Solidification and segregation of Al-Mg alloys and influence of alloy compositionand cooling rate

Abstract

The dependency of solidification and segregation behaviour of Al-Mg alloys on alloy composition and cooling conditions was investigated. In the case of normal solidification, the low magnesium containing alloy had an as cast microstructure of single phase α-Al, with no α-Al + Al₈Mg₅ eutectic structure. With an increase in the magnesium content of the alloy, the eutectic reaction L → α-Al + Al₈Mg₅ occurred at the finally solidified zone. The higher the magnesium content, the greater the amount of the eutectic structure. The formation and growth of the eutectic structure were also influenced by the cooling conditions. As the cooling rate increased, the eutectic fraction increased correspondingly. However, in the fast cooling rate range (> 190 K min⁻¹, the average concentration of magnesium in second phase Al₈Mg₅ decreased gradually. The faster the cooling rate, the smaller the grain size. In respect of the size of second phase Al₈Mg₅, there existed a critical cooling rate of ∼ 60 K min⁻¹, at which the size of the eutectic phase was a maximum. Slower and more rapid cooling rates produced a finer second phase structure.