Microstructure and Mechanical Properties of 2A14-T4 Aluminum Alloy T-joints Prepared by Stationary Shoulder Friction Stir Welding

The 2A14-T4 Al-alloy T-joints were prepared by stationary shoulder friction stir welding (SSFSW) at different welding speeds. A smooth T-joint surface was obtained with the optimized welding process parameters. The results show that the microstructure of weld nugget zone (WNZ) consists of fine equiaxed grains caused by full dynamic recrystallization. The average grain size of the second weld nugget zone (WNZ2) is the largest, that of the weld nugget overlap zone (WNOZ) is in the middle, and that of the first weld nugget zone (WNZ1) is the smallest. The recrystallization mechanism in WNZ is mainly the geometric dynamic recrystallization accompanied by partial continuous dynamic recrystallization. The weak {100}<001> texture is formed in the WNOZ after two times of stirring, and the weak {111}<110> texture appears in WNZ1 and WNZ2. The thermo-mechanically-affected zone (TMAZ) undergoes plastic deformation, while the heat-affected zone (HAZ) is only affected by weld thermal cycles. No plastic deformation or the dynamic recrystallization of grains occurs in HAZ. The hardness of WNZ is high, and the zone with the lowest hardness is located in HAZ which is close to TMAZ. With increasing the welding speed, the ultimate tensile strength is firstly increased and then decreased. The mixed brittle/ductile fracture dominates the fracture mode of skin and stiffener plates.