Explore the mechanism of high fatigue crack propagation rate in fine microstructure of friction stir welded aluminum alloy

Fatigue crack propagation (FCP) at different locations of friction stir welded AA6N01 joint was investigated. FCP experiments were performed to study crack propagation rates in the Paris region. The highest FCP rate was in the stir zone (SZ) which composed of much finer grain compared with the other areas of joint. Fracture surfaces at different locations were examined. More brittle fracture characteristic was observed in SZ. To further investigate the mechanism of high FCP rate in SZ, differential scanning calorimetry (DSC) analysis was conducted. The results showed that there was relatively high energy stored in SZ. After releasing the stored energy, the resistance of FCP in SZ improved significantly while the grain size kept the same, the fatigue fractographs had changed from quasi-cleavage to plastic fatigue striation fracture. These proved that the stored energy affected fatigue crack propagation in FSW joint significantly.