Friction stir welding was used to weld 2.75 mm thick 2519A aluminum sheets. The sheets were welded in conventional condition (in air) and flowing water condition. The influence of cooling conditions on. the microstructure and mechanical properties of the joint was investigated. The results indicate that with the rotation speed of 2 700 r/min and welding speed of 60 mm/min, the water cooling welding results in better joint, and improves the joint strength to 340 MPa. In water cooling condition, the softening region becomes remarkably narrow, and its hardness value is improved. Moreover, the grain size of the nugget zone is much smaller than the grain size in air cooling condition. In water cooling, the distribution of the hardness in the nugget zone is almost a straight line, while in air cooling, the hardness in nugget zone firstly becomes higher and then lower, and it changes significantly. The average hardness in the nugget zone by water cooling is smaller than the average hardness in air cooling condition, because the air cooling can improve the solution degrees and promote the aging process.%采用搅拌摩擦焊焊接厚度为2.75 mm的2519A铝合金板材.分别研究在空气和水流中冷却对焊接性能和焊缝组织的影响.实验结果表明:在旋转速度为2 700 r/min,焊接速度为60 mm/min条件下,水冷焊接得到美观并无明显缺陷的接头,提高了接头强度,抗拉强度达到340MPa;在水冷条件下,热影响区的范围变窄,热影响区平均硬度提高,焊核区晶粒粒度比空冷条件下的更加小,硬度几乎呈直线分布；而在空冷条件下,焊核区硬度呈先上升后下降趋势,变化明显,水冷焊核区平均硬度低于空冷条件下平均硬度,这是因为空冷焊接能够提高固溶度并促进随后的时效发生.
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