Creep effects in diffusion bonding of oxygen-free copper

Diffusion is the transport of atoms or particles through the surrounding material. Various microstructural changes in metals are based on the diffusion phenomena. In solid metals the diffusion is closely related to crystallographic defects. In single-component metals the dominant mechanism of diffusion is the vacancy mechanism. Diffusion bonding is a direct technological application of diffusion. It is an advanced solidstate joining process in which the surfaces of two components are brought to contact with each other and heated under a pressing load in a controlled environment. During the process, the contact surfaces are bonded by atomic diffusion across the interface and as a result, one solid piece is formed. The condition of high temperature and low applied stress combined with relatively long process duration enables the creep effects to take place in bonded metals. Furthermore, creep causes unwanted permanent deformations in the bonded components. Some authors suggest that there could be a threshold for stress below which the creep mechanisms do not activate. In case study, an experimental test on diffusion bonding of the CLIC accelerator structure disks conducted in CERN is introduced with a corresponding finite element simulation. Also a comparison between the results of the experimental test and the simulation is presented. The accelerator structure disk material is oxygen-free electronic (OFE) copper. The observed permanent deformations of the disks in the experimental test were very low or negligible but the bonding joint was partially defected. In turn, some deformations were observed in the simulation. It is possible that the used bonding pressure was just below a threshold stress. In the finite element simulation the creep strain-rates are calculated even for the lowest stresses, which could explain the observed deformations. Based on the results of this and previous experimental tests conducted in CERN it can be concluded that the threshold stress for creep exists and that the value of the threshold is around 0.05 MPa in this case.