Evolution of Fe-Rich Intermetallics in Al-Si-Cu 319 Cast Alloy with Various Fe, Mo, and Mn Contents

In the present work, the precipitation and preferential selection of Fe-rich intermetallics in Al-Si-Cu 319 cast alloys with two Fe contents (0.3 and 0.7 wt pct) and various Mo contents (0 to 0.4 wt pct) were investigated. The results showed that two types of platelet β-Fe (defined as eutectic β-Fe and pre-eutectic β-Fe) and α-Fe can precipitate depending on the alloy compositions, such as the Fe and Mo contents. Generally, the addition of Mo promotes the formation of α-Fe instead of β-Fe. However, its effect on the phase competition between β-Fe and α-Fe is greatly related to the Fe content. In alloys with low Fe content (0.3 pct), an addition of 0.37 pct Mo can promote the complete precipitation of α-Fe and suppress the formation of eutectic β-Fe. However, in alloys with high Fe content (0.7 pct), only the pre-eutectic β-Fe amount decreases with increasing Mo addition, leaving the eutectic β-Fe almost unchanged, while both pre-eutectic β-Fe and eutectic β-Fe can be fully suppressed with a combined addition of Mn (0.24 pct) and Mo (0.4 pct). Compared with Mn, Mo exhibits a stronger effect on the promotion of α-Fe. The combined addition of Mn and Mo can achieve better modifications of both pre-eutectic and eutectic β-Fe in 319 alloys with high Fe content.