Friction stir welding (FSW) is an emerging manufacturing technology increasingly used in the fabrication of aerospace structures. National Research Council Canada, Institute for Aerospace Research, undertook a major initiative to manufacture a large scale representative structural element from an aircraft, such as a fuselage panel, using FSW. This multi-pronged study includes the aspects of structural design, process optimization, mechanical properties, NDE and robotic processing.

 The present study investigates the effects of process parameters on the weld quality of 1.5 mm 7075-T6 stringers lap-joined to 2.3 mm2024-T3 skins. Weld quality in terms of; the presence of hooking, top plate thinning, void or kissing bond defects, was assessed by optical microscopy and bending tests. Different FSW tool geometries, such as smooth, threaded, pyramidal and truncated pins, were used. Moreover, the effects of pin length, roll angle, forge force, welding and rotation speeds were investigated. Advancing and retreating side locations on the joint configuration were also alternated to determine optimal design arrangement.

 It was found that: i) the tool shape did not have a significant impact on the weld quality, except that the smooth pin led to the presence of excessive voids, ii) the increase of the welding speed or the decrease of the rotational  speed caused a reduction of the hooking size and an  increase in the extent of kissing bonds, iii) double pass welds by rastering improved the weld quality significantly by overriding the hooking defect, iv) changing the rotational direction to counter clockwise with a left-threaded pin significantly reduced top plate thinning.