Inhibiting microstructure segregation and softening in a 700 MPa grade Al-Zn-Mg-Cu alloy by a novel underwater-assisted dual-rotation friction stir welding method

700 MPa grade ultra-high strength Al-Zn-Mg-Cu alloys containing over 10 wt.% Zn exhibit significant potential in lightweight materials design, but its weldability remains a major obstacle in practical applications, even employing solid-state friction stir welding (FSW). The substantial increase in Zn content not only reduces the material's plastic deformation capability but also promotes distinct microstructural segregation during FSW. Here, we report superior weld appearance and enhanced joint strength achieved in a 700 MPa grade Al-Zn-Mg-Cu alloy using an innovative underwater-assisted dual-rotation friction stir welding (UWDR-FSW) method. Excellent weld appearance and homogeneous microstructure were obtained by applying an extremely low shoulder rotation rate of 200 rpm combined with water cooling due to the elimination of overheating. High solute solubility and fine recrystallization grains improve the microhardness values of the nugget zone (NZ). Conversely, the dramatically reduced peak temperature and increased cooling rate suppressed precipitate coarsening in the heat-affected zone (HAZ), resulting in a higher microhardness value than even that of the NZ. This new UWDR-FSW process successfully eliminated the microstructural segregation and traditional low-hardness zones in the HAZ, significantly improving joint strength to 514 MPa compared to the conventional FSW joint (414 MPa). This work contributes a novel approach for welding ultra-high-strength aluminum alloys with high alloying content, supported by comprehensive experimental data and theoretical analysis.