Quasi-static and fatigue behavior of 3D-printed polymer-metal hybrid joints by AddJoining technique

Additive manufacturing can enable the production of end-use parts with complex geometries. the AddJoining technique - a new 3D printing method for metal-polymer hybrid parts (pat. appl. DE 102016121267.9) - has been developed inspired by additive manufacturing and joining technology principles. Using fused deposition modeling (FDM) to add layers upon layers of polymer or polymer composite onto a metal substrate, AddJoining allows for the manufacturing of metal-polymer layered structures. FDM-based AddJoining was selected to manufacture 2 mm aluminum 2024-T3 / polyamide-6 / carbon fiber-reinforced polyamide-6 single-lap joints. The metal part was pre-coated with PA6 to promote adhesion of the first deposited polymer layer. Joint quasi-static mechanical performance and microstructure were studied. AddJoining hybrid joints showed ultimate lap shear strength of 12.3 ± 0.1 kN and displacement at break of 2.5 ± 0.1 mm, respectively 19% and 15% higher than the adhesively bonded reference joints. From the microstructure point of view, proper mechanical interlocking was achieved between the coated metal substrate and the deposited polymer. Moreover, a optimal bonding of the subsequent 3D-printed layers was achieved, suggesting the occurrence of intermolecular diffusion. The fatigue performance of AddJoining hybrid single-lap joints was evaluated under tension-tension cyclic loading. The fatigue life assessment was performed using a two-parameter Weibull distribution. The method was used to assess the fatigue data scattering and predict the S-N curves under different reliability levels. During fatigue cycling, stiffness degradation was also monitored to help understanding fatigue damage development. AddJoining hybrid joints fulfilled the commonly accepted aircraft design requirement for new manufacturing technologies, i.e. they reached 47% of the ULSF (i.e. 5.8 kN)at 10⁵cycles. This work has shown that AddJoining has the potential to allow for the production of future high-specific strength composite-metal layered structures with tight dimensional and damage tolerances.