## 3 Results
### Microstructural Analysis  
The optical microscopy images of the thin-walled components are given in Figure 6, and the average grain size is calculated. On the one hand, when the oscillation frequency is the same, it can be seen that the grain size gradually increases from the bottom to the top. The main reason is that as the deposition height rises, the heat dissipation conditions become worse, heat accumulates in the component, and the temperature gradient at the top is smaller, which is favorable to grain growth. On the other hand, when the sample position is the same, the grain size is negatively correlated with the oscillation frequency. For example, in the middle of the thin-walled component, the average grain size decreases from 27 \(\upmu\)m to 19 \(\upmu\)m with a 27.89% reduction as the oscillation frequency increases from 0 Hz to 300 Hz. It can be concluded that the increase in frequency helps to reduce the grain size. The SEM micrographs in different positions of the thin-walled components from 0 Hz to 300 Hz are given in Figure 7. It can be seen that there are white particles of different sizes distributed on the dark substrate. As shown in Figure 8a, the phase composition was tested and calibrated using XRD, and three main phases, \(\alpha\)-Mg, Al\({}_{8}\)Mn\({}_{5}\), and Mg\({}_{17}\)Al\({}_{12}\), were detected. Due to the low content of Al and Mn elements in AZ31 magnesium alloy, the heights of the diffraction peaks of the Al\({}_{8}\)Mn\({}_{5}\) and Mg\({}_{17}\)Al\({}_{12}\) phases were very dwarfed. As shown in Figure 8b-d, the compositions of different microstructures were examined using EDS, and the detailed chemical compositions are shown in Table 3. The dark substrate is composed of \(\alpha\)-Mg and a small amount of solidly solved Al atoms. The atomic ratio of Al and Mn elements in the large granular precipitates is about 1.51, which is similar to that of Al\({}_{8}\)Mn\({}_{5}\). The small granular precipitation is the Mg\({}_{17}\)Al\({}_{12}\) phase because no Mn elements are detected, but the Zn content is high for the reason that some Zn atoms can replace Al atoms to form the Mg\({}_{17}\)(AlZn)\({}_{12}\) phase [24]. Therefore, the EDS test results confirm the XRD results. In addition, a large amount of Mg elements was detected in all of the particles, which is because the spot range of EDS is larger than the size of the precipitates. Further, the size difference between the precipitated phases is due to the direct precipitation of the Al\({}_{8}\)Mn\({}_{5}\) phase from the liquid phase, in which Al and Mn atoms can aggregate rapidly, while the Mg\({}_{17}\)Al\({}_{12}\) phase is precipitated from the supersaturated \(\alpha\)-Mg solid solution, in which relatively few Al atoms can aggregate. As shown in Figure 9, the granular precipitated phases in Figure 7 were counted. It can be seen that for the same component, the proportion of precipitates gradually decreases  
Figure 5: The relationship curves between pore size and porosity and oscillation frequency. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Element (Au/\%)** & **Mg K** & **Al K** & **Mn K** & **Zn L** \\ \hline Dark substrate (\(\alpha\)-Al) & 60.09 & 23.05 & 15.26 & 1.06 \\ Large precipitates (Al\({}_{8}\)Mn\({}_{5}\)) & 63.66 & 25.72 & — & 10.61 \\ Small precipitates (Mg\({}_{17}\)Al\({}_{12}\)) & 96.66 & 3.34 & — & — \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The EDS results for different microstructures in thin-walled components. from the bottom to the top. Further, at the same position, the proportion of precipitates gradually increases with the increase in frequency. For example, as the frequency increases from 0 Hz to 300 Hz, the precipitate content increases from 1.42-1.61% to 2.55-3.32%. Figure 6: OM grain size of the AZ31 alloy at various positions from 0 Hz to 300 Hz. (**a–c**) 0 Hz; (**d–f**) 100 Hz; (**g–i**) 200 Hz; (**j–l**) 300 Hz. Figure 7: SEM-SE micrographs of precipitates at various positions from 0 Hz to 300 Hz.