Variations of the microstructure and mechanical properties for the thick aluminum friction stir welding joint along the thickness direction

Through the hierarchical method, microstructure and mechanical properties in different areas of the weld nugget zone for 30 mm thick 7A05-T6 aluminum alloy friction stir welding join have been studied. The results show that, the size of recrystallization grain in the near area of surface is largest.When the rotation speed is 360 r/min and the welding speed is100 mm/min, for the area of T₁ to T₅, the tensile strength separately is 326, 354, 342, 334 and 307 MPa, the yield strength and elongation are also increased gradually. The fracture micrographs show that, there are a lot of mesh dimples in the fracture surface, and they are deeper on slicing T₂ especially. The hardness profile is not central symmetric, and hardness value of top area is larger than that of lower area. Friction heat will decrease with the increase of thickness, and that is the reason why the performance decline at the middle and bottom of workpiece joint.