The 2219 aluminum alloy with 4 mm thickness,which was used in the launch vehicle's booster tank widely,was taken as the research object.The floating bobbin tool was developed,and the internal plastic metal flow pattern had been analyzed.The matching welding heat input of the weld seam with high quality could be obtained.The process exploration and optimization test results verified the plastic metal flow pattern inside the weld and the process speculation directly.The results of macroscopic morphology analysis of the joints show that the onion ring characteristics can be observed in the macroscopic morphology of the weld cross section at different welding speeds, and the onion ring characteristics increase with the welding speed increasing.This phenomenon validats the design idea of the bobbin use for the thin-plate aluminum alloy effectively.The microhardness distribution of the cross-section of FSWed thin plate aluminum alloy is U-shaped,and the lowest point of the microhardness of the joint is located at the junction of the weld nugget zone and the RS thermo-mechanical affected zone.The mechanical properties test results show that the tensile strength of the joint increased gradually with the welding speed increasing,and the tensile strength of the joint reach the highest value when the welding speed reachs 350 mm/mim.The floating bobbin FSWed aluminum alloy joints has a high elongation,and the welding speed has little effect on it.The floating bobbin FSWed joints can achieve 180° without cracks in both front and back bending tests.Based on the vertical longitudinal seam friction stir welding system,the floating bobbin friction stir welding of 2 m long test piece is realized successfully.The cumulative weld length reachs 60 m and the floating bobbin tool is intact with no cracks,distortion or other damage.