Papers by Author: Keun Yong Sohn

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Authors: Jong Kwan Hwang, Keun Yong Sohn, Kyung Hyun Kim, Dae Min Kang
371
Authors: Hee Jin Kim, Yong Sul Song, Won Wook Park, Keun Yong Sohn
Abstract: The effect of heat treatment and binder additions on the magnetic behavior of P/M(powder metallurgy) processed Fe73.5Si13.5B9Cu1Nb3 alloy has been investigated. FeSiBCuNb amorphous ribbons produced by plannar flow casting were annealed at temperatures between 480°C and 620°C for 20 to 60 minutes. The annealed specimens were milled using a pin crusher and a hammer mill. The powder was then mixed with mineral or polymer binders to press into a toroidal shape of inductor core at room temperature. Fe73.5Si13.5B9Cu1Nb3 alloy showed maximum permeability when annealed at 540°C for 40 minutes. The microstructure of the annealed ribbon has a ultra-fine α-Fe(Si) grain, ranging from 10 to 20 nm in diameter. Inductor cores produced using a glass binder showed better magnetic properties than polymers or oxide binders. The use of mineral binders in producing nanocrystalline inductor cores significantly improved magnetic properties, compared to a commercial moly-permalloy powder core.
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Authors: Su Gui Tian, Keun Yong Sohn, Kyung Hyun Kim
Abstract: The effects of antimony addition on the microstructures and creep behavior of AZ31 have been studied. Constant load creep tests were carried out at 200°C and an initial stress of 50MPa for AZ31 alloys containing antimony up to 0.84% by weight. Results show that Small additions of antimony to AZ31 effectively decreased the creep extension and steady state creep rates. The steady state creep rate of AZ31 was reduced 2.5 times by the addition of 0.84% of antimony by weight. The main reason for the higher creep resistance is due to the presence of high volume fraction (~20%) of second phases including Mg3Sb2, which effectively hindered the movement of dislocations during the elevated temperature creep.
641
Authors: Jong Kwan Hwang, Keun Yong Sohn, Dae Min Kang, Young Su Shin
Abstract: In this paper, the distribution of temperatures on specimen and the variations in punch forces on friction coefficients during press forging of AZ31 sheets have been analyzed by Finite Element Analysis technique. Results show that the top of rib and boss areas showed lower temperatures than other areas because of the heat dissipation to air by radiation. The foot regions of the rib and boss showed greater temperature rise due to greater plastic flow. The applied punch force significantly increased at 473K or lower temperatures indicating the forming temperature of AZ31 sheet should not lower than 473K. The friction condition of the punch influenced more significantly on the materials flow pattern compared to those of the die. Using a lubricant on the contact area between the punch and specimen would effectively reduce the punch force and enhance the forming capability.
457
Authors: Ling Wang, Su Gui Tian, Keun Yong Sohn, Kyung Hyun Kim
Abstract: The mechanical properties and deformation features of AZ31-x%Sb alloys have been studied by means of the measurement of the ultimate tensile properties (UTS) and TEM observation. Results show that the UTS of AZ31 alloy is effectively enhanced to 297 MPa from 222 MPa, by additions of 0.84% Sb element, at room temperature, and the ultimate tensile strength of the alloy is still maintained up to 189MPa as temperature elevated to 200°C. Contrast analysis shows that the deformation mechanisms of AZ31-0.84%Sb alloy are twins and dislocations activated on basal and non-basal planes. The alloy displays the different deformation features at different deformation conditions.
497
Authors: Su Gui Tian, Keun Yong Sohn, Kyung Hyun Kim
Abstract: Mechanical properties and high temperature deformation behavior of AZ31-0.84%Sb alloy have been studied. The results show that small additions of antimony to AZ31 alloy effectively enhanced the creep resistance and the ultimate tensile stress of the alloy at elevated temperature. One of the reasons for the improved high temperature properties is the high volume fraction (~20%) of the precipitates, as Mg3Sb2, which effectively hindered the movement of dislocations during the elevated temperature deformation. Considerable amounts of twins were formed during creep failure of AZ31-0.84%Sb alloy. Within the twins, many dislocation pile-ups were present on both basal and non-basal plane of magnesium matrix.
771
Authors: Su Gui Tian, Keun Yong Sohn, Hyun Gap Cho, Kyung Hyun Kim
Abstract: Creep behavior of AM50-0.4% Sb-0.9%Gd alloy has been studied at temperatures ranging from 150 to 200°C and at stresses ranging from 40 to 90 MPa. Results show that the creep rate of AM50-0.4%Sb-0.9%Gd alloy was mainly controlled by dislocation climb at low stresses under 50 MPa. The activation energy for the creep was 131.2 ± 10 kJ/mol and the stress exponent was in the range from 4 to 9 depending on the applied stress. More than one deformation-mechanism were involved during the creep of this alloy. Microstructures of the alloy consist of a–Mg matrix and fine particles, distinguished as Mg17Al12, Sb2Mg3, and Mg2Gd or Al7GdMn5 that were homogeneously distributed in the matrix of the alloy, which effectively reduced the movement of dislocations, enhancing the creep resistance. Many dislocations were identified to be present on non-basal planes after creep deformation.
749
Authors: Keun Yong Sohn, Dong Woo Suh, Sang Yong Lee
Abstract: The microstructures and electrical conductivity of newly developed Cu-Ca alloys for semi-solid forming have been investigated. High purity calcium was added to molten copper up to 1.4% by weight and mold-cast into a rod. Thermomechanical treatment (TMT) has been carried out to evaluate the variation in electrical conductivity and microstructures of Cu-Ca alloys. The electrical conductivity of copper was reduced linearly with the concentration of calcium by , where k is a constant having the values ranging from 16.7 to 20, depending on the processing condition. The introduction of prestrain significantly reduced the grain size during subsequent heating by recrystallization, influencing the electrical conductivity of Cu-Ca alloys.
685
Authors: Keun Yong Sohn, Ryeo Sun Ha, Min Cheol Kang, Kyung Hyun Kim
Abstract: In this study, the effect of thermomechanical treatment (TMT) on the aging and mechanical behavior of Mg-Al-Zn alloys has been investigated. Three Mg-Al alloys AZ31, AZ61, and AZ91 were mold cast into a block under a CO2 and SF6 gas mixture atmosphere. The cast specimens were hot-swaged to a rod after homogenizing treatment at 400°C for 4 hours. The prestrain was applied by cold swaging up to 10% RA after solution treatment. From the aging curves obtained at 150°C, it was identified that the hardness of TMT-processed AZ31 and AZ61 did not increase during aging, while that of AZ91 remarkably increased. The tensile strength and elongation of the TMT-processed AZ31 and AZ61 remarkably increased depending on the amount of applied prestrains, indicating the introduction of dislocations prior to aging significantly improved the mechanical properties.
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