Effects of scandium on hypoeutectic aluminium copper microstructures under low solidification rate conditions

This work addresses the microstructures and mechanical properties of Al-4.5 wt-% Cu alloys containing 0.1, 0.2 and 0.4 wt-% Sc that were solidified under low cooling rate conditions (0.01, 0.08, 0.3 and 0.8°C s^?1) and then heat treated. The solidification and heat treatment processes were carried out in a differential scanning calorimeter. Two different approaches were used for heat treatment. The first approach, involved the traditional heat treatment sequences including solutionising for 18.5 h at 535°C then quenching, followed by aging at 240°C for 2 h. The second approach consisted of direct aging of the as-solidified samples. Under low solidification rate conditions, Sc is found to modify the grain morphology from elongated dendrites to equiaxed structures but is neither a grain refiner nor a strengthener. Indeed, much of it is tied up with Cu in the form of the intermetallic W-phase. Consequently, there appears to be no benefit to adding Sc, in the hypoeutectic composition, to Al-4.5 wt-% Cu if the traditional heat treatment route is followed. However, a minor improvement in hardness is observed upon aging immediately after solidification when 0.4 wt-% Sc is added, owing to excess Sc dissolved in the matrix and not precipitating as the W-phase.