Carbon fibre reinforced polymer (CFRP) components are usually assembled using fasteners. These are typically glued into the CFRP component once it has been cured and drilled. The consortium behind the CarboLase project came up with a new method, using an ultrashort pulsed laser to drill the holes for the fasteners in the textile preform with micrometre-scale accuracy. Integrating the fasteners in these high-precision cut-outs before the CFRP component is cured saves time by shortening the production process.
Carbon fibre-reinforced plastics (CFRP) are one of the most versatile composite construction materials. They combine the positive mechanical properties of their constituent parts – a polymer matrix reinforced with high-strength carbon fibres – to create a solution that offers high strength, high stiffness and low density. So why are CFRPs still struggling to achieve a real breakthrough at a time of increasing concerns about energy and resource efficiency? One reason is their high production costs – and another is the difficulty of machining and processing CFRP components.
The conventional way of assembling carbon fibre-reinforced polymer components is to drill holes in the fabricated CFRP module and then glue in metal fasteners such as threaded inserts. Replacing conventional parts with lightweight components requires connections between the CFRP part and the conventional parts that are both detachable and secure.
The CarboLase project, started by Fraunhofer in 2017 pursued a different approach by integrating the fasteners in the textile preforms. The final CFRP is then produced with an additional curing process that includes the fasteners. This can significantly shorten production process chains. However, this method only works if the cut-outs for the fasteners in the textile preform are drilled with extreme precision.
The project team developed a CFRP component manufacturing process that checked all the boxes by opting for a three-pronged approach of CNC cutting, laser processing and automated handling. They combined the technologies for these individual process steps in a single robot cell and automated all the steps in between. First, the preform is created by cutting, stacking and assembling the textiles. Next, an ultrashort pulsed (USP) laser drills high-precision cut-outs in the preforms for the metal fasteners.
The USP laser offers a good alternative to conventional manufacturing – but only if the laser is integrated into the robot cell. In a traditional set-up, the ultrashort pulses would be guided to their destination using mirrors, but this is hardly practical on a robot arm. To tackle this problem, experts from Fraunhofer ILT and AMPHOS GmbH worked together to develop a novel technology for coupling the USP laser beam in and out. The USP laser source is connected to the scanner on the robot arm via a hollow-core fibre.
To test the new method and demonstrate its technical feasibility, the project partners produced a demonstrator of a B-pillar component and subjected it to extensive mechanical testing, which it passed with flying colours. In a series of both pullout and torsion tests, the joints produced using the CarboLase method performed better than those in CFRP components produced by conventional means. Thanks to the interlocking connection between the inserts and the matrix material, the CFRP components produced using this new method can withstand a maximum pullout force up to 50 per cent higher than conventionally manufactured components with glued-in inserts. Depending on the component design, this improvement in mechanical performance offers the potential to reduce the overall component thickness and weight.
The CarboLase method offers designers considerably more creative freedom when it comes to defining fastener size and position. Robots and scanners can move much more freely on both the meter and micron scales than static mechanical machining centres. This paves the way for efficient mass customization of CFRP components that goes beyond the current state of the art. The dynamic USP laser drilling process is of particular interest for lightweight components in the aerospace and automotive sectors, offering the potential to reduce the process and material costs of CFRP component manufacturing.