To achieve high-quality friction stir welding (FSW) of the rocket fuel tank, experiments of FSW 18 mm thick 2219 aluminum alloy are carried out, the microstructure and mechanical properties of the joints are studied, and the influence law of the tool rotation speed on microstructure, microhardness and tensile strength of the welded joints are explored. The welding speed used in the experiment is 100 mm/min, and the tool rotation speed is 350 r/min, 400 r/min and 450 r/min, respectively. The microstructure of the weldments is analyzed by optical microscopy and scanning electron microscopy. The mechanical properties are measured by tensile tests and Brinell hardness, and it is found that the mechanical properties of the joint first increased and then decreased with the increase of the tool rotation speed. Due to the uneven temperature distribution along the thickness direction of the weldment, the crystal grains in the nugget zone (NZ) are small equiaxed crystals as a whole, with different local sizes and shapes. The size of the crystal grains seems relatively large in the upper layer and relatively small in the bottom layer. As the tool rotation speed increases, so does the heat generation, and thus, the size and degree of the deformation of the crystal grains increase. The defect appears when the tool rotation speed is 450r/min. As the tool rotation speed increases, the tensile strength and microhardness of the joints increase first and then decrease. The maximum tensile strength of the overall weld is 270 MPa. The microhardness shows a trend of ‘‘U’’ distribution, and the hardness in the NZ is slightly higher than that of thermos-mechanically affected zone (TMAZ) and heat-affected zone (HAZ). The fracture location of the joints is the NZ. The fracture mode is ductile fracture. There exist a large number of second phase particles Al2Cu in the dimples.

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