*Article* **Effect of Liquid-Solid Volume Ratio and Surface Treatment on Microstructure and Properties of Cu/Al Bimetallic Composite**

**Zhiyuan Wu 1, Lijie Zuo 1,\*, Hongliang Zhang 2,\*, Yiqiang He 1, Chengwen Liu 1, Hongmiao Yu 2, Yuze Wang <sup>1</sup> and Wen Feng <sup>1</sup>**

<sup>1</sup> Jiangsu Institute of Marine Resources Development, Jiangsu Ocean University, Lianyungang 222005, China

<sup>2</sup> Zhongtian Technology Submarine Cables Co., Ltd., Nantong 226010, China

**\*** Correspondence: zuoliji@126.com (L.Z.); zhanghl@chinaztt.com (H.Z.)

**Abstract:** Due to exceptional conductivity, lightweight nature, corrosion resistance, and various other advantages, Cu/Al bimetallic composites are extensively utilized in the fields of communication, new energy, electronics, and other industries. To solve the problem of poor metallurgical bonding of Cu/Al bimetallic composites caused by high-temperature oxidation of Cu, different coating thicknesses of Ni layer on Cu rods were used to fabricate the Cu/Al bimetallic composite by gravity casting. The effect of liquid–solid volume ratio and coating thickness on microstructure and properties of a Cu/Al bimetallic composite were investigated in this study. The results indicated that the transition zone width increased from 242.3 μm to 286.3 μm and shear strength increased from 17.8 MPa to 30.3 MPa with a liquid–solid volume ratio varying from 8.86 to 50. The thickness of the transition zone and shear strength increased with the coating thickness of the Ni layer varying from 1.5 μm to 3.8 μm, due to the Ni layer effectively preventing oxidation on the surface of the Cu rod and promoting the metallurgical bonding of the Cu/Al interface. The presence of a residual Ni layer in the casted material hinders the diffusion process of the Cu and Al atom. Therefore, the thickness of the transition zone and shear strength exhibited a decreasing trend as the coating thickness of the Ni layer increased from 3.8 μm to 5.9 μm. Shear fracture observation revealed that the initiation and propagation of shear cracks occurred within the transition zone of the Cu/Al bimetallic composite.

**Keywords:** Cu/Al bimetallic composite; liquid–solid volume ratio; coating thickness
