Abstract
Corrosion is a critical concern in high-strength Al–Zn–Mg–(Cu) alloys and is strongly influenced by microstructural features such as grain boundary precipitates and precipitate-free zones (PFZs) [1,2]. Welding induces complex thermal cycles that modify the microstructure of the heat-affected zone (HAZ). Previous studies report that HAZ I (adjacent to the weld) experiences peak thermal exposure, resulting in a high precipitate density, whereas HAZ II (between HAZ I and the base material) is subjected to intermediate temperatures and contains fewer, coarser precipitates, leading to softening due to precipitate coarsening and reduced dislocation pinning [3]. A study on MIG-welded 7020 alloys found the deepest corrosion penetration at the interface between HAZ II and the base material, although this observation was not further discussed [4]. To address this gap, the present study examines a corrosion-prone region approximately 15 mm from the weld centerline, termed the Corona. Elemental distributions in PFZs near grain boundaries and within grain interiors were analyzed using transmission electron microscopy with energy-dispersive spectroscopy mapping, while microstructural features were assessed by annular dark-field scanning transmission electron microscopy imaging. Results demonstrate that PFZs in the Corona are more enriched in Zn, Mg, and Cu compared with the base material, indicating solute release from intergranular precipitates. Increased Cu content in both PFZs and grain interiors further suggests dissolution of an observed Cu-rich intergranular film which is present in the base material. Elucidating welding-induced microstructural transformations in the HAZ is essential for understanding the increased corrosion susceptibility of the Corona region.