*2.4. Microstructure of the Bonding Interface*

Subsequently, the bonding interface microstructure of the AZ91D/A5052 clad strip was observed using an OLYMPUS BX60M optical microscope. To clearly observe the diffusion layer of the bonding interface, the observation surfaces of the specimens were abraded in turn using #400, #800, #1500, and #2000 sandpaper. The specimens were then polished to a mirror finish using 6.0 and 0.25 μm diamond polishing solutions, in order. Then, the polished surface was sequentially chemically etched with a citric acid solution (10 g of C6H8O7 + 90 g of H2O) and sodium hydroxide solution (1 g of NaOH + 100 mL of H2O). Then, the composition of the intermetallic compounds at the bonding interface was analyzed by ultra-low acceleration voltage scanning electron microscopy (JSM-7100F).

#### **3. Results and Discussion**
