An evaluation of calcium as a eutectic modifier in A357 alloy

Abstract

Modification of the eutectic silicon in Al-Si alloys is traditionally performed by sodium and strontium addition to alter the form of the silicon from an acicular shape to a fine fibrous one. This modification results in a considerable improvement in mechanical properties, especially elongation. Although not commonly used as a modifier, calcium also has the potential to modify the eutectic silicon in Al-Si alloys. Metallographic observation and thermal analysis studies in A357 alloy indicate that, when the cooling rate is quite low, Ca modifies the eutectic silicon, but the effect of Ca as a modifier is not as strong as is Na or Sr. When the cooling rate is high, a fully modified structure can be obtained with calcium. Recovery of Ca in the melt is high and certain. It does not fade with long holding times, and retains its effect after repeated remeltings.Calcium addition does increase porosity in the cast alloys. The reduced pressure test (RPT) shows that the density of samples decreases with calcium addition, and observation of polished sections confirms the porosity increase. Tatur tests indicate that Ca modification increases microporosity, and decreases macroporosity.Calcium and strontium interact together to reduce the effectiveness of each other as a modifier. This interaction can be seen by thermal analysis as well as by microstructural studies.