Microstructure and Mechanical Properties of 7A60-T6 Ultrahigh Strength Aluminium Alloy Friction Stir Welded Joints

Microstructures and mechanical properties of the whole specimen in 6 mm thickness TA60-T6 aluminium alloy plate joints obtained by friction stir welding (FSW) were studied. The results show that fine equiaxed and recrystallized grains could be observed in the weld nugget zone(WNZ), and the segregation phases mostly dissolved into the matrix. The microstructure of the heat affected zone(HAZ) consisted of coarse bar-shape recovery grain, and a large number of segregation phases appeared at the grain boundaries. Microstructure of the thermo-mechanically affected zone(TMAZ) on advancing side consisted of highly deformed grains, and small deformation on retreating side. When the rotating speed was 300 r·min-1 and the welding speed was 100 mm·min-1 or the rotating speed was 600 r·min-1 and the welding speed was 300 mm·min-1, the good weld was achieved, and the tensile strength of the joint reached 465 MPa and 464 MPa, about 70% of the ultimate strength of the base metal. The fracture analysis show that there were lots of low mesh dimples, the fracture mode of the joint was ductile/brittle mixed fracture. The micro-hardness of the top surface of the FSW joint was higher than that of the transversal section, and the micro-hardness of the weld nugget zone presented an ascending trend form bottom to top.