**1. Introduction**

Due to a combination of the high electrical and thermal conductivity of Cu [1] with the light weight of Al [2–6], Cu/Al bimetallic composites [7–14] were widely used in the electric power transportation industry. Cu/Al composite materials can replace copper materials in generators and aluminum materials in external power grids, as well as the contact surfaces between the two, thereby reducing the use of copper resources and reducing the accident rate in power generation and supply. Although the compound casting method has exhibited great superiority in fabricating irregular shapes of Cu/Al bimetallic composites, it still has some drawbacks in the aspects of the rapid growth of intermetallic compounds and the oxidation of the solid Cu substrate. The hard and brittle transition zone was formed at the interface of the Cu/Al bimetallic composite, which reduced the mechanical properties of the material. Therefore, optimization of the transition zone has become a hot topic in the research and development of Cu/Al bimetallic composite.

It was widely reported that the transition zone played an important role in the microstructure and mechanical properties of Cu/Al bimetallic composites [15]. There have been extensive studies on the formation mechanism of the transition zone. For example, Wang et al. [16] used synchrotron X-ray technology to study the interfacial diffusion behavior and microstructure evolution of Cu/Al bimetals. Cu and Al first diffuse to each other and then form α-Al dendrites and intermetallic compounds (IMCs) between the matrix.

**Citation:** Wu, Z.; Zuo, L.; Zhang, H.; He, Y.; Liu, C.; Yu, H.; Wang, Y.; Feng, W. Effect of Liquid-Solid Volume Ratio and Surface Treatment on Microstructure and Properties of Cu/Al Bimetallic Composite. *Crystals* **2023**, *13*, 794.

https://doi.org/10.3390/ cryst13050794

Academic Editors: Andrea Di Schino and Claudio Testani

Received: 27 March 2023 Revised: 7 May 2023 Accepted: 8 May 2023 Published: 9 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The liquid–solid ratio was one of the key factors for the formation of the transition zone in the preparation of bimetallic composite by gravity casting. For example, it has been found that the bonding quality of AZ91 and AZ31 alloys was better when the liquid–solid ratio was larger [17]. Other studies have shown the preparation of high chromium cast iron and medium carbon steel bimetals by gravity casting. With the increase of liquid–solid product ratio, the diffusion activity of elements increased, leading to the increase of the interfacial transition zone width and shear strength [18]. It has been reported that the transition zone consisted of the intermetallic compounds and the remelting zone, and the cooling rate influenced the thickness of intermetallic compounds, the microstructure of the remelting zone, and the morphology of the remelting zone/Al interface [19]. Tavasoli et al. [20] reported the effect of pouring temperature on the transition zone, and the results showed that an increase in Al melting temperature resulted in a gradual increase in the thickness of intermetallic compounds and interfaces. Chen et al. [21] found that pouring temperature, cooling mode of Cu plate surface, and starting time of forced cooling after pouring had no effect on the microstructure species of the transition zone.

In addition, during the preparation of the Cu/Al bimetallic composite by gravity casting, an oxidation reaction occurred on the surface of Cu during preheating [22], and the formation of an oxide film reduced the metallurgical bonding property of the interface between Cu and Al. Therefore, it was particularly important to cover the surface of the copper with a protective film to prevent oxidation [23]. A suitable protective film can not only prevent the surface oxidation of Cu, but also promote the metallurgical bonding between Cu and Al. Boucherit et al. [24] achieved friction stir welding of Cu/Al using a zinc interlayer and found that Zn can significantly reduce the formation of intermetallic compounds such as Al2Cu and Al4Cu9, thereby improving the shear lap tensile strength of the joint. Ye et al. [25] adopted a new Zn-Al-Si filler metal to braze Cu/Al and found that Si could inhibit the growth of intermetallic compounds, thus significantly improving the corrosion resistance of Cu/Al bimetallic materials. Breedis et al. [26] found that adding a certain amount of Ni to copper alloys can inhibit the growth rate of Cu/Al intermetallic compounds, effectively reduce the content of intermetallic compounds, improve the microstructure of copper alloys, and effectively improve its properties. It was found that by depositing Ni-P coating on a copper substrate by electroless plating, the intermediate coating acted as a protective film, which could reduce the rate of intermetallic compound generation [27]. If Ni was used as the intermediate layer during the pouring process of Al and Cu, it can be seen from the Cu-Ni binary phase diagram that due to the infinite solubility of Cu and Ni, intermetallic compounds will not be generated and copper matrix oxidation can be prevented. Liu et al. [28] reported that a uniform Ni protective layer on the electroplating of the copper matrix can prevent the surface oxidation of the copper matrix, and the Ni layer dissolved during the interfacial reaction during the pouring process, promoting the metallurgical combination of copper and aluminum.

At present, the research on interface processing of Cu/Al composite materials mainly focuses on coating materials and coating methods, while there are few studies on the effect of coating thickness on the microstructure and properties of Cu/Al composite materials. In this study, the fabrication of Cu/Al bimetallic composite was achieved by gravity casting, and the effect of liquid–solid ratio and coating thickness on the microstructure and properties of Cu/Al bimetallic composites was discussed. The appropriate thickness of the transition zone will significantly improve the mechanical properties of the Cu/Al bimetallic composites.

#### **2. Materials and Methods**

The Cu/Al bimetallic composite was fabricated by pure copper rods and aluminum rods. In order to prevent oxidation of the copper while being kept at elevated temperature, a layer of nickel was plated on the surface of the copper rods before casting. The electroplating solution consisted of 840 g Ni2SO4, 100 g NiCl2·6H2O, and 3 L deionized water. The nickelplating voltage was 4V, and the nickel-plating time was 10, 25, and 40 min, respectively. The electroplating device was shown in Figure 1. Both the casting mold and nickel-plated copper rod were kept at 500 ◦C with the resistance furnace at the beginning of the test. The pure aluminum rod was melted and refined in a steel crucible at approximately 740 ◦C. The melt was left to stand at 720 ◦C for about 10 min to ensure the equilibrium temperature after the refining slag was skimmed. Then, the aluminum melt was cast into the steel mold equipped with the nickel-plated copper rod, and after waiting until it had completely cooled and solidified, it was removed from the mold.

**Figure 1.** Electroplating Operation Console.

The metallographic specimen was first polished on different grit sandpaper, then with a combination of mechanical polishing and hand polishing on a velvet polish cloth with a solution of 0.5 μm aqueous magnesium oxide. The microstructure of Cu/Al bimetallic composite was observed by AXIO-type metallographic microscopy (OM). The Ultima IV X-ray diffraction (XRD) was used to analyze to the types of intermetallic compounds in the transition zone of materials, with a voltage of 35 kV and a scanning speed of 10◦/min; diffraction angle range was 10◦ ≤ θ ≤ 45◦. Jade 6.0 software was then used to perform phase calibration on the acquired dates. The sampling locations of hardness and shear samples were shown in Figure 2. The hardness (HV) of the Cu/Al bimetallic composite were tested on the HXD-1000TM digital microhardness tester with a load of 200 g and loading time of 15 s. The cylindrical shear specimen with diameter of 16 mm and height of 6 mm was fabricated by the electric spark machine. The shear specimen was performed on HXD-1000TM electronic universal material testing machine with the speed of 1.0mm/s. Then, the FEI Scios-2 HiVac scanning electron microscopy (SEM) was used to analyze the shear surface of the Cu/Al bimetallic composite.

**Figure 2.** Casting mold drawing and shear material: (**a**) casting mold diagram; (**b**) Cu/Al bimetallic composite; (**c**) schematic diagram of shear strength test.
