Improving the microstructure and mechanical properties of 2A14-T4 aluminum alloy thick T-joints via stationary tool shoulder friction stir butt welding

The double-pass butt welding process was carried out on 9-mm-thick 2014-T4 aluminum alloy T-joints by stationary tool shoulder friction stir welding (STS-FSW). Microstructure changes were analyzed by grain and EBSD, combined with the evolution of the precipitation phase, to explore the influence of regulations of different process parameters on T-joints’ mechanical properties. Defect-free T-butt joints were obtained within the rotational speed range of 400–500 rpm and the welding speed range of 100–140 mm/min. Due to severe dynamic recrystallization, the weld nugget zone exhibits fine equiaxed grain characteristics, with a grain size of approximately 10 μm. The degree of recrystallization and deformation in the weld nugget overlapped zone (WNOZ) did not significantly change, and the grain size of WNOZ was approximately 4 μm. T-joints’ hardness has an uneven distribution, and the hardness is greatest in the weld nugget zone at 110–125 HV. The T-butt joints have the weakest hardness at the skin and stringer in the TMAZ, approximately 70–80 HV. The ultimate tensile strength of the skin and stringer in the T-butt joints is equivalent to 85