The utilisation of composite materials has the potential to play a vital role in the development of lightweight structures for future generations of aircraft, with the objective to reduce emissions. Ultrasonic welding is a process that has been proven to exhibit advantageous qualities, including the capacity to achieve welds with a comparatively short process time. Furthermore, its capacity to function as both a static and a continuous process makes it a viable candidate for facilitating the realisation of this objective. The present study investigates the potential of a novel explicit modelling approach for the static ultrasonic welding process to more accurately represent the welding process by incorporating a more precise representation of the hammering effect. The hammering effect describes the partial loss of contact between the sonotrode and the upper adherend. The model’s validation was achieved through a multifaceted approach that incorporates high-speed camera recording, encompassing digital image correlation, laser displacement sensor measurements, and static ultrasonic welding experiments. These experiments encompassed varying welding times, followed by fracture surface analysis. The findings showed that an explicit time-domain model can effectively represent the static welding process of unidirectional materials utilising a film energy director. The experimental validation demonstrated a high degree of correlation between the thermal behaviour of the welding interface and the simulation results. The study demonstrated that the neutral position of the sonotrode exhibited an increase during the initial phase of the welding process due to dynamic stresses. This phenomenon enables reduced constraint movement of the adherends and the energy director, which results in the disconnection of the sonotrode from both the upper adherend and the energy director, as well as the adherends and the anvil. The higher neutral position of the sonotrode was then implemented in an explicit simulation of the static ultrasonic welding process.

Sequential ultrasonic spot welding is an interesting joining method for overlapping thermoplastic composite structures. In the framework of the EU Clean Aviation Multi-functional Fuselage Demonstrator (MFFD) and the lower shell SmarT multifunctional and INteGrated TP fuselage (STUNNING) projects, SAM XL and TU Delft Aerospace Engineering collaboratively developed and demonstrated a robot-based sequential ultrasonic spot welding process for the sub-assembly of structural frames and clips in a fuselage section demonstrator. This full-scale thermoplastic composite fuselage section demonstrator, which was recently awarded the 2025 JEC Innovation award, measures 8.0 m in length and 4.0 m in diameter. Our robot-based sequential ultrasonic spot welding technology played an important role in ensuring the joining of structural clips and frames in the stiffened fuselage skin of the demonstrator, through the use of more than 1600 spot welded joints with an average welding time of approximately 10 s per spot, thereby significantly reducing cycle times as compared to traditional joining methods such as fastening or riveting. This paper provides a comprehensive overview of the technology development process and highlights the results achieved during the sub-assembly of the demonstrator, as well as the challenges encountered.

Structural reuse is a promising alternative to recycling of composite materials; it preserves material composition while liberating the materials for reuse in secondary applications. Thermoplastic reinforced composite materials have the potential to expand reuse opportunities by adapting their shape, or reversing them to a laminate blank. In this study, we evaluated reverse forming of glass fibre-polypropylene (GF-PP) laminates by developing a processing method, testing material properties and the effect of three design parameters: forming strain, laminate architecture and material type. Forming strain relates to the deformation mechanism of inter-ply slip, and is imposed through varying the contour depth and bending radius. Laminate architecture relates to resin redistribution, and is imposed by using an orthogonal as well as quasi isotropic layup. Finally, the material type affects both Inter-ply slip as well as resin redistribution, and is imposed by using plain and twill weaves. GF-PP blanks were prepared using a heated platen press and subsequently formed and flattened using convection heating (<165 °C) and vacuum pressure in a novel moulding process. The samples had typical values for flexural strength of 91 - 113 MPa and flexural modulus of 9–16 GPa. Using a Design of Experiments analysis the process was deemed robust for the given boundary conditions. These results demonstrate the feasibility of reverse forming for cases where inter-ply slip is the governing deformation mechanism. The presented reverse forming process and design parameters can be used to create new thermoplastic composite parts, anticipating for structural reuse through reverse forming.

