13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- steps are applied corresponding with the three phases. Dwell phase means the heat source dwells at the initial position until the temperature reaches to the value required for the welding. Traverse phase means the rotating tool moves along the welding line, this is the most important phase during FSW. During the cool phase, the temperature of workpiece drops to initial value (25 ℃) and then the fixture will be removed; accurate residual stress of FSW joint will be obtained after this phase. The temperature distribution during the second load step is shown in Fig.9. The melting temperature of aluminum alloy 2524 is about 550℃, and the maximum temperature at the welding line is well below it. Figure 9. Temperature distribution during load step 2 The distribution of residual stress (X direction) after the third load step is shown in Fig.10. The residual stress distribution of middle site is different from that of the beginning and ending site. Because the FSW joint specimens are cut out from a big workpiece, the residual stress distribution of specimens can be considered as that of middle site. Figure 10. Distribution of residual stress (X direction) Figure 11. The residual stress distribution of line 1 Fig.11 shows the distribution of residual stress of line 1 (Fig.10). The maximum stress is 115MPa at the point of 2.5mm from the weld center, which shows a good agreement to the fracture position of FSW specimen. 3.2 FEA of lap joint
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