Continuous ultrasonic welding (CUW) of thermoplastic composites is a novel joining technique able to produce long welded seams at high welding speeds. Our state-of-the-art welding setup entails an industrial robot on a track and an in-house developed end effector. Currently, no quality inspection methodology has been implemented; therefore, it is not known how the weld quality can be correlated to sensor and generator data. Such a method would enable the prediction of the weld quality after the process and be a stepping stone for closed-loop controlled welding minimizing defects. The goal of the current study is to use two off-the-shelf sensors for in-situ monitoring of the welding process and correlating the sensor data to the weld quality.

A laser line scanner is introduced behind the consolidator, measuring the step height between the top surfaces of the overlapping adherends. A forward-looking infrared (FLIR) camera is introduced, looking at the region of the top adherend behind the consolidator. This camera captures the residual heat present in the top adherend. The main findings from this study are that it is possible to use these two sensors to monitor the welding process and that the data acquired from them can be used to make predictions of the weld quality along its length. The thickness of the welded interface, which can be measured from the laser line scanner data, can give an indication of whether the interface is unwelded, welded, or overheated.

Regarding the FLIR camera, variations in the temperature at the top surface do correspond to variations in the quality of the weld interface, and furthermore, the rate at which the measured temperature changes is positively correlated to the magnitude of the power consumed by the ultrasonic generator. The FLIR camera can therefore be a powerful sensor to determine weld quality and its consistency throughout the length of the weld. ... Read more

This study investigates the positional accuracy and deformation behaviour of a robotic system performing simulated Continuous Ultrasonic Welding of thermoplastic composites. Thermoplastic composites are attractive due to their high specific properties and can be processed with lower-cost and faster assembly methods. Using a custom-developed end effector on a KUKA KR560 robot, a series of welding motion tests were conducted to analyse lateral deviation and tilt errors during CUW, without activating ultrasonic vibrations. High forces involved in the process result in consistent horizontal slip and tilt at motion initiation, attributed to static friction and stored mechanical energy in the system. Measurements from a laser tracker, force/torque sensors, and triangulation sensors revealed that robot pose and weld-line distance significantly influence these deflections. Notably, deflections were consistent and repeatable, suggesting potential for compensation via calibration or feedback control. Additionally, internal deformation within the end-effector structure and Force Torque sensor assembly contributed to the observed positioning errors. Recommendations include improving frame symmetry and stiffness to reduce mechanical distortion. These findings aim to support more accurate and reliable Continuous Ultrasonic Welding implementation in aerospace-grade composite manufacturing. ... Read more

To meet high production rate demands for single-aisle aircraft, this paper looked into automated fiber placement for composite parts and continuous ultrasonic welding for fast assembly, both leveraging rapid heating rates. We highlighted some of the challenges and opportunities in manufacturing thermoplastic composites with these
advanced methods. Fast heating can change the microstructure of tapes (increased voids and surface roughness) prior to consolidation. Due to the high temperature and fast speeds, limited consolidation times are available to cool down effectively or resolve the changed microstructure, resulting in poor quality (intimate contact). This
could be measured in the cooling phase, through a reduced cooling rate with tapes with low intimate contact. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

One of the most promising welding techniques for thermoplastic composites is ultrasonic welding. It is mainly known as a spot welding technique. The relatively new technology continuous ultrasonic welding of thermoplastic composites makes it possible to obtain large fully welded seams with higher load carrying capabilities. However, at the start of the research conducted for this dissertation very little knowledge on the process was available. The weld quality in the state-of-the-art research was significantly lower than the weld quality of the statically welded counterpart in terms weld uniformity as unwelded and overwelded areas were present simultaneously, the single-lap strength, and the presence of voids. Hence, before continuous ultrasonic welding can be industrially applied the weld quality needs to be improved. Consequently, the main objective of this dissertation was to acquire a deeper understanding of the continuous ultrasonic welding process of thermoplastic composites to promote its development for future industrial applications. ... Read more

