## 1 Introduction  
The 5A06 aluminum alloy, as one type of high-strength Al-Mg alloy, has been widely used in automobiles, shipbuilding, aerospace engineering, and high-speed trains, due to its high strength-to-weight ratio [1, 2, 3]. Laser welding has the advantages of lower heat input, high energy intensity, narrow heat-affected zone (HAZ) and high welding speed, which is very suitable for joining dissimilar alloys and large-scale manufacturing areas [4, 5, 6, 7]. However, the intensive susceptibility to pores restricts the application of Al alloy laser welding [8, 9, 10], especially for load-bearing structures with lock bottom welds, which are the main joint form for ensuring the consistency of weld morphology and quality in manufacturing of pipelines and missile structures [11, 12]. In the lock bottom structure, the thick side of the base metal is prepared with the process step as the back support for the thin side of the base metal. For this non-penetration welding structure, the keyhole induces a closed vortex at the root, resulting in bubbles in this area being unable to escape by the root melt flow as effectively as in welds with full penetration, and it is therefore easier to form pore defects [13]. [PERSON] et al. studied 5A06 Al alloy lock bottom welds using laser welding with fixed laser power. The results showed that the weld had uneven surface forming and internal pore defects [14]. In addition, because the Mg element is the most important and easiest to burn in 5A06 aluminum alloy, laser welding may lead to the deficiency or segregation of Mg element [15]. Therefore, how to solve these issues has been a research hotspot in the field of aluminum alloy laser welding. In recent years, some scholars have proved that oscillating laser welding is helpful to solve the pore problem of aluminum alloys with non-penetration welds. [PERSON] et al. found that the fluctuation of the melt flow at \(f\) = 100 Hz is the smallest in oscillating laser welding of 7075 aluminum alloy, thereby improving the keyhole stability and reducing the porosity [16]. In the study of sinusoidal oscillating laser welded 6061/5182 Al alloy sheets, [PERSON] et al. obtained porosity-free welds in the non-penetration state [17]. [PERSON] et al. used online X-ray imaging to detect the bubble movement in oscillating laser welding of AlMgSi alloy. They observed that bubbles in the molten pool were captured by the oscillating keyhole and disappeared at \(f\) = 100 Hz [18]. The previous research of our team revealed that the keyhole scanning speed threshold, which can effectively suppress the pores of non-penetration 5A06 Al alloy welds, decreased with the increasing oscillation radius [19]. Moreover, oscillating laser welding has positive impacts on the microstructure and properties of Al alloy welds. For the butt welding, [PERSON] et al. found that beam oscillation can realize the comprehensive control of melt flow and temperature field in the laser welding of Al-Li alloys, thus reducing element segregation [20]. [PERSON] et al. found that oscillating laser welding of AA2060 and AA6061 with a circular pattern can reduce the width of the equiaxed grain zone at the AA2060 side and increase the tensile strength, because the convection generated by the laser beam stirring effect can displace the heterogeneous nucleation particles from the edge to the weld center [21]. [PERSON] et al. revealed that the strong stirring effect of the laser beam on the molten pool can increase the equiaxed grains content inside the 6061 Al alloy weld, and significantly improve the tensile elongation [22]. For the lock bottom welds, [PERSON] et al. found that the infinity oscillating mode was the most conducive to reducing the porosity of 5A06 Al alloy and increasing tensile strength. However, the effects of porosity and microstructure evolution on fracture behavior with different oscillating parameters were not involved in this study [23]. [PERSON] et al.