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Du‑Youl Choi,Ashutosh Sharma,Sang‑Ho Uhm,Jae Pil Jung 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.1
Liquid metal embrittlement (LME) caused cracking of Galvanized transformation induced plasticity steels was investigatedduring resistance spot welding (RSW). Effect of electrode force (3–5 kN) for a weld time of 400, and 800 ms on crack resistanceof TRIP steel was examined in relation to LME phenomenon. The microstructural characteristics of spot weld jointsand LME cracking tendency were investigated using dye penetration tests, optical microscopy, scanning electron microscopytogether with energy dispersive X-ray spectroscopy. It was found that as the electrode force increases, the crack size decreasesdue to a fast increase in crack tip temperature which rises further with more holding time up to 800 ms in high temperaturezone. Least amount of crack size was observed at 5 kN electrode force and 400 ms of welding time. Finally, the experimentalresults have also been simulated by finite element modeling (FEM) to find suitable mechanism of crack formation, and acombination of 4 kN and 400 ms was suggested for the crack free and less thermal deformation in the spot welded TRIP steel.
고Si DP980강 스폿 용접 특성에 미치는 Phosphorus (P) 및 in-situ 후열처리 펄스 조건의 영향
최두열(Du-Youl Choi) 대한용접·접합학회 2015 대한용접·접합학회지 Vol.33 No.6
Recently, application of UHSS(Ultra High Strength Steels) whose tensile strength is over 1000MPa to car body structure are growing due to great needs for light weighting and improved crash worthiness. However, their poor weldability is one of obstacles to expand selecting to car body. In this study, effect of Phosphorus contents on resistance spot weldability of high elongation DP980 steel whose Si content is over 1% was investigated. The cross tension strength (CTS) was decreased showing partial interface fracture as Phosphorus content increase because of solidification segregation of Phosphorus. In order to improve resistance spot weldability by modification of welding condition, in-situ post-weld heating pulse was introduced after main pulse. The optimum cooling time between main and post pulse and post-pulse current condtion were determined through FEM welding simulation and DOE tests. The CTS was increased about 1.5 time showing plug fracture. The decrease of Phosphorus segregation was found to be a major reason for weld ductility and CTS improvement.
다중효용 담수설비용 열압축기의 디퓨져 형상에 관한 연구
최두열(Du-youl Choi),정한식(Han-shik Chung),정효민(Hyo-min Jeong),김진화(Zhen-hua Jin),김필환(Pil-hwan Kim) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.10
A thermocompressor is the equipment which compresses a vapor to a desired discharge pressure. Since it was first used as the evacuation pump for a surface condenser, it has been widely adopted for energy saving systems due to its high working confidence. In the present study, the geometrical analysis of the shape between the jet nozzle and the diffuser inlet, the drag force was calculated by means of the integrated equation of motion and the computational fluid dynamic (CFD) package called FLUENT. The computer simulations were performed to investigate the effects by the various suction flow rates, the distance from jet nozzle outlet to the diffuser inlet and the dimensions of the diffuser inlet section through the iterative calculation. In addition, the results from the CFD analysis on the thermocompressor and the experiments were compared for the verification of the CFD results. Furthermore, in this study, a special attention was paid on the performance of the thermocompressor by varying the diffuser convergence angle of 0.0°, 0.5°, 1.0°, 2.0°, 3.5° and 4.5°. With the increase of the diffuser convergence angle, the suction capacity was improved up to the degree of 2.0° while it was decreased over the degree of 2.0°.