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. @en

This study aims at assessing the sensitivity of the ultrasonic welding process for joining epoxy- to thermoplastic-based composites sensitivity to the heating time. For that, carbon fibre (CF)/epoxy adherends with a co-cured PEI coupling layer were ultrasonically welded to CF/polyetheretherketone (PEEK) adherends at different heating times. Process-induced changes in the meso and microstructure of these welds were identified and correlated to the weld strength. Subsequently, a processing interval, i.e., a range of heating times resulting in less than 10% decrease of weld strength, was defined. As, expected, the dissimilar composite welded joints were more sensitive to the heating time than the CF/PEEK to CF/PEEK welded joints. However, this effect was less pronounced than expected, since a relatively wide processing interval could be obtained provided that the coupling layer had a sufficient thickness.

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This paper addresses the sensitivity of the ultrasonic welding process for joining dissimilar composites to variations in either the welding force or amplitude of vibrations. For that, carbon fibre (CF)/epoxy specimens were welded to CF/polyetheretherketone (PEEK) specimens, through a polyetheretherimide (PEI) coupling layer co-cured with the CF/epoxy material. It was found that reducing either the welding force or the amplitude of vibrations caused an increase in the heating time and maximum temperatures between the coupling layer and CF/epoxy adherend. In addition, local signs of thermal degradation were found in the CF/epoxy adherend even at welding conditions that resulted in the highest strength. However, such alterations were not significant enough to have an apparent effect on the maximum lap shear strength of the welded joints.

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Ultrasonic welding is a highly promising technique for joining thermoplastic to thermoset composites. A neat thermoplastic coupling layer is co-cured on the surface to be welded to make the thermoset composite ‘weldable’. A reliable bond is attained when miscible thermoplastic and thermoset materials are chosen. For welding carbon fibre/polyetheretherketone (PEEK) to thermoset composite samples, a PEEK film is not preferable due to its immiscibility with epoxy resins. On the other hand, polyetherimide is an excellent candidate, since it is known to be miscible to most epoxy systems at high temperatures and PEEK polymers. This study focusses on two main subjects; firstly, the nature of the material of the energy director, i.e. a flat thermoplastic film used to promote heat generation at the interface. In this case, the energy director can be either polyetherimide, as in the coupling layer or PEEK material, as in the matrix of the thermoplastic composite adherend. It was found that both materials can produce welds with similar mechanical performance. This study focusses secondly on the thickness of the coupling layer. Due to the high melting temperature of the PEEK matrix, a 60-µm-thick coupling layer was seemingly too thin to act as a thermal barrier for the epoxy resin for heating times long enough to produce fully welded joints. Such an issue was found to be overcome by increasing the thickness of the coupling layer to 250 µm, which resulted in high-strength welds.

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With its short heating times, ultrasonic welding is a highly promising technique for joining thermoplastic (TPC) to thermoset (TSC) composites, to prevent thermal degradation of the thermoset adherend. A neat thermoplastic coupling layer is co-cured on the surface to be welded to make the TSC “weldable”. For welding CF/PEEK to a TSC adherend, it would be logical to use PEEK as the coupling layer. However PEEK and epoxy are not miscible with each other, therefore a bond created after co-curing of these two materials is not reliable. PEI on the other hand is known to be miscible to most epoxy systems at high temperatures and PEEK polymers, hence it is an excellent candidate for the coupling layer material. The other necessary element for ultrasonic welding is the energy director (ED), a neat TP film placed at the interface to help promote heat generation through preferential frictional and viscoelastic heating. Usually EDs are made from the same material as the TP matrix, but in this case ED can be either PEI or PEEK. Mechanical testing and fractographic analysis showed that the usage of a PEEK ED is the most successful approach. This research is part of the European project EFFICOMP.

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In ultrasonic welding of thermoplastic composites an energy director (ED) (i.e. neat thermoplastic film), is used between the two adherends to be welded, to promote frictional and viscoelastic heating. For welding of thermoset composites (TSC), a thermoplastic coupling layer is co-cured on the surface to be welded as typical procedure to make the TSC “weldable”. This study focuses on investigating whether a polyetherimide (PEI) coupling layer by itself has the potential to promote heat generation during ultrasonic welding of CF/epoxy and CF/PEI samples, without the need for a separate ED, and if so, what thickness should that coupling layer be. The main findings were that welding without a loose ED resulted in overheating of the CF/PEI adherend and/or coupling layer due to the inability of the latter to promote heat generation efficiently. However, welding of CF/epoxy and CF/PEI samples with the use of a loose ED resulted in high-strength welds.

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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.

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