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- Volume 1(2); April 1994
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- Microstructure and Flexural Strength of Hardmetals
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Koji Hayashi
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J Korean Powder Metall Inst. 1994;1(2):119-198.
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- The characteristics of various important microstructural factors of WC-base hard- metals (cemented carbides) such as the amount of Co metal binder phase, the carbide grain size, the microstructural defects acting as a fracture source, the solid solubility of tungsten in the binder phase affected by the carbon content, the precipitation of Co_3W, the domain size of binder phase, the formation of beta-free layer or Co-rich layer and CVD or PVD coated layer, and the effects of these factors on the flexural strength of the hardmetals are reviewed.
- 제2차 분무성형(Spray Forming) 국제 학술대회
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J Korean Powder Metall Inst. 1994;1(2):129-134.
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- Properties of SiC Powders Prepared by SHS Method and Its Sintered Bodies
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J Korean Powder Metall Inst. 1994;1(2):135-144.
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- Silicon carbide powder was prepared from mixtures of Sangdong silica sand and carbon black by SHS (Self propagating High temperature Synthesis) method which utilizes magnesiothermic reduction of silica. In the powder preparation process, the reacted powder was leached by chloric acid to remove the magnesium oxide and was subsequently roasted to remove free carbon. The impurities were mostly eliminated by hot acid treatment. The resultant SiC powder showed the mean particle size of 0.22 µtextrmm and the specific surface area of 66.55 m2/g. The SiC powder was mixed with 1 wt% of boron and of carbon to increase densification rate. The mixed powder was pressed and sintered pressurelessly at 2100°C for 30 min in argon gas. The sintered body showed the hardness of 2550 kgcdotf/mm2 and the fracture toughness, KIC of 3.47 MN/m3/2.
- Powder Production of CuAINi Base Alloy via Rotating Disk Atomization
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J Korean Powder Metall Inst. 1994;1(2):145-152.
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- Atomizing mode and powder characteristics of CuA1Ni base shape memory alloy in rotating disk atomization were investigated in accordance with disk materials and additional elements. Produced powders were classified into two types of spherical and flake shape. In the case of CuAlNiBTi and CuAlNiZr alloy, high yield rate and fine powder were obtained. This tendency was same when we used oxide coated disks. We concluded that this results were steno from the wetting characteristics change between molten metal and disk surface. Especially, due to the reactive properties of Ti and Zr with ceramic disk, the change of atomizing appearance and powder characteristics were noticeable.
- Role of Alloyed Components on Densification of Mixed and Prealloyed High Speed Steel Powder Compacts
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J Korean Powder Metall Inst. 1994;1(2):153-158.
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- The effect of the role of alloyed components on the densification of two kind of high speed steel (mixed and prealloyed powder), which were sintered at 1403~ 1573 K for 7.2 ks in vacuum, was investigated. The results obtained were as follows. (1) Without the presence of Vanadium (V), the relative density of sintered compacts (Ds) could not reached the density of 100% regardless of the. elements in the compacts. (2) The addition of V up to 2 mass% did not result In the complete densification when the carbon content was fixed at 2% in the compact. (3) With the fixed amount of V of 7%, Ds decreased with the increase of the carbon content. (4) The addition of mixed fine powder to the prealloyed powder in the range of 20 to 40% provided the complete densification and carbide panicles of 1~2 mutextrmm through the solid phase sintering. (5) The V element played important role in controlling the complete or incomplete densification of the sintered compacts in the alloyed component because of its formation of stable oxide and carbide as well as the low equilibrium pressure of CO gas.
- Effects of Vacuum Hot Pressing Conditions on Mechanical Properties and Microstructures of SiC_w/2124Al Metal Matrix Composites
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J Korean Powder Metall Inst. 1994;1(2):159-166.
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- The variation of the microstructures and the mechanical properties with varying vacuum hot pressing temperature and pressure was investigated in PyM processed 20 vol%) SiCw/ 2124Al composites. As increasing the vacuum hot pressing temperature, the aspect ratio of whiskers and density of composites increased due to the softening of 2124Al matrix with the increased amount of liquid phase. The tensile strength of composite increased with increasing vacuum hot pressing temperature up to 570°C and became saturated above 570°C, To attain the high densification of composites above 99%, the vacuum hot pressing pressure was needed to be above 70 MPa. However, the higher vacuum hot pressing pressure above 70 MPa was not effective to increase the tensile strength due to the reduced aspect ratio of SiC whiskers from damage of whiskers during vacuum hot pressing. A phenomenological equation to predict the tensile strength of SiC_w/2124AI composite was proposed as a function including two microstructural parameters, i.e. density of composites and aspect ratio of whiskers. The tensile strength of SiC_w/2124AI were found more sensitive to the porosity than other P/M materials due to the higher stress concentration and reduced load transfer efficiency by the pores locating at whisker/matrix interfaces.
- Microstructure and Physical Properties of Cu-l0wt%W Sintered Material Fabricated by Mechanical Alloying Method
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J Korean Powder Metall Inst. 1994;1(2):167-173.
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- Cu-10wt%W composite powders have been manufactured by a high energy ball milling technique. The composite powders were pressed at 250 MPa and sintered in a dry hydrogen at 1030°C for 4 hours. After sintering, Cu-10wt%W composite materials were forged. And the arc-resistance of forged materials which have the same relative density of 94% has been tested. Composite particles, i.e. tungsten particles distributed homogeneously in the copper matrix, was formed after 480 min mechanical alloying. Densities of these sintered materials were ranged from 74 to 84%. Densification degree was due to the formation of composite powders. As the mechanical alloying time increased, the hardness was increased and tungsten particle size was decreased. Arc loss of the forged specimens was decreased as increasing the mechanical alloying time.
