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With more than 100 years of experience in welding metals, the French Institute of Welding (IS) group IS emerging as a leader in welding thermoplastic composites. IS group has developed the "dynamic induction welding" process used to connect carbon fiber/polyether ketone (PEKK) unidirectional truss band and fuselage skin in the Aero-thermoplastic composites demonstration project at Airbus' STELIA Aerospace.
Although the process was successful, the lack of sensors at the interface limited the radius properties of the bonding truss and the global heating of the panels. A sensor is a material placed between two bonded materials in a thermoplastic composite welded joint heated by an induction coil in the welded joint. The receptor may be a resistively heated conductive body or a hysteresis heated magnetic body, melting the substrate at the welding interface and pressing it together to form a welded joint with high strength. Sensors used in induction welding thermoplastic composites are initially a metal screen or mesh, sometimes impregnated with a polymer.
The IS Group has formed a partnership with Thermoplastic material supplier Arkema to jointly develop and acquire a patented technology called welding Innovation Solutions.
The foundation of innovative welding solutions
The innovative welding solution is based on the use of sensors to heat the welding interface, but this is a removable sensor connected to the welding joint. The sensor allows the process to perfectly position the heated area of the weld, and the head with the sensor is mobile, so there is no residue in the interface that will not interfere with the performance of the welded structure. In the early iterations of induction welding, the metal mesh receptor remained in the weld, but this was not the desired result. Because carbon fibre in ordinary aerospace laminates is conductive, the latest technology has been able to eliminate the receptors, allowing the use of carbon fibre as a sensor.
Another feature of innovative welding solutions is the use of a pure thermoplastic matrix or low fibre layering at the welding interface to increase resin fluidity. The melt temperature and viscosity of the interfacial layer can be adjusted and functionalized to provide electrical conductivity or isolation to prevent electrical corrosion, such as between carbon fiber and aluminum or steel.
Results of innovative welding solutions
The joint coefficient of the solution is 80% ~ 90%. The joint coefficient corresponds to the welding strength. It is used in metals, plastics and composites. In a single lap shear test of two pre-cured plates welded together using the solution, 80 to 90 per cent of the performance of the unwelded, hot-pressed tank cured reference plates was obtained. The trials used unidirectional strips made of carbon fiber by Hextow AS7 and Kepstan 7002 PEKK.
Innovative welding solutions can be used to weld any kind of substrate: PE, PA, PEKK, PEEK, and carbon, glass, or array-fiber reinforced thermoplastic composites. Moreover, components with copper grids can be welded to protect against lightning strikes, which are key to the construction of aeronautical structures. Innovative welding solutions are designed to be fully automated, with welding heads mounted on a 6-axis robotic arm.
Welding temperature control
A common problem of metal mesh sensors subjected to magnetic field is the uneven temperature distribution of welded parts. The solution controls this by using a sensor to melt the welding interface, sensing the temperature using a laser pyrometer that actually measures the sensor's edge from the side. So you know the exact temperature at the interface. Cooling methods are also used to help control the temperature and ensure that the thermoplastic material is fully crystallized throughout the welding process.
Girder skin welding test
Airbus's STELIA was one of the first customers for the induction welding process. The IS group and Arkema conducted a special study for STELIA in which seven carbon /PEKK beams were welded to 14 layers of skin and covered with copper mesh to prevent lightning strikes. The ultimate goal is a structure with a welding length of 30 meters and a straight and double bending cross section. Components were fabricated using a 194 GSM unidirectional band consisting of Tenax HST45 carbon fibre and Kepstan 7002 PEKK. STELIA specifies a homogeneous weld with mechanical properties greater than 85% of the reference material solidified in the thermocompression tank, without degradation of the thermal or mechanical properties of the adhesive. STELIA also requested the development of a robust process to change the thickness of the adhesive. The IS group conducted chemical and performance tests on the welded components.
The IS group and Arkema were able to meet STELIA's requirements, achieving 85% greater than the single lap shear and interlaminar shear strength performance compared to the thermal tank cured reference laminates. No dispersal or degradation in component laminates or lightning resistant grids. The only downside is speed, STELIA requires welding speed of at least 1 m/min. Currently, the solution has a speed of 0.3 meters per minute. In terms of the thickness of the base material that can be welded, the typical thickness of aerospace structures can be welded and 5 mm thick parts can be welded to 5 mm substrates.
Technological opportunities and challenges
The IS Group and Arkema are co-owners of innovative welding Solutions technology and have secured this technology through a reliable patent portfolio that has included five French and international patent applications. Innovative welding solutions can be used in conjunction with any thermoplastic composite substrate and IS demonstrating the technology through a programme of collaboration with European and Us companies. For Arkema, the focus is on PEKK, which has formed a strategic alliance with Hershey in 2018 to develop carbon/thermoplastic belts for future aircraft, focusing on providing customers with lower costs and faster production speeds. As part of the partnership, France will establish a joint research and development laboratory.
The €13.5 million, 48-month highly automated integrated composite material project for adaptive structures is a continuation of the Strategic alliance between Arkema and Hershel. The project will optimize the design and manufacture of materials for the production of composite parts in order to achieve competitive costs. It will also develop a more productive composite placement/placement technology and a new system with online quality control to weld the final parts together. The targeted applications include the main structures of aircraft, structural components for the automotive industry and pipelines for the oil and gas industry. The recyclability and sustainability advantages offered by thermoplastic materials are also important to these markets and will be demonstrated and quantified in the project.
Compared to the dynamic induction welding process in 2017, one of the benefits that the innovative welding solution can provide is a more than 50% reduction in power requirements. With conventional induction, large power is required to heat the surface, but with sensors at the interface, the surface is much smaller and requires much less energy. This also helps to prevent the beam radius from loosening, as too much heat can soften the material in the radius and allow the fibers to move. However, there are still cooling problems. For flat shapes, thermal control is very simple, but as shape complexity increases, it becomes more challenging. At present, the main objective is to continue to develop and achieve the typical scale of skin beam welding, and the emphasis is still on introducing the technology into the new aircraft development program.