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조규섭 ( Gue Serb Cho ),임정규 ( Jung Kyu Lim ),장훈 ( Hoon Jang ),최경환 ( Kyeong Hwan Choe ) 대한금속재료학회 ( 구 대한금속학회 ) 2010 대한금속·재료학회지 Vol.48 No.5
Carbon Nanotubes (CNTs) have recently emerged as a material with outstanding properties. It has shown promising potential for applications in many engineering fields as electronic devices, thermal conductors, and light-weight composites. Researchers have investigated their use as reinforcements in themetal matrix composites of CNTs. In the present work, we decorated CNTs with Ni particles by electroless plating. The CNTs were wet-ball milled for various milling times with a nickel sulfate solution. The precipitated Ni particles were observed mainly by FESEM. In this study, the dispersion of the CNTs and Ni particles was improved with the addition of the surfactant. Also, as the CNTs were shortened and widened by an increased ball milling time, the size of the precipitated Ni particles increased. It was estimated that the CNTs were deformed and caused some defects on their surface during the ball milling process. Those defects were assumed to be heterogeneous nucleation sites for the Ni particles.
급속응고법으로 제조한 과공정 Al-Si 합금분말 압출재의 마멸특성
안영남,조규섭,나형용 ( Young Nam Ahn,Gue Serb Cho,Hyung Yong Ra ) 한국주조공학회 1994 한국주조공학회지 Vol.14 No.5
N/A Wear resistance and wear mechanism of hypereutectic Al- (15∼40 ) wt%Si alloys were investigated. Primary Si particles under 20㎛ size were formed in hypereutectic Al-Si alloy powders due to rapid solidification. But the Si particles of extruded bars were finely distributed in smaller size than that of atomized powders. The wear mechanism of hypereutectic Al-Si alloys was divided into three types of wear phenomena, which were abrasive wear, delamination wear and severe adhesive wear according to sliding speed and load. At low sliding speed and load, wear mechanism was abrasive wear, so Al-15wt%Si alloy showed the best wear resistance. At high sliding speed and load, wear mechanism was adhesive wear, and Al-40wt%Si alloy showed the best wear resistance.
계장화 압입시험을 이용한 냉간단조 금형의 예압력 평가기술 개발
김경일(Kyung Il Kim),조규섭(Gue Serb Cho) 한국생산제조학회 2021 한국생산제조학회지 Vol.30 No.6
In cold forging molds, the life of the mold is determined by several factors. Recent studies report that the pre-stress that occurs when the mold is press-fitted to the outer ring has a significant effect on the lifetime of the mold. However, there are many limitations in accurately evaluating the pre-stress of the mold. In this study, we developed a technology to evaluate the pre-stress in the process of sintering, polishing, and press-fitting a WC-Co cold forging mold using the instrument indentation technique, a non-destructive method, referring to the residual stress evaluation technique. In addition, the changes in the stress state after the mold is used in the industrial field were confirmed, and the method to improve the mold life with the evaluation technique proposed in this study was also discussed.