Our aim with this work was to evaluate how the thermoplastic resin used in the composite adherends and on the energy director affected the static ultrasonic welding process in both parallel and misaligned configurations. Polyetheretherketone (PEEK) and low-melt polyaryletherketone (LMPAEK) were the resins used and their thermomechanical properties were characterized via dynamic-mechanical analysis and modulated differential scanning calorimetry. With parallel adherends, neither the welding time nor the through-thickness heating in the adherends vary significantly. This similarity was attributed to a larger heat capacity of the PEEK energy director counterbalancing its higher viscoelastic heating rate. With misaligned adherends, the welding time was larger for PEEK welds than for LMPAEK welds and LMPAEK adherends presented a larger though-thickness heating. These effects were attributed to the larger bulk viscoelastic heating rate of carbon fibre reinforced/LMPAEK adherends adding up to the lower heat capacity of LMPAEK.

In the framework of CleanSky2 (CS2)-Large Passenger Aircraft project, a consortium led by Airbus consisting of universities, research centers and industrial partners across Europe investigated various fuselage system, cabin & airframe technologies for commercial aircraft. The goal has been to strongly contribute to the environmental objectives of 20-30% CO2 reduction whereas at the same time performance increase through weight reduction and improved competitiveness through less recurring cost and industrial ramp up capabilities had to be demonstrated. In total, more than 40 different technologies have been further developed by the European consortium consisting of 14 partners in order to finally deliver an 8m large scale flagship in 2024: the Multifunctional Fuselage Demonstrator. Novel fuselage design and built concepts are demonstrated as well as thermoplastic composite material testing & modelling, elementary parts manufacture and assembly for large scale fuselage structures incl. “zero airframe customization” concepts for cabin installation. Final results and major achievements of this European funded program will be presented and discussed in this paper.

Ultrasonic welding of fibre-reinforced thermoplastics is a joining technology with high potential for short welding times and low energy consumption. While the majority of the current studies on continuous ultrasonic welding have so far focused on woven reinforcements, unidirectional materials are preferred for highly loaded aerospace components due to their better mechanical performance. Therefore, this paper investigates the influence and interdependence of the welding speed, amplitude, and energy director thickness on the weld quality of adherends made of unidirectional composites. The quality of the welded joints is assessed by a single-lap shear strength and fracture surface analysis complemented by the microscopic analysis of cross-sections and comparison to a co-consolidated reference. The results showed that the welding process is highly affected by changing welding speeds for a given amplitude. Furthermore, while lower amplitudes lead to significant scatter in the welding quality, higher amplitudes result in increased heating rates and a fully molten energy director even for high welding speeds. Nevertheless, insufficient consolidation at high welding speeds results in porosity in the weld line. Finally, it was observed that thicker, and therefore more compliant, energy directors lead to more uniform melting of the energy director and less deviation in the weld quality for a wider range of welding speeds.

This paper evaluates the potential of changing the welding force and the compliance of the energy director (ED) to reduce the effects caused by misaligned adherends, which were: increased through-thickness heating, reduced size of welded area and increased heating time. In the methodology that was followed, we welded adherends misaligned by approximately 4.5° in different scenarios: with higher welding force; with increased ED compliance by the use of a thicker ED and; with increased ED compliance by the use of a discontinuous ED. The most significant reduction of the effects caused by misaligned adherends was obtained when combining the use of both increased welding force and discontinuous ED. Such improvement derives from the imposed parallelism caused by the use of a higher welding force and from a more efficient concentration of heat generation at the weld line that occurs when a discontinuous ED is used.

Continuous ultrasonic welding is an attractive welding technique for thermoplastic composite structures. In this process, a metallic sonotrode connected to a piezoelectric transducer and to a press moves along the parts to be welded applying ultrasonic vibrations and a static welding force on the welding overlap. Thus far, the research carried out on this topic makes use of sonotrodes featuring a flat contact surface with the parts to be welded, which limits the use of the process to the welding of overlaps with no curvature in the welding direction. With the final aim of assessing whether this process can also be applied to curved structures, this paper explores the feasibility of using a rounded sonotrode for continuous ultrasonic welding of thermoplastic composites. The main conclusions drawn from the results obtained in this research is that it is indeed possible to continuously weld thermoplastic composite panels with a rounded sonotrode and that high-quality welds can be obtained from such a process. Furthermore, the use of a rounded sonotrode has the positive effect of lowering the temperatures at the welding interface as well as the temperatures within the adherends. On the other hand, the use of such sonotrode leads to a decreased, although still competitive, welding speed and, potentially, an increased welding force, thereby setting boundary conditions that need to be considered for each specific application.

