*3.1. Calculation of the Experimental Solidification Constants*

The temperature measurement results at point A showed that the surface temperature of AZ91D was between approximately 491 ◦C and 520 ◦C, while the surface temperature of A5052 was between approximately 600 ◦C and 626 ◦C. Because the metal surface was in a semi-solidified state, the thermocouple did not perfectly measure most of the surface temperature of the metal. Therefore, the measured temperature could only be used as a reference. In addition, during the experiment, the temperature at point B increased from the initial 22 ◦C to 30 ◦C. Compared to the temperature of the molten metal, the effect of this temperature difference was negligible.

The experimental results showed that AZ91D and A5052 could be cast continuously using a single roll, and the average thickness of the solidification layer was measured using a micrometer. The relationship between the roll speed and solidified layer thickness is shown in Figure 3a. With a higher roll speed, the thickness of the solidification layer was thinner [35]. The effect of the roll speed on the solidified layer thickness was more obvious when the roll speed was below 12 m/min, which indicated that the control of the solidified layer thickness of molten metal B, and consequently the cladding ratio of the cladding material, was easier to achieve under roll speed conditions below 12 m/min. The relationship between the square root of the solidification time and the solidification layer thickness is shown in Figure 3b. According to the figure, the experimental solidification constants of AZ91D and A5052 were 62 mm/min0.5 and 34 mm/min0.5, respectively.

**Figure 3.** (**a**) Relationship between the roll speed and solidification layer thickness; (**b**) relationship between the square root of the solidification time and the thickness of the solidification layer.

The duration of the experiment was difficult to quantify. Currently, we have observed the approximate time from the pouring to the end of casting by video, which was not accurate. This made it difficult to quantify the duration of the experiment because the amount of molten metal poured at the beginning potentially did not reach the height of the melt pool, and the amount of molten metal in the melt pool slowly decreased after the end of pouring, resulting in thin ends at the front and back of the continuous sheet, with uniformity in the middle. In addition, calculating the solidification time was reasonable. For example, when the roll speed was 6 m/min, the solidification time was 0.5 s for a solidification length of 50 mm, assuming no relative sliding of the solidified molten metal and casting roll.

#### *3.2. Effects of Roll Speeds and Pouring Sequences on the Surfaces of the Clad Strips*

The experimental results under different roll speeds and pouring sequences are shown in Table 4. The first pouring sequence involved pouring the high melting point A5052 material into the upper nozzle and the low melting point AZ91D material into the lower nozzle. As a result, the remelting of AZ91D on the lower roll side occurred under different roll speed conditions, as shown in Figure 4, which also shows the experimental results of the first pouring sequence at a roll speed of 9 m/min. The upper roll contact surface of the strip showed a good surface condition; however, the lower roll contact surface was uneven. The front portions of the clad strips showed that it was possible to form AZ91D sheets without the A5052 covering. After A5052 covered the AZ91D layer, the underlying AZ91D was dissolved and mixed together, forming grey intermetallic compounds. In addition, two types of heat transfer were associated with AZ91D in this experiment: one with A5052, and the other with the lower copper roll. Because the solidus temperature of A5052 was higher than the liquidus temperature of AZ91D, heat could only be transferred from the A5052 side to the AZ91D side. When the heat absorbed by AZ91D from A5052 was greater than that absorbed by the copper roll, the basic conditions for AZ91D solidification were not present. This resulted in the remelting phenomenon shown in Figure 4b.

**Table 4.** Experimental results under different roll speeds and pouring sequences.


×: bad, Δ: not good.

**Figure 4.** Surfaces of the A5052/AZ91D clad strips at a roll speed of 9 m/min. (**a**) Upper roll side, (**b**) Lower roll side.

The surfaces of the AZ91D/A5052 strips cast using the second pouring sequence at different rolling speeds are shown in Figure 5. The upper roll contact surface (AZ91D side) of the AZ91D/A5052 clad strip contained numerous ripple marks and some black oxides at a roll speed of 6 m/min. This black oxide could be easily removed by acid washing. When the roll speed was increased to 9 or 12 m/min, the ripple marks disappeared, and the surface condition was good with a smooth surface. However, we observed that the roll speed had a significant influence on the lower roll contact surface (A5052 side) of the AZ91D/A5052 clad strips. When the roll speed was 6 m/min, most of the area on the A5052 side was AZ91D and only a small portion of the area was A5052, and cracks were observed. The A5052 side showed a bright Al alloy surface at a roll speed of 9 m/min; however, intermetallic compounds appeared on both the surface and edges of the strip. Specifically, at 12 m/min, the A5052 side was directly covered with intermetallic compounds. We could speculate about these compounds. Because Mg and Al were in direct contact inside the nozzle, the interface output compound likely consisted of a liquid Mg alloy, an Al-Mg intermetallic compound, and a mixture of both.

