Hybrid structural joints between PBF-LB manufactured Ti6Al4V and CFRP – role of surface morphologies

Highly effective metal-composite joints can be manufactured by co-curing a composite component with an additively manufactured metal part featuring printed pin-structures in the joining area. In this work, the mechanical performance of such hybrid joints between laser powder bed fusion (PBF-LB) processed Ti6Al4V and a carbon fiber reinforced polymer (CFRP) was investigated for tensile shear loading. Initially, the performance of co-cured hybrid joints with as-built titanium surfaces was compared to secondary bonded joints with properly pre-treated surfaces. Subsequently, the influence of different titanium surface conditions on the hybrid joint behavior was evaluated, considering three cases: as-built, release-agent-coated and laser treated surfaces. Surface morphologies were characterized using scanning electron microscopy and confocal laser scanning microscopy, while the mechanical joint performance was assessed by single step-lap joint testing. Compared to the secondary bonded joints, the co-cured hybrid joints exhibited, on average, a 44.4% higher ultimate tensile shear strength and a threefold increase in fracture shear strain. However, the limit load (i.e. load at damage initiation) of the co-cured joints was substantially lower due to premature adhesive bond failure at the overlap ends, caused by insufficient adhesion of the as-built titanium surface. A highly similar mechanical performance was found for the hybrid joints with as-built and release-agent-coated surfaces. In contrast, the laser treated titanium surface significantly enhanced joint stiffness, limit load, and energy absorption, while the ultimate tensile shear strength increased moderately by at least 9% relative to the other surface conditions.