*3.2. Effect of Coating Thickness on the Microstructure and Properties of Cu/Al Bimetallic Composite*

3.2.1. Effect of Coating Thickness on the Microstructure of Cu/Al Bimetallic Composite

The coating thickness of the Ni layer on the Cu rod was 1.5 μm, 3.8 μm, and 5.9 μm with prolonging of the electroplating time from 10 min to 40 min, respectively. Then, the Al melt was cast into the mold equipped with an Ni-plated Cu rod, and the liquid–solid volume ratio was 50. The interfacial metallographic structure of Cu/Al bimetallic composite with different electroplating time was displayed in Figure 10. The transition zone width increased from 246 μm to 286 μm with the coating thickness varying from 1.5 μm to 3.8 μm. It was found that the thickness of the Ni plating was very uneven according to the metallographic observation of the Cu rod with Ni plating time of 10 min. Even some parts of the Ni layer had fallen off while kept at 500 ◦C. Then, the anti-oxidation effect of the Ni layer was sharply reduced, and part of the surface of the Cu rod was oxidized before casting the Al melt. This accounted for the smaller transition zone thickness for the electroplating time of 10 min.

**Figure 10.** Interface microstructure of Cu/Al bimetallic composite with different electroplating time: (**a**) 10 min, (**b**) 25 min, and (**c**) 40 min.

The coating thickness and the width of the transition zone were summarized in Table 1. As shown in Table 1, the transition zone width first increased and then decreased with increasing the coating thickness of Ni from 1.5 μm to 5.9 μm. Exactly, the transition zone width decreased from 286 μm to 268 μm with the coating thickness varying from 3.8 μm to 5.9 μm. SEM image and chemical elements mapping of Al, Ni, and Cu of the Cu/Al bimetallic composite fabricated with 3.8 μm coating thickness was shown in Figure 11 Combined with Figure 10, it was inferred that the Ni layer on the Cu rod was dissolved by pouring in the high temperature Al liquid at the beginning of the casting. Subsequently, part of the Cu rod in contact with the Ni layer began to dissolve. Then, the Cu atoms diffused through the liquid Ni layer to the liquid Al, and the Al atoms diffused through the liquid Ni layer toward to the Cu side. At the same time, the Ni atoms diffused to both sides, which promoted the metallurgical combination of the Cu and Al. As the temperature dropped, different intermetallic compounds began to precipitate and transition zones gradually formed. As displayed in Figure 11, the 3.8 μm Ni layer had almost diffused out at elevated temperature. Therefore, the 3.8 μm Ni layer only partially limited the diffusion of the Cu and Al atoms, resulting in a peak thickness of the transition zone.

**Table 1.** Coating thickness and transition zone width of fabricated materials after different electroplating time.


**Figure 11.** SEM image and chemical elements mapping of Al, Ni, and Cu of the Cu/Al bimetallic composite fabricated with 3.8 μm coating thickness.

The SEM image and chemical elements mapping of Al, Ni, and Cu of the Cu/Al bimetallic composite fabricated with 5.9 μm coating thickness was demonstrated in Figure 12. It was shown that there were still Ni layers distributed at the transition zone of the Cu/Al bimetallic composite with increasing the thickness of the Ni layer to 5.9 μm in Figure 12. The Ni layer was effectively limited the diffusion of the Cu and Al atoms at elevated temperature, which reduced the transition zone width to 268 μm.

**Figure 12.** SEM image and chemical elements mapping of Al, Ni, and Cu of the Cu/Al bimetallic composite fabricated with 5.9 μm coating thickness.

The content of Ni in the transition zone increased with increasing the coating thickness of electroplating Ni. When the thickness of the Ni layer was 1.5 μm, the Ni layer had been diffused in the transition zone during the casting process. Therefore, the transition zone of the Cu/Al bimetallic composite with Ni layer thickness of 3.8 μm and 5.9 μm was characterized. Table 2 summarized the interface Al, Cu, and Ni elements content of Cu/Al bimetallic composite in Figures 11 and 12. The results also indicated that with increasing the thickness of the Ni layer, the diffusion of the Cu and Al was limited.


**Table 2.** Interface element content of Cu/Al bimetallic composite (wt, %).

3.2.2. Effect of Coating Thickness on the Properties of Cu/Al Bimetallic Composite
