• Medium-sized MgSi clusters (10–19 atoms) at grain boundaries strengthen the boundary, suppress crack initiation, and inhibit intergranular cracking during hemming.
• The Si/Fe atomic ratio in Fe-containing phases is decisive: a ratio of ~0.71 retains ductility, while 0.90 induces distinct brittle fracture.
• Pre-aging treatment reduces the yield strength of AA6016 Al alloy to ~120 MPa by promoting solute dispersion, thereby enhancing hemming performance.
• Pre-aging also promotes the formation of finer Mg–Si phases at grain boundaries during bake hardening, contributing to improved mechanical response.