This study aims to evaluate the effect of grain refinement on slurry formation and surface segregation in semi-solid castings produced by the Rheometalä process. The effect of two grain refiners, Al-8B and Al-5Ti-1B, on the slurry a-Al grain size, shape factor and solid fraction was evaluated. The results suggest that the addition of a grain refiner can affect the solid fraction obtained in the Rheometal TM process and, consequently, reduce the solute content near the casting surface. Grain refiner addition resulted in a larger fraction of a-Al grains £ 60 lm for the refined alloys compared with the unrefined alloy. Additionally, the growth of a-Al slurry globules was greater for the unrefined alloy compared with the refined alloy during solidification in the die-cavity. A more homogeneous and finer microstructure was observed near the surface in the grain-refined castings compared with the unrefined castings. Evidence of significant liquid penetration was identified in some a-Al globules, indicating that disintegration of a-Al globules may occur during the Rheometalä casting process.

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