Effect of Fe-content on fatigue crack initiation and propagation in a cast aluminum–silicon alloy (A356–T6)

The effect of Fe-content on the fatigue damage evolution in a cast A356–T6 alloy was investigated both experimentally and through microscale finite element simulations. High cycle fatigue tests indicated that iron content has little influence on the fatigue life in the short lifetime regime (<105 cycles) but it significantly reduces the fatigue life in the long lifetime regime (>106 cycles). At high applied stress levels in the short lifetime regime, fatigue life is dominated by the crack propagation stage. The large plate-like Fe-rich intermetallic particles in high Fe-content castings were observed metallographically to retard the growth of small cracks through crack branching and meandering. For the long lifetime regime, the crack initiation stage is important. Fracture surface examination and finite element analysis revealed that in the absence of other defects such as porosity or oxide films, the large plate-like Fe-rich intermetallic particles in high Fe-content castings promote crack initiation by raising the stress-strain concentration in the eutectic region. Based on experimental observations and finite element analysis, a microstructure-based model was developed involving crack initiation and propagation, which allows quantitative assessment of the influence of Fe-content on the fatigue life. Good agreement was obtained between the model and experimental results.