Ultrasonic welding is a fast and promising joining technique for thermoplastic composite parts. Understanding how changing the part thickness affects the process is crucial to its future upscaling and industrialization. This article presents an initial insight into the effect of the adherend’s thickness on the near-field ultrasonic welding of CF/LMPAEK thermoplastic composites. Different thicknesses of the top and bottom adherend were welded and analyzed using the output data of the welding equipment, temperature measurements, and other visual characterization techniques. Increasing the thickness of both the top and the bottom adherends showed to increase the power consumed during welding. An overshoot in the power needed at the onset of the welding process for increased thickness of the top adherend precluded welding beyond a threshold thickness of 4.72 mm. In the case of the thicker top adherends, there was also melting of the energy director and early fiber squeeze-out within the top adherend as a result of increased bulk heating. Increased bulk heating was hypothesized to be caused by increased hammering, as indicated by the amplitude readings for thicker adherends. Welding with a higher force, which is known to reduce hammering, corroborated this hypothesis as fiber squeeze-out within the top adherend was not observed. It is believed that hammering contributes to heating by causing an oscillatory impact excitation that is close to the natural frequencies of the system, which would result in amplification of the cyclic strain and subsequent increase in the viscoelastic heating in the adherend.

In thermoplastic composite joints with a circular ultrasonic spot weld, the damage growth is located at the interface between the joined components. This means that any damage in these joints is invisible from the outside. This experimental study compares three different in-situ methods to measure the damage growth indirectly. The motivation lies in the potential benefits of using multi-spot welded joints for increased damage tolerance and the need to prove the damage arresting and damage progression behaviour for certification. The current study focused on measuring the damage in single-spot welded Single Lap Shear (SLS) joints during fatigue test with the global specimen compliance, the local out-of-plane displacement and the local strain on the surface. Digital Image Correlation (DIC) measurements were used to obtain the local quantities. The results showed that the local quantities are better suited to obtain detailed information on the damage state. The local surface strain showed more distinct features that facilitate the recognition of damage locations. The benefits and challenges of all methods are discussed as well as the difference in the level of detail, generalization and the prior knowledge necessary to obtain the damage state.

The need for robust joining methods for thermoplastic composites increases since the usage of these materials expands steadily in the aerospace industry. Continuous ultrasonic welding has been demonstrated in the recent years as one of the most promising joining techniques for thermoplastic composites to fulfill this need. This paper presents our state-of-the-art research conducted on continuous ultrasonic welding and aims to provide insight into the future steps needed to obtain an industrialized robotic welding process.

Continuous ultrasonic welding is a promising high-speed and energy-efficient joining technique for thermoplastic composite structures. However, in the current state-of-the-art research on the topic numerous deconsolidation voids could be identified at the welding interface, which results in a strength knock-down. The aim of this study is, therefore, to improve the quality of continuous ultrasonically welded joints by adding a consolidation shoe to the welding setup. To determine the required consolidation pressure, the size of the shoe, and its distance from the sonotrode a stepwise approach was followed based on the static ultrasonic welding process. The closest consolidation distance, best representing the static welding conditions, did not improve the weld quality as significant porosity was still found in the weld line and in the adherends. However, for the furthest consolidation distance high-quality continuous welds were obtained with almost no porosity and a high strength.

Ultrasonic welding is a promising joining technique for thermoplastic composite parts. On the way towards upscaling and industrialisation of this technology, it is crucial to understanding how it is affected by manufacturing tolerances. The objective of the research presented in this paper was to investigate the influence that misaligned adherends have on static ultrasonic welding of thermoplastic composites. Different angles were created by changing the clamping configuration. The results showed that increasing the angle between adherends decreases the power consumed and increases the process time while decreasing the weld uniformity and increasing the risk of overheating. These effects were associated to the impact of the different clamping configurations on the cyclic strain in the energy director and adherends during the welding process. Decreasing the top adherends bending stiffness by increasing the clamping distance was found to significantly reduce the adverse effects of adherend misalignment on weld quality.

