Issue34

A. Tajiri et alii, Frattura ed Integrità Strutturale, 34 (2015) 347-354; DOI: 10.3221/IGF-ESIS.34.38 351 D ISCUSSION igher crack initiation resistances of the FSPed specimens could be easily attributed to the elimination of casting defects as clearly revealed from the fractographic analyses near crack initiation sites (Fig.4 and 5). But the reason for the lower crack growth resistance of FSPed specimens is unclear. The macroscopic appearances of the crack paths in the as-received, L and H specimens are shown in Fig.7. Fatigue crack, initiated from casting defect, grew macroscopically perpendicular to the loading direction in the as-received specimen (Fig.7(a)). On the contrary, the crack paths are macroscopically curved in L and H specimens. Especially, in H specimen, the shape of fatigue crack path corresponds well with the onion ring patterns in Fig.2(b). It indicates that the fatigue crack growth path is affected by the microstructure, such as texture, induced by severe plastic deformation by FSP. Figure 7 : Crack growth paths just before final fracture: (a) as-received, N =0.78 N f , (b) L specimen, N =0.98 N f , (c) H specimen, N =0.92 N f . N f is number of cycles to failure. To investigate the effect of texture induced by FSP on the fatigue properties, EBSD analyses were performed along the fatigue crack path. Fig. 8(a) indicates the macroscopic fatigue crack path of L specimen. EBSD analyses were conducted in the areas (A) ~ (F) in Fig.8(a), and corresponding pole figures are shown in Fig.8(b). FCC unit cells in accordance with pole figures are also indicated in Fig.8(b). The macroscopic fatigue crack path of H specimen, pole figures and FCC unit cells are shown in Fig.9. Texture evolution is clear along the crack path in H specimen (Fig.9(b)). Morita et al. investigated texture evolution in wrought Al-Mg-Si alloy FSW joint, and proposed the rotation of unit cells around the probe as shown in Fig.10 [10]. Fig. 11 indicates the schematic illustration of the rotation of unit cells around the probe, resulting in the texture evolution in A356. The unit cell rotation in A356 is slightly different from that in wrought Al-Mg-Si alloy (Fig.10), due to the different processing conditions and chemical composition. It is clear that the fatigue crack path is dependent on the orientations of unit cells. In L specimen, however, unit cell rotations are less clearer as shown in Fig.8(b) than in H H 300μm 300μm 300μm (a) (b) (c)