## 3 Results
### Microstructure and Properties
#### 3.3.2 Microstructure  
The 1\(\#\), 10\(\#\), 17\(\#\) samples were selected from the as-deposition WAAM alloys. The micro porosity (Figure 10), microstructure (Figure 11), the Fe-rich phase (Figure 12), and microstructure after T6 heat treatment (Figure 13) of the WAAM alloys were observed. It can be seen that there were mainly spherical porosity defects in the as-deposited WAAM alloy. The pore size was mainly tens of microns, and the distribution was random. The size and number of pores did not change significantly with the increase of heat input. Furthermore, the micropores in the WAAM alloy have no effect on the ultimate tensile strength and yield strength [19]. The microstructure of the as-deposition WAAM alloy is shown in Figure 11. It can be seen that the WAAM alloy can be clearly divided into the PZ, HAZ and ACZ. It should be noted that the observed height of the HAZ (\(h1\)) did not increase with heat input, due to the fluctuation of the arc caused by environmental interference (such as air flow and magnetic bias blowing) and the uncontrolled flow of the liquid alloy during WAAM processing, resulting in the mutual mixing between adjacent layers. Nevertheless, the hierarchical structure can be clearly observed. The HAZ of the as-deposition WAAM alloy is shown in Figure 11d-f. Compared with the ACZ, the eutectic silicon structure was obviously spheroidized, and the structural morphology of the HAZ was similar under the condition of low heat input, such as that observed in Figure 11d,e. However, under the condition of high heat input (Figure 11f), the heat affected zone showed slight overburning, as shown in Figure 11f. The ACZ of the as-deposition WAAM alloy is shown in Figure 11g-i. It can be seen that with heat input increasing,  
Figure 10: Porosity and distribution as-deposition WAAM alloy and SEM image of porosity. Figure 9: WAAM alloy model. the \(\alpha\)-Al dendrites in the ACZ became coarser, the number of dendrites decreased, and the secondary dendrite arm spacing (SDAS) increased gradually. After amplifying the ACZ, the fine-needle-like or short-bar-like Fe-phase was observed. The EDS analysis results are shown in Figure 12. It is known that the observed phase is the \(\pi\)-Fe phase (Al\({}_{8}\)Mg\({}_{3}\)FeSi\({}_{6}\)) [26], and its size gradually increases with heat input. This is because the Al-7 Si-0.6 Mg alloy is a hypoeutectic alloy. \(\alpha\)-Al precipitates first in the solidification process. With the decrease in temperature, eutectic silicon and the second phase particles (Fe phase) form between \(\alpha\)-Al dendrites. Under the condition of high heat input, the solidification process of the alloy becomes relatively slow, resulting in sufficient time for silicon and Fe phases in the eutectic region to aggregate and grow. Therefore, the SDAS of the as-deposition WAAM alloy increases gradually, and the Fe phase transforms and coarsens with the increase in heat input. Figure 11: Microstructure of the as-deposition WAAM alloys. (**a–c**) The microstructure near PZ of WAAM alloy of 1\(\#\), 10\(\#\) and 17\(\#\); (**d–f**) the microstructure in HAZ of WAAM alloy of 1\(\#\), 10\(\#\) and 17\(\#\); (**g–i**) the microstructure in the ACZ of WAAM alloy of 1\(\#\), 10\(\#\) and 17\(\#\); (**j–l**) the microstructure amplification structure in the ACZ of WAAM alloy of 1\(\#\), 10\(\#\) and 17\(\#\). The microstructure of the WAAM alloy after T6 heat treatment is shown in Figure 13. There were mainly \(\alpha\)-Al, Si particles and Fe-rich phases in the WAAM alloy (T6); the EDS analysis results are shown in Figure 14. It can be seen that the sizes of the \(\alpha\)-Al and the eutectic silicon grains in the alloy tended to increase with the increase in heat input. The size increment is attributed to the small SDAS of the alloy at low heat input: the shorter the average free path of diffusion and migration of the second phase particles, such as Mg2 Si and Si, the easier the homogenization.