Investigation on the microstructure and mechanical properties of the turbulent zone in friction stir welded 7075 aluminum alloy medium thickness plate joints

Friction stir welding of 7075 aluminum alloy plate with a thickness of 10 mm was carried out with a self-designed stirring tool, and after welding, an irregular transitional zone between the tool shoulder zone and the nugget zone, known as the turbulent zone, was discovered on the advancing side of the weld. The formation mechanism of the turbulent zone was analyzed by observing the transverse and longitudinal sections of the joint cross section. The grain size, the degree of dynamic recrystallization, the misorientation angle distribution in the turbulent region were evaluated by electron backscatter diffraction (EBSD). The grain size in the turbulent zone is large with an average grain size of 2.98 μm. The proportion of substructure is as high as 75.1 %, which makes the hardness value higher than that of other areas of the weld. In addition, layered tensile mechanical test results show that the tensile strength of the turbulent zone layer (342 MPa) is comparable to that of the central layer of the weld nugget zone (345 MPa).