Journal of Powder Materials

Processing and Properties of Engine Valve-shaped TiAl-Mn Intermetallics by Reactive Sintering: J Korean Powder Metall Inst. 1997;4(4):243-251.

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Abstract PDF: Engine valve-shaped TiAl-Mn intermetallics containing 43.5 to 47.5at%Al (Mn/Al=0.036) are successively fabricated by reactive sintering the elemental powder mixtures near-net shaped by extrusion and die forging. A duplex structure consisted of lamellar grains and equiaxed gamma grains is developed for all compositions, and the areal fraction of the lamellar grains(or equiaxed gamma grains) decreases (or increases) with increasing Al content. As Al content increased, the elongation increases with accompanying decrease in yield strength and ultimate tensile strength at both room temperature and 800°C. This indicates that the suitable composition is Ti-45at%Al-1.6at%Mn in considering the balance of ambient and elevated tensile properties. The reactive-sintered Ti-45Al-1.6Mn alloy shows superior oxidation resistance not only to the plasma arc melted one but also to the heat resistance steel STR35(representative exhaust valve head material for automotive engine). The reactive-sintered Ti-45Al-1.6Mn alloy coated with an oxidizing scale exhibits a better wear resistance than induction hardened martensitic steel STR11(representative exhaust valve tip material for automotive engine).

Columnar Structural Growth in Molten Filler Metal during Brazing of Dissimilar Materials: J Korean Powder Metall Inst. 1997;4(4):252-257.

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Abstract PDF: Cu-brazed layer between the sintered-cam(Fe-5Cr-lMo-0.5P-2.5C, wt%) and seamless steelpipe(0.25-0.35C, 0.3-1.0 Mn, bal Fe, wt%) in the camshaft shows a columnar structure of gamma-phase growing from the steel pipe. Liquid phase sintered 60Fe-40Cu alloys are carburized to simulate the brazing process giving rise to the columnar growth. Liquid film migrations and columnar growth of gamma-grains are observed in the carburized regions. The gamma-grains grow in the same direction as the C-diffusion. Fe-solubility in the liquid of carburized region is higher than in the uncarburized by about 0.3 at%. The columnar growth is driven by the gradient of the supersaturated Fe-solute in the liquid between two adjacent gamma-grains.

Development of Bevel Gear by Powder Forging Process: J Korean Powder Metall Inst. 1997;4(4):258-267.

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Abstract PDF: The powder forging process is an attractive manufacturing route for bevel gears. It offers beneficial material utilization and the minimization of finishing operations over that of conventional hot forging. The paper describes the process conditions for the powder forging of bevel gear, for example, powder alloy design, preform design, deformation of sintered preform, forging processes. The characteristics of prototype gear are investigated with microstructure, the density distribution, surface roughness of tooth, bending strength test of tooth, etc. The results of the bending strength test may prove the mechanical properties of powder forged gear.

Characteristics of Plasma Nitriding and Nitrocarburizing of Steam Treated Sintered Steels: J Korean Powder Metall Inst. 1997;4(4):268-274.

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Abstract PDF: Characteristics of plasma nitriding and nitrocarburizing for steam treated sintered steels were studied. Fe-0.8%C powder containing Ni, Cu were sintered at 1120°C and steamed at 520°C. Temperature of plasma nitriding and nitrocarburizing was varied from 500°C to 600°C. Gas mixture of nitriding was set at N_2 : H_2 =80:20 (vol.%), but CH_4gas was added 1~2 vol.% for nitrocarburizing. Steam treatment for sintered steels brought not only the formation of oxide layer but also decarburizing near the surface. Decrease in hardness near the surface resulted from the formation of ferrite due to decarburizing. Thus, the low hardness was recovered not with plasma nitriding but with plasma nitrocarburixing. Wear resistance properties of steamed specimens and ni-trocarburized specimens were better than those of nitrided specimens according to the pin-on-disk wear test. On the other hand, the fatigue life of steamed specimen was shorter than that of nitrocaiburized specimen.

Development of Sintered Oilless Bearing using Fluid-Dynamic Mechanism For High-RPM Application: J Korean Powder Metall Inst. 1997;4(4):275-282.

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Abstract PDF: Sintered oilless bearing using fluid-dynamic mechanism for high revolution of over 10,000 rpm was developed by Powder metallurgy Process. Developed bearing was composed of Fe-40-60%Cu of which inner face consisted of six-regular-prominence-and-depression. Inner face has controlled pore size and amount according to application. Jitter and friction properties were tested under assembled conditions with housing and motor. Bearing-own properties and precision were also tested because of demanding for assembled properties. Measurement-skill as well as mould-production-skill were investigated for the precision of bearing. Final precision reached 2 µm in inner diameter tolerance and 5 µm in coaxiality under assembled conditions.

