Investigation into the friction stir welding of thick section aluminium alloys

Friction stir welding (FSW) is a solid-state joining technology with a growing range of applications, many of them in safety critical service. These include aircraft wing spars and fusion reactor components, where the heat input associated with thick section components and long-term service performance are becoming increasingly important. However, there is a limited pool of published knowledge relating to FSW of thick section materials due to the previous industrial demand surrounding thin section welding. The primary aim of this research project was to investigate the influence of FSW using three different techniques joining 50 mm thick aluminium alloys AA5083-H111, AA6082-T651 and AA7050-T6451. These techniques, namely Weld-Flip-Weld (WFW), Simultaneous Double Sided (SDS) and Supported Stationary Shoulder (SSS) each have a different thermo-mechanical weld input and effect on the weld zone which have yet to be extensively investigated and quantified. The stir zone experienced large deformation and recrystallization resulting in an equiaxed and refined grain structure. Grain size was influenced by energy input with a greater input resulting in a larger grain structure. In each weld, constituent particles were identified with common elements of iron and aluminium being observed in all cases. Further particles of Mg2Si were found in AA5083 and AA6082 welds while Silica was recorded welds of AA7050. Simultaneous Double Sided FSW was shown to be an effective process and arguably the most appropriate for a wide range of industrial uses. In non-heat treatable AA5083 the process not only used less energy but also retained higher strength (298 MPa) than Weld-Flip-Weld (277 MPa). The retention of strength in the SDS weld in heat-treatable alloy AA7050 (352 MPa) and was not as high as that in the WFW weld (419 MPa), x however, coupled with the other benefits, such as faster production time and lower energy input, can be considered the most industrially suitable variant for AA7050- T7451.