Microstructure Evolution and Recrystallization Behavior of Friction Stir Welded Thick Al-Mg-Zn-Cu alloys: Influence of Pin Centerline Deviation

Four tools with different pin centerline deviations were fabricated to friction stir weld (FSW) thick plates of Al-Mg-Zn-Cu alloys. The results show that, compared with the pin without pin centerline deviation, the applying of pin centerline deviation is favorable to improve the flowability of plastic material, enlarging the size of nugget zone, refining the grains and reprecipitated phase particles, enhancing the degrees of dynamic recrystallization and quantity of high angle grain boundaries owing to higher temperature and bigger eccentric force, resulting in the increase of strain hardening. Especially for the used pin with a centerline deviation of 0.2 mm, the highest average hardness and best metallurgical properties of thick FSW joints are produced, and which are consistent with the microstructure evolution and recrystallization behavior. Moreover, the fracture mode of the joints produced by the pins with centerline deviation from 0 mm to 0.3 mm changes gradually from a brittle fracture in the nugget zone (NZ) to a ductile failure in the HAZ.