(1) Shear Strength

The shear strength in transition zone of the Cu/Al bimetallic composite fabricated with different coating thickness was depicted in Figure 13. With increasing the coating thickness from 1.5 μm to 3.8 μm, the shear strength in transition zone increased from 18.6 MPa to 30.3 MPa, an increase of 62.9%. The shear strength decreased from 30.3 MPa to 26.7 MPa with the coating thickness varying from 3.8 μm to 5.9 μm. As discussed above, when the coating thickness was 1.5 μm, the Ni layer cannot effectively prevent the oxidation of the Cu rod, which results in poor metallurgical bonding of the Cu/Al interface. When the coating thickness was 3.8 μm, the Ni layer was effective in preventing the oxidation of the copper rod and promoting the mutual diffusion of Cu and Al atoms. Furthermore, due to the partial diffusion of the Ni layer, the binary phase structure in the transition zone

changed into the ternary phase structure, which resulted in a great improvement in the metallurgical bonding property of the Cu/Al interface. When the coating thickness was 5.9 μm, the Ni layer was not only effective in preventing the oxidation of the Cu bar, but also greatly limiting the mutual diffusion of Cu and Al, which had an adverse effect on the metallurgical bonding of Cu/Al interface.

**Figure 13.** Shear strength of Cu/Al bimetallic composite fabricated with different coating thickness.

As illustrated in Figure 14, the phase composition of the transition zone changed with increasing the coating thickness of the Ni layer. For example, the diffraction of AlCu phase decreased with increasing the coating thickness of the Ni layer, indicating that the proportion of AlCu phase in the transition zone decreased. The diffraction of Al3Ni2 phase was found with the coating thickness of the Ni layer reached to 5.9 μm. Above all, the XRD results were consistent with the analysis of the influence of the Ni layer on the diffusion of Cu and Al.

**Figure 14.** XRD results of shear fracture surface of Cu/Al bimetallic composite fabricated with different coating thickness.

Figure 15 depicted metallographic observations of shear fracture in the transition zone (Cu side) of Cu/Al bimetallic composite fabricated with different coating thickness. As shown in Figure 14, the content of residual intermetallic compounds at the Cu side increased with increasing the coating thickness of the Ni layer.

**Figure 15.** Shear fracture morphology of Cu/Al bimetallic composite fabricated with different coating thickness: (**a**) 1.5 μm; (**b**) 3.8 μm; (**c**) 5.9 μm.

The SEM morphology of the shear surface of the Cu/Al bimetallic composite fabricated with different coating thickness was displayed in Figure 16. Foam-like fluffy holes and cracks were found on the shear fracture surface in Figure 16a, which was consistent with poor metallurgical bonding due to surface oxidation of the Cu rod. As shown in Figure 16b, the foam-like fluffy holes disappeared and the length of cracks become shorter, indicating better shear strength than the former. As for Figure 16c, the Ni layer of 5.9 μm limited the diffusion of Al atom to the Cu side, resulting in a relatively low content of intermetallic compounds in the Cu side of the transition zone. Therefore, the morphology resembling a slip band appeared on the dissociation surface.

(2) Microhardness

**Figure 16.** SEM morphology of shear surface of Cu/Al bimetallic composite fabricated with different coating thickness: (**a1**,**a2**) 1.5 μm; (**b1**,**b2**) 3.8 μm; (**c1**,**c2**) 5.9 μm.

Observations and statistical results of the interfacial microhardness of Cu/Al bimetallic composite fabricated with different coating thickness were described in Figure 17. The results showed that the micro-hardness of the Cu/Al bimetallic composite demonstrated a slight decreasing trend after a small increase, while the hardness of the interfacial compounds did not increase significantly. Ni elements were dissolved and diffused to the matrix under different electroplating times and different coating thickness, and the hardness of the electroplated Ni layer is 160–180 HV, which was close to the microhardness of the transition zone without the coating Ni layer. Therefore, it can be inferred that the Ni layer thickness has little effect on the microhardness of Cu/Al bimetallic composites.

**Figure 17.** Microhardness of Cu/Al bimetallic composite with different coating thickness: (**a**) Indentation metallographic observation; (**b**) Hardness distribution.

#### **4. Conclusions**

The Cu/Al bimetallic composite was successfully fabricated by gravity casting. The effect of surface treatment and liquid–solid volume ratio on the microstructure and properties of the Cu/Al bimetallic composite were studied. The thickness of the transition zone was directly related to the mechanical properties of the Cu/Al bimetallic composite. Therefore, the quantitative regulation method of the thickness of the transition zone may become a new research hotspot in Cu/Al bimetallic composite fabrication. The following results and conclusions can be drawn:


(4) The initiation and propagation of shear cracks occurred in the transition zone of Cu/Al bimetallic composite.

**Author Contributions:** Conceptualization, H.Y., Y.W. and W.F.; Methodology, H.Z. and Y.H.; Writing original draft, Z.W.; Writing—review & editing, L.Z.; Project administration, C.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (grant number 20KJB430015), the Open Fund of Jiangsu Institute of Marine Resources Development (grant number JSIMR202208), the Natural Science Foundation of Jiangsu Province, (grant number BK20201467), and the Scientific Research Funding Project of "333 High-level Talents Training Project" of Jiangsu Province (grant number BRA2020260).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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