*3.1. Effects of the Vibration Acceleration on Microstructure of the Mg–Al Bimetal*

Figure 4a–c shows the interface morphology of the Mg–Al bimetal obtained with different conditions. Without the vibration, the outline of the interface is relatively flat. After applying the vibration, the thickness of the interface changes, and its morphology becomes irregular. When the vibration is not applied, there is a boundary in the Mg–Al interface at the location marked in Figure 4a. In the Mg–Al interface, many long dendrites grow towards the AZ91D matrix on the left side of the boundary, as shown in Figure 4a. On the right side of the boundary, the distribution of the grey precipitates in that area is not uniform. Near the A356 matrix, fewer precipitates can be observed.

**Figure 4.** *Cont*.

**Figure 4.** Microstructures of the Mg–Al bimetals obtained with different conditions: (**a**) without vibration; (**b**) with the vibration acceleration of 0.3 g; (**c**) with the vibration acceleration of 0.9 g; (**a1**–**a3**) microstructures under high magnification in the regions a1–a3 in (**a**), respectively; (**b1**–**b3**) microstructures under high magnification in the regions b1–b3 in (**b**), respectively; (**c1**–**c3**) microstructures under high magnification in the regions c1–c3 in (**c**), respectively.

When the vibration with 0.3 g is adopted, a clear boundary can still be observed in the Mg–Al interface at the location marked in Figure 4b, and there are obvious dendrites on the right side of it. However, the dendrites in the interface zone decrease compared with that without vibration. As the vibration acceleration increases to 0.9 g, no obvious boundary can be observed in the Mg–Al interface, and the dendrites in the Mg–Al interface decrease further. According to the above observations, three regions are selected for further observation to analyze the effect of vibration acceleration on the microstructure of the Mg–Al interface. Figure 4(a1–c3) shows the microstructures under high magnification in the regions marked in Figure 4a–c, respectively. EDS point analysis is used to analyze the composition of the phase existing in the Mg–Al interface, and the results of the corresponding points are shown in Table 1. The possible phases of the analyzed points are identified and indicated in Table 1, combing the EDS point analysis results with the Al–Mg [36] and Mg–Si [37] binary phase diagrams. According to these results, the Mg–Al interface can be divided into three regions: layer I (composed of Al3Mg2 and Mg2Si phases), layer II (composed of Al12Mg17,

and Mg2Si phases), and layer III (Al12Mg17 + δ-Mg eutectic). Layers I and II can also be collectively called the IMCs layer because their substrates are the Al–Mg IMCs, and layer III can be named the eutectic layer. Layer I is mainly composed of the Mg2Si precipitates and Al3Mg2 substrate. There are many Mg2Si bulks and bars when the vibration is not applied, as shown in Figure 4(a1). After applying the vibration with the acceleration of 0.3 g, the Mg2Si phase is dispersed into granular form, as shown in Figure 4(b1). When the acceleration increases to 0.9 g, a large number of fine Mg2Si particles can be observed in the Al3Mg2 substrate, and the size of the Mg2Si phase is refined, as shown in Figure 4(c1).


**Table 1.** Results of EDS analysis at different locations in Figure 4.

Figure 4(a2–c2) shows the microstructures of the Mg–Al interfaces in layer II, which is composed of the black Mg2Si phase and the Al12Mg17 substrate. These results indicate that the vibration also affects the distribution and size of the Mg2Si phase in the Al12Mg17 substrate. Without applying the vibration, the aggregation of the Mg2Si phase is observed in Figure 5b. As shown in Figure 5b,c, fewer large Mg2Si bulks are observed in the Al12Mg17 substrate when the vibration is applied. It indicates that the Mg2Si phases in layer II are dispersed and refined after the mechanical vibration is brought to the manufacturing process. As to the microstructures in layer III, there is no significant difference in the δ-Mg and Al12Mg17 eutectic structure, after applying the vibration.

