The fatigue behaviour of friction stir processed (FSPed) cast magnesium alloy AZ91 was studied. Fully reversed axial fatigue tests have been performed using as-cast, T5-aged and their FSPed specimens (as-cast/FSP and T5/FSP). Both FSPed specimens showed considerably higher fatigue strength than the as-cast and T5-aged specimens. By applying FSP, β-phases with irregular shape from which cracks initiated in the as-cast and T5-aged specimens were finely dispersed and grain refinement was achieved, thereby the crack initiation resistance and the crack growth resistance were significantly improved, resulting in the higher fatigue strengths of the FSPed specimens.
