Friction stir processing (FSP) was applied to cast aluminium alloy, AC4CH, to modify the microstructure and fully reversed axial fatigue tests have been performed in the as-cast and FSPed conditions in order to study the effect of FSP on the fatigue behaviour. Two different FSPed conditions were evaluated ; as-cast/FSP for which FSP was applied to the as-cast material and FSP/T6 for which as-cast/FSP was post heat treated. In the as-cast/FSP condition, the fatigue strength decreased in the finite life region, but the fatigue limit i.e. the fatigue strength at 107 cycles significantly increased, compared with the as-cast condition. The fatigue strength in the FSP/T6 condition in the finite life region was higher or slightly lower than in the as-cast/FSP and as-cast conditions, respectively, while the fatigue limit was lower than in the as-cast/FSP condition, but still higher than in the as-cast condition. These complicated fatigue test results obtained were discussed on the basis of microstructural modification resulting from FSP and observations of crack initiation and small crack growth.
