**3. Results and Discussion**

*3.1. Effect of Liquid–Solid Volume Ratio on the Microstructure and Properties of Cu/Al Bimetallic Composite* 3.1.1. Effect of Liquid–Solid Volume Ratio on the Microstructure of Cu/Al Bimetallic Composite

The Ni electroplating time for this part was 25 min. The metallographic structure of the Cu/Al bimetallic composite prepared at different liquid–solid volume ratio was shown in Figure 3. It was found that a distinct transition zone was formed at the junction of Cu and Al when casting at 720 ◦C. The thickness of the transition zone increases with the increase of the liquid–solid volume ratio, as the solidification time increases with the increase of the high-temperature liquid volume ratio, which means that Al and Cu atoms have a longer diffusion time. Figure 4 displayed the statistical results of dissociation thickness of the Cu and the thickness of the transition zone. With increasing the liquid–solid volume ratio of Cu and Al from 8.86 to 50, the thickness of the transition zone increased from 242.3 μm to 286.3 μm and the dissolved thickness of Cu increased from 74.3 μm to 90.3 μm. The ratio between the thickness of the transition zone and the dissolved thickness of Cu was 3.26 and 3.17 with increasing the liquid–solid volume ratio. Then, the ratio of Cu to Al in the transition zone was constant with increasing the liquid–solid volume ratio. Therefore, it is reasonable to infer that the phase composition of the transition zone did not change with the liquid–solid volume ratio, which was consistent with the literature [28].

**Figure 3.** Metallographic structure of Cu/Al bimetallic composites prepared with different liquid–solid volume ratios: (**a**) VR8.86; (**b**) VR50.

**Figure 4.** Thickness of the copper matrix and transition zone in composite fabricated with different liquid–solid volume ratio.


The shear strength of the Cu/Al bimetallic composite was described in Figure 5. The shear strength of the Cu/Al bimetallic composite increased with increasing the liquid–solid volume ratio. As shown in Figure 5, the shear strength was 17.8 MPa and 30.3 MPa for VR8.86 and VR50, respectively. With the increasing volume ratio from 8.86 to 50, the shear strength of the Cu/Al bimetallic composite increased 70%. As discussed above, the thickness of the transition zone was increased with the liquid–solid volume ratio. In addition, the microhardness of the phase (Al2Cu, AlCu, and Al4Cu9) in the transition zone was higher than the pure Cu. This may account for the fact that the shear strength increased with the thickness of the transition zone. To further confirm this phenomenon, metallographic and SEM observation of the shear fracture of the Cu/Al bimetallic composite were performed.

**Figure 5.** Shear strength of Cu/Al bimetallic composite fabricated with different liquid–solid volume ratio.

Figure 6 depicted the metallographic observation of the shear fracture of Cu/Al bimetallic composite fabricated with different liquid–solid volume ratio. It was shown that the yellow was Cu and some intermetallic compounds remained at the edge of Cu after the shear test. In addition, the content of the remaining intermetallic compounds remained at the edge of Cu increased with the thickness of the transition zone. This phenomenon indicated that the shear fracture was directly related to the transition zone.

**Figure 6.** Metallographic observation of shear fracture of Cu/Al bimetallic composite fabricated by different liquid–solid volume ratio: (**a**) VR8.85; (**b**) VR50.

As displayed in Figure 7, XRD results of shear fracture surface of Cu/Al bimetallic composite indicated that the Al2Cu, Al4Cu9, and AlCu phase remained at the surface of the Cu rod after the shear test. The SEM photograph of the shear fracture of the Cu/Al bimetallic composite fabricated by different liquid–solid ratio was shown in Figure 8. Combined with energy spectrum analysis and XRD results, the phase calibration of the shear fracture was illustrated in Figure 8. In summary, it was concluded that the initiation and propagation of shear cracks occurred in the transition zone of the Cu/Al bimetallic composite.

(2) Microhardness

**Figure 7.** XRD patterns on shear fracture of Cu/Al bimetallic composite fabricated with different liquid–solid volume ratio.

**Figure 8.** SEM photograph of shear fracture of Cu/Al bimetallic composite fabricated with different liquid–solid volume ratio: (**a1**,**a2**) VR8.86; (**b1**,**b2**) VR50.

The distribution of microhardness indentations and statistical results for the interfaces of Cu/Al bimetallic composite fabricated with different liquid–solid volume ratio were depicted in Figure 9, from left to right: Al, transition zone, Cu. It was reported [29] that the hardness of Al2Cu was 400–500 HV, and the microhardness of [α(Al) + Al2Cu] eutectic structure was about 150–200 HV. The microhardness of the transition zone of Cu/Al bimetallic composite was 140–180 HV, which was higher than Cu and Al. In addition, with the increase of liquid–solid volume ratio, the thickness of the intermediate transition layer increased, the content of the mesophase was higher, and the microhardness was increased.

**Figure 9.** Microhardness of composite fabricated by different liquid–solid volume ratio: (**a**) indentation metallographic observation; (**b**) hardness distribution.
