Influence of porosity on the fatigue limit of die cast magnesium and aluminium alloys

High cycle fatigue properties of high-pressure die-cast magnesium alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and of similarly produced cast aluminium alloy AlSi9Cu3 have been investigated. Ultrasonic fatigue tests up to 10 ⁹ cycles show mean fatigue limits of approx. 38-50 MPa (magnesium alloys) and 75 MPa (AlSi9Cu3) in the tested casting condition. Fatigue cracks initiated at porosity in 98.5% of the samples. Considering porosity as initial cracks, specimens fail, if critical stress intensity amplitude, K _cr is exceeded. K _cr of the magnesium alloys range from 0.85±0.05 to 1.05±0.05 MPa√m, and 1.85±0.10 MPa√m was found for AlSi9Cu3. Below K _cr, fatigue cracks may initiate at porosity, however, do not propagate until failure. Using K _cr, the statistical distribution of defects is linked to the fracture probability at different stress amplitudes.