Chemical Bonds in Laser Welded Aluminum and Polyamide

Automotive industry is showing an increasing interest towards polymer/metal
assemblies, essentially in order to increase fuel efficiency through the reduction of car
body weight. In parallel, these assemblies are interesting for biomedical applications,
because of the potential to obtain improved or new properties e.g. for implants. Laser
welding is considered one of the most promising methods of joining dissimilar materials
because of its unique advantages; the process is fast, can be adapted to complex
geometries and is totally solvent free, which is a major asset for biomedical applications
[1]. A strong adhesion between polyamide 6.6 (PA-6.6) and aluminum (Al) plates was
obtained from laser welding in optimized conditions [2]. However, the root causes of
this adhesion are not yet understood. Several effects may come into play, such as
covalent binding, electrostatic binding, interdiffusion and mechanical interlocking. This
is further complicated in “real life” samples by the high roughness of the metal sheet,
the additives contained in the polymer, the Al and PA-6.6 surface composition and the
difficulty to reach the metal-polymer interface. In order to gain information on the
chemical binding between the oxydized Al surface and the PA-6.6, model samples were
prepared by spin coating ultrathin PA-6.6 films on polished Al (99.999 % purity) plates
(Fig. 1.). The two materials were subsequently welded by laser irradiation. FT-IR, XPS
and ToF-SIMS have been used to carry out this study. The interface was reached by
sputtering the polymer with low energy Cs+ ions in ToF-SIMS and Ar clusters GCIB in
XPS. In this preliminary study, ions binding the Al with polymer were identified in both
the positive (AlCH3O+, AlNH+, AlNH2
+, Al2NH+) AlCNO+, AlONH3
+) and negative
polarities, (AlN-, AlNO-, AlCO-, AlCNO-). Depth profiles and images near the interface
were obtained. Results obtained on our model samples will be compared with laser
joints obtained on “real” samples. Ultimately, this work aims at providing guidelines for
improving the mechanical resistance of the weld.