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고대훈(Dae Hoon Ko),김태정(Tae Jung Kim),이봉상(Bong Sang Lee),임학진(Hak Jin Lim),이정민(Jung Min Lee),김병민(Byung Min Kim) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.5
The purpose of this study predicts the residual stress of Al6061 during T6 heat treatment according to conditions of solid solution and ageing time. The General T6 heat treatment of AI6061 is composed of solid solution(530℃, 2hr) and artificial ageing(175℃, 9hr) process. This study conducts the prediction and measurement of residual stress during T6 heat treatment. The prediction of residual is analyzed by elasto-plastic nonlinear FE-simulation and the residual stress relaxation of artificial ageing is investigated by applying a Zener-Wert-Avrami function. In order to consider the relaxation of residual stress during artificial ageing, FE-simulation is used in DEORM-3D V6.0 with USER SUBROUTINE. Also, residual stress is measured by X-ray diffraction(XRD) which is conducted to confirm the result of FE-simulation. Residual stress predicted by FE-simulation is in good agreement of with measured residual stress.
극저온 열처리에 의한 Al6061 합금의 잔류응력 제거 및 열처리 특성 평가
고대훈(Dae Hoon Ko),박기정(Ki Jung Park),조영래(Young Rae Cho),임학진(Hak Jin Lim),이정민(Jung Min Lee),김병민(Min Byung Kim) 대한기계학회 2011 大韓機械學會論文集A Vol.35 No.10
본 연구의 목적은 극저온 열처리를 통해 Al6061 의 열처리 잔류응력을 제거하는 것이다. 이를 위해 유한요소해석을 이용하여 열처리 잔류응력을 예측하였으며, 열처리 조건에 따른 각 단계별 대류 열전달계수를 T6 와 극저온 열처리 실험을 수행하여 결정하였다. 예측된 잔류응력 결과는 X 선 회절법(XRD)으로 측정된 잔류응력 결과와 비교하여 유한요소해석 결과의 타당성을 확인하였다. 또한 T6 와 극저온 열처리에 대해 각각 전기 전도도와 경도를 측정하여 기계적 특성을 평가하고 TEM 관찰과 XRD 회절 분석을 통해 석출물의 크기 및 성분을 파악하였다. 이를 통해 Al6061 의 T6 열처리와 비교하여 극저온 열처리를 적용함에 따른 잔류응력, 기계적 특성 및 미세조직변화를 조사하였다. The purpose of this study is to relieve the residual stress of Al6061 using cryogenic heat treatment. Experimental T6 and cryogenic heat treatments were carried out to define the convective heat-transfer coefficient, which was then applied in the finite-element method (FEM) to predict the residual stress. The predicted residual stress was compared with the residual stress measured by X-ray diffraction (XRD), and the results were in good agreement. The mechanical properties were estimated by measuring the electrical conductivity and hardness. In addition, the size and formation of the precipitations were observed by TEM and XRD analysis for both T6 and cryogenic heat treatments. The effects of the cryogenic heat treatment on the residual stress, mechanical properties, and precipitation of Al6061 alloys were thus confirmed.
