Fatigue behavior of a rolled AZ31 magnesium alloy after surface treatment by EP and BB conditions

Rotating bending fatigue tests have been performed for AZ31 Mg alloys. Characteristic of fatigue was evaluated and discussed on the basis of fracture surface analysis. Electrical polishing (EP) as well as ball burnishing (BB) was performed on two groups, to evaluate the best conditions for the fatigue life. The microstructure and tensile properties of roll cast AZ31 were investigated. The fatigue strength of 107 cycles was around 100 MPa for BB while it was 40 MPa for EP. The effect of BB conditions on the hardness of the surface through to the core was investigated. The two procedures improved the fatigue performance, but a higher value was found in the BB condition. The growth of small cracks initiated at the surface coincided with the fatigue crack propagation that is characteristic after allowing for crack closure for large cracks, but the operative fracture mechanisms were different between small and large cracks. At the subsurface smooth facets were always present regardless of the applied stress.