Composite are now widely used in the aircraft industry. The use of light-weight, high-temperature resistant composite materials will allow for aircraft to have high-performance and economical designs. The composites used in the aircraft industry can be classified into two types: Thermosets and thermoplastics. Thermosets are polymers that are obtained by curing, where there are chemical cross-links between the polymer chains. Wings, fuselages, and bulkheads are some applications in aerospace which are made of thermoset composites. The interest in and the application of thermoplastic composites is increasing in the aerospace industry due to their advantages over thermoset composites. They have the advantage that they can be re-molten and reshaped upon heating, that they are recyclable, and have a high material toughness. The first advantage can lead to major cost-benefits since it makes efficient forming and welding techniques possible. As a result, different welding techniques have been developed for joining thermoplastic composite structures such as resistance, induction, and ultrasonic welding. Ultrasonic welding is generally the fastest and most energy-efficient welding technique.

This thesis explores the continuous ultrasonic welding (CUW) of unidirectional carbon fiber-reinforced thermoplastic composites, with a focus on the impact of adherend thickness on the welding process and joint quality. As the demand for lightweight, high-performance materials in industries such as aerospace and automotive continues to rise, efficient and scalable welding techniques for thermoplastic composites are becoming essential. CUW presents a promising method for joining large composite structures, offering speed and energy efficiency advantages.

Through experimental investigations, this study evaluates the effects of increasing adherend thickness on defects such as overheating and fiber squeeze-out. Two different clamping setups, the bar clamp and picture frame jigs, were used to assess their impact on weld quality. It was concluded that the welds made on the bar-clamped jig overall had a much higher energy requirement compared to those made on the picture frame jigs. The picture frame jig, due to its lack of constraints on the thick adherends, resulted in much better parallelism throughout the process. The welds made using the picture frame jig also experienced less overheating at the interface compared to the bar-clamped jig. The resulting weld uniformity and strength were also higher when using this jig. However, one issue with the picture frame jig was the extent of top surface scraping, which was found to be prevalent regardless of the weld speed. The changes in process parameters were also studied. Contrary to findings from static welds, increasing the welding force resulted in through-the-thickness heating when the thickest adherend was welded and did not have much effect on the weld strength of the thickest adherend on the picture frame jig. The findings from this thesis result in the need for further research to mitigate the effects of the thicker adherends.

The effect of creating parallel welds in a single overlap using continuous ultrasonic welding on the weld quality has been investigated, with the goal of creating three high-quality parallel welds. To achieve this, multiple aspects were examined to understand the creation of high-quality parallel welds. The weld quality is highly influenced by a change in boundary conditions. The number of welds, the order of welding, and the energy director strategy all change the boundary conditions. Increasing the number of parallel welds will make it more difficult to create high-quality welds. The order of welding has a large effect on the quality of the welds. The order of welding will determine if a low- or high-quality weld is created with constant welding parameters. Two energy director strategies were compared: a ‘full overlap energy director’ and ‘energy director strips’. Both can create high-quality welds, but energy director strips create an overall better-quality weld.

The goal of the project was to produce slotted single-lap shear specimens using continuous ultrasonic welding, which had not been done before. These aim to provide more reliable lap shear strengths than currently used coupons due to increased assembly stiffness and eliminated squeeze-out. By increasing the weld width and stacking smaller rotated adherends, fully welded coupons could be extracted and subsequently slotted. However, high-quality welds were not yet obtained consistently due to pre-heating of the adherends, resulting in overheating during about the second half of these welds. The produced slotted coupons showed the viability of using this configuration, the best achieving 1% under-welding and 90% of the co-consolidated slotted coupon reference lap shear strength, despite not being fully optimized. However, slot precision is essential. In contrast, welded conventional coupons likely yield non-conservative strengths, reaching 109% of the reference, possibly due to the squeeze-out and adherend geometrical distortions resulting from welding.

Research and development of thermoplastic composites has been ever increasing owing to the advantages brought by it over conventional materials such as metals and thermoset composites. Higher processing rates, automation, the possibility to recycle, weldability and better mechanical and chemical properties are some of them. Fusion bonding of TPCs that involves application of heat and pressure at the localised bonding area promises faster assembly times and weight reduction and is thus attractive to the aerospace industry. When comparing different welding techniques, ultrasonic welding has the lowest welding times and energy consumption and also overcomes certain challenges that come with the other techniques. The ultrasonic welding process has been a part of the plastic industry for decades, and with time, the research has also shown promising results in welding advanced thermoplastic composites. Heat generation in this mechanical/frictional welding technique occurs due to surface friction and viscoelastic heating caused due to high frequency and low amplitude vibrations. The process creates quality welds in seconds and can be controlled in-situ, ensuring robustness and repeatability. Scaling up the static process to a continuous process enables continuous sealed welds having a more uniform distribution of load and higher load-carrying capability. However, the continuous process brings challenges. The state of the art continuous ultrasonic welding (CUW) equipment at TU Delft has demonstrated quality welds for CF/PPS fabric laminates. However, problems like excessive through the thickness heating (TTH) and deconsolidation of welds need to be understood and overcome.

