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전기선폭발법으로 제조된 텅스텐 분말의 산소 조성이 방전플라즈마소결 거동에 미치는 영향
김철희,이성,김병기,김지순,Kim, Cheol-Hee,Lee, Seong,Kim, Byung-Kee,Kim, Ji Soon 한국분말야금학회 2014 한국분말재료학회지 (KPMI) Vol.21 No.6
Effect of oxygen content in the ultrafine tungsten powder fabricated by electrical explosion of wire method on the behvior of spark plasma sintering was investigated. The initial oxygen content of 6.5 wt% of as-fabricated tungsten powder was reduced to 2.3 and 0.7 wt% for the powders which were reduction-treated at $400^{\circ}C$ for 2 hour and at $500^{\circ}C$ for 1h in hydrogen atmosphere, respectively. The reduction-treated tungsten powders were spark-plasma sintered at $1200-1600^{\circ}C$ for 100-3600 sec. with applied pressure of 50 MPa under vacuum of 0.133 Pa. Maximun sindered density of 97% relative density was obtained under the condition of $1600^{\circ}C$ for 1h from the tungsten powder with 0.7 wt% oxygen. Sintering activation energy of $95.85kJ/mol^{-1}$ was obtained, which is remarkably smaller than the reported ones of $380{\sim}460kJ/mol^{-1}$ for pressureless sintering of micron-scale tungsten powders.
지역규모 대기질 모델 결과 평가를 위한 통계 검증지표 활용 - 미세먼지 모델링을 중심으로 -
김철희,이상현,장민,천성남,강수지,고광근,이종재,이효정,Kim, Cheol-Hee,Lee, Sang-Hyun,Jang, Min,Chun, Sungnam,Kang, Suji,Ko, Kwang-Kun,Lee, Jong-Jae,Lee, Hyo-Jung 한국환경영향평가학회 2020 환경영향평가 Vol.29 No.4
본 연구에서는 3차원 기상 및 대기질 모델의 입출력 자료를 평가하는 데 필요한 통계 검증지표를 선별하고, 선정된 검증지표의 기준치를 조사하여 그 결과를 요약하였다. 여러 국내외 문헌과 최근 논문 검토를 통해 최종 선정된 통계 검증지표는 MB (Mean Bias), ME (Mean Error), MNB (Mean Normalized Bias Error), MNE (Mean Absolute Gross Error), RMSE (Root Mean Square Error), IOA (Index of Agreement), R (Correlation Coefficient), FE (Fractional Error), FB (Fractional Bias)로 총 9가지이며, 국내외 문헌을 통해 그 기준치를 확인하였다. 그 결과, 기상모델의 경우 대부분 MB와 ME가 주요 지표로 사용되어 왔고, 대기질 모델 결과는 NMB와 NME 지표가 주로 사용되었으며, 그 기준치의 차이를 분석하였다. 아울러 이들 통계 검증지표값을 이용하여 모델 예측 결과를 효과적으로 비교하기 위한 표출 도식으로 축구 도식, 테일러 도식, Q-Q (Quantile-Quantile) 도식의 장단점을 분석하였다. 나아가 본 연구 결과를 기반으로 우리나라의 산악지역의 특수성 등이 잘 고려된 통계 검증지표의 기준치 설정 등의 추가연구가 효과적으로 진행될 수 있기를 기대한다. We investigated statistical evaluation parameters for 3D meteorological and air quality models and selected several quantitative indicator references, and summarized the reference values of the statistical parameters for domestic air quality modeling researcher. The finally selected 9 statistical parameters are MB (Mean Bias), ME (Mean Error), MNB (Mean Normalized Bias Error), MNE (Mean Absolute Gross Error), RMSE (Root Mean Square Error), IOA (Index of Agreement), R (Correlation Coefficient), FE (Fractional Error), FB (Fractional Bias), and the associated reference values are summarized. The results showed that MB and ME have been widely used in evaluating the meteorological model output, and NMB and NME are most frequently used for air quality model results. In addition, discussed are the presentation diagrams such as Soccer Plot, Taylor diagram, and Q-Q (Quantile-Quantile) diagram. The current results from our study is expected to be effectively used as the statistical evaluation parameters suitable for situation in Korea considering various characteristics such as including the mountainous surface areas.
