On Sc Addition in A206 Alloy: The Formation of W-AlCuSc Phase and Its Strength-Weakening Effect in Sand Castings

Scandium (Sc) is attracting attention in aerospace and special-vehicle industries due to its potential benefits, but research on the effect of Sc addition on the phase formation and mechanical properties of Al-Cu casting alloys (i.e., 2xx.x series) have been conflicting. To explore the potential of Sc in the Al casting industry, Al-Cu-based A206 alloys with different Sc contents were used to cast cylinder heads of special vehicles in an industrial sand casting environment. The results showed that Sc addition promoted the formation of W-AlCuSc rods precipitates within α-Al grains and the large W-AlCuSc intermetallic particles at α-Al grain boundary by redistributing Cu atoms from the α-Al grain interior to the boundary. The formation of the W-AlCuSc phase and the redistribution of Cu atoms reduced the yield strength of both as-cast and T6-state A206 alloys. Solidification calculations indicate that W-AlCuSc phase evolves from primary Al₃Sc phase or from supersaturated α-Al phase. It has also been experimentally verified that the formation of the W-AlCuSc phase is dependent on the cooling rate and can be suppressed by increasing the cooling rate. These findings provide a reference for the efficient utilization of Sc in casting industries.