**1. Introduction**

Bimetallic materials have received close attention in recent years because of their unique superiority in combining the advantages of both different materials. Aluminum and magnesium alloys are two commonly used structure materials for lightweight applications for economic savings and ecological protection [1,2]. The former has the characteristics of good formability, excellent corrosion resistance, high specific strength, and stiffness [3]. The latter has low density, good castability, and high damping capacity [4]. Mg–Al bimetal, which consists of these two components, combines these advantages, and is widely applied in the automobile, aviation, and aerospace fields [5,6].

At present, a large number of technologies have been used to fabricate Mg–Al bimetal, such as welding [7], rolling [8], and casting [9]. The rolling process is an effective way to produce laminated or rodlike material, but it is challenging to manufacture complex parts. The welding methods are usually used to prepare the bimetallic products with a simple shape with high efficiency. Compared to the rolling and welding processes, compound casting has the advantage of being suitable for the production of bimetals with large and complex geometry. Moreover, the production procedure of the compound casting process is simple and low cost. Recently, much attention has been attracted to compound casting technology. Mg–Al bimetals have been prepared by various compound casting methods. Zhu et al. [9] fabricated AM50–Al6061 bimetallic products by a compound die casting method, followed by a low-temperature (200 ◦C) annealing schedule. The shear strength of the AM50–Al6061 bimetal reached the maximum value of 8.09 MPa, after a three-hour annealing. Their research found that the thickness of the Mg–Al interface

**Citation:** Guan, F.; Fan, S.; Wang, J.; Li, G.; Zhang, Z.; Jiang, W. Effect of Vibration Acceleration on Interface Microstructure and Bonding Strength of Mg–Al Bimetal Produced by Compound Casting. *Metals* **2022**, *12*, 766. https://doi.org/10.3390/ met12050766

Academic Editor: Emin Bayraktar

Received: 20 March 2022 Accepted: 26 April 2022 Published: 29 April 2022

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greatly affected its shear strength. He et al. [10] used a solid–liquid compound casting process to manufacture the arc-sprayed Al–AZ91D bimetal via casting AZ91D melt into the molds deposited by arc-sprayed aluminum coating. Hajjari et al. [11] used compound casting to produce a pure Al–pure Mg bimetal joint. The Mg–Al bimetal obtained in this experiment has a thick interface of about 1 mm, and the interface consists of multiple layers of different microstructures. The shear strength of the Mg–Al interface is about 23 MPa. Emami et al. [12] produced Mg–Al bimetal by both conventional and lost foam compound casting. The research also found a thick Mg–Al interface, achieving a millimeter level in the contact area of the two kinds of metals. In our previous research, lost foam compound casting was used to fabricate the A356–AZ91D bimetal, and key parameters such as pouring temperature and liquid–solid volume ratio were systematically studied. The results show that the improvement of the bonding strength of Mg–Al bimetal can be achieved by adjusting the processing parameters [13–16]. The properties of the Mg–Al bimetal are still poor due to the massive brittle and hard Al–Mg intermetallic compounds (IMCs) phase in the Mg–Al interface [10–12], and the Al2O3 film on the surface of the solid Al alloy, which hinders the direct bonding of the solid and liquid alloy and brings down the wettability [17,18].

In the recent research, the metal coating is usually prepared on the solid matrix to strengthen the mechanical properties of the bimetal prepared by compound casting [19,20]. Mola et al. [21] prepared a Zn coating with a thickness of 0.1 mm on the surface of the 6060 inserts by diffusion bonding. Then, the coated insert was used for the compound casting. After introducing the Zn element in the interface, the microstructure of the Mg–Al interface changed from Al–Mg IMCs to Mg–Al–Zn ternary IMCs, and the shear strength of the Mg–Al joint increased significantly, from about 8 MPa to about 42 MPa. Liu et al. [22] fabricated the Mg–Al bimetallic composites by a compound casting process. Moreover, a Ni interlayer was coated on the aluminum insert to hinder the direct reaction between the solid aluminum and liquid magnesium. As a result, the bonding strength of the Mg–Al bimetal was improved from 17.3 MPa to 25.4 MPa after adopting the Ni interlayer. However, the preparation process of the coating will make the preparation process of the bimetal more complicated and greatly increase the energy consumption and production cost, thus producing more pollution.