Synthesis and Densification of Ti_5Si_3-base Intermetallic Compounds by Reactive Sintering and Electro-Pressure Sintering: J Korean Powder Metall Inst. 1997;4(4):283-290.

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Abstract PDF: Ti_5Si_3 intermetallics containing 0-6 wt% of Cu were made by reactive sintering (RS) under vacuum using elemental powder mixtures (Process 1), electro-pressure sintering (EPS) using RS'ed materials (Process2), and EPS using elemental powder mixtures (Process 3). Relatively low dense titanium silicides were gained by process 1, in which porosity decreased with increasing Cu content. For example, porosity changed from 42 to 19.4% with the increase in Cu content from 0 to 6 wt%, indicating that Cu is a useful sintering aid. The titanium silicides fabricated by Process 2 had a higher density than those by Process 1 at given composition, and porosity decreased with increasing Cu content. For example, porosity decreased from 38 to 6.8% with the change in Cu content from 0 to 6 wt%. A high dense titanium silicides were obtained by Process 3. In this Process, porosity decreased a little by Cu addition, and was almost insensitive to Cu content. Namely, about 9 or 7% of porosity was shown in 0 or 1-6 wt% Cu containing silicides, respectively. The hardeness increased by Cu addition, and was not changed markedly with Cu content for the silicides fabricated by Process 3. This tendency was considered to be resulted from porosity, hardening of grain interior by Cu addition, and softening of grain boundary by Cu-base segregates. All these results suggested that EPS using elemental powder mixtures (Process 3) is an effective processing method to achieve satisfactorily dense titanium silicides.

Synthesis of TiC and TiC-Al Functionally Graded Materials by Electrothermal Combustion (ETC): J Korean Powder Metall Inst. 1997;4(4):291-297.

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Abstract PDF: Titanium cabide, TiC-x mole% Al composites, and functionally-graded materials (FGMs) of TiC-x mole% Al were synthesized by an electrothermal combustion (ETC) method. TiC-70 mole% Al composite was not ignited by indirect tungsten coil heating, but can be synthesized by an electrothermal combustion. The velocity of the combustion wave decreased with increasing addition of Al and increased with an increase in the applied electric field. Functionally-graded TiC-Al materials were made from reactant layers with compositions of Ti+C+x moles Al with x ranging from zero to 70 by an electrothermal combustion. In the FGM products a nearly linear change in composition in the graded region was observed in samples with 0≤ x ≤ 70 with x being the mole% Al.

Observation of Densification Behavior during the Sintering of Ni-added MoSi_2 Powder Compacts: J Korean Powder Metall Inst. 1997;4(4):298-303.

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Abstract PDF: The activated sintering behavior of MoSi_2 powder compacts with addition of 0.5 and 1.0 wt.%Ni during the sintering under As atmosphere was studied. The shrinkage was measured and the microstructures were observed by SEM (scanning electron microscopy) and BEI (backscattered electron image) along with the phase analysis by EDS during heating up to 1550°C and holding for various time at 1550°C. The most of shrinkage occurred upon heating and 92% of theoretical density was attained after sintering for 1 hr at 1550°C. However, little shrinkage ensued even for prolonged sintering over 1 hr at 1550°C. A liquid film formed at about 1350°C along necks and grain boundaries. The polyhedral grain structure composed of (Mo,Ni)_5Si_3and Ni_2Si across the MoSi_2 grain boundary developed at 1550°C. It was concluded that the activated sintering of MoSi_2 powder by Ni led to the diffusion of Si into Ni decreasing the liquidus temperature and the enhanced diffusion of Mo and Si through such a liquid phase and/or interboundary of (Mo,Ni)_5Si_3.

Synthesis and Characteristics of W-l5wt%Cu Nanocomposite Powder by Oxide Reduction: J Korean Powder Metall Inst. 1997;4(4):304-309.

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Abstract PDF: The synthesis of W-l5wt%Cu nanocomposite powder by hydrogen reduction of ball milled W-Cu oxide mixture was investigated in terms of powder characteristics such as particle size, mixing homogeneity and micropore structure. It is found that the micropores in the ball milled oxide (2-50 nm in size) act as an effective removal path of water vapor, followed by the formation of dry atmosphere at reaction zone. Such thermodynamic condition enhances the nucleation of W phase but suppresses the growth process, being in favor of the formation of W nanoparticles (about 21 nm in size). In addition, the superior mixing homogeneity of starting oxide mixture turned out to Play a significant role for forming extraordinary chemical homogeneity of W-l5wt%Cu nanocomposite powder.

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