Joining and connecting composites: Tomorrow’s alternatives for proven mechanical processes

Joining composites is a challenge, since these materials are inherently hard to join. To date, thermal processes have generally proved unsuccessful and researchers have predominantly focused on mechanical joining technologies including crimping, gluing, riveting or screwing. The upcoming Composites Europe show, November 6–8, 2018, in Stuttgart, Germany, will showcase the advantages and drawbacks of each of these processes for the fiber composite in question.

Composite joining technologies will feature at Composites Europe this November in Stuttgart, Germany.

Depending on the process, various factors have to be taken into account. During riveting, for example, the fiber layers are damaged by delamination during the pilot drilling. Drilling also weakens the component. Furthermore, the power transmission between the components to be joined is very localized. In addition, in bonded parts, the gap width can cause problems.

Nevertheless, riveting is the standard process for joining fiber composites due to the uniform power transmission that makes optimum use of the material properties. There are, however, alternatives under development. For manufacturing the CRP body of its i3 and i8 e-vehicles, BMW foregoes mechanical joining processes and relies exclusively on a special gluing technology, which has now developed into a standard process for these model series. This process avoids mechanical damage to the carbon fiber reinforced plastic components thereby increasing component stability while saving costs at the same time.

The adhesion of the glue here is determined by the relevant surface priming. Due to the versatile properties of composites, this priming varies by application–in terms of both the matrix and the fibers used. On top of this, process parameters and materials also influence the adhesion quality and durability of the bonded connection. On account of this complexity, research and evaluation of proven processes and new technologies for joining fiber composites and other hybrids is at the center 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 together. 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.

For its part, 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. In this way, the thermoplastic melts, penetrates the textures and adheres to the surface. Joining guns specifically developed for this purpose produce strong connections within seconds.

For the form-fitted and substance-bonded connection of fiber-reinforced thermoplastics (organic sheeting) with metal, the IWS experts have also developed the so-called slot-web principle. The organic sheeting only serves as a web plate, a the metal sheet as a slot plate. A fiber laser is used for bonding. It makes for a very finely adjusted heat input and heats the protruding part of the fiber-reinforced web plate–contactless and at the precise position. The two-dimensional and high-frequency beam deflection by means of scanner lenses allows uniform heating of the plastic. Here the right heating concept ensures the quality of this sensitive process.

In another development, the Fraunhofer Institute for Manufacturing Engineering and Applied Material Research (IFAM) in Bremen has developed a test line for the automated bonding of fiber composite boards for aircraft construction partnering with CFK-Valley Stade for a project. The process saves costs over conventional methods and is also important for all industries that require lightweight, dimensionally stable and low-cost components.

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 developed. With the help of the decentralised concept even 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 are focusing on a combination of 3D printing and organic sheeting from unidirectional semi-finished parts. To optimize the load-bearing capacity, the 3D printed components are joined with a fiber 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 minimizes cut-off waste and results in marked savings considering the high energy consumption involved in carbon fiber production.

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