Effect of Annealing on the Interfacial Structure of Aluminum-Copper Joints

The aim of this study is to investigate the structure development and growth kinetics of the interfacial structure of cold roll bonded Al/Cu bimetal sheet. An interfacial structure is developed during the annealing process. The characteristics of the constituent phases at the interface of Al/Cu bimetal are studied by means of scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The results indicate that an obvious multi-layers interdiffusion structure is developed at the Al/Cu interface. The diffusion layer is consisted of four intermetallic compounds; Al2Cu, AlCu, Al3Cu4 and Al4Cu9. The growth of these intermetallics during annealing can be achieved by the diffusion process. The activation energies of Al2Cu, AlCu + Al3Cu4, Al4Cu9 and the total intermetallic layer are found to be 97.504, 107.46, 117.52 and 107.85 kJ/mol, respectively. These intermetallics generally possess higher hardness values than those of the corresponding base metals. AlCu and Al3Cu4 exhibit much higher hardness than that of Al2Cu and Al4Cu9, which implies lower fracture toughness. The observation of crack propagation paths shows that fracture mainly occurs in the intermetallic compound layers of AlCu and Al3Cu4, which are located between Al2Cu and Al4Cu9.