Laser technology for easier flying

Significant weight savings

In the Large Passenger Aircraft (LPA) framework program, the Fraunhofer IWS succeeded in demonstrating the welding of long joining seams on large-volume thermoplastic aircraft fiber composite structures using a CO2 laser beam source for the first time in the world as part of the MFFD project. © Clean Aviation

The innovative process makes the use of mechanical connecting elements superfluous and also dispenses with material doublings as with classic riveted overlap joints. The fuselage buoy made of welded, thermoplastic composite material therefore weighs significantly less than conventional sections. This marks an important step in aircraft construction using new types of high-performance materials, as it enables the joining of high-strength and weldable large fiber composite components. The challenge was to process materials such as PAEK, which has a comparatively high heat resistance and temperature resistance for plastics.

Aircraft half-shells welded by laser

On the left side of the MFFD, the process approach developed at the Fraunhofer IWS produced the final longitudinal seam connection between the upper and lower halves of a section of Carbon Fiber Reinforced Thermo-Plastics (CFRTP) measuring eight by four meters - in original size. © Clean Aviation

The key to success for the team was to join the upper and lower shells of the aircraft body together step by step by continuously placing several laminate strips, known as straps, on top of each other. The strips, which became progressively wider from 60 to 360 millimetres with each work step, were automatically placed in a stepped geometry (stock) on the surfaces of the half-shells. The overlap joints created in this way restore the initially interrupted force flow of the fiber composite material between the half-shells and form a reliable load-transferring bond.

"Another special feature of this process is the wavelength of the_{CO2 laser system} used," adds Dr. Langer. The Contijoin process offers the unique advantage that the wavelength of 10.6 micrometers in the relevant plastic part of the fiber composite material has a significantly higher coupling (absorption) of the laser radiation than the conventionally used fiber lasers with 1.06 micrometers. "This allows the required energy input in the area of the interfaces to be joined between the individual components to be reduced to a minimum, completely eliminating the subsequent process steps that are typically required today."

Monitoring and control of the welding process in real time

The conceptual development and implementation of the entire plant and control system, including the human-machine interface, was based on an in-house development by the Fraunhofer IWS. Commercially available system components were also used. © Clean Aviation

Another essential technology component is the ESL2-100 module, which was also developed at the Dresden institute. "This allows us to process a wide variety of sensor signals and implement corresponding control algorithms derived from them," explains Peter Rauscher, Group Manager High-Speed Laser Processing at the Fraunhofer IWS. "This enables both monitoring and adaptive control of the welding process in real time. This would not be possible with conventional control electronics. For example, in addition to controlling the welding temperature along the welding gap, we are also able to take into account the position, width and curvature of the aircraft half-shells."

The setup also consisted of two interacting movement units, the so-called end effectors. The task of the straphandling end effector was to precisely guide the laminate to be applied during continuous deposition and to press it onto the aircraft half-shells in a contour-true, width-dependent manner. The second end effector made it possible to guide the laser beam and pyrometrically measure the temperature in the joining zone. Each end effector moved synchronously with the other on its own linear axis system in order to decouple the transmission of possible vibrations or deformations caused by the pressing of the laminate strips from the optical beam guidance of the laser system. While the conceptual development and implementation of the entire plant and control system, including the human-machine interface, was based on a completely in-house development, commercially available industrial components were used for the other system components used, such as the laser beam source, pyrometer or X-Y scanners.

Open up further areas of application

In this way, it was possible to demonstrate both the technology development and the scaling and application of the process using large structures made of thermoplastic fiber composite material such as the MFFD. The next step is now to increase the technology readiness level (TRL) and thus take a further step towards qualifying for aviation suitability. Dr. Langer explains: "The Contijoin technology developed is not only interesting for aircraft construction, but also for other industries. In addition to aviation, the process solution developed could also be interesting for applications in shipbuilding, truck and trailer construction as well as in rail transport or in the further development of modern wind turbines." One challenge is to establish the acceptance and use of both the thermoplastic composite materials and the corresponding processes in the various industries.

The researchers will present their results and the system technology at the International Aerospace Exhibition ILA 2024 in Berlin.