Joining-by-hydroforming of aluminum and poly(ether ether ketone) – A model experiment

Joining-by-hydroforming is a process in which components are joined through expansion under internal pressure. Depending on the required fluid pressure and application rate, this process can be technically demanding and challenging to implement on industrial equipment. To address this, a simplified experimental setup was developed to investigate the fundamental joining mechanisms. In this setup, aluminum 6061-T6 (AA6061-T6) and poly(ether ether ketone) (PEEK) rings are force-fitted using a conical punch and segmented conical expansion elements, enabling controlled radial expansion. The resulting assemblies are subsequently separated in a dedicated push-out test. Experimental results show that the required separation force increases with rising elastic strain in the polymer, attributable to an increase in contact pressure according to Coulomb’s friction law. This effect diminishes once plastic deformation of the thermoplastic initiates. Furthermore, stress relaxation in PEEK causes a time-dependent decrease in joint strength, reaching a quasi-equilibrium after approximately 10 4 s, as confirmed by relaxation experiments on PEEK coupons. To analyze thermal effects, the entire ring assembly is preheated to defined temperatures in a laboratory furnace. An inverse correlation between joining temperature and joint strength is observed, consistent with the trend identified in the dynamic-mechanical-thermal analysis (DMTA) of PEEK. The proposed experimental method enables rapid identification of the most influential parameters for joining-by-hydroforming, without requiring dedicated hydroforming equipment or production machine time.