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최경환 ( Kyeong-hwan Choe ),서정혁 ( Joung-hyck Seo ),김수황 ( Su-hwang Kim ) 한국주조공학회 2017 한국주조공학회지 Vol.37 No.2
A new form of austenitic heat-resistant ductile iron was developed and its microstructures and elevated temperature properties were compared to those of Ductile Ni-Resist D5S. According to JMatPro calculations, it was predicted that Mo-rich carbides would be crystallized before the eutectic reaction starts in the developed alloy. At the austenite cell boundaries of the developed alloy, both Mo-rich carbides and Cr-rich carbides were found. In addition, Ni-silicides were found adjacent to Cr-rich carbides in D5S specimen and were identified as Ni2Si. The developed alloy also had greater yield strength and lower tensile strength levels with less elongation due to the dissolution of Mo atoms into the austenite matrix and the precipitation of Mo-rich carbides. From the results of elevated temperature tensile tests and stress-rupture tests, it was found that the developed alloy had elevated temperature properties superior to those of D5S. This was due to the pinning effect of the dissolved Mo atoms in the austenite matrix.
최경환 ( Kyeong Hwan Choe ),장훈 ( Hoon Jang ),임정규 ( Jung Kyu Lim ),김상섭 ( Sang Sub Kim ),조규섭 ( Gue Serb Cho ) 대한금속재료학회(구 대한금속학회) 2013 대한금속·재료학회지 Vol.51 No.10
Carbon nanotubes (CNTs) have recently emerged as a reinforcement for metal matrix composites (MMCs), but the problem of dispersion of CNTs and the interface between the metal matrix (MM) and CNTs makes it difficult to utilize MM-CNTs composites. In this study, in order to decrease the floatation and clustering of CNTs and improve the wettability between CNTs and an Al melt, CNTs were decorated with Cu particles by a metal displacement reaction. To investigate the effects of their morphology on the metal displacement reaction, flake-and spherical-type Zn powders were respectively added to precipitate Cu particles in a Cu salt solution. The flake-type Zn powders were fully displaced by Cu in the solution, but in case of the spherical-type Zn powders, Cu was deposited on the surface only. The reaction temperature did not affect the characteristics of the metal displacement reaction. This result was attributed to the fact that the rate of the metal displacement reaction was decreased in ethanol due to an increase in viscosity in diffusion of Cu2+ from the solution to the metallic Zn surface. Cu/CNTs nanocomposite powders were manufactured by the addition of copper chloride (CuCl2) to an ethanol suspension containing pre-treated CNTs. The CNTs were mixed with precipitated Cu nanoparticles and others were entrapped between the Cu particles or agglomerates. It was attributed to the lack of the active sites on the CNTs surface for the nucleation of Cu crystal. (Received November 1, 2012)
초음파 진동에 의한 A328 알루미늄 합금 용탕의 탈가스
최경환 ( Kyeong Hwan Choe ),장훈 ( Hoon Jang ),임정규 ( Jung Kyu Lim ),김상섭 ( Sang Sub Kim ),조규섭 ( Gue Serb Cho ) 한국주조공학회 2011 한국주조공학회지 Vol.31 No.6
A328 alloy is an attractive candidate for recycle-friendly aluminum alloy in the recycling of automotive components. In this study, A328 alloy melt was degassed by ultrasonic vibration and the effect of treatment time on the density, fluidity and mechanical properties was investigated. Experimental results reveal that a constant value of density can be reached after less than 180 seconds of ultrasonic treatment time, but the density decreased when the treatment time was 300 seconds. Ti which was dissolved from the horn during ultrasonic treatment reduced the fluidity of melt. After degassing by ultrasonic vibration for 180 seconds, tensile strength increased from 201MPa to 250MPa, and elongation increased from 2.38% to 3.50%, however, further treatment deteriorated the mechanical properties.
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)
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.
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.