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.