*2.1. Materials and Fabrication Process*

The commercial A356 aluminum alloy (Al-6.81Si-0.44Mg-0.21Fe-0.02Ti wt.%) and AZ91D magnesium alloy (Mg-9.08Al-0.62Zn-0.23Mn wt.%) were used to fabricate the Mg–Al bimetal. The aluminum rods with a diameter of 10 mm and a height of 110 mm were obtained from the commercial aluminum ingot by wire-electrode cutting. Figure 1a shows the original microstructures of the A356 rods. They were mainly composed of the Al substrate and the Si phase dispersed in the substrate. Figure 1b,c shows the microstructure zoom in areas b and c in Figure 1a, indicating that the original shape of the Si phase in the A356 rods is mostly needle-like or slate-like.

**Figure 1.** (**a**) Original microstructures of the A356 insert; (**b**) microstructures zoom in area b in (**a**); (**c**) microstructures zoom in area c in (**a**).

Before being used in the experiment, the as-cast A356 rods were polished with silicon carbide sandpapers, followed by cleaning with acid (50% HNO3 + 48% HF + 2% water) and lye (20 g/L NaOH, 5 g/L ZnO) to remove the oxide film on the surface. The treated A356 rods were assembled with the foam pattern. Then, the foam patterns were used for the LFCC process, of which the schematic diagram is illustrated in Figure 2. The foam pattern was placed in the sand flask, which was vibrated through the vibration table, while the loose sand was added to the sand flask. Under vibration, the loose sand in the sand flask was compacted. Then, a plastic film was placed over the surface of the loose sand. Finally, the vacuum pump was launched, and the sand flask was vacuumized and maintained at a vacuum of −0.03 MPa during the experiment. Under atmospheric pressure, the sand mold became tough and could tolerate the applied vibration. During the LFCC process, the pouring temperature of the AZ91D magnesium alloy was 720 ◦C. The 35 Hz vibration with different peak accelerations of 0.3 g (with the peak–peak displacement of 0.2 mm) and 0.9 g (with the peak–peak displacement of 0.6 mm) was applied to the manufacturing process of the Mg–Al bimetal by the vibration table under the sand flask, as shown in Figure 2, to investigate the effect of the vibration acceleration on the microstructure and bonding strength of the Mg–Al bimetal. During the casting process, the solidification temperature curve of the Mg–Al bimetal was measured at a sampling frequency of 75 Hz by the thermocouple, placed against the surface of the A356 insert, as shown in Figure 2.

**Figure 2.** The schematic diagram of the experiment.
