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- Volume 4(3); June 1997
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- Variations in Carbon Content and Sintered Density of M3/2 Grade High Speed Steel Powders on Metal Injection Molding Process
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J Korean Powder Metall Inst. 1997;4(3):170-178.
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Abstract
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- An investigation was performed to apply the M3/2 grade high speed steel for metal injection molding using both prealloyed and elementally blended powders. The injected samples were subjected to a debinding step in H_2/N_2 gas atmosphere at a ratio that affected the carbon content of the material. The carbon content ranged from 1.4wt.% to 1.43wt%. with increasing H_2 content up to 80% H_2 in H_2/N_2 atmosphere for the prealloyed powders. The carbon contents of the elementally blended powders exhibited 1.44wt.% and 1.62wt.% at 10% H_2/N_2 and 20% H_2/N_2 gas, respectively. This level decreased to 0.17wt.% upon increasing the H_2 content. The sintered density of both powders increased rapidly as the temperature reached the liquid phase forming temperature. After forming the liquid phase, the density rapidly increased to the optimum sintering temperature for the prealloyed powders, whereas the density of mixed elemental powders goes up slowly to the optimum sintering temperature. The optimum sintering temperature and density are 1260°C and 97.3% for the prealloyed powders and 1280°C and 96.9% for the elementally blended powders, respectively. The microstructure of the specimen at the optimum sintering temperature consisted of fine grains with primary carbides of MC and M_6C type for the prealloyed powders. The elementally blended powders exhibited coarse grains with eutectic carbides of MC, M_2C and M_6C type.
- Effect of Milling Medium Materials on Mechanical Alloying of Mo-65.8at%Si Powder Mixture
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J Korean Powder Metall Inst. 1997;4(3):179-187.
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Abstract
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- Milling media of steel and zirconia were used to produce MoSi_2 by mechanical alloying (MA) of Mo and Si powders. The effect of milling media on MA of Mo-65.8at%Si powder mixture has been investigated by SEM, XRD, DTh and in-situ thermal analysis. The powders mechanically alloyed by milling medium of steel for 8 hours showed the structure of fine mixture of Mo and Si, and those mechanically alloyed by milling medium of zirconia for longer milling time showed the structure of fine mixture of Mo and Si. The tetragonal alpha-MoSi_2 Phase and the tetragonal Mo_5Si_3 phase appeared with small Mo peaks in the powders milled by milling medium of steel for 4 and 8 hours. The alpha-MoSi_2 phase and the hexagonal beta-MoSi_2 phase were formed after longer milling time. The alpha-MoSi_2 phase appeared with large Mo peaks in the powders milled by milling medium of zirconia for 4 hours. The phases, alpha-MoSi_2 and beta-MoSi_2. were formed in the powders milled for longer milling time. DTA and annealing results showed that Mo and Si were transformed into alpha-MoSi_2 and Mo_5Si_3, while beta-MoSi_2 into alpha-MoSi_2. In-situ thermal analysis results demonstrated that there were a sudden temperature rise at 212 min and a gradual increase in temperature in case of milling media of steel and zirconia, respectively. The results indicate that MA can be influenced by materials of milling medium which can give either impact energy on powders or thermal energy accumulated in vial.
- Prediction of Density Distribution in Sintered Metal Powder Compacts by Indentation Force Equation
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J Korean Powder Metall Inst. 1997;4(3):188-195.
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Abstract
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- In most of sintered metal powder compacts, the sintered density distribution is controlled to be as high and uniform as possible to ensure the required mechanical properties. In general, the density distribution in the compacts is not uniform and not easy to measure. In the present study, a method for measuring the density distribution was developed, based on the indentation force equation by which the hardness and the relative density were related. The indentation force equation, expressed as a function of strength constant, workhardening coefficient and relative density, was obtained by finite element analysis of rigid-ball indentation on sintered powder metal compacts. The present method was verified by comparing the predicted density distribution in the sintered Fe-0.5%C-2%Cu compacts with that obtained by experiments, in which the density distribution was directly measured by machining the compacts from the outer surface progressively.
