Study On Processing, Microstructure And Properties Of Cu/Al Laminated Composites

Metallic laminated composites, composed of several layers of metals with different properties, can combine the advantages of various metals and obtain relatively high strength, corrosion resistance, electrical and thermal conductivity if the components are properly designed. They can be applied in special environments. At present, Cu/Al laminated composites, having great electrical and thermal conductivity, low cost and high specific property, are widely used in heat exchangers, transmission systems and important sections of electric equipments such as cooling fins, collector tubes, transition joints and so on.However, as the reaction between Al and Cu atoms of comparable electronegativity at elevated temperature, some brittle intermetallic compounds form in Cu/Al interface, such as Cu9Al4, CuAl and CuAl2, which could deteriorate the bonding strength of the interface. Several methods including diffusion welding, roll bonding and extrusion bonding which are used to fabricating the Cu/Al laminated materials are all not be able to avoid such metallic atomic action in the interface. Fortunately, the asymmetrical roll bonding and annealing technique has profound prospects in producing metallic laminated composites for its effective control of the atomic action in the hot-working process and effective approaches to dealing with rolling pressure and equipment condition in the cold rolling process.In this paper, the asymmetrical roll bonding at room temperature and annealing are taken to prepare the Al/Cu laminated composites. The evolutions of the microstructure and phase composition of the Cu/Al interface are studied by scanning electron microscope (SEM)? transmission electron microscope (TEM)?X-ray diffraction (XRD)?energy dispersive X-ray detector (EDX) and selected area electron diffraction (SAED). In addition, the influence of the interface to the mechanical properties of Cu/Al are characterized and evaluated through tension tests at room and elevated temperature, tensile test at high strain rate, peeling test and three-point bending test. The main results of the paper are summarized as follows:1) A large shear deformation caused in the Cu/Al interface during the asymmetrical roll bonding promotes the surface cracks and extrusion of underlying metals. Compared with the composites produced by symmetrical roll bonding, the asymmetrical rolled composites annealed at 350? have a tight bonding interface and uniform interlayer. The tensile strength of laminated composites can be enhanced due to interface strengthening during quasi-static tensile tests at room temperature. The tensile strength increases but the fractured elongation decreases with increasing the strain rate. As a result, the Cu/AI interface exhibits the high strain rate sensitivity. The tensile strength of Cu/AI reduces due to the the rapid growth of interlayer in the tensile tests annealed at 200?.2) Comparing with the composites using roll bonding in several passes, a single pass with a large thickness-reduction can produce the Cu/AI composites with a good interface which have an optimal bonding strength without any voids and stress concentration. In the asymmetrical roll bonding, Cu/AI interface touching the high-speed roll gets a larger deformation than the Al/Cu interface. Therefore, much more Al can be extruded and adhered on the interface, which definitely influences the interfacial bonding strength of composites.3) The grains in matrixes and interfacial zone show distinct elongations and finally become the ultra-fined grains during the asymmetrical roll bonding process. The formation and growth of intermetallic compounds in the interface are affected by the plastic deformation and atomic diffusion. The growth kinetics indicates that the growth of interfacial interlayer follows the Arrhenius law. The interfacial interlayer first forms in the point with high deformation energy, and then expends into the whole interface. The magnetic field promotes the atomic diffusion in the interface. When the magnetic field reaches 12 T, the interfacial intermetallic compounds grow rapidly and the recrystallization of matrixes tends to increase.4) The interfacial interlayer plays a buffer effect to the mismatch deformation between dissimilar metals in the quasi-static tensile tests. The interface only with solid solution is better for the buffer effect. The cracks first occur in the interface of composites and finally cause the plastic instability of matrix layers until the complete fracture of laminated composites due to the formation of interfacial intermetallic compounds.5) The tensile tests with strain rates ranging 0.001 s-1-0.1 s-1 show that the tensile strength is improved annealed at 400? because of the strengthening of the strain rate. However, the interface is destroyed by the deformation of dissimilar matrices and the increase of elongation is then induced. The tensile strength increases significantly but the elongation decreases from 20% to 7% with increasing the strain rate in the range of 10 s-1-500 s-1. A combined action of strengthening by strain rate and softening by deformation heat is caused to the laminated composites under dynamic loading. Twin transformation occurs in the Cu matrix. A deformation compatible between matrixes and interlayer occurs and retains a good interfacial bonding in the fractured composites. Some defomed microstructure with fine grains form in the Cu and Al matrix along the tensile direction.6) Asymmetric interfaces form in the laminated composites produced by the asymmetrical roll bonding and directly influences the three-point bending performance of the composites, which can be used to improve the interfacial resistance to the tension stress in the bending process.