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Composites are considered hard to join and researchers have predominantly focused on mechanical joining technologies including crimping, gluing, riveting or screwing. The Composites Europe exhibition in Stuttgart, Germany, on 6-8 November will show the advantages and drawbacks of each of these processes.
Depending on the process, various factors have to be taken into account. During riveting, for example, the fibre layers are damaged by delamination during the pilot drilling. Drilling also weakens the component. Furthermore, the strength transmission between the components to be joined is very limited locally. In bonded parts, the gap width can cause problems. Nevertheless, it is the standard process for joining fibre composites due to the uniform strength transmission that makes optimum use of the material properties. What the individual processes look like will be demonstrated by the Institute for Welding and Joining Technology (ISF) at RWTH Aachen, Oxford Advanced Surfaces and Weiss Chemie + Technik, three exhibitors at Composites Europe.
BMW focuses on adhesive technologies for e-vehicles
For manufacturing the carbon fibre reinforced plastic (CFRP) body of its i3 and i8 e-vehicles BMW relies exclusively on an adhesive technology. This process avoids mechanical damage to the carbon fibre components thereby increasing component stability while saving costs at the same time.
The adhesion of the glue is determined by the surface priming. Priming varies by application, in terms of both the matrix and the fibres used. Process parameters and materials also influence the adhesion quality and durability of the bonded connection. On account of this complexity the research and evaluation of proven processes and new technologies for joining fibre composites and other hybrids are at the centre of numerous scientific studies.
Experts at the Karlsruhe Institute for Technology (KIT) have developed a novel, strong and low-cost joining technology for gluing structural components. This hybrid process combines inorganic and organic adhesive layers and is therefore substantially cheaper and more hard-wearing. In joining technology this is particularly suitable for connecting structural components and therefore applicable in numerous sectors such as wind power, construction but also in automotive and mechanical engineering.
The Fraunhofer Institute for Material and Beam Technology (IWS) Dresden seeks to replace bonding processes entirely with the HeatPressCool-Integrative (HPCI) process. This so-called thermal direct joining presses laser-textured metal with thermoplastic components and heats them locally. The thermoplastic melts, penetrates the textures and adheres to the surface. Joining guns specifically developed for this purpose produce strong connections within seconds.
Fibre lasers permit contact-free joining
For the connection of fibre-reinforced thermoplastics (organic sheeting) with metal, the IWS has also developed the slot-web principle. The organic sheeting serves as a web plate, a metal sheet as a slot plate. A fibre laser is used for bonding. It makes for a very finely adjusted heat input and heats the protruding part of the fibre-reinforced web plate – contactless and at the precise position. The 2D high-frequency beam deflection by means of scanner lenses allows uniform heating of the plastic.
The Fraunhofer Institute for Manufacturing Engineering and Applied Material Research (IFAM) in Bremen has developed a test line for the automated bonding of fibre composite boards for aircraft construction, partnering with CFK-Valley Stade. The process saves costs over conventional methods and is relevant for all industries that require lightweight, dimensionally stable and low-cost components.
Combining 3D printing and organic sheeting
With this move, the project partners have succeeded in replacing the so far manual frame assembly by filling the slot in a completely automated process. With a view to future mass production, the required drive intelligence has also been provided for. With the help of the decentralised concept a high number of drives can be concentrated on a small footprint and adjusted and efficiently controlled in a modular way with little wiring.
In the LightFlex project scientists from the Fraunhofer Institute for Production Technology (IPT) in Aachen focus on a combination of 3D printing and organic sheeting from unidirectional semi-finished parts. To optimise the load-bearing capacity the 3D printed components are joined with a fibre composite component. For this, custom-sized organic sheets are used which are produced on a so-called PrePro line with a near-net shape. This approach minimises cut-off waste and results in marked savings considering the high energy consumption involved in carbon fibre production.
Image provided by Composites Europe
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