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Friction stir (FS) welding is a relatively new solid-state welding processthat offers high levels of joint performance with minimal preparation and little post-weld dressing. The high levels of plastic work induced in the weld zone produce a veryfine grain size in the stirred region of the weld (e.g. the nugget), while the low heatinput limits residual stresses to a low fraction of the proof strength of the weld metal.These effects are generally beneficial to weld dynamic performance. The peculiarthermomechanical history in the FS weld region leads, however, to particular defectswith some unusual effects on crack path, whose occurrence partly depends on crackspeed, or growth rate. This paper presents observations regarding specific influencesof the friction stir welding process on crack paths and dynamic performance for 5083-H321 and 5383-H321 aluminium alloys, and proposes an explanation for theobservations in terms of the weld microstructures and thermomechanical history. Theinsights presented in this paper can be used to inform optimisation of the weld processparameters, through on-line feedback and control of tool geometry, force footprint,torque and temperature.