As the industry moves toward welding larger and more complex composites structures such as stringers, frames and skin joints, the CUW process becomes more attractive. The primary thermoplastic structures would involve the use of unidirectional materials. The material that has been proposed for the future structures is the Carbon fiber/ Low Melt - PolyArylEtherKetone composite. LM-PAEK polymer exhibits comparable properties to the other advanced thermoplastic materials like PEEK and PEKK but has a lower processing temperature enabling faster processing. To the author's knowledge, no literature is currently available related to CUW of this material. It was believed that when welding UD / LM-PAEK, the problem of excessive through the thickness heating might be present, leading to fibre and polymer squeeze-out and porosity in the adherends and the need for longer consolidation time. Therefore, this research was focused on investigating the extent of TTH in the ultrasonic welding of this UD material and the solutions to mitigate the effects of TTH on the welds. Static and continuous welding experiments were conducted, and temperature readings were obtained from the adherends and the interface. Along with this, different characterisation techniques such as lap shear tests, fractography and cross-sectional microscopy were used to observe the effects of TTH.

CUW showed overall higher TTH. It was observed that changing the fibre architecture from fabric to UD exacerbates the effects of excessive TTH in the adherends. Also, the change in the boundary conditions from that of a static weld to that of a spot or a continuous weld brings in the differences in the viscoelastic bulk heat generation in the top adherend. The cause of this difference is yet to be understood. Changing the process parameters and shape of the sonotrode helped in mitigating the overall TTH in the CUW. The use of a round sonotrode helped in reducing the preheating during welding and the lower welding speeds used, brought the advantage of consolidating the welds for longer times. In conclusion, it was found that changes in the welding and consolidation parameters, and the equipment, can help mitigate the effects of excessive TTH on the adherends. Overcoming the problem of excessive TTH is one of the first steps toward enabling CUW of UD/LM-PAEK composites for future applications.

Thermoplastic composites are gaining prominence in the aerospace industry owing to their higher damage tolerance, cost-efficient means of manufacturing, and the possibility to be recycled. One of the major benefits of thermoplastics compared to thermosets is their ability to be welded, and one of the most promising welding techniques for thermoplastic composites is ultrasonic welding. Ultrasonic welding is the fastest welding technique currently known, with typical weld times of a few hundred milliseconds.

Studies have been conducted in plenty regarding static ultrasonic welding of thermoplastic composites. But the industrialisation of the process involves the development of a robust continuous ultrasonic welding process which can weld the entire span of the joints, thus enabling higher load transfer and reduced stress concentrations. However, the state-of-the-art continuous ultrasonic welded joints contain voids at various locations within the weld which are assumed to appear due to a lack of consolidation during the welding process. Unlike the static ultrasonic welding, where the sonotrode can both transfer the vibrational energy to the adherends being welded and provide consolidation force, the continuous ultrasonic welding requires a separate consolidation device to provide consolidation pressure application. This makes it necessary to expand the understanding of the consolidation process to improve the weld quality and increase the Technology Readiness Level (TRL) of the ultrasonic welding process before it can be industrially used. While a lot of research to date focused on the vibration phase of the process, not much information is available regarding the consolidation phase. This research project thus explores the effect of consolidation pressure and time on (de)consolidation in ultrasonic welding of thermoplastic composites.

An experimental study was carried out on the consolidation in static welding of CF/PPS test coupons, and the knowledge obtained was extended to the continuous ultrasonic welding process. The consolidation in the continuous ultrasonic welding process was provided by a separate consolidation device or ”consolidator” placed behind the sonotrode. Various characterisation techniques including lap shear strength, void content assessment and fracture surface analysis were used to analyse the results obtained. The experiments revealed that for semi-crystalline polymer PPS, consolidation should start when the polymer is in its melt state and extend until the interface temperature of the weld drops below the crystallisation temperature of the polymer. The results obtained indicated that the voids in ultrasonic welding were formed due to a combination of shrinkage due to crystallisation, fibre decompaction, the choice of the clamps used and excessive squeeze out of the resin. In continuous ultrasonic welding, the location of the consolidator behind the sonotrode and the consolidation pressure was found to influence the weld quality. The research conclusions serve as a first step towards developing a robust consolidation process in continuous ultrasonic welding of thermoplastic composites.