Modification-related porosity formation in hypoeutectic aluminum-silicon alloys

A series of aluminum-10 wt pct silicon castings were produced in sand molds to investigate the effect of modification on porosity formation. Modification with individual additions of either strontium or sodium resulted in a statistically significant increase in the level of porosity compared to unmodified castings. The increase in porosity with modification is due to the presence of numerous dispersed pores, which were absent in the unmodified casting. It is proposed that these pores form as a result of differences in size of the aluminum-silicon eutectic grains between unmodified and modified alloys. A geometric model is developed to show how the size of eutectic grains can influence the amount and distribution of porosity. Unlike traditional feeding-based models, which incorporate the effect of microstructure on permeability, this model considers what happens when liquid is isolated from the riser and can no longer flow. This simple “isolation” model complements rather than contradicts existing theories on modification-related porosity formation and should be considered in the development of future comprehensive models.