Ultrasonic welding of epoxy- to thermoplastic-based composites

Welding is a promising alternative to mechanical fastening, as currently used, to join dissimilar (i.e., thermoset- to thermoplastic-based) composite parts in modern aircraft. Thermoset composites can be indirectly welded through a thermoplastic coupling layer co-cured on the surface of the laminate that needs to be welded. One of the main challenges when welding thermoset to thermoplastic composites, is the high welding temperatures that are needed to melt the thermoplastic matrix, especially when high-performance thermoplastic polymers are used such as in aerospace applications. The most efficient way to overcome this challenge is by ensuring very fast and localized heating in order to prevent thermal degradation mechanisms from occurring. Out of the currently most developed welding methods, ultrasonic welding can offer exceptionally short heating times of even less than 500 ms, which makes it an excellent candidate for joining thermoset and thermoplastic composites. However, further understanding of the process as applied to dissimilar composite joints is still lacking in order for it to be utilized in actual applications. The aim of this PhD thesis was to further the knowledge on ultrasonic welding of thermoset to thermoplastic composites by firstly identifying suitable practices for successfully welding the dissimilar composites and secondly assessing the robustness of the ultrasonic welding process with respect to changes in process parameters. The comparable strength of the welded, dissimilar composite joints to both co-cured, dissimilar composite joints and to welded, thermoplastic composite joints, demonstrated that ultrasonic welding is a very promising joining technique. Moreover, this process was proven to be robust (with respect to the variations in the heating time), since despite the sensitivity of the thermoset composite adherend to the high welding temperatures, a relatively wide processing interval, i.e., range of heating times that result in a certain mechanical performance, could be obtained. Additionally, the weld strength presented a certain degree of insensitivity to changes in the process parameters, i.e., welding force and amplitude of vibrations.