Crater array generated on Al alloy surface by nanosecond laser: Surface characteristics and bonding strength

Abstract

The strength of aluminium (Al) alloy adhesive joints is critical for aerospace and automotive applications. This work uses an infrared nanosecond laser to create a crater array (CA) and thereby modify the substrate bonding area and enhance the strength of 7075-T6 Al alloy joints bonded by epoxy resin adhesives. We study how the parametrisation (crater overlap ratio β and pulse energy density Ф) of the CA affects the surface texture features (surface roughness Sa and fractional increase Sdr in surface area), wettability, chemical properties, and shear strength τ. The mechanism by which CA affects τ is revealed by the surface characteristics and failure modes. Sandblasted specimens were used as a reference. The results show that, for laser treatment, Sdr, Sa, and τ are closely related to β but not to Ф. When β = 30 % or 60 %, the CA surface is superhydrophilic. Regardless of β and Ф, the substrate absorbs carbon-containing pollutants from the air, so that the contact angle of the CA surface gradually increases over time until, 15 days after the laser treatment, the CA surface becomes hydrophobic. Zn, Cu, and Fe are uniformly distributed over the alloy surface, whereas C, O, Mg, and Al are unevenly distributed. The C content decreases with increasing β but is not correlated with Φ. The O content increases with the increasing β and Φ. When β = 30 %, the area of cohesive failure in the substrate bonding area is maximal. When β ≤ 0, the untreated area outside the craters is susceptible to adhesive failure. When β ≥ 60 %, the substrate bonding area is covered by a thin layer formed by molten residue, which is susceptible to substrate failure. Laser treatment is more effective than sandblasting in modifying the texture, wettability, and chemical properties of the bonding area, which can further improve the bonding strength. The maximum strength of the bonded joint obtained by laser treatment is 26.89 MPa at Φ = 21.6 J/cm2 and β = 30 %, which is 16.2 % stronger than sandblasting.