As part of the MuMaK joint project, a new joining technology was developed for even lighter and more resilient vehicle bodies. To this end, technological solutions were developed for the high-strength and durable joining of fiber-reinforced plastics and metal components.
The aim of the Fraunhofer IWS subproject was to develop a process for laser structuring of fiber-reinforced plastic composites (FRP). This surface pretreatment ensures optimal bonding of the subsequently generated metallic spray coating. This creates the necessary conditions for the final joining of FRP and metal components using a soldering process.
Within the research project, laser-pre-structured fiber composites (carbon fiber-reinforced thermoset) were surface-functionalized by thermal spraying with metallic layers (copper). The possibility of thermally depositing firmly adhering layers on notch-free substrates opens up a wide range of possible applications – even beyond the joining process addressed in the project. For example, plastic-based components can be given metallic properties on their surface to increase wear resistance, media resistance, or electrical conductivity. Subsequent joining processes make it possible to connect metallized lightweight structures (CFRB-based) to metal components in a material-locking manner.
For thermal spraying, substrate pretreatment using sandblasting is the most common state-of-the-art technique. In our development work, we were able to demonstrate that material-specific laser pretreatment leads to a significant improvement in coating adhesion. Compared to sandblasted samples, the adhesive strength in the tensile test was more than doubled, reaching values of 18.1 MPa. Comparable increases were seen in the shear pull test with strengths of 18.2 MPa. The key here is a combination of laser-based surface roughening and trench-shaped structural elements that serve as mechanical anchor points for the coating material.
The advantages of laser pretreatment are also clearly evident in the 4-point bending test on samples coated with copper on both sides. Compared to the sandblasted reference, the laser-structured variants were significantly more resilient. With sandblasted substrate material, the bending stress causes the layer adhesion to fail, resulting in the metal layer flaking off the CFRP. In contrast, with laser pretreatment, the metallic coating remained intact even after the mechanical failure of the CFRP substrate and showed no signs of delamination.
Laser surface pretreatment or functionalization can be implemented both locally and over large areas. For the economical processing of large areas, it is necessary to scale up the structuring process. As part of the research project, initial investigations were therefore carried out to parallelize the process using diffractive optical elements (DOE), which significantly reduced the processing time. Investigations into the soldering of coated CFRP elements to steel components revealed a further advantage of laser-pretreated, metallized CFRP samples. Laser pretreatment allows the flux to be expelled more efficiently during the soldering process, leaving smaller amounts of flux residue in the joined component. This is expected to have positive effects on long-term durability. Corrosion tests showed that the joints produced were resistant.
The technology chain developed in the project made it possible to demonstrate a soldered connection between CFRP and steel or aluminum for the first time. This joining technology represents a promising alternative to adhesive bonds, which are susceptible to aging. Property rights have been registered for parts of the developed technology.
Fraunhofer Institute for Material and Beam Technology IWS