EFFECTIVENESS OF FULLY ALLOYED PRECURSOR POWDERS AND FINE PARTICLE SIZES IN SINTER GROWTH CONTROL OF IRON-COPPER-NICKEL-GRAPHITE COMPACTS

摘要

The cost effective production of iron/copper PM parts requires as-sintered parts to be close enough to near net shape that no secondary processing is necessary. In order to achieve this, it is necessary to maintain dimensional stability during sintering. This work represents two approaches towards this goal. The use of smaller particle sized copper, and the use of a copper source which is alloyed with another element in the system. To this end, a series of pressed iron samples with 2% copper and 0.8% graphite were produced using a standard coarse particle copper, a fine particle copper, and a fine particle iron/copper alloy as the copper source. In addition, another similar series of pressed iron samples was produced containing 2% nickel, using coarse copper powder, fine copper powder, iron/copper alloy and nickel/copper alloy powders, as the source of copper. For the first set of samples, results demonstrate little variation in strength with the use of finer or coarser copper particles and little variation in growth from sintering, except at low compaction pressures. The use of the iron/copper alloy lowered growth by 15%, and reduced strength by 10% as compared to the conventional copper additions. For the sample series containing nickel, the use of finer copper powder decreased growth by 50%, and increased strength by 8% relative to the coarse copper powder. The use of the copper/iron alloy as the copper source lowered growth by 71%, and increased strength by 5% over the use of conventional coarse powder copper additions. The copperickel alloy samples showed a decrease in growth from sintering of 57% with an increase in strength of 1.3%. The reason for improved properties with the use of finer particle copper powder in the nickel containing sample series is explored by examination of the microstructure. The copper is found in high concentration regions, as it does with the coarser copper powder, but these regions are much smaller and greater in number. The growth from sintering is reduced by limiting the amount of copper available for any grain boundary, while the strength is increased by a greater number of inter-granular spaces being filled and supported by copper. As for the samples which use prealloyed powder for their copper sources, the most probable explanation for their behavior is the bonding of the copper to the alloys controls its movement, resulting in smaller and better distributed religions of copper in the iron matrix. This allows a tailoring of the microstructure much as is seen with the fine copper powder, where there is less material in the iron grain boundaries causing less growth during sintering.