이동규(Dong-Gyu Lee),김경택(Kyung-Taek Kim),조규섭(Gue-Serb Cho),김경일(Kyung Il Kim) 한국철도학회 2024 한국철도학회논문집 Vol.27 No.3
본 연구에서는 고속철도용 상용 제동디스크의 최소 성능기준을 도출하고자 상온(RT) 및 고온(H/T)에서의 인장특성과 저주기 및 초고주기 피로특성을 평가하였다. 인장특성 중 항복강도와 인장강도는 온도가 증가함에 따라 감소하였으며, 연신율은 온도가 증가할수록 증가하였다. 상온에서의 피로한도는 저주기 및 초고주기 영역에서 각각 10<SUP>6</SUP>사이 클에서 340MPa을 나타내었으며, 10<SUP>9</SUP>사이클에서 240MPa을 나타냈다. 200℃에서의 피로한도는 저주기 및 초고주기 영역에서 각각 10<SUP>6</SUP>사이클에서 225MPa을 나타내었으며, 109사이클에서 80MPa을 나타내었다. 피로파단기구는 저주기영역의 경우, 시편 표면 부근에 존재하는 개재물 또는 산화물 등에서 피로균열의 발생 및 전파에 의한 파단기구를 보였으며, 초고주기 영역의 경우, 시편 표면 부근에 존재하는 수축결함에서 시작된 피로균열의 전파에 의한 파단기구와 초음파 피로시험편의 전형적인 파단기구인 Fish-eye 영역이 관찰되었다. In this study, the tensile, low-cycle fatigue, and ultra-high cycle fatigue properties at room temperature(RT) and high temperatures(H/T) were evaluated to derive the minimum performance standards for commercial brake discs for high-speed railways. Among the tensile properties, yield strength and tensile strength decreased as temperature increased, and elongation increased as temperature increased. The fatigue limit at room temperature was 340MPa at 10<SUP>6</SUP> cycles and 240MPa at 10<SUP>9</SUP> cycles in the low-cycle and ultra-high cycle regions, respectively. The fatigue limit at 200°C was 225MPa at 10<SUP>6</SUP> cycles and 80MPa at 10<SUP>9</SUP> cycles in the low-cycle and ultra-high cycle regions, respectively. In the case of the lowcycle region, the fatigue fracture mechanism was shown to be caused by the occurrence and propagation of fatigue cracks in inclusions or oxides existing near the surface of the specimen; in the case of the ultra-high cycle region, fracturing a shrinkage defect existing near the surface of the specimen. A fracture mechanism caused by fatigue crack propagation and a fish-eye region, which is a typical fracture mechanism of ultrasonic fatigue test specimens, were observed.
김기영 ( Ki Young Kim ),오돈석 ( Don Suk Oh ),최경환 ( Kyeong Hwan Choe ),조규섭 ( Gue Serb Cho ),이경환 ( Kyung Whoan Lee ) 한국주조공학회 2004 한국주조공학회지 Vol.24 No.2
N/A Gas porosity which is a common defect in aluminum alloy casting, is also thought to be severer in aluminum alloy castings produced by lost foam process due to the pyrolysis of the polystyrene foam pattern during pouring. Fundamental experiments were carried out to evaluate the effect of process variables such as the melt treatment, the cooling rate and pouring temperature on the density and mechanical properties in A356.2 castings with simple bar shape. The density of grain refined specimen was slightly lower than that of degassed one, but was higher than that of no treated one and that of shot ball packed specimen was higher than the other specimens. The tensile strength and elongation were in the ranges of 200∼230MPa and 0.5∼1.5% respectively. The density and hardness of lost foam cast specimens decreased with increase in pouring temperature.
이강래 ( Kang Rae Lee ),최경환 ( Kyeong Hwan Choe ),조규섭 ( Gue Serb Cho ),이경환 ( Kyong Whoan Lee ),김명호 ( Myung Ho Kim ),임경화 ( Kyung Hwa Rim ),김기영 ( Ki Young Kim ) 한국주조공학회 2004 한국주조공학회지 Vol.24 No.5
N/A Vibrational motions of the compaction table were investigated to select the optimal operation conditions of sand filling and compaction for the EPC process. Their modes were measured at the nine points of the table with changing the relative rotation angles between the two eccentric mass vibrators which were attached parallel beneath the table. Well-defined vibration modes were measured at the center of the table but those of left and right sides of the table were distorted regardless of rotational angle differences. The distortion of vibration modes at both sides of the table were caused by the moment generated by offset positions of two eccentric masses. It was found that the uniform vibration modes would be gathered by controlling the relative distances between the rotating axis and the center of gravity in the compaction system at the various conditions of vibration modes and rotational angle differences.
