Assessment of the Effect of Mg Addition on the Solidification Behavior, Tensile and Impact Properties of Al–Si–Cu Cast Alloys

The purpose of this study was to determine the influence of different amounts of Mg on the microstructures of experimental and industrial 319 alloys. Thermal analysis was carried out for the various alloy compositions to determine the reactions corresponding to the formation of various phases. These phases were identified by examining the corresponding microstructures of the as-cast alloys. The results indicated that the addition of Mg leads to the segregation of the copper phase, resulting in the formation of the block-like form of the CuAl 2 phase rather than its finer eutectic-like form. This makes it more difficult to dissolve the CuAl 2 phase during solution heat treatment. It was also observed that the degree of modification achieved in the microstructures of the 319 alloys, irrespective of the alloy source, is greatly enhanced at 0.6 wt% Mg content. Addition of Mg also leads to the precipitation of the Al 5 Mg 8 Cu 2 Si 6 phase, which normally precipitates after the CuAl 2 phase. However, when the Mg level exceeds 0.4 wt%, the precipitation of the Al 5 Mg 8 Cu 2 Si 6 phase also takes place in another reaction, before the precipitation of the CuAl 2 phase. The morphology of the Al 5 Mg 8 Cu 2 Si 6 phase in this case is script-like rather than the irregular-shaped particles normally observed. Tensile and impact properties of bars aged at different temperatures/times were reported as well.