발간논문

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The effects of powder size and heating rate on various properties of sintered Cu-10% Sn was investigated. Experiments were carried out on four combinations of copper and tin powder sizes by mixing copper powders of －150 mesh and －270 mesh with －230 －＋325 mesh and －400 mesh tin powders. The sintering temperatures were 797℃ and 807℃ with sintering times ranging from 0 to 15 minutes. Generally the growth of Cu-Sn compacts occurred within the first 1 minute of sintering with subsequent slow shrinkage. The growth was largely dependent on the size of tin powder with larger growth was largely dependent on the size of tin powder with larger growth occurring for lager tin powder size. Faster heating rate caused larger growth. The observed dependence of growth of particle size and heating rate can be most consistently explained by the penetration of tin-rich liquid phase into the copper particle boundaries. The amount of open porosity as indicated by the amount of oil impregnation was larger for samples of larger tin powder. Slow heating reduced the open porosity significantly while the total porosity is little affected. Radia crushing strength was larger for fine tin powder samples. Microstructures show that larger grain growth occurred around the pores which were originally tin particle site and the amount of tin rich phase decreased with fine powder size. Therefore, more uniform structures can be obtained with finer powders.

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The effect of cooling rate of sintering on the strength of sintered Fe-Cu has been studied. The copper conteet was varied between 0 and 12 weight %. The tensile test specimens were prepared by usual powder metallurgy techniques. The compacts were sintered at 1120℃ for 40 minutes in hydrogen atmosphere and cooled to room temperature at various cooling rates over several temperature ranges. The strength of specimens with more than 2% Cu increased greatly with increasing cooling rates between 850℃ and 600℃ the eutectoid transformation range. For 8% Cu, the ultimate tensile strengths for the fast cooled and slow cooled specimens were about 35 and 20 ㎏/㎟ respectively. This observation is explained by the decreasing size of the precipitated Cu-rich phase with fast cooling rate in the eutectoid transformation. For 2% Cu specimens, the strength increased with slow cooling rate below 600℃. This is attributed to the precipitation hardening effect of the Cu-rich phase. In general the strength of sintered Fe-Cu increased with copper content reaching the maximum strength for different Cu contents at various cooling rate. The effect of cooling rate on dimensional change was found to be negligible.

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The effects of green density and sintering time on various properties of sintered Fe-20% Cu with and without graphite addition were investigated. Sintering was carried out at 1140℃ in hydrogen atmosphere. The specimens were cylindrical ring shape and dimensional changes in O. D. and I. D. after compacting and sintering were carefully measured and compared. Fe-20% Cu compacts showed rapid growth during first 10 minutes of sintering with subsequent densification. The samples with higher green density showed larger dimensional growth and smaller shrinkage during the initial and subsequent stages of sintering. Observations of the microstructures indicate that the results can be best explained by the penetration of the iron particle boundaries by liquid copper at the beginning of the sintering process. Addition of graphitereduces the growth but has little effect on the shinkage. The effects of green density and sintering time on and degree of oil impregnation and radial crushing strength wera also investigated. The results closely follow general expectations. With increased sintering time the amount of open porosity decreased while the closed porosity did not show much change.

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