Fig. 13 Fracture behavior of SZ of DS-FSW joint: (a) Lamellar structure in SZ, (b) Crack initiation in SZ, (c) Crack propagation in SZ, (d) Complete fracture of SZ
Fatigue life of SZs relied on the three stages during fatigue test. The crack initiation stage greatly determined the fatigue life of SZs. The rolling process did not change the lamellar structure. Therefore, the lamellar structure contributed to the crack initiation in the as-welded SZ and rolled SZs. Lamellar structure induced fatigue crack initiation. Chen et al [27] investigated the banded structure (lamellar structure) of the Mg-FSW joint and its effect on fatigue crack initiation behaviors. The results showed that the band structure was the alternate layers of fine grain layer and coarse grain layer. The thickness of the two layer was approximately 100 μm. At lower stress, the plastic deformation accumulated in the alternate layers and contributed to random failures in banded structure. In this investigation, the thickness of thin fine grain layer was approximately 10μm. Between two thick coarse grain layers as shown in Fig. 10. The fine grain layer was the just the dark line in optical images (in Fig.9 (c)-(e) and Fig. 10 (a)). From Fig. 9 (c)-(e) and Fig. 10 (a), the cracking occurred along the dark line. Therefore, the debonding of lamellar structure depended on the cracking of fine grain layer in this investigation.
According to the fracture surface (in Fig. 11 (d) and Fig. 12 (e)), the propagating region of fatigue crack (region 2) was narrow. The fatigue crack propagation left the fatigue striation on region 2[28, 29]. When the crack inner SZ expanded to a certain degree, the residual connection inner the SZ cannot supported the high stress anymore. The complete fracture instantaneously occurred.