Periodic sub-grain structure in the stir zone and its effect on strain hardening behavior of multi-pass friction stir welded Al-Zn-Mg-Cu alloy joints

Al-Zn-Mg-Cu alloy joints were fabricated by 4 passes friction stir welding. The effect of the different passes on the grain size, sub-grain structure, kernel average misorientation of the stir zone (SZ) were characterized by electron backscatter diffraction. Tensile properties and strain hardening behavior of these joints were evaluated. Results reveal that the welding passes nearly have no effect on the grain size of SZ. The fraction of the sub-grain structure in the SZ is 36.9%, 33.4%, 30.4% and 41.3%, respectively when the welding pass increases from 1 to 4. It is very interesting that a periodic sub-grain structure in the SZ can be found when compared 1 pass joint with that of 4 passes joints. 3 passes joint with the lowest sub-grain structure proportion in SZ possesses the best tensile properties and highest strain hardening rate in the stage III.