## 3 Results and Discussion
### Mechanical Properties  
Figure 7 is the cross-sectional hardness distribution of welds in different beam oscillation modes. The averages of three measurements is shown as one point in Figure 7. It is determined from Figure 7  
Figure 5: Microstructures of butt joints: (**a**) fusion zone without oscillation; (**b**) fusion zone with transverse oscillation; (**c**) fusion zone with circular oscillation; (**d**) heat-affected zone with circular oscillation. Figure 6: X-ray diffraction profile of the joint welded with transverse oscillation. that no significant difference is identified between the hardness of welds in the three beam oscillation modes, and also no difference between the hardness in different regions' base material (BM), fusion zone (FZ), and heat-affected zone (HAZ). In particular, the average microhardness of the fusion zone of the circular oscillating weld is max. 69.50 HV vs. min. 68.71 HV, compared to the weld without oscillation. Moreover, the change in microhardness change in the fusion zone of the weld is greater with oscillation than with circular or transverse oscillation. This confirms that beam oscillation has a beneficial effect on the uniform distribution of the fusion zone microstructure. In addition, the microhardness value of the annealed base material before welding is 66.83 HV. In summary, the microhardness of the weld is well matched to the metallographic microstructure, and the hardness of the weld and the heat-affected zone are both higher than those of the base material. An explanation for this is that the heat input is concentrated, the heating and cooling times are extremely short, and the weld structure formed is very small during laser welding, thus increasing the hardness. In other words, the circular oscillating weld has the finest and most dispersed dendrites and thus the largest microhardness, which confirms the Hall-Petch relationship. Figure 7: _Cont._  
The transverse tensile test represents an effort to assess the tensile strength of the butt joints. The stress-strain curves of welds are displayed in Figure 8. The tensile strength of joints welded with or without beam oscillation is 212-215 MPa, which lends support to the idea that the beam oscillation and weld surface conditions exert only minor influences on the weld tensile strength. However, the beam oscillation is an important predictor of weld ductility. The circular oscillating weld has the biggest strain of about 6.0%, 17.6% higher than that of the weld without oscillation (5.1%). The elongation of a transversely oscillating weld is measured at about 5.8%. According to [24], this is due to the increased proportion of equiaxed grains in the weld under circular oscillation. Figure 9 is a graph showing the fracture effect of a tensile specimen. As indicated in Figure 9a,d, the specimen without oscillation starts to fracture from the undercut, and ends at the weld fusion zone after tension. A large pore is seen in the fracture, the presence of which reduces the force area and thus increases the tensile strength of the weld. The cause of the pores in the welds is the instability of the keyhole [33]. Porosity is a typical effect during the laser welding of aluminum. A similar fracture pattern is established after tension in a specimen welded with transverse oscillation, compared to one without oscillation, as shown in Figure 9b. However, for the circular oscillating specimen, the fracture starts from the heat-affected zone of the weld, and ends at the weld fusion zone, as indicated in Figure 9c. In addition, no obvious pore is observed in the fractures of the welds with transverse oscillation and circular oscillation, as evidenced in Figure 9e,f. This sheds light on the fact that the undercut defect of the surface weld is a weak part of the welded joint, and can be improved by the  
Figure 8: Stress-strain curves of the welded specimens. Figure 7: Microhardness profiles of butt joints: (**a**) without oscillation; (**b**) transverse oscillation; (**c**) circular oscillation. application of circular oscillation.