- Joining of AIN Ceramics to Metals: Effect of Reactions and Microstructural Developments in the Bonded Interface on the Joint Strength
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J Korean Powder Metall Inst. 1997;4(3):196-204.
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Abstract
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- Joining of AIN ceramics to W and Cu by active-metal brazing method was tried with use of (Ag-Cu)-Ti alloy as insert-metal. Joints were produced under various conditions of temperature, holding time and Ti-content in (Ag-Cu) alloy Reaction and microstructural development in bonded interface were investigated through observation and analysis by SEM/EDS, EPMA and XRD. Joint strengths were measured by shear test. Bonded interface consists of two layers: an insert-metal layer of eutectic Ag- and Cu-rich phases and a reaction layer of TiN. Thickness of reaction layer increases with bonding temperature, holding time and Ti-content of insert-metal. It was confirmed that the growth of reaction layer is a diffusion-controlled process. Activation energy for this process was 260 KJ/mol which is lower than that for N diffusion in TiN. Maximum shear strength of 108 MPa and 72 MPa were obtained for AIN/W and AIN/Cu joints, respectively. Relationship between processing variables, joint strength and thickness of reaction layer was also explained.
- The Effect of Y_2O_3 Addition on the Mechanical Alloying of Ni_3Al
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J Korean Powder Metall Inst. 1997;4(3):205-213.
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Abstract
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- Mechanical alloying of Ni_3Al and Y_2O_3 added ODS Ni_3Al from elemental powders was investigated by the X-ray diffraction, differential scanning calorimeter, transmission electron microscopy and optical microscopy. The steady states of Ni_3Al and ODS Ni_3Al powders were reached after mechanical alloying with the condition of the ball-to-powder input ratio of 20:1 for 20 hours and 10 hours, respectively. The addition of nano-sized Y_2O_3 particles enhanced cold working and fracture, and subsequently accelerated MA of Ni_3Al powders. DSC results of MAed Ni_3Al powders showed four exothermic peaks at 140°C, 234°C, 337°C and 385°C. From the high temperature X-ray diffraction analysis, it was concluded that the peaks were resulted from the recovery solution of unalloyed Al in Ni, the formation of intermediate phase NiAl, and LI_2 ordering of MAed Ni_3Al powders.
- Effect of Sintering Atmosphere on the Densification and Grain Growth of Uranium Dioxide at the Final-Stage Sintering
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J Korean Powder Metall Inst. 1997;4(3):214-221.
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Abstract
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- The densification and grain growth mechanisms of UO_2+x in H_2 and in CO_2 have been investigated. Uranium dioxide powder compacts were sintered at 1700°C in H_2 or at 1100°C in CO_2 for various times from 0.5 h to 16 h. The grain size and density of the specimens were measured. From the measured data, the mechanisms of the densification and grain growth were determined by use of available kinetic equations which express the relations between densification and grain growth. In both atmospheres, it has been found that the densification was controlled by the lattice diffusion and the grain growth by the surface diffusion of atoms around pores. It appears that the surface diffusivity as well as the lattice diffusivity increase considerably with the increase in O/U ratio in the specimen.
- Effect of Heat Treatment on the Mechanical Properties of P/M High Speed Steel
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J Korean Powder Metall Inst. 1997;4(3):222-229.
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- A P/M high speed steel of ASP 30 grade was austenitized, gas quenched and tempered at various conditional. The mechanical properties such as hardness, bend strength and fracture toughness were evaluated after heat treatment. The microstructure and the type and volume fraction of carbides were analyzed by an optical microscope, image analyzer and XRD. The primary carbides after the heat treatment were MC and M_6C type. The volume of the total carbide varied from 10 to 15% depending on the austenitizing and tempering temperature. The tempering temperature for maximum hardness was at around 520°C. But the maximum bend strength was obtained at about 550°C. The fracture toughness was largely affected by the presence of retained austenite after gas quenching and secondary hardening during tempering.
- 비철금속분말의 국내활용현황
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J Korean Powder Metall Inst. 1997;4(3):230-235.
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