Enhanced ductility in Al-Si-Cu-Mg foundry alloys with high Si content

It has recently been suggested that the β-Al₅FeSi and ϑ-Al₂Cu intermetallic particles are refined and dispersed in the presence of high silicon, thereby improving the ductility of Al-Si-Cu-Mg alloys. However, limited metallographic evidence was presented to support these claims. Therefore, a study of the effect of Si content in the range of 4.5 to 9 pct on the morphology and distribution of Fe-rich and Cu-rich intermetallic phases has now been conducted. It is shown that Si, indeed, exerts a refining effect on the iron-containing particles (α and β) and disperses clusters of intermetallics (including the Cu-rich particles). In alloys with low Si content, the Fe- and Cu-rich particles form long and closely intertwined clusters. Microcracks originating from cracked intermetallic particles extend and propagate along the clusters with little plasticity, resulting in the low ductility of the alloys. At a high Si content, the intermetallic phases appear more dispersed and the clusters of particles are small and isolated from each other. Microcracks resulting from the cracked intermetallics are short and are isolated, as well, thereby increasing the ductility of the alloys. The mechanisms by which the refinement and dispersion of intermetallic phases occur are discussed. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA.