Joining-by-hydroforming of AA6061 and PEEK tubes

This study investigates interference fit joining-by-hydroforming of tubes made from AA6061-T6 and PEEK. The experimental program consists of joining experiments, and subsequent tests on the produced connections to measure transferable axial force as functions of post-joining time, in order to determine the acting contact pressure. These connections are intended for use in fuel systems of commercial aircraft, therefore accurate prediction of joint strength is essential to ensure safe operation in flight. The investigations show that stress relaxation within PEEK causes a progressive reduction in joint strength after depressurization of the working fluid. After about 10,000 s an equilibrium state is reached and the joint strength does not decrease further. An analytical calculation model is developed that enables the joint strength at 10,000 s in the equilibrium state to be computed from the circumferential strain achieved at the maximum working fluid pressure. The model adopts an elastoplastic material description for the aluminum tube and a viscoelastic description for the thermoplastic partner, based on a generalized Maxwell model. The analytical model provides a transparent link between process-induced deformation and residual contact pressure in the joint at equilibrium. The outcome is a predictive capability that supports parameter selection for joining-by-hydroforming in aerospace-relevant applications.