How the Surface Morphology of Electron Beam Melted Additive Manufactured Grade 5 Titanium Affects Adhesive Bonding Shear Strength
Electron Beam Melting (EBM) is a powder bed fusion (PBF) additive manufacturing (AM) process, capable of easily manufacturing complex shapes out of strong materials, such as Grade 5 titanium. One way to utilise this process while avoiding its downsides (its high cost and small chamber size – 350mm x 380mm (Ø/H)) is through producing large, stiff and strong structures made of small metal EBM AM nodes adhesively bonded to carbon fibre tubes. These structures offer time and cost savings whilst still being incredibly strong. However, the strength of these structures is only as good as the strength of the adhesive bonds holding it together. Therefore, this thesis aimed to understand the relationship between different as-built surface morphologies producible by EBM AM, and the resulting adhesive bonding shear strength of each surface.
This study tested three EBM as-built surfaces, defined as vertical, horizontal, and trabecular meshed surfaces. These surfaces were shear strength tested using the ASTM D1002 (eccentrically) and D3165 (concentrically) loaded single-lap-joint (SLJ) standards. The samples were macro and micro-roughness tested using a profilometer, with the nano surface roughness features classified and defined by scanning electron microscope. Both a smooth and NaTESI anodised vertical surface were also tested for bonding strength. Finally, a simulation was carried out to analyse the mechanics behind the SLJ failure, showing how the bonding could be improved.
The results showed that the vertical surface had the best micro-roughness and outperformed the epoxy’s rated strength by 11.2% (17.9 MPa) in the ASTM D3165 experiments. For the NaTESI anodised vertical samples, no changes occurred to the macro or micro surface roughness, yet the nano surface roughness increased significantly. This resulted in the strongest bond, which was 15% stronger than the epoxy’s rated strength, reaching 18.7 MPa. Further, the NaTESI bond failed through 100% cohesion failure of the epoxy adhesive. Therefore, a stronger adhesive would resist even more force. Further, the trabecular meshed surfaces created for this study had a high macro-roughness and porosity, with its struts having a similar micro-roughness to the vertical surfaces. Due to these characteristics, the bond was equivalent to the epoxy’s rated strength (15.6 MPa) in the D3165 experiments. The horizontal and smooth surfaces bonded poorly, with the samples reaching only 12.9 MPa and 3 MPa respectively, and both failing through 100% adhesion.
Overall, of the surfaces investigated, the surface with the highest fractal roughness was the most effective for epoxy adhesive bonding. It was proposed that this occurred through increased micro and nano surface roughness impeding the adhesive’s deformation motion at the interface. This would then stress relieve the interface, diverting the loading forces into the bulk of the epoxy adhesive. Additionally, the surface roughness increased the specific adhesion contact area between the epoxy polymer chains and the titanium substrate.
Finally, the simulation found that both the D1002 and D3165 SLJs began failing at the lapping edges due to peeling stresses; the lapping edges were identified as stress concentration points. The D3165 SLJ was exposed to less peeling stresses than the D1002 SLJ due to its loading acting concentrically through the bond line. For both the D1002 and D3165 SLJs, bending moments were present, but increasing the substrate thickness significantly reduced the principal and peeling stresses on the lapping edge, strengthening the joint.