Competitive relationship during fatigue-crack initiation of friction-stir-welded Al alloy

Highlights: • Surface treatment will affect the competition during fatigue crack initiation. • SMRT leads to internal fatigue cracking occurring in advance at the 10⁶ cycles. • Internal fatigue cracking in Al alloy also presents a typical fish-eye morphology. • FGA can be used to calculate the ΔK_th for interior-crack growth. • The internal fracture of as-welded joints can be inferred from the ΔK and S–N curves. In the present work, uniaxial fatigue tests (R = 0.1) showed that for high-strength aluminum alloys, the crack origin also presented a tendency of transition from the outside to the inside with the stress decreasing. The interior crack-initiation mode has a characteristic of a fish-eye embracing fine granular area (FGA) originated from an inclusion. Moreover, the formation mechanism of FGA was discussed. The values of the stress intensity factor (SIF) range of a surface mode and an internal mode were calculated. The results showed that ΔK_FGA keeps constant with an average value of 1.84 MPa• m^1/2, which was close to the interior crack-growth threshold (ΔK_th,i), 1.911 MPa• m^1/2. Paris-Hertzberg law was used to predict the life of fatigue-crack initiation and propagation during internal fracture. The crack-growth rate within FGA and fish-eye was also estimated. Furthermore, the competitive relationship between the interior and surface cracks initiation of welded joints was discussed through the SIF range.