Hierarchical surface features for improved bonding and fracture toughness of metal–metal and metal–composite bonded joints

Structural adhesive joints involving Selective Laser Melting (SLM) titanium bonded to titanium or to a composite material have significant potential for weight and cost saving in aerospace and other industries. However, the bonding potential of as-manufactured SLM titanium is largely unknown, and the use of additional hierarchical surface features has not been explored or characterised. Here we demonstrate with the use of SLM that a hierarchy of two surface features at different length scales can improve the fracture toughness of metal–metal and metal–composite bonded joints. At one length scale (10–15 μm), we established through fracture toughness testing that the intrinsic irregular roughness of the SLM surface maximises the bonding potential for both metal–metal adhesive joints (KIc=1.38 kJ/m2) and hybrid metal–composite co-cured joints (KIc =1.20 kJ/m2). We then combined this with surface features at a larger length scale (200 μm). For metal–composite joints, the use of groove surface features was found to deflect the crack path, which increased the fracture toughness of the joint by up to 50% for outward protruding grooves to a value of KIc=1.65 kJ/m2. We identified the rise in fracture toughness as a combination of an increase in the crack path length and a shift from pure mode I to a mixed-mode crack growth. We found that the relative contributions of these two factors were approximately equal. This work demonstrates that SLM-manufactured titanium can have significant advantages over conventional titanium for bonded joints. In comparison with conventional techniques, SLM surfaces can be used in adhesive bonds without the need for expensive and time-consuming surface preparation, and the design freedom allows for surface features that can significantly improve performance.