Grain size dependence of fatigue properties of friction stir processed ultrafine-grained Al-5024 alloy

The main objective of the present study was to examine the effect of grain size on the fatigue properties of 5024 Al alloy. Two ultrafme-grained (UFG) and one fine-grain microstructures were produced via friction stir processing (FSP). Fatigue properties were investigated using a fully reversible bending fatigue testing machine. Microstructural evolution during fatigue was analyzed by carrying out interrupted fatigue testing with subsequent orientation imaging microscopy. Emphasis was given to grain size on the evolution of deformed structure, intergranular dislocation density, and overall S-N curve. A comparison with literature demonstrates the importance of the processing route to obtain the UFG microstructure. This difference is highlighted through an analysis of Basquin equation parameters. The results indicate that FSP can be utilized as an effective route to obtain stable UFG microstructure. A fractographic study was done to understand the effect of microstructure on fatigue crack initiation and propagation rates. Crack growth rates in stage-II has been compared for the three different grain sizes.