The influence of Sr, Mg and Cu addition on the microstructural properties of a secondary AlSi9Cu3(Fe) die casting alloy

• Mg reveals positive effects on reducing porosity in Sr-modified and Cu-enrich alloys.
• SDAS, eutectic Si and secondary intermetallics are more affected by the cooling rate.
• Sr refines and modifies eutectic Si, Mg and Cu inhibit the benefits of Sr refinement.
• Sr, Mg and Cu addition reduces SDAS and grain size of die cast AlSi9Cu3(Fe) alloys.
• The volume fraction of intermetallics increases as Cu and Mg content increases.

The influence of Sr, Mg and Cu content on the microstructure of a high-pressure die cast AlSi9Cu3(Fe) alloy is reported. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring at different Sr, Mg and Cu levels. The results reveal that the Sr and Cu increase the amount of microporosity in the die castings, while Mg counteracts this effect. The secondary dendrite arm spacing and the grain size slightly decrease by the addition of the alloying elements. Compared with the base AlSi9Cu3(Fe) alloy, the Sr-modified alloy shows significant refinement and morphological modification of eutectic Si particles in the central regions of the castings. In contrast, such mechanisms at the casting surfaces are substantially driven by more rapid solidification. The Mg and Cu addition annihilates the benefits of refinement of eutectic Si particles by Sr modification, while it seems to not affect their morphology. The combined addition of Cu and Mg determines an increase of Cu- and Mg-rich intermetallic compounds, while the Sr-modified alloy shows lower fraction of intermetallics, even if comparable to the base alloy in terms of other alloying elements.