Microstructure and properties of friction stir welded joints for 6061-T6/7075-T6 dissimilar aluminum alloy

Friction stir welding technique was applied to 2 mm thick 7075-T6/6061-T6 dissimilar aluminum alloys. The influence of rotating speed and material placement on the joint formation, microstructure and properties were studied. The results indicated that when the 7075-T6 aluminum alloy was placed on the advancing side, it is more conducive to material migration. The plates were joined successfully without welding defects and good surface formation when the welding speed is 150 mm/min and the rotating speed is 1000 r/min. Compared with the base metal, the thermo-mechanically affected zone had an increased content of low-angle boundaries. In the nugget zone, the low-angle boundaries converted to curve high-angle boundaries because of dynamic recrystallization. With the increase of rotating speed, the tensile properties of the joint firstly increased and then decreased. The ultimate tensile strength of the joint reached 231 MPa, and the elongation reached 4.0%. the fracture locations of the joints all located in the nugget zone at the 6061-T6 side, which coincided with the position with the minimum hardness.