The use of plastic-metal hybrid composites is steadily increasing in various industrial manufacturing applications as they offer beneficial functionalities such as high load-bearing capacity, electrical and thermal insulation, vibration damping as well as enhanced design options. In this context, the combination of high-alloy steel and polyamides is of particular interest. However, this pairing poses specific challenges for both the resulting composite as well as the applied joining process, given their distinct chemical and physical properties. In this context, application of reactive multilayer systems (RMS) inserted directly into the joining zone offers particular advantages as the underlying reaction mechanism of so called self-propagating high-temperature synthesis (SHS) provides rapid exothermic reaction propagation with well-known exothermic heat of reaction. Short term and local application of thermal energy significantly reduces the requirements for welding temperature, pressure and time. Therefore, thermal load on the joining partners, especially the more sensitive plastic, is reduced to a minimum. The basic suitability of RMS of the Al/Ni type for joining hybrid assemblies of polyamide 6 and austenitic stainless steel has already been demonstrated in previous work, where the expected temperature-time regime and a resulting heat-affected zone were estimated [1,2]. However, the use of reactive multilayers as an energy carrier directly in the joining zone also has the disadvantage that the reacted foil remains within the composite after joining and must be regarded not as a barrier layer or contaminant but as an additional bonding partner.
Within this work, joining of hybrid overlap composites between semi-crystalline polyamide 6 and pre-structured austenitic stainless steel X5CrNi18 10 (1.4301) by means of reactive Al/Ni multilayer foils of the type Indium NanoFoil® is investigated. Main objective is the metal surface structuring as this is responsible for a local fixation of the reacting multilayer, which counteracts any shrinkage effects that occur and enables cracking of the foil interface. Resulting foil openings have a beneficial effect on the resulting composite quality, as they promote additional path for material wetting of the metal surface. Through combination of structure geometry, structure orientation, structure density and applied joining pressure, a regular crack pattern and therefore, opening of the foil at predetermined positions is obtained. Resulting joints are examined with respect to joint strength, the characterization of failure behaviour at the fracture interface under shear tensile stress and the occurring bonding mechanisms in the composite. Performed investigations allow to derive a model approach that describes the multilayer behaviour during the reaction and afterwards cooling phase.

This study is supported by the Deutsche Forschungsgemeinschaft (DFG grants DFG BE3198/7 1 (project: 426339810) and Scha 632/29 (project: 426206394)). Support by the Center of Micro- and Nanotechnologies (ZMN), a DFG-funded core facility (DFG Scha 632/27 “DFG-Gerätezentrum”) of the TU Ilmenau, is gratefully acknowledged.