Influence of tiny concentrations of copper and zinc on the microstructure and intergranular corrosion of Al-Mg- Si alloys

In recycling, scrap composition will determine the amount of scrap that can be added to the recycled products without leading to possible quality degradation in their mechanical properties and/or corrosion resistance. Therefore, this thesis details the research work focused on understanding the influence of a minor amount of Cu and/or Zn (as impurities) on the microstructure, precipitates structures, and intergranular corrosion (IGC) resistance of Al-Mg-Si alloys. The influence of Cu and Zn on the IGC resistance was systematically studied using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray computed tomography (CT). Moreover, atomic force microscopy (AFM) coupled with scanning Kelvin probe force microscopy (SKPFM) was conducted to measure the surface potential of the grain boundaries. Furthermore, atom-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was used to study the influence of Cu and Zn on the precipitates structures.

The investigation detailed in this thesis is mainly structured into three parts. The first part focused on the influence of Cu and/or Zn on the microstructure and IGC resistance of Al-Mg-Si alloys in the as-received condition (Chapters 5-8). The second part is focused on the influence of aging time and temperature in the presence of low Cu and/or Zn on the microstructure and IGC resistance of Al-Mg-Si alloys (Chapters 9 and 10). The main focus of the third part of this thesis is to investigate the impact of a minor amount of Cu and/or Zn on the hardening precipitate structures. The detailed results related to the impact of Cu and/or Zn on the microstructure, precipitates structures, and intergranular corrosion of Al- Mg-Si in different aging conditions are presented and thoroughly discussed.

The optical investigation revealed a duplex microstructure compromised of a deformed center layer sandwiched between two recrystallized layers. The EBSD results from the surface layers showed that the fraction of the random high-angle grain boundaries in the explored alloys was ~91 %. It was found that low Cu and/or Zn concentrations can negatively influence the IGC resistance of Al-Mg-Si alloys. The IGC extension and the penetration depth were noticed to increase by increasing the Cu additions. However, Zn addition showed a more noticeable increase in the IGC extension than penetration depth by increasing Zn concentrations. The results also revealed that aging time and temperature can significantly influence the IGC resistance of Al-Mg-Si alloys. Moreover, the results evidently showed that minor addition of Cu and Zn can significantly influence the precipitate crustal structures. The STEM results indicated the presence of Cu/Zn-rich films along some investigated grain boundaries.