## 1 Introduction  
Because of their low density, high strength ratio, and excellent electromagnetic shielding properties, magnesium alloys have been widely used in railway transportation, automotive, and aerospace industries [1, 2]. Additive manufacturing is one of the most popular manufacturing technologies for metal components nowadays [3, 4, 5]. However, magnesium alloys are highly susceptible to macroscopic defects and internal porosity during the rapid non-equilibrium solidification process of the additive manufacturing process due to their low melting/boiling point, high thermal conductivity, high expansion coefficient, and low surface tension [6, 7, 8]. Therefore, how to improve the quality of magnesium alloy additive components is one of the current urgent issues to be solved.  
The traditional magnesium alloy additive manufacturing methods are selective laser melting (SLM) and wire arc additive manufacturing (WAAM). Between these, the SLM process is easier to control and can manufacture precise and complex structural parts. However, limited by the volume of the forming cavity, it is difficult to meet the requirements of efficient manufacturing for large parts. In addition, magnesium alloy powder is flammable and explosive, which also results in there being little research on the SLM of magnesium alloys [9, 10, 11]. Relatively, WAAM has the characteristics of a high deposition efficiency and unrestricted forming size; thus, it has received wide attention [12]. However, the macroscopic morphology is poor, and grain coarsening is serious in the WAAM of magnesium alloy due to the large heat input from the arc [13]. For example, [PERSON]. Et al. studied WAAM using a self-developed wire, but the fabricated cubes were very rough due to the low viscosity of the liquid magnesium alloy [14]. [PERSON] et al. used cold metal transition (CMT) to manufacture thin-walled components of magnesium alloy, and the macroscopic formation of the components was significantly improved owing to the reduction of heat input [15]. It is worth noting that the CMT arc reduces the deposition efficiency with a wire feed rate of only 3 m/min. So, a new method needs to be developed to ensure excellent forming along with high deposition efficiency.  
Laser arc hybrid additive manufacturing is a new additive manufacturing technology developed in recent years, which contributes to improved component accuracy and quality with less heat accumulation and high production efficiency [16; 17]. [PERSON] et al. have found that laser can stabilize the arc and improve the surface quality effectively in LHAM of aluminum alloy [18]. However, pores are easily formed in the component due to the instability of the laser keyhole [19; 20]. To reduce the number of pores, [PERSON]. et al. developed the oscillating laser-arc hybrid additive manufacturing process (O-LHAM), and as a result, the surface roughness of the components obtained using O-LHAM was found to be reduced by 20%, the deposition efficiency was improved by 4 times, and the internal porosity was significantly reduced compared with WAAM [21]. Obviously, O-LHAM has a positive effect on improving the quality of components.  
As mentioned above, O-LHAM may have great potential in the additive manufacturing of magnesium alloys, but related studies have not been reported until now. In this paper, the feasibility of using O-LHAM to manufacture thin-walled components of high-quality AZ31 magnesium alloy was proven, and the effect of the oscillating frequency on the macroscopic defects and microstructure of the components was systematically investigated, which helps to deepen the understanding of magnesium alloy O-LHAM and expand its application scope.