Water diffusion behaviour in the interface of laser-structured aluminium-epoxy adhesive joints

With the possibility of joining dissimilar materials with an areawide load distribution in the interface of the bonding partners, structural adhesive bonding enables the development of new lightweight solutions for the aerospace and automotive industry. Nevertheless, joining defects, an overall lower mechanical strength compared to other joining methods, e.g. bolts or welding and a general susceptibility to aging in moist environments still limit a widespread application of bonding in structural applications [1]. Surface pre-treatments, especially with pulsed lasers, can help to enhance the mechanical strength and aging resistance of metal joints bonded with thermoplastic or thermoset polymers [2, 3]. However, the mechanisms responsible for the enhanced joint performance are challenging to resolve. Laser structuring of AW 6082-T6 surfaces prior to adhesion can result in different surface structures (Figure 1) with similar chemical compositions but different mechanical performance [3]. The laser-treated surfaces also improve the aging characteristics of the joints, implying that either chemical interactions with laser pre treated surfaces are different or that the structured surface influences the diffusion characteristics of water at the interface. In order to determine the extent to which the surface structures influence the diffusion behaviour of water at the interface of AW 6082-T6 – E320 epoxy joints, an experiment based on µ-XRF analysis was designed and carried out. The laser pre-treated adhesive joints that were sealed off on all but one side were hydrothermally aged in a diluted solution of the chemical tracer SrBr2 in H2O at 80 °C. Cross-sections of aged joints with different laser pre-treatments were subsequently prepared and mapped with µ-XRF and the resulting diffusion lengths of strontium and bromine were determined. An analysis of these degradation experiments in the light of mechanical studies of the joint performances of differently pre-treated surfaces will be presented. References [1] R. A. Pethrick, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials, Design and Application, 2015, 229 (5), 349-379. [2] M. Löbbecke et al., International Journal of Adhesion and Adhesives, 2023, 120, 103282. [3] M. Irfan et al., International Journal of Adhesion and Adhesives, 2022, 119, 103271