Effect of the surface morphology of 3D printed titanium substrates on the mode I fracture toughness of metal-metal and metal-composite bonded joints

• Adhesive joints with 3D printed Ti6Al4V adherends are studied herein. • The mode I fracture toughness of metal–metal and metal-composite joints is assessed. • As-printed joints have comparable fracture toughness to surface treated joints. • Sandblasting is not crucial, but leads to higher percentage of cohesive failure. • High temperature oxidation due to thermal treatment hinders fracture toughness. Parts manufactured with Laser Powder Bed Fusion (LPBF) are drawing interest in the adhesive joints research because of their high surface roughness, which is usually associated with good adhesion. This work aims to assess the adhesion strength of the inherent surface morphology of LPBF manufactured titanium. Double Cantilever Beam (DCB) tests were carried out to determine the mode I fracture toughness of joints comprising as-printed titanium (Ti6Al4V) adherends, namely titanium-titanium secondary bonded and titanium-Carbon Fibre Reinforced Polymer (CFRP) co-bonded joints. The effect of high-temperature oxidation on the fracture toughness was also evaluated by testing a batch of joints in which the titanium underwent a post-printing thermal treatment. The as-printed specimens were compared to the same type of joints but with sandblasted titanium adherends to evaluate the effect of this surface pre-treatment on the value of fracture toughness. The results indicate that non-oxidised titanium joints with untreated adherends had an average of 11% higher fracture toughness than their sandblasted counterparts. On the other hand, sandblasting proved beneficial for oxidised joints, increasing the fracture toughness by 64% on average over the untreated samples.