**Figure 5.** Surface of the AZ91D/A5052 strips cast using the second pouring sequence with different roll speeds.

The large size of these intermetallic compounds and the rapid solidification of the alloy were not very well connected. In the twin-roll experiment, the upper and lower sides of the sheet were close to the upper and lower roll faces, respectively, and could achieve rapid solidification. Therefore, the surface condition was good; however, the middle part of the sheet was far from the upper and lower rolls and did not solidify well. This led to incomplete solidification in the middle part of the sheet, and excess liquid metal or metal compounds were then squeezed out under pressure of the double rolls and flowed around the sheet. The large size could be explained by Figure 6. The excess liquid metal or metal mixture at the bonding interface flowed to the lower surface of the sheet and accumulated between the individual drums of the drum conveyor under the influence of its own gravity. After cooling and solidification, large-size intermetallic compounds formed.

**Figure 6.** Schematic diagram of intermetallic compound output.

Figure 7 shows the casting process at different roll speeds. At low roll speeds, there was a significant increase in the thickness of the solidification layer in both molten metals. The increased thickness of the solidification layer of the Al alloy caused the lower nozzle to become blocked and the solidified Al alloy layer could not be smoothly brought out. Thus, only a small portion of the Al alloy was bound. However, the increased thickness of the solidification layer of the Mg alloy created a downward squeezing force. This inhibited the formation of the Al alloy in the lower nozzle, and this was consistent with the experimental results shown in Figure 5 (6 m/min). At high roll speeds, the thickness of the solidification layer in both molten metals was significantly reduced. Theoretically, with a constant roll gap, most of the intermediate gap would be filled by the molten Mg and Al alloys in a semi-solidified state. In this study, the bonding interface between the Mg alloy and Al alloy occurred at a high temperature, because the higher the temperature, the higher the energy of the atoms, the easier the migration, the higher the diffusion coefficient, and the faster the diffusion. Therefore, the mutual diffusion movement of the Mg and Al atoms was very strong, which would intensify the generation of intermetallic Mg and Al compounds. The experimental single-roll casting results showed that the surface temperature of A5052 before bonding was higher than 600 ◦C. In addition, the AZ91D in the upper nozzle was in a liquid state and the temperature of the bonding interface was the liquidus temperature (595 ◦C); thus, the average temperature of the bonding interface was higher than 595 ◦C. Because the temperature of the bonding interface was higher than 595 ◦C, the excess molten Mg alloy and intermetallic compounds remained in a liquid state and were extruded to both sides of the strip under the pressure of the twin rolls, and finally flowed along the edge of the Al layer to the lower roll contact surface of the strip via gravity. In addition, more intermetallic compounds were produced at the bonding interface at high roll speeds. This also resulted in more compounds flowing to the lower roll contact surface of the Mg/Al strips, which was consistent with the experimental results shown in Figure 5 (12 m/min).

Figure 5 shows that the surface condition of the Mg/Al clad strips was closest to being successful at a roll speed of 9 m/min with a 5 mm roll gap. The generation of intermetallic compounds was caused by excessive contact between the molten AZ91D and A5052 at the bonding interface. According to Equation (2), extending the solidification length at a certain roll speed increased the thickness of the solidification layer, which in turn reduced contact between the molten AZ91D and A5052. Theoretically, this could reduce the output of intermetallic compounds, which we verified experimentally. The upper solidification length was extended from the original 50 mm to 100 mm, and the experiments were carried out. Figure 8 shows the surface of the AZ91D/A5052 clad strips at an upper solidification length of 100 mm, showing the good overall surface condition of the AZ91D/A5052 clad strip. The picture of the lower roll side showed that the intermetallic compounds were only present at the edges of the AZ91D/A5052 clad strips. The effectiveness of extending the upper solidification length in reducing the generation of intermetallic compounds was verified. In this study, the surface of A5052 was smooth and had the metallic luster of the Al alloy, and the average thickness of the AZ91D/A5052 clad strip manufactured at a roll speed of 9 m/min and solidification length of 100 mm was 4.9 mm.

