Mechanical property and characterization of 7A04-T6 aluminum alloys bonded by friction stir welding

The 7A04-T6 aluminum (Al) alloys were bonded via a friction stir welding (FSW) technique. The tensile strength of this FSW joint was the highest when the welding speed was 120 mm/min and the rotation speed was 1000 r/min, which reached 77.93 % of that of the base material (BM). The nugget zone (NZ) consisted mainly of the fine grains; the grain boundary misorientation angle in NZ was about 15 degrees, indicating that the propagation of microcrack was hindered to a certain degree, improving the toughness of this joint. The S-N curve of this joint showed that the fatigue strength decreased with an enhancement of the fatigue life when N was below 10(5), also N was above 10(6). The fatigue limit (141 MPa) of the joint was calculated through an up-and-down test method, the fatigue fracture position occurred in HAZ, exhibiting the ductile fracture. Then a laser local heat treatment (LLHT) process was carried out on this weld seam, which refined the microstructure, strengthening the mechanical property of this FSW joint.