## 3 Results and Discussion
### Fracture Mechanism  
According to the reported literature, the factors affecting the fracture are as follows: (1) Non-uniform stress distribution occurs at the defect section (such as pores), resulting in micro-cracks at the edge of the defect [30]; (2) Grain refinement can improve the ability of the weld to resist crack initiation and propagation [31]; (3) Welding structure and stress state. For example, in the T-joint, the initiation of cracks is mainly owed to the concentration of weld-induced residual stresses at the weld toe under shear stress [32]. Figure 10 shows the mechanism diagram of different fracture modes of lock bottom welds. It was found that, no matter what kind of fracture mode, the crack originated from the lock step. This was due to the special structure of the lock bottom weld. The BM thick side was prepared with process steps as the back support for the thin side of the BM; in the tensile process, the stress concentration occurred near the weld root of the BM  
Figure 8: Fracture position of lock bottom welds, (**a**) \(f\) = 0 Hz, (**b**) \(f\) = 50 Hz, (**c**) \(f\) = 200 Hz, (**d**) \(f\) = 350 Hz. thin side, which was shown in Figure 10a. It was the crack initiation position because the coarsened CGZ and overheated HAZ were the main microstructures near the weld fusion line. Notably, the weld actually bore the shear forces, because the stress center of the lock bottom weld was not on the same axis at the state of an uneven thickness. Moreover, the shear plane was located above the lock step, and the crack tended to expand upward after it was generated. Figure 10: Fracture mechanism diagram, **(a)** fracture in EQZ, **(b)** fracture in CGZ, **(c)** fracture in HAZ. Figure 9: Morphologies of fracture surfaces, overviews of weld **(a)** #1, **(c)** #2, **(e)** #5 and **(g)** #8, details of **(b)** P1, **(d)** P2, **(f)** P3 and **(h)** P4. As shown in Figure 10a, when 0 Hz \(\leq\)_f_ \(<\) 50 Hz, there were large pores in the weld, and the weak area around the pores was more likely to attract crack expansion. When the crack initiated from the lock step, it expanded in the direction of large pores. At the same time, the presence of large pores reduced the real load area during the tensile test, and the stress concentration was also prone to initiate cracks. Finally, the weld was completely broken in the EQZ. As shown in Figure 10b, when 50 Hz \(\leq\)_f_ \(\leq\) 200 Hz, the angle between the fusion line and the lock step increased with \(f\). Although there were pores in the weld, the size of the pores was small and located below the normal line of the fusion line, which meant that the pores were not in the direction of crack propagation. At this time, the influence of pores on crack propagation was reduced. After the crack initiation, it first propagated along the columnar grain boundary with the direction parallel to the normal direction of the fusion line. It kept the upward trend under the effect of shear stress as the crack went through the CGZ. However, the resistance of crack propagation increased because of the weld center forming fine equiaxed grains and possessing a high-volume fraction of grain boundaries [33]. Therefore, the crack propagated along the CGZ boundary for a certain distance and then re-entered the CGZ, and the weld finally fractured in the CGZ near the fusion line. As shown in Figure 10c, when \(f\) \(>\) 200 Hz, the pores in the lock bottom weld were eliminated and the width of the CGZ decreased to the minimum under the influence of beam oscillation. When the crack was generated, it propagated along the columnar grain boundary to EQZ, and then went directly through the CGZ to the HAZ in the direction of 45\({}^{\circ}\) under the maximum shear stress, showing a typical pure shear fracture characteristic. The weld finally fractured in the HAZ.