The effect of mixedness on the dimensional change during sintering of Cu-10% Sn compact and a possible mechanism for the abnormal expansion near the peritectic point(798℃) are investigated.
In the first series of experiments cylindrical copper and tin powder compacts with the center part consisting of Cu-20% Sn mixture surrounded by an outer part consisting of pure. Cu powder were prepared and sintered in hydrogen atmosphere at temperatures below and above the peritectic point(798℃), where the abnormal expansion in known to occur. When sintered at 770℃ below the peritectic point the cetner core of Cu-20% Sn mixture shows only slight expansion. On the other hand when sintered at 10℃ above the peritectic point the core shows large expansion and a number of large spherical shaped cavities. The expansion and volume of cavities increase with sintering time at 10℃. These results suggest that the abnormal expansion observed near the peritectic point during sintering of copper-tin mixture is caused by sudden evolution of a gas.
In the second series of experiments well mixed and poorly mixed specimens of Cu-10% Sn mixture compacts were prepared by mixing for 80 and 2 minutes, respectively. Commercial grade electrolytic copper powder of about 270 mesh average size and an atomized tin powder of about -400 mesh were used.
The compacts were heated at rapid constant rates to 820℃ and sintered for various times. With sintering time the dimensional growth of the well mixed specimen reaches the maximum rapidly and the compact begins to shrink with further sintering. In the poorly mixed compact the dimensional growth occurs more slowly as the sintering time increases and the maximum expansion appears much later than the well mixed specimens. An analysis of models and the microstructures of the sintered specimens suggest that the growth during sintering is controlled by the flow of tin rich liquid phase which appears temporarily above the peritectic point. The flow model is also used to explain the previous observations on the effect of powder size on the expansion during sintering of copper-tin mixture.
