Silicon segregation in aluminium casting alloy

摘要

The objective of the present study was to find anexplanation for the observation made by earlier researchers that thedistribution of silicon across dendrite branches in an aluminium-silicon casting alloy is sometimes anomalous, in that theconcentration gradient is I the opposite direction to that predictedby solidification theory. Small specimens of aluminium alloyA356 were solidified to give a similar dendritic microstructure.One specimen was quenched from a temperature just above theeutectic temperature, giving the silicon distribution expected fromtheory, while a second specimen was cooled more slowly to givethe anomalous silicon distribution, suggesting that it is caused bysomething occurring in the solid state. It was noted that there is arapid decrease in solubility of silicon in aluminium withdecreasing temperature below the aluminium0silicon eutectictemperature, so that a substantial amount of silicon is expected tocome out of solution at temperatures above 500degC on cooling.An estimate of the diffusion rate of silicon in aluminium showedthat, for normal cooling rates, this can occur by diffusion rate ofsilicon to the interdendritic silicon particles formed during thefinal stages of freezing, thereby removing silicon directionallyfrom the dendrite branches and producing the observed anomaloussilicon distribution.