Multi-spot sequential ultrasonic welding is a promising joining technique for thermoplastic composites in an overlap configuration. In the framework of the EU Clean Sky 2 Multi-functional fuselage demonstrator (MFFD) and the lower shell SmarT multifUNctioNal and INteGrated TP fuselage (STUNNING) projects, a robot-based sequential ultrasonic spot welding process has been developed for joining of large thermoplastic structural components, based on process development steps reported in previous work [1]. The technology is being demonstrated on a full-scale thermoplastic composite fuselage section of 8 m length and 4 m shell radius. The fuselage skin is being joined to longitudinal stringers and circumferential frames through welded clips, in the lower shell of the fuselage demonstrator. This paper presents an overview of the robotic sequential welding technology developed at SAM|XL in collaboration with Delft University of Technology and the ongoing sub-assembly process of the fuselage demonstrator, led by GKN Aerospace.

Ultrasonic spot welding is a joining technique for thermoplastic composites with great potential regarding processing speed and cost. To investigate the damage tolerance and possible inherent damage arresting behavior of multi-spot welded joints, a technique is necessary to measure damage growth in the joints under cyclic loading. Visual inspection is not possible because the damage is not located on the outside surface and conventional techniques such as C-scan are not practical during a fatigue test because the specimen would have to be removed from the setup. This paper details a methodology for quantifying damage growth rates in singlespot welded joints using surface strain measurements made by Digital Image Correlation. This represents the first step towards developing a methodology for quantifying damage progression behavior in complex multi-spot welded joints.

Continuous ultrasonic welding (CUW) is one of the most efficient integration methods of thermoplastic composites. Researchers from our group have already utilized CUW method by using both frame based and robotic welding platforms to achieve sufficient amount of joint strength for aerospace applications [1-3]. On the other hand, the implementation of this method into an industrial manufacturing process still requires the ability of consistent and high-quality welding. It is obvious that a sophisticated monitoring system, which is developed for ensuring the highest-level of weld quality, will play a key role to transform CUW method into a commonly relied on industrial tool. Typically, experimental techniques like micrograph, mechanical testing and fracture surface examination are used to determine the performance of a welded composite joint, which provide the opportunity of correlating the process parameters and other process data with the welded joint performance. Within the context of this study, several experiments are conducted using the welded composite plates by robotic CUW system. It’s seen that the overall weld-line thickness, among other parameters, indicates a remarkable correlation with lap shear strength of welded joints. The results show that for weld line thicknesses above the original energy director thickness, considerable voids can be found in the weld interface. On the other hand, samples with a lower thickness than the original energy director show less voids and improved lap shear performance. After evaluating the aforementioned experimental outcomes, a weld monitoring system is designed for continuous measurement of weld line thickness for in-situ monitoring purposes. Weld monitoring system is built on a frame based continuous ultrasonic welding platform as seen in Figure 1, where the laser sensors are utilized to perform very precise thickness distribution analysis along the weld line during the continuous monitoring applications. A specialized python code is created to analyze the raw sensor data for monitoring purposes. Different monitoring system iterations and python code pairs tested and compared to achieve the most accurate monitoring experience. Results indicate that laser sensor based monitoring system provides very sensitive weld line thickness measurements, which can be related to the weld quality for industrial applications.

Ultrasonic welding is a promising technology to join fibre-reinforced thermoplastic composites. While current studies are mostly limited to fabric materials the applicability to unidirectional materials, as found in aerospace structures, would offer opportunities for joining primary aircraft structures. However, due to the highly anisotropic flow of a molten unidirectional ply undesired squeeze flow phenomena can occur at the edges of the weld overlap. This paper investigates how the fibre orientation in the plies adjacent to the weld line influences the welding process and the appearance of edge defects. Ultrasonic welding experiments with different layups and energy director configurations were carried out while monitoring temperatures at different locations inside and outside the weld overlap. The joints were characterized by single lap shear tests, analysis of corresponding fracture surfaces and microscopic cross-sections. Results showed that the anisotropic flow and the anisotropic thermal conductivity of the plies adjacent to the weld line have a distinct effect on the appearance and location of edge defects. By using energy directors that cover only part of the weld overlap area a new approach was developed to mitigate edge defects caused by the highly directional properties of the unidirectional plies.

