The implementation of wire-arc additive manufacturing for fabricating complex structures requires detailed knowledge of process-structure-property relationships. Results of scanning electron microscopy, wide-angle X-ray scattering, small-angle X-ray scattering, and microhardness measurements are presented to identify the mechanisms that govern the microstructure formation of an Al-Mg-Zn-Cu crossover alloy during wire-arc additive manufacturing. These analyses provide evidence on the formation of Mg-, Zn- and Cu-rich phases on different length scales spanning from ~10 µm (microsegregations), down to a few nm (bulk) following the intrinsic heat treatment. Future alloy concepts should build on the presented findings.