High-temperature heat treatment attenuating the influence of micron-sized inclusions on the microstructure and properties of recycled Al-Zn-Mg-Cu alloy sheet

In the recycling process, aluminum scraps, characterized by a considerable specific surface area, yield a remelt abundant in inclusions. While advanced melt purification systems proficiently extract hydrogen and large inclusions, micron-sized inclusions remain challenging to remove. Despite previous findings suggesting nominal effects of inclusions on the microstructure and properties of aged 7xxx-series plates, the products' airworthiness demands further improvement. This research aims to decipher the influence of micron-sized inclusions on the microstructure and properties of recycled 7xxx aluminum alloy. We prepared thin sheets of varied cleanliness and utilized SEM, EBSD, and TEM to characterize their microstructure at different heat treatment stages. Additionally, high-temperature tensile and bending properties, together with exfoliation corrosion behaviors, were examined. Our results point to the causative role of inclusions in localized dislocation accumulation post-rolling, which annealing can mitigate but not fully eliminate; however, a solid solution can effectively address this issue. The inclusions significantly disrupt the microstructure and properties of the annealed recycled sheet, but their impact on the solid solution-aged recycled sheet is minor. This study's findings can guide the high-end transformation of recycled aluminum alloy and lay a theoretical foundation for sustainable aluminum recycling.