Fig. 1 Schematic diagram of FSW a and sampling positions b (RS represents retreading side; AS represents advanced side)

Fig. 2 Microstructures of BM: a inverse pole figure map, b grain boundary map; c recrystallized grain morphology; d TEM image [13] (blue, yellow, and red in c represent recrystallized grains, sub-grains, and deformed grains, respectively. AT represents annealing twin in d)

Fig. 3 Microstructures of SZ: a inverse pole figure map, b grain boundary map; c, d TEM images (pink and blue arrows represent the CDRX and DDRX grains in b, respectively; green arrow represents the recrystallization grain growth direction in c, d; blue lines represent the ATs grains grain growth direction in d; S1 and REX-G1 in c, d represent sub-grain1 and recrystallized grain1, respectively)

Fig. 4 a, d Inverse pole figure maps of P1 and P2, respectively; b, e grain boundary maps of P1 and P2, respectively; c, f recrystallization grain distribution maps of P1 and P2, respectively; g, h local magnified images of a, b, respectively (blue, yellow, and red in b, e represent recrystallized grains, sub-grains, and deformed grains, respectively. Red cycle zones in f represent recrystallized zones)

Fig. 5 a, c, e Inverse pole figure maps of P3, P4, and P5, respectively; b, d, f grain boundary maps of P3, P4, and P5, respectively; g, h local magnified images of c, d, respectively; i pole figure of T1 on the {111} plane in g (pink arrows represent the DDRX grains in b)

Fig. 6 a, c, e Inverse pole figure maps of P6, P7, and P8, respectively; b, d, f grain boundary maps of P6, P7, and P8, respectively

Fig. 7 a, b Local magnified images of Fig. 6a, b, respectively; c pole figure of grains (G1, D1-D4) of {110} and {111} planes in a (G1, D1-D4 in a represent different grains)

Fig. 8 a Variations of LAGBs fraction, ATBs fraction, and average grain size; b recrystallized grain fraction at different locations during FSW

Fig. 9 a, b Local magnification of grain boundary and recrystallization grain distribution at the P5, respectively; c, d local magnification of grain boundary and recrystallization grain distribution at the P1, respectively; e all HAGBs, ordinary HAGBs, and ATBs at the P1-P8 (green represents LAGBs < 15°, red represents HAGBs > 15°, blue represents ATBs with a 60°/<111> orientation relationship, purple represents Σ9 grain boundaries, and indigo represents Σ27 grain boundaries)

Fig. 10 a, b Local grain boundary at the P2; c, d local grain boundary at the P4 (green represents LAGBs < 15°, red represents HAGBs > 15°, blue represents ATBs with a 60°/<111> orientation relationship, purple represents Σ9 grain boundaries, and indigo represents Σ27 grain boundaries)