Al-xMg-CNT 복합분말체의 용융특성에 미치는 마그네슘 첨가와 온도의 영향
임정규 ( Jung Kyu Lim ),최경환 ( Kyong Hwan Choe ),김상섭 ( Sang Sub Kim ),조규섭 ( Gue Serb Cho ) 대한금속재료학회(구 대한금속학회) 2013 대한금속·재료학회지 Vol.51 No.4
We conducted melt-down experiments on Al-xMg-carbon nanotube (CNT) complex powder compacts at 700℃ and 800℃ for carrying the CNTs into the molten Al7021 matrix alloy. The parameters in the melting experiments were the holding temperatures and the amount of Mg content. The microstructures of the melt-down compacts were observed by optical microscopy (OM) and field emission scanning electro-microscopy (FESEM). The several phases obtained from the reaction products within the Al-xMg-CNT compacts were identified by energy dispersive X-ray spectroscopy (EDXS) and X-Ray diffractometry (XRD). The complete melt-down area was increased due to a large addition of Mg in the compacts and outer areas remained as undissolved powder compacts due to the large number of oxidation products such as MgO, MgAl2O4 and Al2O3. CNTs were observed on the surface of Al powders with oxidation products at 700℃ in a melting experiment. However, at 800℃, the CNT agglomerates moved away from the center because of the poor wettability and low density of the CNTs.
용사 코팅한 ZrO2/W/Mo-Graphite 코팅층의 2100 ℃ 열처리 후 미세조직 변화에 관한 연구
안규백 ( Gyu Baek Ahn ),최경환 ( Kyeong Hwan Choe ),김상섭 ( Sang Sub Kim ),조규섭 ( Gue Serb Cho ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.8
A tungsten coating was deposited onto a graphite substrate using the atmospheric plasma spraying (APS) technique. In order to increase the adhesion strength between the metallic tungsten(W) and graphite, a molybdenum (Mo) interlayer was pre-deposited onto the graphite surface by utilizing the APS technique. Also, after deposition of a APS-W coating, a zirconia (ZrO2) was deposited onto the W coating layer. For the APS process, argon and helium were used as the plasma-forming gases, and argon was used as the shield gas to protect the plasma from oxidation. After the APS coating process, heat exposure treatment was performed at 2100 ℃ for 360 h within a sapphire single crystal-growing furnace in order to evaluate the thermal stability of the coatings. After heat treatment, the ZrO2/W/Mo coating layers were bound with the graphite without any peeling off. The microvickers hardness of the APS-W coating layer was increased after heat treatment due to the formation of carbides. Also, carbide phases such as Mo2C, WC, ZrC and Mo3C2 were identified by XRD diffraction and EDS analysis, by analyzing the depths below the coating surface. It was considered that the Mo interlayer served as a good buffer layer between the APS-W coating and the graphite after the heat exposure treatment because the lattice structure of the molybdenum carbide was similar to that of the graphite. (Received September 16, 2015; Accepted Mach 11, 2016)
Al-CNT 복합펠렛의 계면반응에 미치는 마그네슘 첨가 및 제조 조건의 영향
임정규 ( Jung Kyu Lim ),최순열 ( Soon Yool Choi ),최경환 ( Kyong Hwan Choe ),김상섭 ( Sang Sub Kim ),조규섭 ( Gue Serb Cho ) 대한금속재료학회(구 대한금속학회) 2013 대한금속·재료학회지 Vol.51 No.5
In the present study, Al-CNT pellets were investigated to understand the effect of Mg addition and manufacturing conditions on the interfacial reactions between Al and CNTs in Al-CNT pellets. The pellets were heated and held at 700 ℃ and 800 ℃ for 2 hours under nitrogen (N2) atmosphere. To confirm the reactions between Al and CNT in the pellets under different manufacturing conditions, the microstructures were observed by optical microscopy (OM) and field emission scanning electro microscopy (FESEM). And, the composition and reaction phases were analyzed by energy dispersive X-ray spectroscory (EDXS) and X-ray diffractometry (XRD). The presence of oxidation products and Mg on the surface of Al powder in the pellets appeared to prevent the formation of Al4C3. But, Al4C3 reaction products were increased due to the high temperature of 800 ℃, which produced a high amount of reduced aluminium and increased the reaction areas between Al and CNT. The Al-CNT pellets compacted under air atmosphere prohibited the reaction between Al and CNT because of the high amount of oxidation products, such as MgO and MgAl2O4.