## 3 Results and Discussion
### Microstructure  
Figure 4 shows the microstructure near the lock step. All welds were composed of columnar grain zone (CGZ), equiaxed grain zone (EQZ) and heat-affected zone (HAZ). By measuring the width of different regions, it can be found that the width of the CGZ for the weld without beam oscillation were 200 \(\upmu\)m. But as shown in Figure 4c-e, the width of the CGZ gradually decreased with the increasing \(f\). When \(f\) increased to 350 Hz, the width of the CGZ decreased to 54 \(\upmu\)m, which was only 27% of the width of the CGZ without beam oscillation. In order to further analyze the grain size and morphology of the CGZ and EQZ, EBSD was performed, and the results are shown in Figure 5. For the non-oscillating weld, the average grain sizes of the CGZ and EQZ are 66.4 \(\upmu\)m and 64.2 \(\upmu\)m, respectively. When \(f\) increased to 350 Hz, the average grain sizes of CGZ and EQZ are 48.6 \(\upmu\)m and 42.8 \(\upmu\)m, which are 26.8% and 33.3% smaller than those of the non-oscillating weld. The alloying elements in aluminum alloys have a boiling point lower than the molten pool temperature. During the laser welding process, these elements are lost due to evaporation, resulting in insufficient element content in the weld and reducing the performance of the weld. The Mg element is the most important, but also the easiest to burn, alloying element in 5A06 aluminum alloy. To study the distribution characteristics of alloying element Mg, the XRF test is carried out in the transverse and longitudinal directions of the weld, and the degree of segregation is evaluated by the standard deviation. The test position and results are shown in Figure 4 and Table 3, respectively. The results show that the average content of Mg element in the circular oscillating weld is 6.05% in the transverse direction and 6.12% in the longitudinal direction, accounting for 98.2% and 99.4% of the Mg element content (6.16%) in the BM respectively. In contrast, the content of Mg element in the transverse and longitudinal directions in the non-oscillating weld is 5.87% and 5.91% respectively, accounting for 95.3% and 95.9% of the Mg element content in the BM. In addition, the standard deviation of Mg element content in circular oscillating welds is smaller than that in non-oscillating welds. Therefore, circular oscillating laser welding greatly alleviates Mg element burning loss and makes the distribution of Mg element more uniform. Figure 6 shows the effect of beam oscillation on the melt flow. For the weld without oscillation, as shown in Figure 6a, the laser energy was concentrated in the center and the temperature gradient of the molten pool was large, resulting in columnar grains on both sides of the weld. The melt flow was mainly affected by the Marangoni effect, whose speed was slow and the scouring effect on both sides of the weld was not obvious. Thus, the width and grain size of CGZ was large. The lock step was located at the waist of weld. The melt flow moved downward smoothly and the fusion line was straight, thus the angle between the fusion line and lock step was small. As shown in Figure 6b, the range of laser beam oscillation increased significantly with \(f\), making the energy distribution more uniform and reducing the temperature gradient of molten pool greatly. This is also the reason why the burning loss of Mg element is alleviated. The strong stirring effect promoted the remelting and fragmentation of the CGZ, weakened and inhibited the growth of columnar grains, and thereby decreased the width and grain size of CGZ.