Influence of Mg on the Microstructure and Mechanical Behaviour of Recycled Al-Si Alloys

The transition to sustainable manufacturing is driving the increased use of recycled aluminium alloys. However, the variability of residual elements such as Mg, Fe, or Mn poses challenges for achieving reliable microstructural control and mechanical performance. This study investigated the influence of Mg and Mn content on the microstructure and properties of recycled Al-Si alloys. Alloys with varying Mg concentrations (0.2 to 0.5 wt.%) and Mn additions (0.07 to 0.54 wt.‰), representative of recycled feedstock compositions, were prepared by casting. Microstructure analysis revealed that increasing Mg promoted Mg₂Si formation, and also modified the morphology and distribution of intermetallic phases. Mechanical testing showed that increasing Mg from 0.2 to 0.5 wt.% enhanced strength through precipitation and solid solution strengthening while reducing ductility; the yield strength increased from 156 to 250 MPa and the ultimate tensile strength from 242 to 296 MPa, whereas ductility decreased from 7.8 to 2.9 %. Addition of 0.54 wt.‰ Mn did not show a significant effect on strength or ductility in the compositions evaluated. The results highlight the critical role of Mg in recycled aluminium alloys, demonstrating both its strengthening potential and its risk of embrittlement as a function of its composition. The findings provide a pathway for alloy design and process optimisation to enable high-value use of recycled Al alloys in structural applications, supporting a more sustainable circular economy in the aluminium sector.