발간논문

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The original and most widely used P/M self-lubricating bearing material is bronze but iron-based bearings have the merits that they have the higher strength and cheaper than copper-based bearings so that its use has risen recently. So in this study, microstructure and mechanical properties of the Fe-Pb-Cu and Fe-Pb-Cu-Sn bearings sintered at 840∼960℃ and driving performance test of bearings after sizing sintered bearings are investigated. In case of Sn addition with 0.5∼2wt% to Fe-8Pb-2Cu, pore distribution was uniform and Pb segregation was reduced. And the coefficient of friction and temperature-rise were small. Pb is distributed uniformly at interparticle and grain boundarys because the wettability of Pb increased and Sn in iron increased diffusion rate during sintering.

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A new purifying process using the principle of the fractional melting has been developed and tested for the commercial 99% aluminium. The fractional melting of specimen was performed in a molten salt bath, the temperature of which was controlled artificially through out the process. On exuding, the liquid metal was ejected from the solid specimen by centrifugal force. The purifying level of the products was controlled by various processing parameters such as rotating speed, heating rate, the shape of specimens, and starting salt temperature. The nature and the type of the filter were also important parameters to achieve a good result. The wetness, representing the efficiency of purification, was evaluated as 0.07 to suggest an excellent purifying effect during the fractional melting process of Al alloy.

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Effect of heat treatment on mechanical properties and stress corrosion of 2024Al matrix composites manufactured by mechanical alloying process has been investigated in this study. The composite as extruded has a high tensile strength and very short stress corrosion life. Specimens stabilized show susceptibility to stress corrosion equal to that of as extruded. Electron microscopy of the specimens at their peak hardness revealed the S` phase precipitates, which are thought to enhance the age hardenability. It was found that the composite aged at 190℃ for 1000 minutes had the highest hardness valuewhile the one aged at 190℃ for 3000 minutes having the highest resistance to stress corrosion cracking. This means that the maximum susceptibility to stress corrosion cracking of 2024Al/SiC_p composite does not occur in peak aged condition. There was a great difference in stress corrosion characteristics between the composite in this study and the conventional 2024Al alloy.

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Calciothermic reduction of Nd₂O₃ was attempted to produce Nd-Fe mother alloy in the presence of a flux, CaCl₂ at temperatures of 820℃ to 1050℃. Feed material was a powder mixture of Nd₂O₃, Ca, CaCl₂ and Fe, having a given composition. Reduction experiments were carried out under stirring condition and reduced Nd was collected into a molten metal pool of Nd-Fe alloy at the bottom of the crucible. Recovery of the metallic Nd was calculated by the difference of weights of the alloy and slag. Alloy and slag thus made were analyzed by I.C.P., SEM and X-ray diffractometer. Metal recovery was increased with increasing temperature, reaction time and also stirring rate. Experimental results showed that the optimum amounts of metallic Ca and CaCl₂ were 15% in excess than the theoretical amount and 4.0 mole ratio to the theoretical moles of CaO formed, respectively. The alloy composition was in the range of 76.31 to 85.23wt% Nd, 13.90 to 23.41wt% Fe and 0.29 to 0.39wt% Ca depending on the experimental temperature. The best purity of the alloy was analyzed to be 99.55wt%.

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The microstructural characteristics and decomposition behaviour of rapidly solidified Mg-Nd-X(X=Al, Si) alloys have been investigated by using a detailed transmission electron microscopy. Rapid solidification of Mg-8Nd, Mg-8Nd-1Al and Mg-8Nd-1Si produces various types of solidification microstructure depending upon the alloy composition, i.e. fine Mg₃Nd precipitates and α-Mg in as-melt-spun Mg-8Nd, α-Mg cellular structure with secondary solidification phase particles at cell boundaries in as-melt-spun Mg-8Nd-1Al and supersaturated α-Mg grains with fine particles at the grain boundaries in as-melt-spun Mg-8Nd-1Si. Annealing of as-rapidly solidified Mg-8Nd and Mg-8Nd-1Si up to 300℃ results in coarsening of Mg₃Nd precipitates. However, no significant change occurs in Mg-8Nd-1Al with annealing up to 300℃. With annealing at 400℃, the pre-existing phase decomposes forming new phases stable at 400℃, i.e. a fcc phase(a=5.4Å) in Mg-8Nd, a fcc phase(a=8.0Å, Al₂Nd) in Mg-8Nd-1Al and Mg_9Nd in Mg-8Nd-1Si.

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During mechanical alloying(MA) of ductile materials, it is often found that either welding or fracturing becomes so dominant that both very fine layered structure and homogeneously alloyed powder can not be obtained. For this reason, it is necessary to add an appropriate amount of process control agent(PCA). However, it has not been reported yet how PCA affects the MA process. Hence, the present work investigated the precise role of PCA during MA process. The results showed that carbon atoms in PCA molecules suppress the welding of powders and thereby increase the fracturing as the amount of PCA or the number of carbon atoms in PCA increases. On the other hand, the oxygen atoms in PCA seem to delay the phase transformation kinetics. Therefore, the amount of PCA necessary in mecahnical alloying should be added so that PCA suppresses only severe welding and increases the hardness of soft materials.

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