Improved high cycle fatigue property of ultrafine grained pure aluminum

•Ultrafine-grained (UFG) pure Al was prepared by friction stir processing (FSP).•Significantly increased tensile and high-cycle fatigue properties were achieved.•High microstructural stability improved fatigue damage resistance of FSP UFG Al.•Fatigue strength exponent played an important role on fatigue strength of UFG Al. Ultrafine-grained (UFG) pure Al with uniform and stable microstructure produced by friction stir processing (FSP) significantly increased the high-cycle fatigue (HCF) strength compared with coarse-grained and other UFG materials prepared by severe plastic deformation (SPD). There was no obvious surface damage for FSP-UFG pure Al and the improved fatigue damage resistance can be attributed to the uniform microstructure and high microstructural stability. The ring-island stress distribution of FSP-UFG impeded the formation of large-scale shear bands, and enhanced the coordinated deformation during cyclic deformation. In addition to the enhanced tensile strength, increasing the fatigue strength exponent, which is decided by the microstructural stability, is also an effective method of improving the fatigue strength.