*2.2. Control of Clad Ratio*

In this study, because molten metal B in the lower nozzle solidifies before the molten metal A in the upper nozzle, the cladding ratio of the cladding material could be expressed by Equation (1), where the roll gap was the shortest distance between the surfaces of the twin rolls:

$$\text{clad ratio} = \frac{\text{roll gap} - \text{thickness of solidification layer of molten metal B}}{\text{thickness of solidification layer of molten metal B}},\tag{1}$$

The thickness of the solidification layer of molten metal B was controlled by the solidification length and roll speed, and the thickness of the solidification layer is given by Equation (2) [32]:

$$\mathbf{d} = \mathbf{K}\sqrt{\mathbf{t}} = \mathbf{K}\sqrt{\mathbf{L}/\mathbf{V}},\tag{2}$$

where d is the thickness of the solidification layer, K is the experimental solidification constant, t is the solidification time, L is the solidification length, V is the roll speed, and the K values were obtained from single-roll casting experiments. The schematic diagram of the single-roll casting experiment is shown in Figure 2, and the experimental conditions are shown in Table 2. The length of the molten metal in the lower nozzle in contact with the roll was defined as the solidification length (L), and the length of the molten metal in contact with the roll after completion of solidification was defined as the cooling length (Lc). The pouring temperature was the temperature of the molten metal inside the crucible during pouring, while the casting temperature was the temperature of the molten metal inside the nozzle. Because the temperature of the molten metal decreased as a result of the pouring process, the pouring temperature was increased by 5 ◦C to ensure that the temperature inside the nozzle reached the liquidus temperature of the metal material [34]. Because it was difficult to measure the temperature of the bonding interface in the twin-roll composite casting, the surface temperature of the solidified metal immediately out of the melt pool was measured in the single-roll casting experiment. The temperature at point A was approximately equal to the temperature of the metal bonding surface on the lower roll side before the two metals were compounded. The temperature of the metal contact surface on the upper roll side was its liquidus temperature. In addition, the continuous twin-roll caster used in this experiment was not equipped with water cooling. To investigate the variation of the surface temperature of the cast rolls, the surface temperature of point B during the casting process was measured.

**Figure 2.** Schematic diagram of single-roll casting.


**Table 2.** Experimental conditions of the single-roll casting experiments for the cast AZ91D and A5052 alloys.

#### *2.3. Cladding for Mg/Al Clad Strips by Twin-Roll Caster*

The casting conditions for the Mg/Al clad strips are shown in Table 3. The experimental results of single-roll casting showed that the effect of roll speed on the thickness of the solidified layer was small when the roll speed exceeded 12 m/min. Therefore, in this experiment, to study the effect of rolling speed on the Mg/Al clad strip, casting experiments were conducted at rolling speeds ranging from 6–12 m/min, and different pouring sequences were also investigated. In addition, the settings of cooling length and solidification length were kept the same as for single-roll casting, and both were set to 50 mm. The roll gap was set at 5 mm. Because flame retardant gases such as SF6 will destroy the ozone layer and cause environmental pollution, in this experiment, we did not use a flame retardant gas such as SF6. To inhibit the oxidation and combustion of Mg alloys, 0.5 mass% flux (S.K.No.101, TACHIGAWACAST, Japan) was added during the dissolution and refining

stages, and an oxide film formed on the surface of the crucible at the end of refining. This oxide film could prevent direct contact between the molten Mg alloy and the oxygen in the air, preventing further oxidation and combustion. In addition, the oxide film was removed from the surface before the Mg alloy was poured. In this study, the molten metal from the upper nozzle was poured directly onto the surface of the metal solidified by the lower nozzle, avoiding direct contact between the lower nozzle metal and the air. This replaced the traditional vacuum environment of hot rolling bonding. The aim of this work was to maximize the control of strip production costs.

**Materials AZ91D A5052** Pouring temperature [◦C] 600 654 Pouring sequence [upper nozzle/lower nozzle] A5052/AZ91D, AZ91D/A5052 Upper solidification length [mm] 50, 100 Solidification length [mm] 50 Cooling length [mm] 50 Roll gap [mm] 5 Roll speed [m/min] 6–12 Roll surface temperature [◦C] 22

**Table 3.** Casting conditions for the Mg/Al clad strips.
