## 4 Discussion  
### Mechanism of Hump Inhibition Using Beam Oscillation  
According to the experimental results, it can be found that beam oscillation can eliminate defects such as lack of fusion, height difference, and wavy hump in the thin-walled components. Among them, unfused and height difference defects are formed because of the uneven heat accumulation at the head and tail of the components. During the deposition process, the gradual accumulation of heat and the uneven dissipation of heat at the head and tail positions lead to the formation of height differences. However, as the oscillation frequency increases, the height difference in the component gradually changes toward a hump defect. Beam oscillation can reduce the temperature gradient and decrease the local solidification rate [26] so that the melt pool at different locations can spread outward evenly. Due to better spreading of the melt pool, unfused defects are removed, and the height difference shifts toward the hump. The formation and suppression of the hump are related to the flow state of the melt pool changed by the oscillating behavior. Further, the flow state can be characterized by a dimensionless Reynolds number \(R_{e}\)[27] computed with Equation (3) as follows:  
\[R_{e}=\frac{4\rho f\tau^{2}}{\mu} \tag{3}\]  
where \(\rho\) is the density of the melt pool, \(\mu\) is the viscosity of the melt pool, \(f\) is the oscillating frequency, and \(r\) is the oscillating radius. For the magnesium alloy melt pool, the density \(\rho\) is 1.59 g/cm\({}^{3}\), and the viscosity \(\mu\) is 1.2 \(\times\) 10\({}^{-3}\) Pa-s [28]. The stirring theory indicates that the laminar flow shifts to turbulent flow in the molten pool when \(R_{e}\) is higher than 10\({}^{3}\). In this experiment, the oscillating behavior could produce a stable stirred laminar flow in the molten pool (shown by the blue arrows in Figure 13) because the calculated \(R_{e}\) = 143 is much smaller than the limit of turbulence generation. Therefore, the mechanism of beam oscillation to inhibit hump defects is shown in Figure 13. First, the liquid metal in the molten pool is pushed to the tail by the arc pressure and then reversed to form a reflux (shown by the red arrow in Figure 13) [29]. [PERSON] et al. found that the liquid metal could build up and solidify at the tail of the melt pool to form  
Figure 12: SEM-SE fracture surface morphology. (**a–c**) the upper part of the additive wall; (**d–f**) the lower part of the additive wall; (**g–i**) the base. a hump when the reflux was hindered [30], and it can be inferred that the wavy hump in this study may be related to the hindered reflux of the liquid metal. As shown in Figure 13, the effect of the laser's oscillating frequency on the reflux of liquid metal is analyzed. As shown in Figure 13a, the thin-wall component has no hump defects when the laser does not oscillate because there is no stirred laminar flow, and the reflux of liquid metal is smooth in the molten pool. As shown in Figure 13b, the stirred laminar flow is transported backward in the same direction as the liquid metal flow and in the opposite direction of the reflux when the laser begins to oscillate so that the oscillating behavior enhances the liquid metal flow and also hinders its reflux, which leads to the formation of a hump In the thin-wall component. As shown in Figure 13c, when the frequency increases to 300 Hz, the liquid metal flow is so powerful that its reflux will be hardly hindered from flowing smoothly, and therefore, there were no wavy humps formed in the thin-wall components at 300 Hz. In conclusion, at a lower frequency, the stirred melt flow generated using the oscillating laser in the melt pool hinders the reflow, thus forming a hump defect. However, when the frequency is increased to 300 Hz, the stirred melt flow in the melt pool is enhanced, which, in turn, promotes reflow, thus suppressing the hump. Beam oscillation enables the regulation of the melt flow in the melt pool, which affects the macroscopic morphology of the deposited layer.