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A B S T R A C TThe effects of advancing speed and rotational speed on the microstructure and the mechanical properties of friction stir welded 7075-T6 aluminium alloy sheets were studied. The fatigue strength of sound joints was measured and compared to tensile testing results. Macrographs and microhardness maps were carried out to reveal the microstructure transformations. Fractographic observations were made to identify the failure mechanisms. The effects of welding parameters on the fatigue strength are discussed in terms of welding pitch k (mm/rev) and heat input (J/mm). At a high welding pitch, crack initiation at the root of the circular grooves left by the tool on the weld surface is the most detrimental failure mechanism. As the size and the depth of the grooves are related to the welding pitch, the fatigue strength increases when the welding pitch is reduced. However, when the heat input is excessive, the failure is caused by sub-surface defects produced after abnormal stirring and/or by softening of the heat-affected zone. Lateral lips on the weld surface edges also have an effect on the fatigue strength for intermediate welding pitch values. A = Elongation after tensile test FSW = Friction stir welding F X = Longitudinal force applied by the tool during welding F Z = Vertical force applied by the tool during welding H = Heat input HAZ = Heat affected zone k = Welding pitch l 0 = Gauge length of tensile test specimen LOP = Lack of penetration N = Number of cycles at rupture P FORCE = Longitudinal force power P TORQUE = Torque power P TOT = Total power R = Fatigue stress ratio T = Torque TMAZ = Thermomechanically affected zone UTS = Ultimate tensile strength v = Advancing speed Correspondence: F. Gemme.