Shifting toward sustainability and low carbon emission necessitates recycling. Aluminum alloys can be recycled from postconsumer scrap with approximately 5% of the energy needed to produce the same amount of primary alloys. However, the presence of certain alloying elements, such as copper and zinc, as impurities in recycled Al-Mg-Si alloys is difficult to avoid. This work has investigated the influence of tiny concentrations of Cu (0.05 wt %) and Zn (0.06 wt %), individually and in combination, on the precipitate crystal structures in Al-Mg-Si alloys in peak aged and overaged conditions. To assess whether such concentrations can affect the hardening precipitate structures, atomic resolution high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography were adopted. The results indicate that low levels of Cu or Zn have a significant influence. Both elements showed a relatively high tendency to incorporate into precipitate structures, where Cu occupies specific atomic sites, creating its own local atomic configurations. However, Zn exhibited distinct behavior through the formation of extended local areas with 2-fold symmetry and mirror planes, not previously observed in precipitates in Al-Mg-Si alloys. Incorporation of Cu and/or Zn will influence the precipitates' electrochemical potential relative to matrix- and precipitate-free zones and thus the corrosion resistance. Furthermore, the presence of Cu/Zn structures (e.g., β'Cu, Q'/C) enhances the thermal stability of these precipitates and, accordingly, the mechanical properties of the material. The results obtained from this work are highly relevant to the topic of recycling of aluminum alloys, where accumulation of certain alloying elements is almost unavoidable; thus, tight compositional control might be critical to avoid quality degradation.
Keywords: APT; HAADF-STEM; aluminum alloys; crystal structure; hardening precipitates; recycling.