Papers by Author: Jung Hwan Lee

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Authors: Yong Nam Kwon, Young Seon Lee, S.W. Kim, Jung Hwan Lee
Abstract: Mg alloys could be the lightest alloys among the industrially applicable engineering alloys. Since wrought Mg alloy has limited applications due to the poor formability, casting is currently the main processing technique to fabricate Mg components even though wrought alloys are superior in terms of mechanical properties and reliability. While a lot of research and development has been focused on warm forming under the temperature condition of around 250°C where more formability could be expected, superplastic forming could be another way to get over the low formability of Mg alloys. Like other superplastic materials grain boundary sliding is the main deformation mechanism of Mg superplasticity. Accommodation of stress concentration around triple point of grain boundary should be done favorably if grain boundary sliding continues without any fracture. In the present study, superplastic behavior of AZ31 alloys with several grain sizes was examined firstly. Accommodation of grain boundary sliding of AZ31 alloy would be discussed on the basis of grain morphology and texture evolution after tensile deformation.
Authors: Young Seon Lee, Y.C. Kwon, Yong Nam Kwon, Jung Hwan Lee, S.W. Lee, N.S. Kim
Abstract: Internal voids have to be eliminated for defect-free in some open die forging. The FEM analysis is performed to investigate the overlap defect of cast ingots during cogging stage. The measured flow stress data were used to simulate the cogging process of cast ingot using the practical material properties. Also the numerical analysis of void closure is performed by using the DEFORMTM-3D. The calculated results of void closure behavior are compared with the measured results before and after upsetting, which are scanned by the X-ray scanner. From this result, the criteria for deformation amounts effect on the void closure were estimated into effective strain of 0.6 by the comparison of practical experiment and numerical analysis.
Authors: Young Gun Ko, Yong Nam Kwon, Jung Hwan Lee, Dong Hyuk Shin, Chong Soo Lee
Abstract: Cavitation behavior during superplastic flow of ultra-fine grained (UFG) Ti-6Al-4V alloy was established with the variation of grain size and misorientation. After imposing an effective strainup to 8 via equal-channel angular pressing (ECAP) at 873 K, alpha-phase grains were markedly refined from 11 μm to ≈ 0.3 μm, and misorientation angle was increased. Uniaxial-tension tests were conducted for initial coarse grained (CG) and two UFG alloys (ε = 4 and 8) at temperature of 973 K and strain rate of 10-4 s-1. Quantitative measurements of cavitation evidenced that both the average size and the area fraction of cavities significantly decreased with decreasing grain size and/or increasing misorientation. It was also found that, when compared to CG alloy, cavitation as well as diffused necking was less prevalent in UFG alloys, which was presumably due to the higher value of strain-rate sensitivity. Based on the several theoretical models describing the cavity growth behavior, the cavity growth mechanism in UFG alloys was suggested.
Authors: Young Seon Lee, Jung Hwan Lee, M.Y. Lee, Young Hoon Moon, T. Ishikawa
Abstract: Formability of tube in elevated temperature is essential data to design the warm hydroforming process parameters, such as tube diameter, forming temperature and die geometries. Since the quantitative data of forming limit can be used to predict the failure on forming process, formability data available on the FE analysis is one of the very important information for the optimum design. In this study, the effect of heat treatment conditions and deformation temperature on the formability was investigated for the warm hydroforming of Al6061 tube. Full annealing and T6-treatment are applied for the heat treatment of Al6061 tubes. To evaluate the hydroformability, uni-axial tensile test and bulge test were performed at temperature ranges between room temperature and 300oC. The measured flow stresses were used as input parameters for the simulation of warm hydroforming process. The damage value and strain variation during hydroforming are analysed by FEM. A forming limit based on the ductile fracture criteria has been proposed by combining the results of experimental and FE analysis for the estimation of formability and optimization of warm hydroforming process.
Authors: Jonghun Yoon, Oana Cazacu, Jung Hwan Lee
Abstract: In spite of this progress in predicting ductile failure, the development of macroscopic yield criteria for describing damage evolution in HCP (hexagonal close-packed) materials remains a challenge. HCP materials display strength differential effects (i.e., different behavior in tension versus compression) in the plastic response due to twinning. Cazacu and Stewart [1] developed an analytic yield criterion for a porous material containing randomly distributed spherical voids in an isotropic, incompressible matrix that displays tension-compression asymmetry. The matrix material was taken to obey the isotropic form of the Cazacu et al. [2] yield criterion, which captures the tension-compression asymmetry of the matrix material. In this paper, finite element calculations of a round tensile bar are conducted with the material behavior described by the Cazacu and Stewart [1] yield criterion. The goal of these calculations is to investigate the effect of the tension-compression asymmetry on the necking induced by void evolution and propagation.
Authors: Yong Nam Kwon, Young Seon Lee, Jung Hwan Lee
Abstract: Micro-alloyed cold forging steel has been developed to rule out heat treatment process before forging in order to save energy consumption. These non heat-treated cold forging steels utilize the work hardening during cold working to ensure the mechanical properties. In other words, the required strength of forged part is achieved by work hardening with the accumulation of plastic strain during the cold working. Therefore, the plastic deformation characteristics should be carefully understood for successful process design. Evolution of both microstructure and plastic characteristics of micro-alloyed cold forging steel has been investigated in the present study. For the optimization of forging processes, finite element analysis and die life predictions were carried out.
Authors: Yong Nam Kwon, Young Seon Lee, Jung Hwan Lee
Abstract: Industrial application of magnesium alloys has increased significantly recently. However, wrought magnesium alloys still have a lot of technical challenges to be solved for more applications. First of all, low formability of wrought alloys should be improved by optimizing the processing variables. In the present study, the effect of process variables such as forging temperature and forging speed were investigated to forgeability of magnesium alloy. To understand the effect of process variables more specifically, both numerical and experimental works have been carried out on the model which contains upsetting geometry. Forgeability of AZ31 alloy was found to depend more on the forging speed rather than temperature. Forged sample showed a significant activity of twinning, which was found to be closely related with flow uniformity.
Authors: Hyae Kyung Yi, Jung Hwan Lee, Young Seon Lee, Young Hoon Moon
Abstract: Warm hydroformability and mechanical properties of pre- and post- heat treated Al6061 tubes were investigated in this study. For the investigation, as-extruded, fully annealed and T6- treated Al 6061 seamless tubes were prepared. To evaluate the hydroformability, uni-axial tensile test and free bulge test were performed at room temperature and 200ÓC. Also mechanical properties of hydroformed part at various pre- and post-heat treatments were evaluated by tensile test. The tensile test specimens were obtained from hexagonal shaped tube hydroformed at 200ÓC forming temperature. As a result, hydroformability of fully annealed tube is 25% higher than that of extruded tube. The tensile strength and elongation were more than 330MPa and 12%, respectively, when hydroformed part was T6 treated after warm hydroforming. However, hydroformed part using T6 pre treated tube represents low elongation, 8%. Therefore, the T6 treatment after hydroforming for as-extruded tube is proved to be the most cost-effective among various processing conditions.
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