temperature fields and stress fields are very useful for studying the forming mechanism and controlling the accuracy of laser forming. The non-liner finite element solver, ABAQUS, is employed to solve the thermal-mechanical analysis. Using this model, the temperature and stress distribution of DP980 are analyzed. Numerical results agree well with the experimental results.

Abstract: AZO (Aluminum doped Zinc Oxide) is widely used to produce transparent conductive coatings for liquid crystal displays, flat panel displays, plasma displays, touch panels, and electronic ink applications. The densification behavior of AZO powder is a critical factor related to the design of the compaction process. In this study, the densification behavior of AZO powder during cold compaction has been investigated in order to calibrate the modified Drucker-Prager Cap (DPC) model for FE simulations. A compaction test with a cylindrical die was carried out, and two failure tests were performed: the diameteral compression test and the uniaxial compression test. AZO compacts with various densities from the compact tests were used as specimens for the failure tests. Based on the experimental results, the parameters of the modified DPC model were determined through simple manipulations.

Abstract: In this paper, finite element analyses of thread rolling process for the micro-sized screw with the diameter of 800 μm and the thread pitch of 200 μm are firstly carried out. For the finite element analysis of the thread rolling process, hardening behavior of SUS has been estimated by the compression and tensile tests. And then, process design has been conducted with the aid of simulations and the prototype of the micro screw finally has been fabricated. In order to verify the CAE based process design for micro-sized screws, the deformed shape and dimensions obtained from the CAE are compared with those from the experiment. The deformed shapes of the pitch part and the top part of threads can be demonstrated the feature of experimental result.

Abstract: High precision micro screws have been widely required for the fastening parts of electronic storage devices. Teeth’s forming of the micro screw is generally utilized by a cold thread rolling process with a flat die. This process for micro sized parts has some difficulties such as the design of die shape, alignment between dies, and process managements. In this paper, it is focused on the effect of the alignment between dies and material, and process parameters in simulation. The investigated parameters are the friction coefficient and the relative position between stationary and moving dies using a numerical simulation. The simulations provide that the shear friction factor of 0.9 is proper for preventing slip between dies and raw material and the relative position of two dies has to be set to the half length of the pitch for maintaining the continuous thread profiles. The deformed shapes of the pitch part and the top part of threads can be demonstrated the feature of experimental result. The folding phenomenon appeared at the top part of threads, which seemed to be induced by a lack of the metal flow under micro sized deformation.

Abstract: This paper is concerned with formability of AZ31B magnesium alloy sheets at various temperatures. In order to acquire the forming limit curve for quantifying the formability of the magnesium alloy sheet, hemispherical punch forming tests are performed under various temperatures. The initial shape of the blank is rectangular and six different types of specimens are used with their length along the rolling direction of 175mm and widths along the transverse direction of 25, 50, 75, 100, 120 and 175mm. With the equipment of on a SIMADZU 100-ton universal testing machine, hemispherical punch forming tests are performed and a forming limit diagram is constructed at the temperature of 100°C, 200°C and 250°C. FE analyses of hemispherical punch forming tests are also conducted by considering ductile failure model in order to compare the forming limit curve obtained from the experiment and numerical analysis. The comparison examines problems in the current material model and it suggests a future research direction.

Abstract: Mechanical properties of micro-electro-mechanical systems (MEMS) materials are increasingly important with the wide use of miniaturized systems. This paper deals with development of the strain measurement module for micro materials tests. Vision strain method (VSM) and actuating stroke method (ASM) are adopted in the strain measurement module in order to reduce the discrepancies between the measured deformation and actual one in the specimen. With the developed strain measurement system, a number of tensile tests were concluded for 99.9% nickel thin films with the thickness of 500µm. Specimens with various widths from 1 mm to 2mm and with various lengths from 2mm to 4mm were used in the tests. From the experiment, material properties such as yield strength, tensile strength, fracture strength, fracture strain and young’s modulus are analyzed with stress-strain curves of 99.9% nickel thin film.