Fracture Resistance of CFRP Co-Bonded Joints with Additively Manufactured Metallic Substrates - Effect of Surface Pre-Treatment and Additive Manufacturing Parameters

Surface pre-treatments are commonly used to ensure proper adhesion in bonded joints. However, these processes are time-consuming, difficult to repeat consistently, and can leave areas with weak adhesion. This is a key factor limiting their use in critical components, where undetected defects may lead to sudden failure. To address these challenges, this thesis investigates the potential of using untreated Laser Powder Bed Fusion (LPBF) manufactured metal substrates in co-bonded metal-composite joints, focusing on their inherent surface roughness to promote adhesion. Initially, the fracture toughness of as-printed and sandblasted LPBF aluminum (Al\-Si10\-Mg) adherends was assessed in both secondary-bonded metal-metal joints and co-bonded Carbon Fiber Reinforced Polymer (CFRP)/metal joints. The results demonstrated that sandblasting improved fracture toughness by just 3\% over untreated joints. The study then expanded to LPBF titanium (Ti6Al4V) adherends, evaluating the effect of sandblasting and post-printing thermal oxidation on fracture toughness. Untreated titanium exhibited 10\% higher fracture toughness than sandblasted titanium for non-oxidized joints, whereas sandblasting significantly increased fracture toughness in oxidized joints (+73\%). Finally, co-bonded joints with LPBF Ti6Al4V and CFRP substrates were investigated to analyze the influence of printing parameters, specifically laser scan speed and build angle. The build angle had the most significant effect on adhesion strength, with vertically printed (90° with respect to the build platform) Ti6Al4V adherends exhibiting a 3.4-fold increase in mode I fracture toughness compared to joints comprising LPBF titanium printed at 110° and 130°. Therefore, this work highlights the potential of LPBF substrates to achieve reliable adhesion without surface treatments, contributing to the development of lightweight and, at the same time, cost-efficient multi-material structures.