The French Institute of Welding (IS) group has more than 100 years of experience in welding metals and is currently becoming a leader in welding thermoplastic composite materials. IS Group developed the "dynamic induction welding" process, which was used to connect carbon fiber/ polyetherketoneketone (PEKK) unidirectional belt stringers and fuselage skin in the aerospace thermoplastic composite demonstration project of Airbus's STELIA Aerospace .
Despite the success of this process, there is no susceptor at the interface, so there are limitations in the radius performance of the bonded stringers and the global heating of the wall panels. The susceptor is the material placed between the two bonded objects of the thermoplastic composite welded joint, and the material is heated by the induction coil in the welded joint. The susceptor can be a conductive body heated by resistance or a magnetic body heated by hysteresis. It melts the base at the welding interface and presses the base together to form a high-strength welded joint. The susceptor used for induction welding of thermoplastic composites was originally a metal screen or grid, sometimes impregnated with polymer.
IS Group has established a partnership with Arkema, a supplier of thermoplastic materials, to jointly develop and obtain a patented technology called Welding Innovation Solution.
The basis of innovative welding solutions
The basis of innovative welding solutions is the use of susceptors to heat the welding interface, but this is a movable susceptor connected to the welding head. The susceptor allows the process to perfectly locate the heating area of the weld, and the welding head with the susceptor is mobile, so there is no residue in the interface and will not interfere with the performance of the welded structure. In the early iterations of induction welding, the metal mesh susceptor remained in the weld, but this was not the desired result. Since the carbon fiber in ordinary aerospace laminates is conductive, the latest technology has been able to eliminate the susceptor, which also enables the use of carbon fiber materials as the susceptor.
Another feature of the innovative welding solution is the use of a pure thermoplastic matrix or a layer with low fiber content at the welding interface to increase resin fluidity. The melting temperature and viscosity of the interface layer can be adjusted, and it can also be functionalized to provide conductivity or isolation to prevent electrical corrosion, such as electrical corrosion between carbon fiber and aluminum or steel.
The results of innovative welding solutions
The joint coefficient of this solution is 80%~90%. The joint coefficient corresponds to the welding strength and is used for metals, plastics and composite materials. In a single-lap shear test on two pre-cured plates welded together using this solution, 80% to 90% of the performance of the unwelded, autoclaved reference plate was obtained. These tests used unidirectional belts made of Hextow AS7 carbon fiber and Arkema Kepstan 7002 PEKK.
Welding innovative solutions can be used to weld any kind of substrate: PE, PA, PEKK, PEEK and carbon, glass or aramid fiber reinforced thermoplastic composites. Moreover, components with copper grids can also be welded to prevent lightning strikes, which is the key to the manufacture of aerospace structures. The design of the innovative welding solution is fully automated, with the welding head mounted on a 6-axis robotic arm robot.
Welding temperature control
A common problem of metal mesh susceptors when subjected to a magnetic field is the uneven temperature distribution of the welded parts. The solution controls this by using a susceptor to melt the welding interface, using a laser pyrometer to sense the temperature, which actually measures the edge of the susceptor from the side. Therefore, the exact temperature at the interface can be known. Cooling methods are also used to help control the temperature and ensure that the thermoplastic material can fully crystallize throughout the welding process.
Stringer skin welding test
STELIA, a subsidiary of Airbus, is one of the first customers for this induction welding process. IS Group and Arkema conducted a special study for STELIA, welding 7 layers of carbon/PEKK stringers to 14 layers of skin and covering them with copper grids to prevent lightning strikes. The ultimate goal is to weld a structure with a length of 30 meters, a straight line and a double curved section. The components are made with 194 gsm unidirectional tape including Tenax HST45 carbon fiber and Kepstan 7002 PEKK. STELIA specifies a homogeneous weld with mechanical properties greater than 85% of the reference material cured by the autoclave, and the thermal or mechanical properties of the adherend will not be degraded. STELIA also required the development of a robust process for changing the thickness of the adherend. IS Group conducted chemical and performance tests on welded components.
IS Group and Arkema were able to meet STELIA's requirements, achieving greater than 85% of the single-lap shear and interlaminar shear strength performance compared with the reference laminate cured by autoclave. There is no spreading or degradation in the component laminate or lightning protection grid. The only shortcoming is the speed. STELIA requires the welding speed to be greater than or equal to 1 m/min. Currently, the speed of this solution is 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 a 5 mm base.
Technical opportunities and challenges
IS Group and Arkema are the co-owners of innovative welding solutions technology and have protected the technology through a reliable patent portfolio, which already includes five French and international patent applications. The innovative welding solution can be used with any thermoplastic composite matrix, and IS Group is demonstrating the technology through a plan to cooperate with European and American companies. For Arkema, the focus is on PEKK, which formed a strategic alliance with Hexcel 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 euros, 48-month highly automated integrated composites project for adaptive structures is a continuation of the strategic alliance between Arkema and Hexcel. The project will optimize the design and manufacturing of materials used in the production of composite parts in order to achieve competitive costs. It will also develop a more efficient composite material placement/layout technology, as well as a new system with online quality control to assemble final parts by welding. Target applications include the main structure of aircraft, structural parts in the automotive industry, and pipelines in the oil and gas industry. The recyclability and sustainability advantages provided by thermoplastic materials are also important to these markets and will be demonstrated and quantified in the project.
Compared to 2017, one of the benefits that innovative welding solutions can provide is a reduction of more than 50% in the power required compared to the dynamic induction welding process. With conventional induction, high power is required to heat the surface, but with susceptors at the interface, the heated surface is much smaller and the energy required is much less. This also helps to prevent the stringer radius from vibrating. If heating is too much, it will soften the material on the radius and move the fibers there. However, there is still a heat dissipation problem. For flat shapes, thermal control is very simple, but as the complexity of the shape increases, it becomes more challenging. At present, the main goal is to continue to develop and achieve typical-scale skin upper beam welding, and the focus is still to introduce the technology into the new aircraft development plan.