Continuous ultrasonic welding is a promising technique for joining thermoplastic composites structures together. The aim of this study was to gain further insight into what causes higher through-the-thickness heating in continuous ultrasonic welding of thermoplastic composites as compared to the static process. Thermocouples were used to measure temperature evolutions at the welding interface and within the adherends. To understand the mechanisms causing the observed temperature behaviours, the results were compared to temperature measurements from an equivalent static welding process and to the predictions from a simplified heat transfer model. Despite the significantly higher temperatures measured at the welding interface for the continuous process, viscoelastic bulk heat generation and not thermal conduction from the interface was identified as the main cause of higher through-the-thickness heating in the top adherend. Interestingly the top adherend seemed to absorb most of the vibrational energy in the continuous process as opposed to a more balanced energy share between the top and bottom adherend in the static process. Finally, the higher temperatures at the welding interface in continuous ultrasonic welding were attributed to pre-heating of the energy director due to the vibrations being transmitted downstream of the sonotrode, to reduced squeeze-flow of energy director due to the larger adherend size, and to heat flux originating downstream as the welding process continues.

Multi-spot sequential ultrasonic welding is a promising joining technique for fibre-reinforced thermoplastic composites structures (TPC). In existing research on the multi-spot sequential ultrasonic welding process, welds are produced through the use of a static table-top welding machine, at a coupon level. However, in order to apply this joining technology to large structures, the welding process needs to be up-scaled through the use of a robotic platform.
At the Smart Advanced Manufacturing (SAM|XL) automation field lab and TU Delft Aerospace Engineering, a robotic sequential ultrasonic welding system has been developed. The system consists of a welding end-effector (EEF) equipped with various sensors that enable online process monitoring and control, which can be mounted on an industrial robot arm to perform sequential multi-spot welds. The goal of this study was to assess the welding performance of the ultrasonic welding EEF, which was mounted on an industrial KUKA KR210 R2700 Extra 10-axis robot arm, by comparing it to the performance of welds produced through the static table-top machine.
In this study, single and multi-spot welds were produced on thermoplastic composite coupons, based on welding conditions which were defined in a preliminary study. The robot and EEF deflections observed during the welding process were analysed to assess the deviation of the robotic process from the static one. The feedback obtained from the welding equipment in terms of consumed power and tool displacement in both processes was also compared. The weld quality was assessed though single lap shear testing of the welded joints as well as fractography of the failure surface. The results of this study indicate that the developed robotic welding process is quite robust and is capable of producing high-quality sequential welded joints despite significant system deflections observed during the welding process. Slightly lower welded area and weld strength was obtained which can be attributed to the system deflections. Finally, the results indicate that the use of a stiffer robotic platform as well as a stiffer EEF construction will result in better system rigidity and weld spot positioning accuracy, and through this the welding process shows promise for large-scale industrial applications.

Ultrasonic welding is a versatile welding technique for the assembly of thermoplastic composite structures. The research in this paper aims at improving the manufacturing efficiency and the weld attributes, featured by welded area, weld strength and failure mechanisms, of ultrasonic spot welded joints by adopting proper sonotrodes combined with process control techniques. To achieve this goal, the research was carried out in two steps. In the first part, single-spot ultrasonic welding was performed by using sonotrodes with various diameters. The influence of the sonotrode sizes on the welding processes and weld attributes of the as-manufactured welded joints was investigated. Based on the information gathered from the first part, a novel simultaneous spot welding technique to produce two welded spots within a single step was subsequently proposed. The results in both parts indicated that the increase of the sonotrode size could significantly improve the welding efficiency. The weld attributes were also effectively enhanced by adopting proper welding force.