온-오프 미디어간 의제 공유와 역동성: 2014년 서울시교육감선거를 중심으로
고대훈 ( Dae Hoon Ko ),최영 ( Young Choi ) 한국커뮤니케이션학회 2015 커뮤니케이션학연구 Vol.23 No.2
본 연구의 목적은 서울시교육감선거 과정에서 나타난 미디어간 역동성을 동질성, 상호의존성, 의제 공유, 의제 폭발이라는 네 가지 개념에서 접근해 그 과정과 효과를 살펴보는 것이다. 연구 목적을 위해 소셜미디어(페이스북과 트위터), 인터넷 뉴스사이트 및 종이신문에서 데이터를 추출해 콘텐츠를 비교 분석하였다. 연구 결과 인터넷 뉴스와 SNS의 확산과 함께 신문을 포함한 미디어간 의제 공유 현상이 역동적으로 활성화된 것으로 나타났다. 이러한 현상은 신문이나 TV 등 매스미디어가 의제를 독점하던 시대는 끝났음을 의미한다. 또한 페이스북 글에서 출발한 특정 이슈가 현저성을 띠게 되면 인터넷 뉴스 사이트와 신문은 이를 의제로 만들려고 추동하고, 이 과정에서 상호작용성과 융합이 강력하게 발생하는것으로 나타났다. SNS-인터넷 뉴스 사이트-종이신문의 3각 조합은 정보의 폭포를 촉발하고, 선거 결과에 깊은 영향을 미치는 의제 폭발을 생성하였다. 본 연구는 사회이슈의 의제 폭발을 이끄는 협력 저널리즘을 탐구하는 데 기여할 것으로 기대한다. The purpose of this study is to examine the process and the effects of the intermedia dynamics by way of approaching four factors such as homophily, codependence, agenda-sharing and agenda-bombing. In order to reach this purpose, various data from the social media(Facebook and Twitter), a internet news site and a newspaper are collected and comparatively analysed. Results showed that intermedia agenda-sharing was being dynamically and mutually activated as the use of internet news and SNS rapidly spread. Such phenomenon implies that agenda monopoly dominated by traditional media has passed by. As a certain issue initiated by a Facebook post is recognized as salient, the internet news site and the newspaper have driven the issue to make an agenda in which interactivity and convergence powerfully occurs. The result also indicated that the so-called ``triangle combination`` of SNS-internet news-newspaper triggered the informational cascades and generated agenda-bombing which made deep impacts on the result of the election. It is expected that this study would contribute to explore collaborative journalism leading to agenda-bombing of social issue.
Al6061 판재성형에서 핫 포밍 퀜칭의 성형성 및 기계적 특성 평가
고대훈(Dae Hoon Ko),김재홍(Jae Hong Kim),이찬주(Chan Joo Lee),고대철(Dae Cheol Ko),김병민(Byung Min Kim) 대한기계학회 2013 大韓機械學會論文集A Vol.37 No.4
알루미늄 소재의 냉간 판재성형에서는 낮은 성형성과 성형 후 과도한 스프링백에 의한 치수정밀도가 저감되는 문제가 있다. 이에 본 연구에서는 기존 제조공법의 문제점을 개선하기 위해 새로운 제조공법인 Hot forming quenching(HFQ)을 제시하고자 한다. HFQ 은 용체화처리 온도로 가열된 알루미늄 판재를 열간성형하고, 다이 내에 설치된 냉각채널에 의해 퀜칭하여 열간성형과 열처리를 동시에 함으로써 성형성 향상, 스프링백 저감 효과를 얻을 수 있다. 기존의 냉간성형 대비 HFQ 의 성형성 향상효과를 에릭슨 시험을 통해 평가하였다. 스프링백 특성을 파악하기 위해 V-beding 시험에 HFQ 를 적용하여 굽힘 성형된 제품의 치수정밀도를 기존의 냉간성형과 비교하였다. 또한 강도 및 경도를 측정하여 기존 냉간 성형에 의한 값들과 비교함으로써 알루미늄 제품 성형에 HFQ 을 적용하기 위한 타당성을 평가하였다. In aluminum sheet metal forming, the conventional forming methods of T4 or T6 heat-treated sheets result in low formability and dimensional accuracy. This study suggests a new forming method for aluminum sheets called as hot forming quenching (HFQ) that solves the problems faced in the conventional method. HFQ combines the heat treatment and forming processes through the forming die during the quenching of a solid solution. To evaluate the application of HFQ to the sheet forming of aluminum, an Erichsen and V-bending test are performed in this study to measure the dimensional accuracy and formability, which are then compared with those of the conventional forming method. Furthermore, the strength and hardness of the products formed by HFQ are measured to confirm the degradation in mechanical properties compared with the conventional forming method, which shows the validity of the application of HFQ to aluminum sheet metal forming.
