Aluminium alloys based on the hypoeutectic aluminium-silicon system are the most commonly used aluminium foundry alloys. Small additions of either strontium or sodium are frequently used to promote a flake to fibrous transition in the morphology of the eutectic silicon phase. These additions have been associated with increased porosity, although there is no consensus on the fundamental mechanisms responsible. The purpose of this research was to establish the way in which modification affects both porosity formation and the macroscopic and microscopic growth patterns of the aluminium-silicon eutectic phases.
A series of sand castings were produced to investigate the effect of additions of either strontium or sodium on the microstructure and pore distribution in aluminium-l 0 wt% silicon alloys with copper additions of 0 - 3 wt%. Samples taken from permanent-mould castings were also examined to investigate the effect of an increased cooling rate. Thermal analysis experiments were performed as an aid to microstructural interpretation and interrupted solidification experiments were performed to observe the evolution of the aluminium-silicon eutectic phases. Interrupted solidification experiments were also performed with unmodified and strontium modified A356 alloys.
In addition to promoting a transition in the morphology of the silicon phase it was established that modification with either strontium or sodium results in a dramatic decrease in the nucleation frequency of aluminium-silicon eutectic grains. It is suggested that the decrease in eutectic grain density results from the poisoning of phosphorus-based nucleant particles, which were frequently associated with the eutectic phases in the unmodified alloys.
A model was developed to predict the eutectic interface velocity during growth from the bulk thermal undercoolings recorded during thermal analysis. The model predicts significant variations in the velocity of the solid-liquid eutectic interface during growth. The variation in growth velocity results in fluctuations in the silicon morphology, even within a given eutectic grain. The model provides a rationale for interpreting the eutectic grain structure of fully solidified castings and also a physical basis for understanding the formation of partially modified structures.
Iron, copper and magnesium containing intermetallics that solidify after the aluminium-silicon eutectic reaction were shown to segregate to the periphery of eutectic grains. In unmodified alloys, where the eutectic grain size is relatively small, intermetallics were found in small isolated pockets. In modified alloys, where the eutectic grain size is relatively large, intermetallics were coarser and were often found in semi-continuous networks. A modified version of the Scheil equation was used to predict the segregation of both iron and copper intermetallics in the sand cast microstructures.
Modification with either strontium or sodium resulted in a statistically significant increase in the amount of porosity in the sand castings. The increased porosity was the result of the formation of a large number of dispersed, isolated pores during eutectic solidification. A simple geometric model, based on the distribution of liquid during solidification, was used to show how the increase in porosity is consistent with an increase in the eutectic grain size.
The potential of refining the eutectic grain size of modified alloys is discussed as future work. Refining the eutectic grain size is predicted to reduce the amount of porosity and enhance the homogeneity of castings.