**1. Introduction**

Automotive light-weighting has been a hot research topic in recent years to improve fuel efficiency and reduce vehicle exhaust emissions [1–4]. Traditional automotive lightweighting technology has been used to replace steel materials with a single lightweight alloy [5]. However, with the development of new energy vehicles, the characteristics of single-metal materials can no longer meet the new demands of automotive industry development. On this basis, metal additive manufacturing technologies have also started to receive attention [6–9]. Among these, the fabrication of clad sheets has become a current research hotspot [10–14], as they will exhibit the characteristics of each metal alloy, and can realize the complementary advantages and disadvantages of different metal alloys.

As the lightest metal among the practical structural metals, magnesium (Mg) alloys have gained attention as they offer excellent specific strength, vibration damping performance, and recyclability. However, drawbacks such as low corrosion resistance and high production costs have limited their application in the automotive industry [15]. As one of the practical lightweight metals, aluminum (Al) alloys have been widely used in the transportation and aerospace fields due to their active chemical properties, which can easily react with oxygen in the air to produce a dense oxide film and improve their corrosion resistance. Composite sheets prepared by using Mg and Al alloys can improve the corrosion resistance of Mg alloys, while also endowing the composite sheets with the physical properties of two metal sheets [16].

**Citation:** Feng, G.; Watari, H.; Haga, T. Fabrication of Mg/Al Clad Strips by Direct Cladding from Molten Metals. *Metals* **2022**, *12*, 1408. https://doi.org/10.3390/ met12091408

Academic Editor: Wenming Jiang

Received: 1 August 2022 Accepted: 23 August 2022 Published: 25 August 2022

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Many preparation methods for Mg/Al composite sheets have been reported, such as vacuum diffusion bonding [17], in situ hot press bonding [18], cold rolling [19], hot rolling [20–24], explosion welding [25–29], and solid-liquid composites [30,31]. The vacuum diffusion bonding method consists of bonding Mg and Al sheets by heating, pressurizing, and cooling in a vacuum environment. Because oxides on the metal surface can hinder diffusion bonding, it is necessary to clean the oxides before experimentation; however, this method is tedious and has high equipment requirements [17]. Jin investigated an in situ hot press bonding technique to prepare Mg/Al composite sheets. This technique required only four steps, and no vacuum environment or protective gas was needed to avoid oxidation of the metal material. The results showed that the Mg/Al composite sheet had good interfacial strength, while the intermetallic compounds that were generated at the interface had little effect on the Mg/Al composite sheet in compression, but caused clad–core delamination under tensile conditions [18]. Cold and hot rolling is a combination of pressure on Mg and Al alloy sheets using rolling equipment. The advantage of this approach is the low production cost and the fact that the preparation process does not need to be performed under a vacuum. In addition, most of the Mg/Al composite sheets are rolled by hot rolling because of the difficulty of processing Mg alloys at room temperature. Generally, rolled Mg/Al composite sheets also must be annealed. Research has shown that the best bonding strength of Mg/Al composite plates can be achieved under an annealing temperature of 200 ◦C and annealing time of 1 h [21]. Cao et al. investigated the effect of secondary rolling on the interfacial bonding strength and mechanical properties of Al/Mg/Al composite sheets. The results showed that secondary rolling significantly improved the interfacial bond strength. After annealing, the elongation of the composite plate was as high as 21% and the interfacial strength was maintained at 12 MPa [24]. Explosion welding uses the high impact forces generated by the explosion to strongly bond Mg and Al alloy sheets. Wang et al. improved the explosion welding technique by adding an additional thin aluminum plate as a buffer layer between the Mg and Al sheets, while the other sides of the magnesium sheets were fixed with steel plates to avoid deformation. The welding process was also simulated using smoothed-particle hydrodynamics (SPH) simulations, and the bonding interface showed a regular wave shape, which was in agreement with the experimental results [28]. However, explosion welding poses certain safety risks and cannot be produced on a large scale. Solid-liquid composite uses twin-roll casting technology, which differs from traditional sheet production methods in that it significantly reduces production costs due to the use of a twin-roll caster, enabling continuous casting from molten metals. Park et al. successfully fabricated three-layer A5052/AZ31/A5052 clad strips by twin-roll casting and post-treatment, where the thinness of the A5052 alloy sheet resulted in an uneven surface condition of the composite sheet after casting [31]. In addition, Haga et al. successfully cast clad strips consisting of Al alloys in one step using a single-roll caster equipped with a scraper and an unequal diameter twin-roll caster. A theoretical formula for controlling the clad ratio by solidification length is also proposed. Because the second molten metal was located directly over the free solidification surface of the first molten metal, oxidation of the first metal as a result of contact with air was avoided [32,33]. However, relatively little research has been conducted on the liquid–liquid lamination of dissimilar metals.

In this study, AZ91D/A5052 clad strips were fabricated by direct cladding from molten metals using horizontal twin-roll casters, and the effects of experimental parameters such as the roll speed, pouring sequence, and solidification length on the surface conditions of the AZ91D/A5052 strips were investigated. Then, the microstructure of the fabricated strips was investigated using optical microscopy and electron microscopy.

#### **2. Experimental Procedures**

#### *2.1. Materials and Methods*

An AZ91D Mg alloy (Mg-9mass%Al-1mass%Zn alloy), with excellent specific strength, was chosen as the base material, and an A5052 Al alloy (Al-2.5mass%Mg alloy) with good

corrosion resistance and plasticity was chosen as the covering material. Table 1 shows the chemical compositions of the AZ91D and A5052 alloys. Figure 1 shows a schematic illustration of the cladding processes of the two molten metals, which utilized a horizontal twin-roll caster (HTRC). The two molten metals were poured into the upper and lower nozzles separately, cooled rapidly when they made contact with the rolls, and then bonded between the rolls. The rolls were composed of copper and were 300 mm in diameter and 150 mm in width. In addition, both the upper and lower nozzles had a width of 150 mm.

**Materials Mg Al Si Fe Cu Mn Zn Cr** AZ91D Rest 9 0.05 0.002 0.01 0.3 1 - A5052 2.5 Rest 0.09 0.14 0.01 0.01 - 0.25

**Table 1.** Chemical compositions of the AZ91D and A5052 alloys [mass%].

**Figure 1.** Schematic showing the cladding processes of Mg/Al using a horizontal twin-roll caster (1. crucible; 2. molten metal A; 3. molten metal B; 4. upper nozzle, 5. lower nozzle; 6. upper solidification length; 7. cooling length; 8. solidification length; 9. upper roll; 10. lower roll; 11. clad strip).
