2 Experimental procedure
The 6061-T6 aluminum alloy plates with a thickness of 6 mm were joined using DS-FSW. Then, the joined plates were rolled to the thickness of 4 mm, 3 mm, and 2 mm, respectively, as shown in Fig. 1. The fatigue samples of SZ were prepared as shown in Fig. 1. Three samples of a plate were prepared. The stress of fatigue test is listed in Table 1. The fatigue test was performed in HLGP-100 with a stress ratio (R ) of 0.1. The loading frequency was between 60 Hz and 75 Hz. When the instantaneous displacement increment surpassed 50 μm, the fatigue test would be aborted.