Microstructure and Mechanical Properties of Underwater Friction Stir Welded Joints of 7020 Aluminum Alloy

7020 aluminum alloy plate with a thickness of 5 mm was welded by underwater friction stir welding(UFSW), and the microstructure and mechanical properties of the welded joint were studied.The results show that the microstructure of nugget zone consists of fine equiaxed grains due to dynamic recrystallization, and most of precipitates dissolve into the Al matrix. Compared with that of the air-cooling FSW joint, the microstructure of the nugget zone of the UFSW joint are finer, more precipitates exsit in thermo-mechanically affected zone and finer precipitates exists in heat affected zone, the soft region becomes narrow. The ultimate tensile strength of the UFSW joint reaches (418.5±7.4) MPa, equivalent to 90% of the base metal, which is greatly improved compared with that of the air-cooling FSW joint.