The state of the art in resistance and continuous ultrasonic welding

Date：2025-06-11 Source：JEC Hits：158 Comment：0

Core tips：Several techniques are available for joining thermoplastic composites, including resistance welding and ultrasonic welding. While resistance welding uses Joule heating inside an electrically conductive carbon fibre implant, melting the thermoplastic matri

Thermoset composite materials are currently used in different aircraft, such as the Airbus A350 and the Boeing B787, e.g. for the fuselage and the wings. Due to the strong focus on the use of thermoset composites, manufacturing processes have evolved towards a high level of technological maturity in recent years. However, thermoplastic composites are already being used for smaller parts such as the Airbus A320neo engine pylons or the A380 wing ribs. Th ese materials off er several advantages over thermosets such as efficient high-rate manufacturing capabilities when thermoforming recurring parts.

From research to industrial applications

The Centre for Lightweight Production Technology (ZLP), located in Augsburg, is an institution of the German Aerospace Centre (DLR). Focusing on the automation of manufacturing and assembly processes for lightweight structures, mainly in the space and aerospace sector, the ZLP aims to develop automation approaches and processes to a high level of technological maturity (technology readiness level 6 / manufacturing readiness level 6) to enable transfer to industrial applications with reduced eff ort. To this end, a highly automated and flexible research platform is used for validating the technologies developed in a representative large-scale environment (Figure 1). The ZLP has a dedicated research branch that focuses on manufacturing and assembly processes for fibre-reinforced thermoplastics.

Cover photo: Fig. 1: Multifonctional robot cell at the DLR Augsburg

Advantages of resistance and ultrasonic welding

A key benefit of thermoplastics is the ability to apply welding or fusion bonding technologies, generating an intrinsic integral assembly from low-complexity individual parts. This enables a more efficient and streamlined production process, offering significant advantages over traditional composite manufacturing methods. In the context of the targeted production rate of 70 to 100 aircraft for future single-aisle configurations, welding technologies offer the possibility of changing assembly sequences, which in turn enables the use of automation solutions.

In general, the choice of welding methods for thermoplastic composites should consider material properties, geometric parameters, design requirements and manufacturability. It can be assumed that, in principle, a wide portfolio of welding processes can be used to join high-performance thermoplastic composites such as polyphenylene sulfide (PPS), polyetherketoneketone (PEKK), polyetheretherketone (PEEK) and low-melt polyaryletherketone (LM-PAEK). The different welding methods have their own advantages and limitations, and selecting the appropriate one requires a comprehensive understanding of the materials and product requirements. ZLP focuses on Resistance Welding (RW) and Ultrasonic Welding (UW), with a particular emphasis on Continuous Ultrasonic Welding (CUW). These processes offer significant potential for assembling components within the strict quality standards of the aerospace industry while enabling efficient automation.

In resistance welding, an off-the-shelf Carbon Fibre (CF) fabric is the preferred choice for the electrically-conductive implant within the weld line, allowing for consistent welding results. As mentioned before, the electrical current flow induces Joule heating and subsequent melting of interfaces, thus forming a material unity within the assembled structure. In comparison to e.g. ultrasonic or induction welding, the very tangible and simple static process of resistance welding makes it possible to customise the welding profile and to consider the specific needs of the thermoplastic matrices, which finally results in highly controlled and repeatable weld line properties.

Ultrasonic welding is a process that introduces high-frequency vibrations into the workpieces, thus generating heat by local friction and viscoelastic damping. For this purpose, a sonotrode is placed vertically onto the components to be welded and introduces the vibrations into the components. The process can be conducted by spot welding, where the sonotrode remains stationary and is also used to apply pressure to the components during cooling. The sonotrode can also be moved over the parts: the process is then called continuous ultrasonic welding. In order to apply a controlled joining pressure during the matrix solidification phase in the weld line, the sonotrode is followed by a consolidation unit. As the welding infrastructure (end-effector) is guided by a robot, the length of the weld seam is only limited by the robot’s reach. Thus, the process makes it possible to weld seams with a simple horizontal or vertical curvature. The current average welding speed, which varies slightly depending on the matrix material used, is about 1.5 m/min.

Fig. 2: Robot-based continuous ultrasonic welding

Significant R&D efforts

Welding technologies for thermoplastic composites, such as resistance and ultrasonic welding, offer significant potential for a comprehensive approach to assembling composite structures. These technologies allow the creation of a single component from parts that were manufactured separately. Over the last decade, the technical maturity of welding processes has notably advanced due to focused development based on application-oriented full-scale demonstrators such as the Multi-Functional Fuselage Demonstrator (MFFD). The need for demonstrating full-scale capabilities significantly contributed to the maturation of technologies, but also opened application-oriented questions, such as the robot’s absolute accuracy and the forces of the welding process that could push the robot off its path. As the processes were implemented at full scale, suitable solutions could be implemented and validated.

HotStuff project

The HotStuff project focused on the development of a thermoplastic Rear Pressure Bulkhead (RPB) for an A320 that is less costly than the current metallic version. Since the base material is significantly more expensive, the savings must be made through a much simpler assembly process which does not use bolts.

The RPB consists of 8 equally-sized parts with integrated stringers made of CF/ PPS. The parts were assembled using 8 1.5-m resistance welding seams. Previously, predominantly a stainless-steel mesh was used as the state of the art for resistance welding. As part of the project, a welding element based on carbon fibres was successfully used.

Fig. 3: Automated integration of frames using resistance welding

The joining process for connecting the CF/ PPS pressure bulkhead segments using electrical resistance welding was advanced to TRL 5; as a result of which, an extensive test pyramid was passed. The individual stages of this pyramid included coupon, element, and component tests from bottom to top.

This involved further maturation and adapting the welding process for overlap test specimens to meet the specific requirements of the component’s joining task. Despite the glass fabric corrosion protection layer on the parts, Single-Lap Shear strengths (SLS) exceeding 23 MPa were achieved with a 25.4 x 12.7 mm overlap. A full-surface connection of the joining contact area was successfully established, which helps reduce crack initiation and the associated variability in the results.

MFFD project

The MFFD European flagship project combined various manufacturing and assembly technologies to build an 8-m-long fuselage barrel made from Toray TC 1225 CF/LM-PAEK. Th e ZLP built the upper shell for the demonstrator together with Premium Aerotec (PAG), Aernnova (ANN) and Airbus. First, the skin was placed by in situ fibre placement. Then, the Z-shaped stringers were integrated using robot-based continuous ultrasonic welding (Figure 2). Th e frames (Figure 3) and frame couplings were integrated by resistance welding using an in-house developed device for transport, positioning and welding, and the cleats were integrated with a cobot-on-robot based RW welding end-effector. The advantage of this system is that the cobot can detect the corner for the welding position with its force-torque sensors, meaning that the position does not have to be hard-coded. This allows tolerances to be optimally compensated during assembly. Before verifying the respective technology bricks on a full-scale test shell with single-aisle dimensions, intense process development was conducted at the coupon level. SLS values of 37 MPa were achieved for RW and over 40 MPa for CUW, also with full surface connection of the joining contact area.

Conclusion

Both welding processes have reached a high level of technological maturity and have already been demonstrated in various applications. When considering the pure SLS performance, both RW and CUW can achieve repeatable high values.

In addition, it has become clear in all projects that the design philosophy for thermoplastic composite structures should be closely aligned with the chosen assembly method. Th is ensures that the necessary boundary conditions and prerequisites are considered, allowing to unleash the full potential of the assembly to be realised and achieving significant benefits in terms of weight reduction and reduced manufacturing complexity.

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