- Electrical Contact Property of W-Cu Materials Manufactured from Nanocomposite Powder
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J Korean Powder Metall Inst. 1994;1(2):174-180.
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- Electrical contact property of the W-20wt%Cu contact materials manufactured by liquid phase sintering of nanocomposite W-Cu powders was investigated and discussed in terms of microstructural development during performance test. Nanocomposite powders were prepared by hydrogen reduction of ball milled W-Cu oxide mixture. They underwent complete densification and microstructural homogenization during liquid phase sintering. As a consequence, the W-Cu contacts produced from nanocomposite powders showed superior contact property of lower arc erosion and stable contact resistance. This might be mostly due to the fact that the arc erosion by evaporation of Cu liquid droplets and surface cracking remarkably became weakened. It is concluded that the improvement of anti-arc erosion of the composite specimen is basically attributed to microstructural homogeneity.
- Mechanical Properties of Sintered Steel of Pure Iron Powder and Iron Powder Coated with Phosphorus
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J Korean Powder Metall Inst. 1994;1(2):181-189.
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- The compacts of pure and phosphorus-coated iron powder with 0~0.8%C were sintered at 1100°C for 40 min. in cracked ammonia gas atmosphere. The tensile and impact strengths were measured and the relationship of the results with carbon content, phosphorus, quenching and tempering was investigated. The results obtained can be summarized as follows : (1) The tensile strength of sintered compacts increased slowly with carbon content. Increase in tensile strength by heat treatment was evident especially in the low carbon specimen. The specimen with phosphorus showed higher strength compared to pure iron compacts value. (2) No inflection point of elasticplastic deformation on stress-strain curve was observed in sintered steel. The elastic modulus of sintered steel had the same tendency as tensile strength. But the elongation showed the opposite tendency. (3) The impact absorption energy of sintered steel without addition of phosphorus decreased successively with carbon content and by quenching and tempering. On the contrary, addition of phosphorus resulted in an increase of the impact absorption energy. Quenching and tempering did not affect the impact energy especially in high carbon content. (4) The main fracture source was pore in specimen and the propagation of crack occured mostly along the grain boundaries. But the intragranular fracture was also observed in high carbon, quenched and tempered specimen, and especially in the specimen with phosphorus.
- The Effect of Sintering Parameters on the Densification Behavior of PM High Speed Steel
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J Korean Powder Metall Inst. 1994;1(2):190-197.
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- The densification behavior during a sintering of M2 and T15 grade high speed steel powder compacts was reported. Sintered densities over 98% theoretical were achieved by a liquid phase sintering in vacuum for both grades. The optimum sintering temperature range where full densification could be achieved without excessive carbide coarsening and incipient melting was much narrower in M2 than in T15 grade. The sintering response was mainly affected by the type of carbides present. The primary carbides in M2 were identified as M_6C type whereas those in T15 were MC type which provides wider sintering range. The addition of elemental carbon up to 0.3% lowered the optimum sintering temperature for both grades, but had little effect on expanding the sintering range and sintered structure.
- Friction Welding of MA754 ODS Alloy Produced by Mechanical Alloying
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J Korean Powder Metall Inst. 1994;1(2):198-207.
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- In order to find an optimal friction-welding condition for Ni-base ODS alloy (MA 754) produced by mechanical alloying, joint experiments were performed with various conditions of friction pressures (50~500 MPa), friction times (1~5 sec) and upset pressures (50~600 MPa). The optimal friction pressure and upset pressure must be above 400 MPa and 500 MPa, respectively, which are determined by tensile strengths and fracture features of as-welded joints. A maximum stress설h of 975 MPa could be obtained under these pressure conditions at friction time of 2 sec. Microstructural features of bonded interface by optical microscope and SEM revealed that the interface regions of all specimens are consisted with three distinct regions and defects such as voids, cracks and wavy interfaces exist in the joints produced under not-optimized conditions. EDS results showed that these defects include oxides composed with elements of Al, Y and Ti. The hardness on the bonded interface was higher than in the base metal region. Specimens fractured in bonded interface region had lower strength values compared to those fractured in base metal region. Surfaces of the former showed a typical intergranular fracture.
- Effect of Grain Growth Inhibitor on Sintering of Nanophase WC-10wt%Co
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J Korean Powder Metall Inst. 1994;1(2):208-216.
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- A radically new approach to the in situ synthesis of the consituent phases of a composite structure has enabled the production of a new WC/Co materials with an ultrafine microstructure. The process for synthesizing nanophase WC/Co powders consists of spray drying from solution to form a homogeneous precursor powder, and thermochemical conversion of the precursor powder to the nanophase WC/Co powder. Near theoretical density of pure nanophase WC-10 wt%Co has been obtained in only 30 sec at 1400°C. But WC particles were grown up very rapidly with longer sintering time to get full density. To overcome coarsening of WC particle during sintering, VC, TaC and VC/TaC were used as the grain growth inhibitor with different amount respectively. VC/TaC doped WC-10 wt%Co was shown superior hardness and TRS and microstructure was maintained ultrafine scale (average WC size is less than 0.1 µtextrmm).
- 자성재료에 대한 기술 전망
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J Korean Powder Metall Inst. 1994;1(2):217-223.
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- 초미립 분말제조 및 응용현황
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J Korean Powder Metall Inst. 1994;1(2):224-227.
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- 세라믹스 관련 시장 전망과 첨단 세라믹스 분말 개방 동향
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J Korean Powder Metall Inst. 1994;1(2):228-229.
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