Projects

HPCi® technology for thermal direct joining of fiber - reinforced composities to metal

Joining gun
© Fraunhofer IWS

Joining gun

Load-adapted multi-materials combining the material-specific advantages of metal and thermoplastics are becoming more and more important for applications in industry, above all in lightweight construction. For this purpose, it is necessary to find efficient process chains that use - aligned with the specific load - an optimized pre-processing and joining technology, as well as modified tools for process simulation and feature characterization.

Post- and in-mould assembly processes, mechanical joining methods, such as screwing and riveting, and adhesive bonding are techniques that actually  have been commercially available to reliably manufacture joints consisting of heterogeneous materials. Specific limitations of these technologies mainly arise from:

  • Limitations in geometry complexity
  • Local cross section thinning and impaired force flow in fiber reinforced polymers  (FRP)
  • The necessity of using additional materials
  •  Longer process times.

Consequently, new joining techniques that work quickly without the above mentioned limitations were sought.

Solution

Scheme of thermal direct joining
© Fraunhofer IWS Dresden

Scheme of thermal direct joining

Research on surface pre-processing and adhesive bonding technologies, as well as thermally induced joining methods, was done at the Fraunhofer IWS Dresden, based on their extensive materials knowledge and process expertise in laser- and plasma-based production technologies. The thermal direct joining technologies are characterized by short bonding times and the substitution of additional material (e.g. adhesive).

When joining mixed compounds consisting of metal and thermoplastic, the IWS makes use of load-adapted surface structuring, layers to improve adhesion and various heating strategies. Application-appropriate heating is generated by laser beam, heating elements or induction.

The required heat input is adapted to the specific materials combination and the joining part’s geometry as a function of the technology based on numerical simulations in order to provide a sufficient melting volume without damaging the base materials.

Results

Mechanical characterization/ thermal direct joining
© Fraunhofer IWS Dresden

Mechanical characterization/ thermal direct joining

Center armrest of a car
© Fraunhofer IWS

Center armrest of a car

In a first step, the metal surface is structured according to the expected load or is enhanced by structures made by additive manufacturing to enable an optimal bonding of the molten thermoplastic material. Besides the form fit, made by surface structures, the adhesion is only based on weak non-covalent interactions (Van the Waals forces). To significantly increase the strength of these metal-thermoplastic mixed compounds, adhesion promoting layers can be applied. The direct compound created this way combines the active joining principles based on form-fit and adhesive bonding.

At the Fraunhofer IWS Dresden, various application-adapted heating strategies were analyzed in terms of their application potentials and constraints for the profitable joining of highly complex metal and thermoplastic part geometries. Thus, within the scope of the BMBF project” LaserLeichter“ (Laser-Lighter), a technology for laser-based thermal joining of mild steel and aluminum with glass fiber reinforced PA6 material was engineered. This technology ensures joint strength values comparable with those achieved by adhesive bonding – within extremely short joining times. Tests were performed under different loading conditions to model the material characteristics. This makes it possible to exactly reproduce the joint for FEM component simulations.

The results are involved in current demonstrator designs (multi-material battery housing and center armrest) of the project partners and thus enable a stringent multi-material design – from dimensioning up to the serial product.

The research project “LaserLeichter” was carried out under the aegis of the Federal Ministry for Research and Technology BMBF and is registered under the code 13N12878.