Issue 35

Y. Besel et alii, Frattura ed Integrità Strutturale, 35 (2016) 295-305; DOI: 10.3221/IGF-ESIS.35.34 299 Figure 4 : Hardness profiles along midsection of weld. In order to identify fracture location of each weld, cross sections of the fractured samples were metallographically prepared, i.e. polished followed by etching. As clearly seen in Fig. 5, fracture behavior in weld-480 seems different from that in weld-720. While in weld-720 fracture occurred nearly through the center of the weld, weld-480 was fractured on the retreating side. Fig. 6 shows the intersection of JLR and the fracture plane of weld-480 in more detail; the area corresponds to the dashed rectangle in Fig. 5(a). The fracture path in the lower part was smooth and steadily curved while the fracture face in the upper part was jagged but macroscopically nearly straight, see also upper part of fracture face in Fig. 5(a). The lower smooth fracture path presumably originated from JLR; as pointed by an arrow in Fig. 6 fracture face and JLR meet tangentially. Furthermore, taking into account the JLR spatial distribution in the weld seam (see Fig. 3(a)), it can be finally concluded that the fracture in the lower part took place along JLR. Scanning electron microscope (SEM) fractography was performed on the fracture surfaces in the upper and lower areas, as shown in Fig. 7(a) and (b), respectively. The SEM fractographs revealed different fracture behavior in both areas: comparably large dimples were observed in the upper area, while very finely shallow dimples covered the fracture surface in the lower area. The shallow dimples indicate low ductility or weak bonding. Since JLR was formed at the location of originally abutting free surfaces, it can be considered that the shallow dimples in the lower area were attributed to weak or incomplete bonding at JLR. Because more than about 30 % of the fracture path proceeded along JLR (lower part A-A’ in Fig. 5(a)), weld-480 exhibited significantly lower elongation and strengths than the other welds showing JLR independent fracture paths. Heat input in FSW is described to be proportional to tool rotational speed and inversely proportional to tool travel speed [3, 19]. It has been reported that high zigzag line pattern (herein JLR) is not formed in the high heat input welds, since degree of stirring increases with heat input and results in sufficient break-up of the initial oxide layer [4]. In this study, friction stir welding was performed at two different tool travel speed, i.e. two different heat input levels. JLR was formed in all welds, but the distribution of JLR in each weld was different. In the weld with the highest heat input (weld-480), the specimen was fractured partially along JLR which resulted in lower elongation than the other welds with lower heat input. Since the high heat input in weld-480 facilitated the material transport, the spatial distribution of JLR became wider. In the area where the fracture occurred, JLR located around the boundary of SZ and TMAZ, see Fig. 8, which corresponds to Area I marked with square in Fig. 3(a). Since the direct stirring of the material, fine-recrystallized, equiaxed grains were formed in SZ. Meanwhile, the material in TMAZ experienced strong shear deformation that resulted in a unique wavy structure as seen on the right-hand side in Fig. 8. Additionally, JLR at the fracture site lay almost normal to the loading direction and seemed to have comparably weak bonding properties. Furthermore, local inhomogeneity and mismatch of microstructure may induce local strain concentration under mechanical loading. As a consequence, it can be concluded that the fracture factually occurred along JLR in this area in weld-480 although higher heat input was applied than weld- 720 not showing this kind of fracture along JLR.