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.@en

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.@en

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. @en

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.

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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.@en