Characterization of Friction Stir Welded (FSW) AA5083-H111

Friction Stir Welding (FSW) has become the most important evolving welding technology for the proposed integral aluminium structures for the aerospace fuselage to avoid high cost, excessive weight. The fatigue performances of FSW parts are superior to riveting. FSW entails a non-onsumable rotating cylindrical tool, consisting of a shoulder and pin, being pushed in to induce heat to plasticize the material and then traversed along the abutting surfaces of the two plates to be welded. Several factors influence the resulting microstructure and mechanical characteristics, and tool transverse and rotational speed are the most significant. Characterization of selected pioneering FSW components is presented. AA5083-H111 was selected because of its superplasticity, formability, weldability, high strength and corrosion resistance. It has been the preferred material for the manufacturing of pressure vessels for aircraft and spaceships, and vehicle bodies’ structures. A CNC milling machine was converted to weld 3mm thick plates of 360mm by 120mm at combinations of different rotational (400, 500 and 630rpm) and welding speeds (50, 60 and 70mm/min) using a tapered probe and cylindrical shoulder at zero tilt angle. Tensile testing and bend testing were undertaken. The macrostructure and fractured surfaces were observed using an optical microscope and an SEM in secondary electron mode. Stress-strain relationships of the welds were different. Compared to the base material, FSW samples had lower strain-to-failure results, owing to strain localization in the weld. H1, welded at 400rpm and 50mm/s had the highest UTS, 99.3±0.2%, whereas H2, welded at 400rpm and 70mm/s had the highest yield strength, 97.7±0.3%. Welds performed at 400 rpm were generally flaw-free, while higher weld speeds produced welds with tunnels, due to the low forging pressure. In the macrostructure, the weld nugget could be easily distinguished from the TMAZ (Thermo-Mechanically Affected Zone) on the advancing side, because the grains were finer, being recrystallized and equiaxed. However, no distinct boundary existed on the retreating side. The joint-line remnant, which comprises amorphous clusters of alumina, was observed in the nugget, but had no effect on mechanical properties. There was an increase in hardness in the weld zone, towards the retreating side, due to grain refinement and better weld consolidation. Conversely, there was a decrease in hardness in the TMAZ. The weld nugget average hardness of all flaw-free samples was 79.8±4.6HV0.1, with no definite relationship between weld parameters and hardness profiles. Tensile specimens fractured in the TMAZ on the advancing side at 45° to the far-field stress, because of reduced micro-hardness, strain localization and as well as the presence of tunnels in the flawed samples. Generally, failure occurred in a brittle mode, but areas of local ductile shear and quasi-cleavage fracture were observed in the SEM. This research demonstrated that good weld efficiency was obtainable on FSW of AA5083-H111 with a CNC milling machine of sufficient welding power.