Fatigue of friction stir welds in aluminium alloys that contain root flaws

Abstract Although the vast majority of friction stir welds will be free of flaws, it is not always possible to assume that they are. The properties of welds with flaws are needed to enhance confidence in the design and application of friction stir welded joints. The monotonic strength and fatigue behaviour of single-sided butt welds in 6–7 mm thick AA5083-O, AA5083-H321 and AA6082-T6, both without and with root flaws, was investigated. Examination of the root flaw faces showed that there was bonding between the flanks of the flaws but the bonding was of poor quality and incomplete. This meant that the strength and ductility of the flaws were lower than the surrounding material. However, the comparison of the mechanical test results suggests that root flaws up to a certain size are tolerable without a significant loss in performance when compared to nominally flaw-free welds. These data also suggest that even friction stir welds with root flaws exceed the design life for equivalent fusion welds set out in the draft Eurocode 9 and that a higher rating may be warranted. Limited test results produced for this work need to be supplemented with a wider range of tests.