In addition, the researchers also tried a variety of other methods to improve the bonding properties of bimetals produced by the compound casting process. Chen Yiqing et al. [23] added La element to the magnesium alloy melt when preparing Mg–A390 bimetal by casting liquid Mg alloy onto the solid Al alloy panel. Experimental results show after adding the rare earth La into the magnesium alloys, the Al12Mg17 phase at the interface becomes lesser and thinner, and the cast grain is gradually refined. When the magnesium alloys contain 1% rare earth La, the maximal shearing strength of the interface can achieve 88.5 MPa. Wu Li et al. [24] proposed a modified horizontal continuous casting process under the electromagnetic field for preparing AA3003–AA4045 clad composite hollow billets. When rotating electromagnetic stirring was applied, the flow pattern of fluid melt was greatly modified; the temperature field in the interface region became more uniform. As a result, the microstructure of the clad composite hollow billet was refined, and the diffusion of the elements at the interface was promoted. Tayal et al. [25,26] used sandpaper with different mesh sizes to grind the surface of the aluminum alloy to research the influence of surface roughness on the shear strength of the A356–Mg bimetal produced by vacuum-assisted sand mold compound casting. The results show that it is more appropriate to use 800-grit sandpaper to polish the surface of the A356 insert. Babaee et al. [27] machined a special concentric groove pattern on the surface of the Al insert to improve the bonding properties of Al–Al-4.5%Cu bimetal prepared by squeeze casting. The tensile strength of the bimetal increased from 17 MPa to 54 MPa, after applying this method.

Although many studies have been conducted on the preparation of Mg–Al bimetal, few studies have been conducted to improve the performance of the Mg–Al interface by improving its microstructure through external assistance. The dependence of the

mechanical properties on the microstructure of the Mg–Al interface is still insufficient. Vibration-assisted solidification is a technology of applying vibration in metal casting. It can effectively improve the microstructure solidification and the properties of the materials. Meanwhile, it has the advantages of low cost and no pollution. On the one hand, the molten metal was compressed and stretched by periodic force under the vibration, which was beneficial to the degassing and crystallization process [18,19,28,29]. Moreover, mechanical vibrations can form forced convection in the melt. The flow of the melt causes the temperature equalization of the melt, promotes the heat exchange between the molten metal and the mold, and increases the cooling rate of the melt [30]. At the same time, the flow caused by the melt also leads to the transport process of the solid phase and an increase in crystal nucleation. Therefore, it promotes the formation of more uniform fine-grained solidification microstructures [31]. Moreover, the vibration can also affect the distribution and shape of precipitates by promoting the element diffusion and solute exchange in the bimetallic interface [32]. It has excellent application prospects for the bimetals produced by compound casting.

In our previous study, we have investigated some key process parameters of a lost foam composite casting (LFCC) [13–16]. The microstructure of the interface of the Mg–Al bimetal prepared by the lost foam solid–liquid composite casting process was also reported. However, there are still many unsolved problems in our previous studies. For example, the bonding strength of the Mg–Al interface was not strong, the composition of the interface was relatively complex, and the formation process of the interface has not been fully clarified. In this work, the AZ91D–A356 bimetal was produced using the LFCC process. Vibration with different accelerations was applied during the casting and solidification process to enhance the bonding strength of the Mg–Al bimetal, by the vibration table used in the original lost foam casting process. The effect of the vibration acceleration on the microstructures and the bonding strength of the Mg–Al bimetal was investigated.

#### **2. Experimental Procedure**
