## 1 Introduction  
Aluminum alloys are an important engineering material in the automobile, rail, and aerospace industries as the overall performance meets the requirements of mechanical strength and low weight [1; 2; 3; 4]. When aluminum alloy parts are used, a welding process is often needed to join the aluminum alloy parts with other components or structures. The weldability of an aluminum alloys is varied with different types of aluminum alloys. To some degree, the existence of magnesium, zinc, and silicon elements can improve the weldability of aluminum alloys due to the lower thermal conductivity and melting temperature [5]. The 5000-series aluminum alloys have better weldability than other types of aluminum alloys [5; 6]. Various welding techniques are available [7; 8; 9]. In comparison with traditional welding techniques, laser beam welding has the advantages of a high energy density, a high welding speed, a deeper penetration depth, and a small heat-affected zone (HAZ) [10; 11; 12; 13; 14; 15]. Therefore, this paper is especially intended to discuss a laser welding process that can meet the aforementioned requirements of a joining process.  
Beam oscillation technology is proposed during electron beam welding [16], and then transplanted to laser beam welding [17; 18]. Previously, laser beam oscillation was obtained by the mechanical swing of a welding head with low frequency and poor stability. However, a high-power galvanometer scanner was developed and widely used for laser beam oscillation due to its high frequency and high flexibility. Currently, laser welding with beam oscillation is applied widely in remote laser welding for its good weldability [19; 20; 21; 22]. The primary cause of good weldability with an oscillating beam is the dilution degree of the melt pool. Therefore, laser beam oscillating welding is more popular in the industry. [PERSON] et al. [17] demonstrated that a good weld quality without porosity could be achievedwith a weaving laser beam during the laser welding of carbon steel. [PERSON] et al. [23] investigated the influences of beam oscillating parameters on the weld appearance during the laser welding of austenitic stainless steel. [PERSON] et al. [24; 25] found that beam oscillation stabilized the process and improved weld morphology. Furthermore, the elongation of welds was increased. [PERSON] et al. [26] documented that beam oscillation strategies could improve the gap bridge ability in fillet welding. [PERSON] et al. [27] revealed that the equiaxed dendritic grain was prone to be generated without the formation of hot cracks during the laser oscillation welding of aluminum. [PERSON] et al. [28] argued that fiber laser welding with beam oscillation offered possibilities for crack-free welding of dissimilar Al-Cu metals, comparable to the electron beam. [PERSON] et al. [29] observed that the solidification crack disappeared at a weaving frequency of 5 Hz during laser weaking of a self-restraint tapered aluminum alloy. [PERSON] et al. [30] found that the humping defect of aluminum alloy welds could be suppressed by high-frequency oscillation and the welding process could be stabilized under high-speed welding.  
In summary, the research on laser beam oscillating welding is still at an initial stage [31]. Studies have indicated that laser oscillation welding has the advantages of process stability and good welding quality. However, the effects of beam oscillation on the microstructure and mechanical properties of aluminum alloy welds have seldom been studied [24; 32]. The present study makes an attempt to compare and analyze the effects of no oscillation, transverse oscillation and circular oscillation on the performance of aluminum alloy butt welding through a welding test on a 4-mm-thick 5056 aluminum alloy butt joint with a fiber laser and a wobble laser welding head. The weld appearance, metallurgical, and mechanical properties of the butt-welded joints are analyzed comparatively.