**Figure 7.** Casting process at different roll speeds. (**a**) Low speed, (**b**) High speed.

**Figure 8.** The surface of the AZ91D/A5052 clad strip with an upper solidification length of 100 mm. (**a**) Upper roll side, (**b**) Lower roll side.

#### *3.3. Microstructure of the Bonding Interface*

Figure 9a shows the cross-section of the AZ91D/A5052 clad strips cast by the horizontal twin-roll caster. Figure 9b shows the microstructure of region A of the bonding interface in Figure 9a. We clearly observed that the bonding interface was free of voids, with two diffusion layers at the bonding interface of AZ91D and A5052. The thickness of the total diffusion layer on the bonding interface was about 1 mm, which was much greater than the thickness of the diffusion layer obtained by hot rolling [21,22]. Because the solidus temperature of A5052 was higher than the liquidus temperature of AZ91D, when the molten Mg alloy and Al alloy were combined, the temperature at the bonding interface still remained above 595 ◦C. This accelerated the diffusion of Mg and Al elements on the bonding surface. In addition, the thicknesses of the AZ91D layer and A5052 layer were 2.1 mm and 1.8 mm, respectively, and the cladding ratio was approximately 1:1. According to Figure 3a, the theoretical solidification thickness of A5052 at a roll speed of 9 m/min was 2.4 mm, and the thickness of the A5052 layer in the clad strip was reduced by 25%. This was because the surface of A5052 in the lower nozzle was in a semi-solidified state after the end of solidification and before bonding with the molten AZ91D [35]. When the molten AZ91D was poured onto the semi-solidified surface of A5052, the thickness of A5052 on the lower roll side was smaller than the theoretical value due to the gravity of the molten AZ91D and rolling force of the upper roll. Combining the results in Figure 10 and Table 5, it was

clear that the two diffusion layers in the microstructure consisted of the α-Mg+Mg17Al12 eutectic layer near the Mg side and the Mg17Al12+Al3Mg2 compound layer near the Al side. Because the lowest magnification image of the laboratory optical microscope did not show the complete cross-section image of the clad strip, the image in Figure 9a was obtained using a normal camera under the illumination of a vertical light source. In addition, the two etching solutions used in this experiment could not etch the Mg17Al12 and Al3Mg2 layers, which were the two intermetallic compound layers that also showed a mirror effect under polishing and reflected under the irradiation of a vertical light source, causing the photographed Mg17Al12 and Al3Mg2 layers to show dark colors.

**Figure 9.** (**a**) Cross-section of the AZ91D/A5052 clad strips cast by the horizontal twin-roll caster; (**b**) microstructure of region A of the bonding interface in (**a**).

**Table 5.** Results of EDS point scan analysis at different positions of the interface corresponding to Figure 10a.


The trace element content in AZ91D and A5052 was low and most of the trace elements would solidly dissolve into α-Mg and α-Al, respectively, where the content did not change significantly. In addition, the focus of this study was on the content changes of the major elements Mg and Al, and the composition of the compounds. Figure 10 shows an SEM image of the bonding interface of the A5052/AZ91D clad strip and the EDS line scan results of the bonding interface. The SEM image and EDS line scan results indicated that the diffusion layer was divided into three layers. The chemical composition of each layer was analyzed by EDS point scanning and the results are shown in Table 5. The results showed that the layers near the AZ91D side were α-Mg and Mg17Al12, the middle was Mg17Al12, and the layer near the A5052 side was Al3Mg2. The composition of the intermetallic compounds was consistent with the results of previous studies [36,37]. Moreover, reducing the generation of intermetallic compounds has been an effective means of improving the interfacial bond strength of composite sheets [21]. For example, Ni or Zn intermediate layers were inserted between Mg alloys and Al alloys to avoid direct contact between the Mg and Al alloys, which in turn reduced the generation of Al-Mg intermetallic compounds [38–40]. Therefore, the future direction of this study will be to insert an Ni foil in the middle of the upper and lower nozzles to reduce the generation of intermetallic compounds and improve the bonding strength of the composite strip.
