Chemical Bonds between Laser Welded Aluminum and Polyamide?

Hybrid assemblies, like polymer/metal systems, are more and more used for applications in automotive or biomedical industries. Indeed, such assemblies are efficient ways to get lightweight structures, increasing fuel efficiency. They also open a wide range of design opportunities for biomedical applications. The main issue with hybrid systems is their assembly. Different ways of assembling polymers and metals are reported and present many advantages and drawbacks. Among them, laser welding is a promising method, thanks to its fast process, freedom of design and absence of adhesives. Even if there are good achievements with laser welding of hybrid materials, the adhesion mechanisms involved are still unknown. This work aims at getting a deep knowledge on the chemical bonding between a polymer, polyamide-6.6, and a metal, aluminium (Al), after laser welding. Therefore, a study based on a combination of ToF-SIMS and XPS has been successfully carried out. First, model samples were prepared by spin-coating polyamide-6.6 on mirror polished aluminium plates. The two materials were then laser welded, and finally the polyamide-6.6 was dissolved so that the polymer remaining on the surface is strongly (chemically) bonded to the Al, which then could be analysed by ToF-SIMS. A principal component analysis in both positive and negative mode was performed in order to understand chemical modifications brought about by the laser treatment. Different hybrid ions, containing fragments from the polymer bound to Al, were detected, like C2H2OAl+, C2OAl+ or COAl-, AlCONH-, but also characteristic ions of both materials like Al+ and AlO- for the aluminium, and CH4N+, CH2N+ and CNO-, NH- for the polyamide. Chemical binding already occurs on the samples prior to welding, but the results show that hybrid ions are more intense after welding, suggesting that more bonds are created by the laser heat treatment. The results were confirmed by XPS by carefully studying the high resolution spectra of the carbon 1s. Finally, the same approach was applied to real assemblies (thick Al and polyamide plates joined by laser). The interface Al/polyamide was exposed by breaking the assembly followed by dissolution of the remaining polymer. Again, hybrid ions were detected by ToF-SIMS inside the junction area, providing some hints on the binding chemistry.