Grain size control is regarded as an important means of improving the quality of castings. The grain formation mechanisms in hypoeutectic Al-Si alloys have been determined by two experimental methods. The results indicate that crystal formation near the mold walls can play a dominant role in determining the final grain size in a casting. The effect of silicon content and AlTi_5B_1 grain refiner level on grain size was determined using a casting method where the crystals formed near the walls are prevented from contributing to the final grain size. The results show that, in unrefined alloys, the grain size first decreases with increased silicon content, reaches a minimum and then increases upon further silicon additions. The addition of grain refiner, before this minimum, is very efficient, with the grain size being controlled by the number of potent nucleants and the constitution. After the minimum in grain size, and for normal casting alloy compositions, the grain size is controlled by the silicon content, and the addition ofAlTi_5B_1 masteralloy is relatively inefficient. It is proposed that silicon poisons the potency of the nucleants. Recommendations are provided for optimizing the grain size in commercial castings.
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