Effect of different Zn content on microstructure, mechanical, and electrochemical properties of Al–Zn–Mg–Cu Alloys

Zinc content is pivotal in tailoring the properties of Al–Zn–Mg–Cu alloys, yet achieving a multi-performance balance between high static strength, superior fatigue performance, and corrosion resistance remains a persistent challenge. In this study, we systematically investigated the microstructure and comprehensive properties of Al–Zn–Mg–Cu alloys with Zn contents ranging from 4.2 to 8.2 wt%. Uniquely, our results revealed that the microstructural evolution did not follow a linear trend. Instead, the average grain size, precipitate dimensions, and width of precipitation-free zones (PFZs) all exhibited a distinct non-monotonic behavior, which was characterized by an initial refinement and followed by coarsening as the Zn content increased. This microstructural non-monotonicity directly dictated a similar trend in the alloy's macroscopic performance. Specifically, the 6.2 wt% Zn alloy (6.2Zn) emerged as the optimal balanced composition, achieving a synergistic optimization of properties. It demonstrated the highest peak hardness of 182.8 HBW, an ultimate tensile strength (UTS) of 574 MPa, a high-cycle fatigue limit of 114 MPa, and superior corrosion resistance among the investigated alloys. Furthermore, we identified that alloys with the lowest (4.2 wt%) and highest (8.2 wt%) Zn contents developed large “pure matrix regions” devoid of strengthening second phases, a critical microstructural defect that limited their mechanical performance. These findings provide a valuable reference for regulating main alloying elements to develop Al–Zn–Mg–Cu alloys with good comprehensive properties.