Microstructure and mechanical properties of Al/Mg/Al composite sheets containing trapezoidal shaped intermediate layer

In the present study, the Al 6061/Mg ZK60/Al 6061 composite sheets with the trapezoidal shaped intermediate layer were successfully obtained by multi-pass hot roll bonding. The microstructure and mechanical properties of the composite sheets under different rolling passes were investigated. The results demonstrated that the interface of the composite sheets achieved the strong interface metallurgy bonding, without discernible voids, cracks, and other defects. The introduction of the three-dimensional (3D) interface layer significantly improved the interface bond strength of the composite sheets. The interface layer composed of Al3Mg2 and Al12Mg17 was observed at the interface of the composite sheets. The thickness of the intermetallic compounds (IMCs) layer varied with the position of the interface, and the IMCs layer was the thinnest near the center of the sheet. The IMCs layer was continuously coarsening and fragmenting with the increase of rolling deformation, and stress concentration was easily generated at the interface between the Mg layer and the Al layer, so the crystal grains were more refined than that inside of the component sheets, which made the actual strength of the composite sheets higher than the theoretical strength. With the proceeding of the rolling process, the internal grain size of the component sheet gradually increased, and the interface products were coarsened and fragmented, which reduced the tensile properties of the composite sheets. The interface layer with moderate thickness and intact interface layer in the composite sheets appeared uniformly broken when it was under tensile stress, and the direction of microcracks generated was perpendicular to the tensile direction, which hindered the crack propagation and delayed the fracture of the composite materials and thereby enhanced the elongation of the composite sheets. Simultaneously, the trapezoidal shaped intermediate layer made the cracks deflect along the interface, which was conducive to the strengthening and toughening of the interface area, to improve the plasticity of the composite sheets.