Microstructure and bonding properties of hot-rolled 7075/AZ31B clad sheets

Magnesium/aluminum (Mg/Al) bimetallic laminated composites have attracted considerable attention because of their excellent properties. Mg alloys are lightweight structural metals with low density and excellent properties such as high stiffness-to-weight ratio, high strength-to-weight ratio, and good damping capacity. Thus, Mg alloys have considerable potential in automotive and aerospace fields. However, the application of Mg and its alloys is still restricted because of their low corrosion resistance. By contrast, as structural materials, Al alloys are widely used in mechanical and aerospace fields because of their excellent properties, such as light weight, high corrosion resistance, low cost, and good plastic formability. Therefore, Mg/Al laminated composites that combine the advantages of substrates to achieve appropriate coordination, have attracted worldwide attention. To analyze the variation of the bonding strength of hot-rolled Al/Mg clad sheets, various rolling parameters, such as reduction ratio, rolling temperature, and rolling speed, were comprehensively considered in this work. Moreover, 7075 Al/AZ31B Mg composite plates were prepared by single-pass hot rolling. Results show that dynamic recrystallization occurs in the microstructure of the Mg matrix during the rolling process because of heat and strong deformation. Furthermore, the increase in the rolling speed contributed to the complete dynamic recrystallization. At the same rolling temperature, the bonding strength of the Al/Mg composite plates first increased and then decreased with the increase in the reduction ratio. The bonding strength increased because of the increase in the element diffusion width across the interface and the grain refinement near the Mg interface. The bonding strength decreased because cracks occurred near the interface of the Mg matrix due to the strong deformation and excess heat generated by the plastic work, resulting in the growth of the Mg side grains with the increase in the temperature of the Mg matrix. The shear test was conducted on the composite plates. When the shear strength of the Al/Mg composite plates was low, shear fracture occurred at the interface with brittle fracture feature. Although the fracture morphology presented a ductile fracture feature with high shear strength, the fracture occurred on the Mg alloy side.