*4.2. Strengthening Mechanism of the Mg–Al Bimetal*

The IMCs layer in the Mg–Al interface is mainly composed of the IMCs substrate and the Mg2Si phase distributed on the substrate. Compared with A356 and AZ91D matrixes, the IMCs layer has significantly higher hardness and lower plastic deformation ability [45]. Therefore, the thickness of the IMCs layer has an important influence on the number of brittle phases and bonding properties of the Mg–Al bimetal. In the preparation process of the Mg–Al bimetal, reducing the content of these brittle phases is the key to improving the bonding property of the Mg–Al bimetal [46,47]. After applying the vibration, the thinning of the IMCs layer means decreasing the mass of the brittle phases, thereby improving the mechanical properties of the Mg–Al bimetal. Studies have shown that the size and morphology of the Mg2Si phase have an important effect on the properties of the material [48,49]. The refined Mg2Si phase may also contribute to enhancing the shear strength of the Mg–Al bimetal. Without vibration, there are many Mg2Si bulks in the IMCs layer. Since the linear expansion coefficient of the Mg2Si is lower than that of the Al3Mg2 substrate, it will generate compressive stress in the Mg2Si precipitates. Existing research has found that for the large-sized Mg2Si phase, its ability to carry external loads is weaker because the larger size reinforcement particles are more likely to fracture under the action of the residual stress [50]. Therefore, under the action of the stress, the Mg2Si bulks in the IMCs layer are more likely to form cracks when it is under load, leading to the formation of crack propagation channels, which will adversely affect the ability of the Mg2Si phase to withstand and disperse loads [51]. When the vibration is applied, the thickness of the IMCs layer decreases, and the Mg2Si phase is refined. Therefore, the shear strength of the Mg–Al bimetal is improved from 32.2 MPa to 41.5 MPa, after applying the vibration with the acceleration of 0.3 g. As the acceleration increases to 0.9 g, the shear strength of the Mg–Al bimetal continues to rise to 45.1 MPa, due to the further reduction in the thickness of the IMCs layer and the further refinement of the Mg2Si phase.

#### **5. Conclusions**

In the present work, the Mg–Al bimetal was fabricated by the LFCC process, and the effect of the vibration acceleration on the interfacial microstructure and mechanical properties was studied. The main conclusions are presented in the following:


**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/met12050766/s1, Figure S1: The original SEM image., Figure S2: The standard for the measurement., Figure S3: The processed image used for measurement., Figure S4: Space calibration, Figure S5: Select the area to be measured., Figure S6: Using the "perform segmentation" function to convert images to black and white image., Figure S7: The black and white image, Figure S8: Click on the "Count and measure objects" option, Figure S9: Click on the "Select Measurements" option., Figure S10: Setup the measurement options., Figure S11: Measurement results.

**Author Contributions:** Conceptualization, J.W. and G.L.; methodology, Z.Z.; investigation, F.G. and J.W.; writing—original draft preparation, F.G.; writing—review and editing, W.J. and S.F.; visualization, S.F.; Supervision, W.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China [grant number 52075198], National Key Research and Development Program of China [grant number 2020YFB2008304] and State Key Lab of Advanced Metals and Materials [grant number 2021-ZD07].

**Data Availability Statement:** Data presented in this article are available at request from the corresponding author.

**Acknowledgments:** The authors gratefully acknowledge the support provided by the Analytical and Testing Center, HUST.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

