Mechanical properties’ modification of large load friction stir welded AZ31B Mg alloy joint

•3-mm-thick AZ31B Mg alloy plates were successfully joined by LL-FSW.•HAZ was removed due to the significantly improved thermal circle.•Ultrafine grains with abundant dislocations and twins were produced in the weld.•The texture intensity significantly reduced in the weld.•The strength of FSWed Mg joint was enhanced without ductility loss. Friction stir welded (FSW) Mg alloys usually exhibit an undesirable combination of strength and elongation due to its strong texture develops in the weld. Thus, large load FSW associated with an extremely low welding speed and rotation rate were applied to an AZ31B Mg alloy to modify the microstructure, the texture and the mechanical properties of the joint. The twin structure in the weld provided adequate barriers for dislocation motion for strengthening and created more local sites for nucleating and accommodating dislocations, thereby elevating ductility and strain hardening of the weld. This work also provided a simple and effective method to enhance the strength of an FSW Mg joint without ductility loss.