**4. Discussion**

#### *4.1. The Effect of Vibration Acceleration on the Microstructure of Mg–Al Bimetallic Interface*

Figure 6 indicates that the thickness of the IMCs layer in the Mg–Al interface gradually decreases with the increase in the vibration acceleration. To investigate the influence mechanism of vibration acceleration on the thickness of the IMCs layer, the solidification curve of the interfacial region was measured by the thermocouple, located on the surface of the aluminum insert, and the results are shown in Figure 12. Previous research has shown that under the vibration, the disturbance and convection of the molten metal promoted the heat exchange at the solid–liquid interface [42]. As a result, it can increase the cooling rate during the solidification process. It can be observed that the cooling rate of the solidification process significantly increased after the application of the vibration, as shown in Figure 12a. According to the research of Haq et al. [43], the increase in the derivate curve indicates the generation of a new phase. Combining the Al–Mg binary phase diagram with the solidification curve shown in Figure 12, the reaction duration of the IMCs (both the Al3Mg2 and Al12Mg17) phases and the cooling rates during the formation of the IMCs were measured. Then, they are used to estimate the time taken to form the IMCs layer. The cooling rate during the formation of the IMCs was about 0.27 K/s, under the condition of no vibration. After applying the vibration with the accelerations of 0.3 g and 0.9 g, the cooling rates were 0.38 K/s and 0.40 K/s, respectively, during the formation of the IMCs. It increased significantly when the vibration was applied. Without vibration, the reaction duration of the IMCs layer (*t*IMCs) was about 47.7 s. After applying the vibration with the accelerations of 0.3 g and 0.9 g, it decreased to 34.5 s and 28.76 s, respectively. Compared with the Mg–Al bimetal obtained without vibration, it was reduced by 27.7% and 39.7%, respectively, after applying the vibration with the accelerations of 0.3 g and 0.9 g. The decrease in the tIMCs may be the reason why the thickness of the IMCs layer in the Mg–Al bimetallic interface decreased, with the increasing of the vibration acceleration.

**Figure 12.** (**a**) Results of thermocouple measurement; (**b**) Temperature curve zoom in area b in (**a**).

Moreover, the disturbance and convection led by the vibration may also affect the size and distribution of the Mg2Si phase. Figure 13 shows the influence mechanism of the vibration on the size and distribution of the Mg2Si phase. The Si element in the Mg2Si phase comes from the Si phase in the A356 insert. Figure 13a shows the initial state of the Si element in the manufacturing process of the Mg–Al bimetal. In the beginning, the Si existed in the needle-like or slate-like Si phase on the aluminum substrate. After pouring the molten AZ91D alloys, the molten metal filled the position of the foam mold and came into contact with the A356 insert.

**Figure 13.** The influence mechanism of the vibration on the size and distribution of the Mg2Si phase: (**a**) The initial state of the Si element; (**b**) The insert melted to form a molten pool on the surface of the solid insert; (**c**) The diffusion of the Si in the molten pool under the condition of no vibration; (**d**) The interface formed under the condition of no vibration; (**e**) The diffusion of the Si in the molten pool after applying the vibration; (**f**) The interface formed after applying the vibration.

During the casting process, the highest temperature measured in the insert surface region was about 571 ◦C, as shown in Figure 12, close to the Al–Si eutectic reaction temperature of 577 ◦C [35]. The actual temperature of the insert surface area should be higher than the measured temperature, so when the AZ91D melt was in contact with the A356 matrix, the surface area of the insert might be melted. In addition, under the high-temperature condition, the mutual diffusion of the Al and Mg elements occurred between the solid aluminum insert and the molten magnesium alloy. It changed the composition of the

insert surface, decreased the melting point of the insert surface [44], and promoted the melting of the surface region of the solid insert. So, the insert melted to form a molten pool on the surface of the solid insert, as shown in Figure 13b. In the molten pool, there were some Si-rich regions where the eutectic Si phase initially existed. Subsequently, the Si element in the molten pool gradually diffused due to the concentration gradient, as shown in Figure 13c. Without vibration, the diffusion distance of the Si element was relatively short due to the brief solidification time of the interface region during the manufacturing process. Finally, as shown in Figure 13c, the Si element aggregated in a small area and precipitated the large Mg2Si bulks from the molten metal, as shown in Figure 13d.

After applying the vibration, the disturbance and convection led by the vibration promoted the diffusion of the Si element. The diffusion distance of the Si element increased, and the Si element was dispersed into a larger region, as shown in Figure 13e. Finally, the size of the Mg2Si phase was refined, and its distribution became more uniform, as shown in Figure 13f. Therefore, it can be seen from the above experiment results that the size of the Mg2Si phase in the IMCs layer is gradually refined as the vibration acceleration increases to 0.9 g.

However, the improvement of the distribution of the Mg2Si phase in layer I is mainly observed after applying the vibration. Since the A356 insert melts from the outside to the inside during the preparation process, the Si element in layer II has more diffusion time. As a result, the distribution of the Mg2Si phase in that region is relatively uniform. Therefore, the effect of the vibration on the distribution of the Mg2Si phase in layer II is not as significant as that of layer I.