**Figure 5.** The distributions of the Si element in the Mg–Al bimetals prepared with different conditions: (**a**) without vibration; (**b**) with the vibration acceleration of 0.3 g; (**c**) applying the vibration acceleration of 0.9 g.

Figure 5 shows the distributions of the Si element in the Mg–Al bimetals prepared under different conditions. According to the microstructures observed in Figure 4, the Si elements distributed in the Mg–Al interface may mainly exist in the precipitates (the Mg2Si phase) located in the IMCs layer. After the vibration, the distribution of the Si element in the IMCs layer becomes more uniform, and the composition of the Si element in the eutectic layer increases slightly. It indicates that the vibration can improve the uniformity of the Mg2Si phase in the Mg–Al interface.

The above experimental results show that the Mg–Al interface can be divided into two parts, the IMCs layer (composed of layers I and II) and the eutectic layer (layer III). There are a lot of Mg2Si precipitates in the IMCs layer. In the eutectic layer, eutectic structures and some primary dendrites can be observed. These two parts can be distinguished and measured according to their differences in microstructure and contrast. Figure 6 summarizes the measurements of the thickness of the different parts of the Mg–Al interface. Compared with the Mg–Al bimetal without vibration, the thickness of region I decreases by 29.6%, from 914 μm to 643 μm, after applying the vibration of 0.9 g. However, the change of the thickness of the eutectic layer presents a different phenomenon. It decreases when the vibration is applied. Then, it increases when the acceleration of the vibration increases to about 0.9 g.

**Figure 6.** The thicknesses of the Mg–Al interfaces obtained with different conditions.

The microstructures shown in Figure 4 indicate that the Mg2Si phase mainly exists in layers I and II in the Mg–Al interface. To quantify the influence of vibration acceleration on the Mg2Si phase in the Mg–Al interface, we observed and measured the size of the Mg2Si phase in layers I and II, respectively, according to the locations shown in Figure 4a–c. Figure 7 shows the sizes of the Mg2Si phase in the Mg–Al interfaces. The results indicates that the size of the Mg2Si phase in layer II is larger than that in layer I. The size of the Mg2Si phase in the IMCs layer decreases with the enhancement of the acceleration of the vibration. Compared with the Mg–Al bimetal without vibration, the size of the Mg2Si phase decreases from 4.3 μm to 1.8 μm in layer I, and drops from 4.7 μm to 3.3 μm in layer II, after applying the vibration of 0.9 g. Figure 4(a3–c3) shows the microstructures of layer III in the Mg–Al interfaces.

**Figure 7.** The sizes of the Mg2Si phase in the IMCs layers of the Mg–Al interfaces.

To further confirm the phase composition of the Mg–Al interface, the Mg–Al interface was observed by TEM. Figure 8a shows the bright-field image in layer I. There are many large granular phases distributed in the substrate. Figure 8c,d shows the analysis result of the diffraction spots. The results confirm that layer I in the Mg–Al interface is composed of the Al3Mg2 substrate and the Mg2Si precipitates. Research has shown the appearance of the bend contours is related to the strain field led by the residual stresses [38,39]. Since the linear expansion coefficient of Mg2Si (13 × <sup>10</sup>−<sup>6</sup> <sup>K</sup><sup>−</sup>1) [40] is lower than that of the substrate (Al3Mg2, 22 × <sup>10</sup>−<sup>6</sup> <sup>K</sup>−1) [41], it will generate the compressive stress in the precipitates and the substrate. The stress may lead to the deformation of the substrate in the area near the Mg2Si phase. It may result in the occurrence of the bend contours, as shown in Figure 8a. On the other hand, the compressive stress in the Mg2Si phase may lead to a large number of dislocations in the particles, as shown in Figure 8b. These results indicate that the presence of Mg2Si in the interface may lead to the generation of residual stress in the interface. Therefore, the oversized Mg2Si phase may lead to the increase in residual stress and adversely affect the interface properties.

