Microstructure and Strain Hardening of a Friction Stir Welded High-Strength Al–Zn–Mg Alloy

Microstructural evolution and strain hardening behavior of a friction stir welded (FSWed) high-strength 7075Al-T651 alloy were evaluated. The nugget zone was observed to consist of fine and equiaxed recrystallized grains with a low dislocation density and free of original precipitates, but containing uniformly distributed dispersoids. The strength, joint efficiency, and ductility of the FSWed joints increased with increasing welding speed. A joint efficiency of ~91% was achieved at a welding speed of 400 mm/min and rotational rate of 800 r/min, while the ductility remained basically the same as that of the base metal. There was no obvious strain rate sensitivity observed in both base metal and welded joints. While both the base metal and FSWed joints exhibited stage III and IV hardening characteristics, the hardening capacity, strain hardening exponent, and strain hardening rate all increased after friction stir welding.