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The use of a double sided friction stir welding tool (known as a bobbin tool) has the advantage of giving a processed zone in the workpiece which is more or less rectangular in cross section, as opposed the triangular zone which is more typically found when conventional friction stir welding tool designs are used. In addition, the net axial force on the workpiece is almost zero, which has significant beneficial implications in machine design and cost. However, the response of these tools in generating fine microstructures in the nugget area has not been established. The paper presents detailed metallographic analyses of microstructures produced in 25mm AA6082-T6 aluminium wrought alloy, and examines grain size, texture and mechanical properties as a function of processing parameters and tool design, and offers comparison with data from welds made with conventional tools.

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Aircraft skin and stringer elements are typically fabricated from 2xxx and 7xxx series high strength aluminum alloys. A single friction stir welding (FSW) pass using a specially designed tool with shoulder/pin diameter ratio (D/d) of 3.20 is used to produce dissimilar T-butt welds between AA2024-T4 and AA7075-T6 aluminum alloys at a constant travel speed of 50 mm/min and different rotational speeds of 400, 600 and 800 rpm. The AA2024-T4 is the skin and the AA7075-T6 is the stringer. Sound joints are produced without macro defects in both the weld top surfaces and the joint corners at all rotational speeds used (400, 600, and 800 rpm). The hardness value of the nugget zone increases by increasing the rotational speed from 150 ± 4 Hv at 400 rpm to 167 ± 3 Hv at 600 rpm, while decreases to reach the as-received AA2024-T4 hardness value (132 ± 3 Hv) at 800 rpm. Joint efficiency along the skin exhibits higher values than that along the stringer. Four morphologies of precipitates were detected in the stir zone (SZ); irregular, almost-spherical, spherical and rod-like. Investigations by electron back scattered diffraction (EBSD) technique showed significant grain refinement in the sir zone of the T-welds compared with the as-received aluminum alloys at 600 rpm due to dynamic recrystallization. The grain size reduction percentages reach 85 and 90 % for AA2024 and AA7075 regions in the mixed zone, respectively. Fracture surfaces along the skin and stringer of T-welds indicate that the joints failed through mixed modes of fracture.

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