Underwater friction stir welding (FSW) on 7A04-T6 aluminum alloy plates was carried out, and the effect of rotation rate on microstructure and mechanical properties of joints was investigated. The results show that the minimum hardness of underwater FSW joints is located in the thermo-mechanically affected zone. The hardness of welded joints at the high rotation rate of 950r/min exhibits W-shaped distribution, and the average hardness value in the nugget zone is higher than that of welded joints at the low rotation rate of 475, 600, 750r/min. When the rotation rate increases from 475r/min to 750r/min with a constant welding speed of 235mm/min, the precipitated phases in the nugget zone gradually become coarse, and the ultimate tensile strength coefficient of the joint decreases from 89.71% to 82.33%; when rotation rate increases to 950r/min, the precipitated phases dissolve into aluminum matrix during welding, and age after welding. This produces the fine and homogeneous dispersed phases, which results in an increase of the strength coefficient to 89.04% and a certain enhancement of strain hardening capacity and elongation for the joints. All the tensile fracture surfaces exhibit the mixed characteristics of microporous polymerization and cleavage fracture.
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