Impact toughness of Al–Si–Cu–Mg–Fe cast alloys: Effects of minor additives and aging conditions

• Effect of Mn addition on the impact toughness of Al–Si cast alloys.
• Precipitation hardening during aging of Al–Si–Mg alloys.
• Effect of heat treatment on the toughness of Al–Si cast alloys.
• Fracture mechanism of impact-tested 413 alloys.

The effects of Sr modification and aging treatment on the impact toughness of a near eutectic Al–11%Si–2.7%Cu–0.3%Mg–0.45%Fe alloy were investigated. Charpy impact tests were performed on unnotched specimens in the as-cast and heat-treated conditions. It was found that the presence of Fe- and Cu-containing phases increases the alloy brittleness which reduces impact toughness. The eutectic Si phase also plays an important role, where the size/morphology of the Si particles controls the area of α-Al matrix available which affects ductility and toughness. Increasing the Mn content leads to an increase in the volume fraction of the α-Al15(Mn,Fe)3Si2 phase formed and to sludge formation, which facilitates crack initiation and propagation. Crack propagation occurs mainly via the Al2Cu and/or α-Al15(Fe,Mn)3Si2 phases. In the non-modified alloys, the Si phase also plays a considerable role in the fracture process. The impact behaviour of aged alloys is influenced by the amount, size and morphology of hardening precipitates formed in the alloy, depending on the aging conditions. Aging at 240 °C produces a significant increase in the impact energy values of the low Mn-content alloys, as a result of alloy softening. The high Mn-content alloys also show a similar increase in impact energy values, but at a steady level across the same range of aging times, due to the persistence of the α-Al15(Mn,Fe)3Si2 phase.

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