Hierarchical surface features for improved bonding and fracture toughness of metal–metal and metal–composite bonded joints Alex T.T. Nguyen a , Milan Brandt a , Adrian C. Orifici b , Stefanie Feih a,b,c,n a Additive Manufacturing Research Centre, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria 3001, Australia b Sir Lawrence Wackett Aerospace Research Centre, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Victoria 3001, Australia c Joining Technology Group, Singapore Institute of Manufacturing Technology (SIMTech), 71 Nanyang Drive, 638075 Singapore article info Article history: Accepted 1 December 2015 Available online 12 December 2015 Keywords: Selective laser melting Hierarchical surface feature Fracture toughness Finite element analysis abstract 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 indus- tries. 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 (K Ic ¼1.38 kJ/m 2 ) and hybrid metal–composite co-cured joints (K Ic ¼1.20 kJ/m 2 ). 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 K Ic ¼1.65 kJ/m 2 . 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. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction The use of composite materials, and especially carbon fibre- reinforced polymer (CFRP) composites, is increasing rapidly in a range of industry sectors such as aerospace, marine and oil and gas [1,2]. Integrating composite materials with metal alloys can achieve hybrid structures with higher strength-to-weight ratios, longer inspection cycles and hence lower maintenance costs for modern light weight structures. However, the connections between dissimilar materials are problematic due to stress con- centrations, mismatch in thermal expansion and resulting fatigue issues [3,4]. One common joining method for hybrid structures is adhesive bonding, which generally requires extensive surface treatments (e.g. thermal, chemical, mechanical, laser or plasma or combinations thereof) to ensure high strength and durability. Recently, metallic additive manufacturing technologies such as Selective Laser Melting (SLM) have seen increased application due to the enhanced design freedom for highly weight-optimised structures as utilised in aerospace applications. Virtually, any 3D Computer Aided Design (CAD) model can be imported into a SLM system in the form of Standard Tessellation Language (STL) files which are then numerically sliced into many fictitious layers of cross-sectional areas. The additive manufacturing process usually takes place in an inert chamber where a uniform layer of powder particles is first deposited and a focused laser beam then scans the cross-sectional area of the part to selectively melt the powder particles, thereby allowing a single layer of the part to be built. The process is repeated layer-by-layer until the full component is printed. As high thermal energy is used to melt the powder par- ticles, the molten pool at the building focal point dissipates heat Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives http://dx.doi.org/10.1016/j.ijadhadh.2015.12.005 0143-7496/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author at: Joining Technology Group, Singapore Institute of Manufacturing Technology (SIMTech), 71 Nanyang Drive, 638075 Singapore. Tel.: þ65 6793 8378; fax: þ65 6792 5362. E-mail address: feihs@simtech.a-star.edu.sg (S. Feih). International Journal of Adhesion & Adhesives 66 (2016) 81–92