Al 5052, 6061합금에 대한 플라즈마-GMA 용접공정특성 평가
김철희,안영남,최진강,이세헌,Kim, Cheol-Hee,Ahn, Young-Nam,Choi, Jin-Kang,Rhee, Se-Hun 대한용접접합학회 2010 대한용접·접합학회지 Vol.28 No.6
In this study, the bead-on-plate welding were conducted by using GMA welding and plasma-GMA welding on Al 5052 and, Al 6061 plates. For these processes, the maximum welding speeds for full penetration were compared, and various weld qualities such as bead appearance, cross-section, cracks and porosities were examined. Faster welding speed, cleaner bead surface and more stable drop transfer could be achieved by plasma-GMA welding, compared with GMA welding.
김철희,채현병,이창우,김정한,이세헌,Kim, Cheol-Hee,Chae, Hyun-Byung,Lee, Chang-Woo,Kim, Jeong-Han,Rhee, Se-Hun 대한용접접합학회 2006 대한용접·접합학회지 Vol.24 No.1
Laser-rotating arc hybrid welding was introduced by combining $CO_2$ laser and rotating gas metal arc welding. While the arc rotation enhances the weld pool motion, it reduces the undercut formation which is one of most critical weld defects in the conventional laser-arc hybrid welding. This research investigated the bead characteristics according to the welding parameters such as frequency of rotation, welding voltage, shielding gas composition and interspacing distance between laser and we. The welding parameters were selected to reduce spatter generation and ensure sound weld beads fur bead welding and butt welding with various joint gaps. Gap bridging ability was improved, such that the sound weld beads were achieved for butt joint with up to 2mm joint sap, with no adjustment of CTWD(Contact tip-to-workpiece distance) and electrode diameter.
아연도금 강판의 $CO_2$ 레이저-TIG 하이브리드 용접에 관한 연구 Part 2 : 공정변수와 용접성과의 관계
김철희,최웅용,채현병,김정한,이세헌,Kim, Cheol-Hee,Choi, Woong-Yong,Chae, Hyun-Byung,Kim, Jeong-Han,Rhee, Se-Hun 대한용접접합학회 2006 대한용접·접합학회지 Vol.24 No.4
Optimization of process parameters for laser-arc hybrid welding process is intrinsically sophisticated because the process has three kinds of parameters-arc, laser and hybrid welding parameters. In this paper, the relationship between weldability and several process parameters such as laser beam-arc distance, electrode height, welding current and welding speed, were investigated by the full factorial experimental design. Weld quality was evaluated by using weight of spatters which is related with the pore area. It was found that the weld quality was increased with the increases in laser beam-arc distance and welding current, and decreased with the increases in electrode height and welding speed.
아연도금 강판의 $CO_2$ 레이저-TIG 하이브리드 용접에 관한 연구 Part 1 : 용접현상분석
김철희,최웅용,채현병,김정한,이세헌,Kim, Cheol-Hee,Choi, Woong-Yong,Chae, Hyun-Byung,Kim, Jeong-Han,Rhee, Se-Hun 대한용접접합학회 2006 대한용접·접합학회지 Vol.24 No.4
In lap welding of zinc-coated steel, porosity formation is one of most significant weld defects, which is caused by zinc vapor generated between the steel sheets. Various solutions have been proposed in the past years but development of more effective method is a worthwhile subject to be investigated. In this study, autogenous laser welding and laser-TIG hybrid welding was applied to the lap welding of zinc-coated steel without gap, and weld pool behaviors were observed by using high speed camera and the porosity generation mechanism was analyzed. The weld defects were successfully eliminated by laser-TIG hybrid welding. This is because the leading TIG arc partially melted the upper sheet and vaporized/oxidized the coated zinc on the lapped surfaces prior to the trailing laser illuminating the specimen.