With the development and progress of science and technology, the performance requirements of materials are higher and higher. Traditional materials are difficult to meet the requirements of multi performance coupling service environment due to the single structure and insufficient comprehensive performance. However, the composite materials prepared by physical or chemical methods from a variety of materials with different properties can not only maintain the unique properties of each component material, but also make use of each other's strengths to produce synergistic effect, which makes the comprehensive performance of the newly prepared composite far better than that of each member material, so as to meet the requirements of different complex working conditions. There are many similar structures in nature. The multi-layer structure with soft / hard combination can improve its strength, hardness and toughness. The following figure shows the layered structure of shellfish shells.
Fig. 1 microstructure of shellfish shell
Due to their excellent physical, chemical and mechanical properties, the multi-layer structure of dissimilar metals has attracted more and more attention. For example, Zhang  and other scholars prepared a kind of Ag / Cu Bimetallic multilayer composite by rolling and annealing process. After cold rolling, the Cu layer will produce obvious fibrous structure, while the Ag layer is fuzzy equiaxed crystal structure. After annealing, recrystallization occurs in different microstructure layers, and interdiffusion occurs between Cu layer and Ag layer, and Cu diffusion in Ag layer is more obvious.
Fig. 2 microstructure of the composite after room temperature rolling (left), annealing at 400 ℃ for 4.5H (middle) and annealing at 800 ℃ for 4.5H (right)
However, Su  and other scholars found that after annealing treatment, AA1050 / AA6061 ultra-fine grain multilayer composite material after cumulative rolling will show different characteristics under different passes. In the initial three passes, different aluminum alloy base materials can basically synchronize and coordinate deformation, but when the rolling times become more, AA6061 interface will necking, fracture, and eventually interface disorder.
Fig. 3 micrograph of composite after 1 pass (left), 3 passes (middle) and 5 passes (right)
Fig. 4 development diagram of interface morphology: initial state (left) low cyclic deformation (medium) high cyclic deformation (right)
Liu  et al. Prepared mg / Mg and Mg / Al / Mg multilayer composites by warm rolling, and studied the changes of microstructure and properties under different reduction rates. The results show that the critical reduction of mg / Al / Mg is 20% at 400 ℃ up to temperature, which is far lower than the critical value of mg / mg. At the same time, the mg Al intermetallic compounds did not form in the interface due to the short bonding time during warm rolling. The results show that the tensile strength of mg / Mg and Mg / Al / Mg multilayer composites is very high when the applied pressure is 35%.
Figure 5mg / mg30% bending (left) mg / Al / mg30% bending (middle) mg / mg50% bending (right) bending test
At present, the research on multilayer metal composites mainly focuses on the composite of two or more pure metals and their alloys with low melting point, low strength, good deformation matching, good ductility and little difference in strength. They are easy to deform and coordinate with each other, and can form multilayer composites with continuous interface. For example, aluminum aluminum, aluminum copper, aluminum nickel, aluminum titanium, copper niobium, aluminum magnesium, aluminum steel, etc. If the mechanical properties of multi-layer composites are too different or the deformation ability of components is very poor, the coordinated deformation of the whole composite will be affected, and higher requirements for the bonding quality of the interface will be put forward. At present, the main research direction is to prepare multi-layer metal composites with excellent deformation ability and mechanical properties.
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 L. Zhang et al. Behaviors of the interface and matrix for the Ag/Cu bimetallic laminates prepared by roll bonding and diffusion annealing. [J] Materials Science & EngineeringＡ,2004,371(1-2):65-71.
 Su L et al. Ultrafine grained AA1050/AA6061 Ultrafine grained AA4050/AA6061 composite produced by accumulative roll bonding. [J] Materials Science and Engineering A 2013,559:345-351.
 Liu C Y, et al. Microstructures and mechanical properties of Mg/Mg and Mg/Al/Mg laminated composites prepared via warm roll bonding [J]. Materials Science & EngineeringＡ,2012,556:1-8.