Chemical bonding between laser welded aluminum and nylon-6.6

As concerns for climate change are growing, the automotive industry seeks lightweight structures and therefore has a raising interest in polymer/metal assemblies. Such assemblies also find applications in the biomedical field e.g. for implants. There are three ways to assemble such dissimilar materials: by mechanical fastening, adhesive bonding and welding. The mechanical option does not comply to the weight reduction objective, as well as it badly resists to corrosion. Adhesive bonding is doing better in weight reduction, but do not provide an optimum solution for biomedical applications, since adhesives are generally not biocompatible. Welding techniques, especially laser welding, are promising techniques. Indeed laser welding is fast, adaptable to complex geometries and solvent free, which are major assets for industrial applications [1]. Despite the great application potential, the root causes of the adhesion are not well understood. The adhesion theory proposes four mechanisms: mechanical interlocking, diffusion, chemical bonding, and electrostatic interactions. This work focuses on understanding chemical bonding at the interface of aluminum and polyamide-6.6, which are materials commonly used in automotive industry. Using a combination of ToF-SIMS and XPS, three types of samples were analyzed. First the interface of “industrial” like samples were analyzed. To access the interface, the assemblies were broken and polyamide residues on aluminum were dissolved. The results show that a very thin polymer layer remains in the joint area, since typical polyamide-6.6 ions NH-, CN-, CNO-, NH4+, CH4N+, CH2NO+ are detected. Some hybrid ions could also be identified, like CHNAl-, CHNOAl+/-, CH3NO2Al+/-, giving hints of a C-O-Al or a C-N-Al type of bonding. Secondly, the interface of “model” samples were analyzed. The aluminum surface was first mirror polished, then spin-coated with polyamide, welded from the opposite side of the aluminum plate and finally the polymer film was dissolved to access the interface. Here again polymer could be identified in the joint area, and the nature of the bond seems to be of C-O-Al nature, but C-N-Al bonds cannot be excluded. In a third step, N-methylformamide, a molecule similar to the reactive part of the nylon-6.6 molecule, was dip-coated on aluminum surfaces and directly analyzed. Results show that CHNAl+/- ions are originating from recombination effects, so that the only bond possible between both materials is of C-O-Al nature. Finally, a reaction mechanism involving aluminum hydroxide and the amide group of the polymer is proposed.

[1] Amancio-Filho ST, Dos Santos JF. Joining of Polymers and Polymer-Metal Hybrid Structures: Recent Developments and Trends. Polym Eng Sci 2009;47:21–5. doi:10.1002/pen.