Papers by Author: Jong Taek Yeom

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Authors: Jong Taek Yeom, Jeoung Han Kim, Nam Yong Kim, Nho Kwang Park, Chong Soo Lee
Abstract: The dynamic globularization behavior during hot working of Ti-6Al-4V alloy was investigated by high temperature torsion tests. The torsion tests were carried out to investigate microstructure evolution occurring during dynamic globularization in Ti-6Al-4V alloy. The torsion tests were performed under a wide range of temperatures and strain rates with true strain up to 2. The flow curves revealed that the amount of flow softening for the fine alpha-lamellae structure was higher than that for the coarse alpha-lamellae structure under the temperature of 900oC. The effects of hot deformation parameters and initial microstructures on the dynamic globularization were analyzed.
Authors: Jong Taek Yeom, J.H. Park, J.W. Lee, Nho Kwang Park
Abstract: Friction welding of dissimilar materials, Ni-base superalloy IN713LC and oil-quench plus tempered AISI 4140 steel, was investigated. Friction welding was carried out with various process variables such as friction pressure and time. The quality of welded joints was tested by applying bending stresses in an appropriate jig. Microstructures of the heat-affected zone (HAZ) were investigated along with micro-hardness tests over the friction weld joints. DEFORM-2D FE code was used to simulate the effect of welding variables in friction welding process on the distributions of the state variables such as strain, strain rate and temperature. The formation of the metal burr during the friction welding process was successfully simulated, and the temperature distribution in the heat-affected zone indicated a good agreement with the variation of the microstructures in the HAZ.
Authors: Jeoung Han Kim, Jong Taek Yeom, Nho Kwang Park, Chong Soo Lee
Abstract: The high-temperature deformation behavior of the single-phase α (Ti-7.0Al-1.5V) and α + β (Ti-6Al-4V) alloy were determined and compared within the framework of self-consistent scheme at various temperature ranges. For this purpose, isothermal hot compression tests were conducted at temperatures between 650°C ~ 950°C to determine the effect of α/β phase volume fraction on average flow stress under hot-working condition. The flow behavior of α phase was estimated from the compression test results of single-phase α alloy whose chemical composition is close to that of α phase of Ti-6Al-4V alloy. On the other hand, the flow stress of β phase in Ti-6Al-4V was predicted by using self-consistent method. The flow stress of α phase was higher than that of β phase above 750°C, while the β phase revealed higher flow stress than α phase at 650°C. Also, at temperature above 750°C, the predicted strain rate of β phase was higher than that of α phase. It was found that the relative strength between α and β phase significantly varied with temperature.
Authors: Jong Taek Yeom, Chong Soo Lee, Jeoung Han Kim, Dong Geun Lee, Nho Kwang Park
Abstract: A methodology for evaluating and predicting component lives in creep-fatigue interaction region was investigated for Waspaloy. A unified viscoplasticity constitutive equation including multi-back stresses was used to describe cyclic material behaviors. Also, a continuum damage model coupling with the creep-fatigue damage rules was established based on the analysis of creep and low cycle fatigue behavior. Multi-axial fatigue and creep equivalent stress concepts were employed to predict three dimensional component lives. Notched cyclic tests under various stress conditions in the creep-fatigue interaction region were carried out to validate the life prediction methodology with FEM simulation based on the continuum damage model. The comparison of experimental data and prediction results indicates that the continuum damage model is a powerful approach for the prediction of component lives.
Authors: Seon Jin Kim, Yu Sik Kong, Won Taek Jung, Jong Taek Yeom, Nho Kwang Park
Abstract: The purpose of this study is to investigate the high temperature creep life of Waspaloy using the Initial Strain Parameter Technique (ISPT). The creep tests were performed at the elevated temperatures from 550oC to 700 oC. Constant stress creep tests were carried out in the experiment. The initial strain was measured for one minute after loading. The creep life of Waspaloy was calculated using the creep life prediction equation of ISPT. The confidence level between the experimental rupture time and the calculated rupture time using the ISPT is within 95%. So, the results show that the creep life prediction by the ISPT was a good agreement with LMP method.
Authors: Nho Kwang Park, Jong Taek Yeom, Young Sang Na, J.S. Lee, In Ok Shim, S.S. Hong
Authors: Byung Hak Choe, Jeong-Keun Shin, Soo Keun Shin, Je Hyun Lee, Ung Yu Paik, Jong Taek Yeom, Nho Kwang Park
Authors: Jong Taek Yeom, Jeoung Han Kim, Jae Keun Hong, Nho Kwang Park, Chong Soo Lee
Abstract: Microstructure evolution during ring rolling process of a large-scale Ti-6Al-4V ring was investigated with the combined approaches of three dimensional finite element method (FEM) simulation and microstructure prediction model. A microstructure prediction model was established by considering the volume fractions and grain size of  and  phases varying with process variables, and grain growth. In order to perform FE simulation for ring rolling process of Ti-6Al-4V alloy, a constitutive equation was generated by utilizing the flow stress data obtained from hot compression tests at different temperature and strain rate conditions. The volume fraction and grain size of  and  phases during ring rolling were calculated by de-coupled approach between FEM analysis and microstructure prediction model. The prediction results were compared with the experimental ones. Our proposed microstructure simulation module was useful for designing hot forming process of Ti-6Al-4V alloy
Authors: Jong Taek Yeom, Eun Jeoung Jung, Jeoung Han Kim, Jae Keun Hong, Nho Kwang Park, Seung Sik Choi, Chong Soo Lee
Abstract: The high temperature deformation behavior and flow instabilities of Ni-Fe-Co base superalloy, INCONEL alloy 783 during hot working process were investigated with process maps consisting of a power dissipation of dynamic materials model (DMM) and various flow instability criteria. In order to establish the processing map of INCONEL alloy 783, hot compression tests were carried out under different temperature and strain rate conditions, with true strain up to 0.7. On the basis of the comparison between processing maps and microstructural analysis, the reliability of various flow instability criteria was estimated. Finally the useful instability criterion for predicting the forming defects was suggested through the compression test results and experimental observations of actual ring rolling process of INCONEL alloy 783.
Authors: Jong Taek Yeom, Nho Kwang Park, Steve J. Williams
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