Effects of laser processing crater array microstructure on the surface characteristics and bonding strength of Ti6Al4V adhesive joints

Abstract The strength of titanium alloy bonded joints is very important for aerospace applications. Usually, the substrate surface needs to be treated before bonding to improve the strength of the bonded joint. In this work, a nanosecond fiber laser was used to process the crater array microstructure on the substrate to improve the strength of the bonded joint formed by an epoxy adhesive and Ti6Al4V. The effects of energy fluence Ф and crater overlap rate δ on surface chemical composition, surface wettability, surface morphology, and shear strength were studied. The results showed that after laser treatment, Ti and V were uniformly distributed on the surface of the substrate, while Al, C, and O were unevenly distributed. The content of Al decreased with increasing Ф, whilst the content of C and O increased with increasing Ф. Regardless of the choice of Ф and δ, the surface of the substrate after laser treatment was super-hydrophilic, but the surface changed from super-hydrophilic to hydrophobic after 15 days in the air. Surface roughness, surface area, and bonded joint strength were not related to Ф, but closely related to δ. Compared with δ = 50%, when δ = 0, 25 or 75%, the strength of the interlocking structure formed by the adhesive and the substrate was greater, which made the bonding region more prone to cohesive failure. Compared with sanding, laser treatment can lead to a denser microstructure to increase the number of interlocking structures formed by the substrate and the adhesive. When Ф = 6.5 J/cm2 and δ = 75%, the shear strength had a maximum value of 29.2 MPa, which was 11.7% higher compared with sanding.