AddJoining of layered CFRP-aluminum hybrid structures: process feasibility and joint properties

Hybrid structures are necessary to fulfil the market demand for lightweight structures with high specific strength. AddJoining is a new manufacturing technique for metal-polymer layered structures (pat. appl. DE 102016121267.9) inspired by additive manufacturing and joining technology principles. The new technique uses polymer 3D printing – e.g., fused deposition modeling, FDM - to add layers of polymer or composite onto a metal substrate. Main bonding mechanisms are adhesion forces and mechanical interlocking at the metal-polymer interface. FDM-based AddJoining manufactured 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 6.9 ± 0.2 kN and displacement at break of 2.1 ± 0.2 mm, respectively 19% and 15% higher than the adhesively bonded reference joints. The typical microstructure showed strong bond formation at the interface between the deposited polymer and metallic surface. An average void content of 8.1 ± 1.1% (intrinsic to the FDM process) was observed in the printed composites layers. These voids led to local delamination of the polymeric composite part, resulting in a global brittle failure of the overlap joint. 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.