Al6061-T6 열처리 잔류응력의 유한요소해석 및 측정
고대훈(Dae Hoon Ko),김태정(Tae Jung Kim),임학진(Hak Jin Lim),이정민(Jung Min Lee),김병민(Byung Min Kim) 대한기계학회 2011 大韓機械學會論文集A Vol.35 No.7
본 연구의 목적은 Al6061 의 T6 열처리 시 발생되는 잔류응력의 변화를 예측하는 것이다. Al6061-T6 의 일반적인 열처리 조건인 용체화처리(530℃, 2hr)와 인공시효(175℃, 9hr)에서 인공시효 시간에 따라 잔류응력 변화를 고려하였다. 잔류응력 예측은 열처리 실험을 수행하여 대류 열전달계수를 확보하고, 탄소성 모델로 가정한 유한요소해석을 적용하였다. 또한 인공시효와 같이 재료가 고온에서 장시간 유지되는 경우의 Zener-Wert-Avrami 함수를 적용하여 잔류응력 변화를 확인하였다. 잔류응력의 측정은 X 선 회절법으로 측정하고 측정결과는 예측된 유한요소해석 결과와 비교하여 해석의 신뢰성을 확보하고 해석기법을 확립하였다. The purpose of this study is to predict the residual stress in Al6061 during T6 heat treatment. In this study, the variable residual stress in case of the solid solution(530℃, 2h) and artificial ageing(175℃, 9h) of Al6061 subjected to T6 heat treatment is determined at different ageing times. A heat treatment experiment is conducted to determine the heat transfer coefficient, on the basis of which the residual stress during the T6 heat treatment is predicted. In order to take into account the relaxation of residual stress during artificial ageing, a Zener?Wert?Avrami function is used and elasto-plastic nonlinear analysis is conducted through FE-simulation. Further, the residual stress is measured by using the X-ray diffraction(XRD) method, and the result is compared with the result from the FE-simulation. It is found that the residual stress predicted form the FE-simulation is in good agreement with the residual stress measured by using the XRD method.
극저온 열처리가 7075 알루미늄 합금의 잔류응력과 기계적 특성에 미치는 영향
김회봉,정은욱,고대훈,김병민,조영래,Kim, Hoi-Bong,Jeong, Eun-Wook,Ko, Dae-Hoon,Kim, Byung-Min,Cho, Young-Rae 한국재료학회 2013 한국재료학회지 Vol.23 No.1
In this study, the effects of cryogenic treatment cycles on the residual stress and mechanical properties of 7075 aluminum alloy (Al7075) samples, in the form of a tube-shaped product with a diameter of 500 nm, were investigated. Samples were first subjected to solution treatment at $470^{\circ}C$, followed by cryogenic treatment and aging treatment. The residual stress and mechanical properties of the samples were systematically characterized. Residual stress was measured with a cutting method using strain gauges attached on the surface of the samples; in addition, tensile strength and Vickers hardness tests were performed. The detailed microstructure of the samples was investigated by transmission electron microscopy. Results showed that samples with 85 % relief in residual stress and 8% increase in tensile strength were achieved after undergoing three cycles of cryogenic treatments; this is in contrast to the samples processed by conventional solution treatment and natural aging (T4). The major reasons for the smaller residual stress and relatively high tensile strength for the samples fabricated by cryogenic treatment are the formation of very small-sized precipitates and the relaxation of residual stress during the low temperature process in uphill quenching. In addition, samples subjected to three cycles of cryogenic treatment demonstrated much lower residual stress than, and similar tensile strength compared to, those samples subjected to one cycle of cryogenic treatment or artificial aging treatment.
[논문 or 기술논문 택일] Aluminum Tailored Blank의 마찰교반용접을 위한 공구 형상설계
김문생(Moon-Sang Kim),고대훈(Dae-Hoon Ko),김재홍(Jae-Hong Kim),김병민(Byung-Min Kim) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.5
The purpose of this study is to design the tool of friction stir welding (FSW) and evaluate the weldment for tailor welded blank (TWB). In the tool design, tilting angle was used as a main variable to weld the TWB with different thickness. Designed tool shape was applied to perform the experiment of FSW for Al6061 TWB with thickness combination of 1.6mm and 1.0mm. Also, welded specimens were observed and measured to evaluate the macrostructure and hardness of weldment. Experimental result show that tool shape designed in this study is very effective to manufacture the aluminum TWB by FSW process with a fine weldment.