Ultrasonic welding of thermoset to thermoplastic based composites: Effect of the process parameters on the weld strength

Thermoset composites can be made “weldable” by co-curing a neat thermoplastic coupling layer on the surface to be welded. With its exceptionally short heating times, ultrasonic welding is a highly promising technique for joining advanced thermoplastic (TPC) to thermoset (TSC) composites, since it can potentially prevent thermal degradation of the thermoset adherend. Previous research has shown successful results on ultrasonic welding of carbon fibre (CF)/epoxy and TPC using a PEI coupling layer. However, it is still not clear what the processing window is for ultrasonic welding of TSC to TPC, i.e. how much flexibility we have in choosing the process parameters while maintaining good weld strength. Hence, knowing the mechanisms that cause a reduction in weld strength is of great importance for the optimization of ultrasonic welding of dissimilar composites. On top of that, it is important to determine how the thickness of the coupling layer affects the span of the processing window, since precious research has shown that the thickness of the coupling layer affects the welding process and the weld strength. Thus, in this study, a processing window for welding CF/epoxy and CF/PEEK composites (i.e. hybrid welds) was defined and compared to the processing window for ultrasonic welding of CF/PEEK samples (i.e. reference welds). Samples were welded at different displacements of the sonotrode, also known as travel, and the mechanical performance of each batch was determined by single-lap shear tests. Moreover, the effect of the thickness of the coupling layer on the processing window was determined by investigating three different thicknesses. It was concluded that the processing window span is increasing with an increasing coupling layer thickness. Furthermore, the processing window of the hybrid welds is narrower than the processing window of the reference welds.