Laser treatment of PBF-LB manufactured Ti6Al4V parts for structural adhesive bonding

Laser powder bed fusion (PBF-LB) processed Ti6Al4V parts exhibit inherent surface roughness features that are generally favorable for structural adhesive bonding. However, previous studies have shown that effective surface treatment of PBF-LB-Ti6Al4V is still crucial to prevent interfacial failure in adhesive bonds. In particular, laser treatment has demonstrated significant potential as a surface preparation process. Therefore, in this work, the effects of laser treatment on the adhesive bond performance of metal-metal joints were investigated for three different laser fluences. Surface characteristics were evaluated using scanning electron microscopy, confocal laser scanning microscopy and ion beam cross-section polishing. Laser-induced residual stresses in the titanium were measured by X-ray diffraction. The quasi-static bond strength was assessed for tensile loading via centrifugal adhesion testing and for mode I loading by double cantilever beam testing. Moreover, the surface treatment effects on environmental durability and fatigue performance were examined through single step-lap joint testing. It was found that the thickness and uniformity of the laser fluence-dependent nanoporous oxide layer are the decisive factors influencing quasi-static and fatigue adhesive bond performance. The laser fluence impact on the bond strengths was found to be significant, however, not for tensile loading. The overall best bond performance was achieved with the highest applied laser fluence, which produced a thicker and highly uniform oxide layer. Compared to the as-built surface state (i.e., degreased PBF-LB-surface), an increase of up to 52 % in tensile bond strength and around 600 % in mode I fracture toughness was achieved with laser treatment. Moreover, the tensile shear bond strength after hydrothermal aging was improved by up to 79 %, and the fatigue lifetimes were significantly enhanced as well.