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. ... Read more

Continuous ultrasonic welding is a high-speed joining method for thermoplastic composites. Currently, a thin film energy director is used to focus the heat generation at the interface. However, areas of intact energy director remain in the welded seam, which significantly lowers the weld strength, and result in a non-uniformly welded seam. To improve the weld uniformity of continuous ultrasonically welded joints, we changed to a more compliant energy director. A woven polymer mesh energy director was found to give a significant improvement in weld quality. The mesh was flattened in between the composite adherends during the welding process. This flattening promoted a good contact between the energy director and the adherends, fully wetting the adherend surfaces, resulting in a more uniformly welded seam without areas of intact energy director. ... Read more

Continuous ultrasonic welding is a promising high-speed and energy-efficient joining method for thermoplastic composite structures. Our aim was to identify and understand differences between the static and continuous ultrasonic welding process for thermoplastic composites. In particular, melting of the interface, consumed power and energy density, temperature evolution at the weld interface, and optimum welding conditions for both types of processes were investigated. This was done for three combinations of welding force and vibrational amplitude, parameters which are known to have a significant effect in both welding processes. Our results showed that for the continuous process the amount of non-welded area under the sonotrode remains constant, while for the static process the amount of non-welded area gradually decreases to zero. Additionally, the optimum vibration times and welding speeds in both processes are similar. ... Read more

Continuous ultrasonic welding is a new promising high-speed joining technique for thermoplastic composites. At the moment no straightforward method exists to find an optimum welding speed to easily obtain high strength joints. However, for static ultrasonic welding a systematic approach is available. Therefore, the aim of this study is to understand whether the optimum welding time in static ultrasonic welding can be related to the welding speed in continuous ultrasonic welding. The duration of the welding stages in static and continuous welding were compared in order to find similarities and discrepancies. The duration of the welding stages in continuous ultrasonic welding was estimated by analysing the weld interface and measuring the distances of the stages in the fractured interface around the location where the weld was stopped. No significant difference in weld duration was found for the first welding stage in which the mesh energy director is flattened. However, the second stage in which the energy director and adherends melt simultaneously, lasted much longer for the continuous welding process. This was possibly caused by the more constraining boundary conditions in the continuous welding process. In conclusion, the optimum welding speed in continuous welding was lower than the expected optimum welding speed based on the optimum welding time in static ultrasonic welding, because the second welding stage lasted longer in the continuous ultrasonic welding process. ... Read more

The increasing use of fibre-reinforced plastics in the aerospace industry leads to challenges in joining these materials. The traditionally used mechanical fasteners introduce damage when used in composites by disrupting the fibres. However, thermoplastic composites allow for fusion bonding processes to join parts. Ultrasonic welding has shown to be a very promising high-speed fusion-bonding technique for thermoplastic composites. At the moment, a thorough understanding of the process is required to upscale the process for industrial usage. This study focuses on defining a roadmap to develop continuous ultrasonic welding to an industrially applicable level by presenting the current state of the art, ongoing developments, requirements, and challenges. ... Read more

Continuous ultrasonic welding (CUW) is an innovative high-speed joining method for thermoplastic composites. Currently, thin flat energy directors (EDs) are used to focus the heat generation at the weld line. The resulting fracture surfaces exhibit large areas of intact ED, resulting in a non-uniform weld, and significantly lowering the strength. The goal of this study is to improve the weld uniformity of continuous ultrasonically welded joints. In the first part of this paper we found that a 0.20 mm-thick woven mesh ED significantly improved the weld uniformity and strength in comparison to a 0.08 mm thick flat ED. The second part the paper focuses on understanding why the mesh gives this improved weld uniformity by analyzing the feedback data from the welder and by performing a microscopy analysis of the weld line at different moments during the static welding process. It was found that at the beginning of the welding process the mesh filaments expand within the open areas of the mesh while flattening; the mesh is being pre-formed in between the adherends. This pre-forming most likely created a good uniform intimate contact between the ED and adherends, which most likely resulted in a uniform heat generation and therefore created a uniform weld line. Because energy directing meshes make it possible to create uniform weld lines, they are expected to play an important role in the future for the continuous ultrasonic welding of thermoplastic composites. ... Read more