Improving bonding strength of aluminium-PEEK hybrid metal-polymer joints by two-step laser surface treatment

The present study investigates the adoption of laser surface treatments to improve the bonding strength of dissimilar material hybrid structures made by the thermomechanical joining process. Aluminium AA7075 alloy was joined to Polyether-ether-ketone through a friction-assisted joining process. For the surface treatment, a two-step laser treatment was adopted. The latter consists of a first phase, called laser texturing, where a deep square grid texture is obtained by adopting high pulse energy and repetitions, followed by a further laser treatment, called cleaning, carried out with lower energy content, and applied over the overall adhesion surface. Two textures characterised by different grid sizes (200 and 300 µm) were realised. As received, uncleaned, and only cleaned samples were adopted as reference treatments. The results indicated that laser cleaning performed after laser texturing enabled a significant improvement of adhesion irrespective of the previous laser texturing strategy. This was also confirmed by the higher amount of polymer trapped within the laser-texturized paths. The only laser cleaning treatment enabled the successful joining of the substrates with an average shear strength of 12 MPa. On the other hand, when texturing with a hatch distance of 200 μm, laser cleaning performed after texturing increased the shear strength from 18.4 MPa to 25.2 MPa corresponding to an increase of 37%. The increased adhesion between the PEEK and the aluminium was also indicated by the change in the failure mode of the joints. The texturized samples with a hatch distance of 200 μm failed by pull-out; when the laser cleaning was performed on these samples, they failed by cohesive failure.