Welding World's largest CFRP aircraft fuselage segment assembled

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Employees of the German Fraunhofer Society, together with international project partners in the EU-funded Clean Sky 2/Clean Aviation Project Large Passenger Aircraft (LPA), have produced the world's largest thermoplastic CFRP aircraft fuselage segment. Instead of relying on previous riveting processes, the CFRP half-shells were welded dust-free.

In order to save resources, climate-friendly solutions are needed for the production and operation of passenger aircraft. In addition to new propulsion technologies, the focus is also on structural weight and manufacturing effort. Both can be reduced by new construction methods: thermoplastic CFRP materials are one solution. In the Clean Sky 2 / Clean Aviation project Large Passenger Aircraft (LPA) funded by the EU, the project participants have for the first time manufactured a fuselage segment of original size from thermoplastic CFRP materials in order to be able to evaluate feasibility as well as ecological and economical advantages and disadvantages in a well-founded way.

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Rivetless aircraft fuselage construction saves 10 percent weight

To join the two longitudinal seams of the two 8 meter long CFRP half-shells, which have a diameter of about 4 meters, CO2 laser welding for the left longitudinal seam and ultrasonic welding for the right longitudinal seam were selected together with the project coordinator Airbus. Both methods offer the advantage of dust-free joining, which the riveting methods currently used do not possess. However, they have not yet been used in production or research for such large CFRP components and with the specific quality requirements needed here. The need for dust-free joining arises from the first-ever pre-integration of both shells with a large number of also weldingly mounted structural and system components, which does not allow for subsequent removal of dust and chips.

The thermoplastic lower shell, which was solidified in the autoclave under pressure and temperature (consolidated), was contributed from the LPA project Stunning by a consortium of GKN Fokker, Diehl Aviation, Netherlands Aerospace Centre  NLR and Technical University of Delft. The thermoplastic upper shell, which was produced using the tape laying process (in-situ consolidation), is from a consortium of Premium Aerotec, Airbus, Aernnova and German Aerospace Centre – DLR ZLP.

Improved fuel efficiency in operation

Both shells impress, particularly due to their high degree of pre-integration, the largely rivet-less construction, and the weight reduction of 10 percent compared to the status quo. On the one hand, the automated pre-integration opens up high efficiency gains and local flexibility with regard to high-rate production, saving up to 10 percent in costs because, unlike before, all components do not have to be brought into the closed fuselage and manually assembled under cramped conditions. On the other hand, the weight reduction of the aircraft structure leads to improved fuel efficiency in operation.

Ten hexapod robots position CFRP half-shells with sub-millimeter precision

Together with the partner FFT Produktionssysteme, the Fraunhofer Society at the CFK Nord Research Centre in Stade (Germany) has designed and built the automated assembly research platform for the MFFD, including the central plant and process control, in the LPA project Multifunctional automation system for Fuselage Assembly Line (MultiFAL). Other project partners then integrated their technology modules into the platform.

Further tasks of Fraunhofer along the process chain initially included setting up the lower shell using a specially developed holder, which allowed the lower shell to be aligned with high precision in the assembly room for the subsequent processes. After this, the upper shell was inserted using the overhead crane. All other process steps ran fully automated. The sub-millimeter precise positioning of the two shells relative to each other was handled by a field of ten hexapod robots, which, using laser sensors, always adjusted the optimal shape and position of the shells and readjusted if necessary.

Strap handling tool precisely positions CFRP strip on the seam

For the laser welding process, thin strips (butt-straps) of thermoplastic CFK up to 4.5 meters long had to be layered and flush next to and on top of each other on the precisely abutting longitudinal edges of the shells, which were provided with a step joint on the aircraft exterior side. As a result, the straps produced by Fraunhofer were positioned exactly on the seam with a rolling movement using a strap handling tool integrated into the automated process environment. This allowed the oscillating laser beam of the immediately following welding head, directed via mirrors, to continuously melt the contact lines forming between the strap and the shell surface. A pressure unit on the laser welding end-of-arm tool pressed the strap against the upper and lower shell joining points with joining forces of up to a ton, consolidating the weld seam in the same operation. To prevent the high pressure forces during welding from damaging the fuselage structure, a device for force absorption and deflection ran synchronously within the emerging fuselage section.

An online monitor and control system has been implemented in the laser welding process. This allows immediate digital data exchange for process optimization and quality assurance with a digital twin.

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Thermoplastic filler closes gaps between straps

Because the straps and the tiered joining zones of the shells could not join seamlessly due to unavoidable manufacturing tolerances and degrees of freedom required for the positioning processes, small, irregularly wide gaps remained between them. These would impair the quality of the weld joints and therefore had to be completely closed with a so-called thermoplastic filler after the welding process, with any excess material to be strictly avoided.

A compact extruder, guided along the previously generated connection seams just like the laser welding end effector, heated the starting granulate and conveyed it through a screw to the gap. A special nozzle ensured that the material filled the gap before it cured in the air. The local gap volume, which was decisive for the precise filling quantity, was measured just beforehand by a 2D sensor integrated in the same gap-filling end effector and aimed at the open gap, and transferred to the extruder system. This allowed the locally required output to be calculated dynamically during the ongoing process, so that irregular gaps 3 to 20 millimeters wide were filled at each point with the exact required amount of molten thermoplastic material.

Ultrasonic welding for thinner fuselage seam sections

The automated ultrasonic welding method was selected as the joining method for the right longitudinal seam. Compared to laser welding, this can only connect the thinner-walled fuselage seam sections outside the door surroundings. The effort in terms of synchronization of parallel processes, investments, and occupational safety is lower. From the collaboration of the Fraunhofer Society, responsible for the research platform, with the Welding Equipment for optimized, fast and accurate LongituDinal barrEl joint closuRe (Welder) LPA project of partners CT Engineering Group, AIMEN, AITIIP and Dukane, a fast and reliable, automated ultrasonic welding process for the longitudinal seam has emerged. Here, too, online monitoring and control systems enable digital data exchange for process optimization and quality assurance with a digital twin.

The already craned out thermoplastic fuselage section is finalized, among other things, with the installation of a cabin ceiling module at the Center for Applied Aviation Research (ZAL) in Hamburg (also Germany), where it is then used for further tests and for demonstration purposes.