**Figure 8.** *Cont*.

**Figure 8.** TEM analysis results of layer I in the Mg–Al interface: (**a**,**b**) TEM bright-field images; (**c**) SAED pattern of the Al3Mg2 phase; (**d**) SAED pattern of the Mg2Si phase.

## *3.2. Effect of the Vibration Acceleration on Bonding Strength of the Mg–Al Bimetal*

Figure 9a shows stress-displacement curves of the Mg–Al bimetals with different conditions, and the average shear strength of the Mg–Al bimetal prepared with different conditions is presented in Figure 9b. With the increase in the vibration acceleration, the shear strength of the Mg–Al bimetal gradually increases from 32.2 MPa to 41.5 MPa and 45.1 MPa. Compared with the Mg–Al bimetal obtained without vibration, it increases by 30% and 40%, respectively, after applying the vibration with the accelerations of 0.3 g and 0.9 g.

**Figure 9.** (**a**) Stress-displacement curves of the Mg–Al bimetals with different conditions; (**b**) Shear strengths of Mg–Al bimetals with different conditions.

Figure 10 presents the SEM fractographies of the Mg–Al bimetals with different conditions, and the compositions of the phases observed on the fracture surface were analyzed. The cleavage planes and river patterns are observed in the SEM fractographies, as shown in Figure 10b,e,h, demonstrating that the Mg–Al bimetal fractures by a brittle fracture. Figure 10a shows the macroscopic morphology of the fracture surface of the Mg–Al bimetal without vibration. It indicates a noticeable slope on the surface of the fracture. Region b and c, shown in Figure 10a, were selected for observation. The results show that the Mg–Al bimetal fractures in different Mg–Al interface areas. In region b, shown in Figure 10b, the composition of the flat region is the Al3Mg2 phase, and the composition of the granular structure and pit area is the Mg2Si phase, indicating that region b belongs to the IMCs layer. The Mg2Si phase on the fracture surface aggregates and distributes in a reticular form when the vibration is not applied.

**Figure 10.** SEM morphologies of the fracture surfaces of the Mg–Al bimetals at the Mg substrate side: (**a**) macroscopic morphology of the fracture surface of the Mg–Al bimetal without vibration; (**b**,**c**) enlarged images of region b and c in (**a**), respectively; (**d**) macroscopic morphology of the fracture surface of the Mg–Al bimetal with the vibration acceleration of 0.3 g; (**e**,**f**) enlarged images of region e and f in (**d**), respectively; (**g**) macroscopic morphology of the fracture surface of the Mg–Al bimetal with the vibration acceleration of 0.9 g; and (**h**,**i**) enlarged images of region h and i in (**g**), respectively.

In region c of Figure 10a, the surface of the fracture is relatively straight and flat, and the Al12Mg17 + δ-Mg eutectic structure can be observed, indicating that it may belong to the eutectic layer. However, it is noteworthy that no plastic deformation is observed in the eutectic structure, as shown in Figure 10c. The fracture morphology of the Mg–Al bimetal with vibration demonstrates a similar fracture pattern. The Al3Mg2 phase, Mg2Si phase, and the eutectic structure are also found on the fracture surface. Compared with the fracture morphology of the Mg–Al bimetal without vibration, the aggregated Mg2Si phase is dispersed after applying the vibration, as shown in Figure 10e,h. Moreover, the δ-Mg in the eutectic structure is elongated, which is beneficial to improve the ductility of the Mg–Al interface.

After the shear testing, the fragments of broken interfacial structures were used for XRD testing. The fragments were broken and screened again before testing. The XRD testing result, shown in Figure 11, also confirms the presence of the Al3Mg2, Al12Mg17, and Mg2Si phases in the IMCs layers. Because the IMCs is more brittle, it is more likely to break and fall off during shear fracture. Due to the presence of a large number of δ-Mg phases in the eutectic structure, its plasticity is relatively better, and it is not easy for it to break and fall off, so it may not be included in the tested samples. This may explain the result that δ-Mg was not detected in the Figure 11.

**Figure 11.** XRD diagram of the IMCs layer of the Mg–Al interface.
