Sustainability and energy savings are becoming increasingly important issues that all industrial sectors have to face. A frequent answer to this problem in many technical areas is a change in common designs in the sense of lightweight construction. Fibre composite materials enable enormous weight savings. However, due to their anisotropic properties, fibre composite materials cannot be used everywhere and do not achieve the necessary strength and ductility to completely replace metals. [1]

Hybrid casting procedures could serially combine the positive properties of fibre composite materials and metals. Elaborate joining processes that influence the material properties would therefore be eliminated as well as time-consuming bonding preparations.

In an earlier project at the Fraunhofer IFAM the production of a multi-material system between CFRP and aluminium in an HPDC process using the high performance material PEEK as matrix material in combination with different PEEK films with varying thicknesses was investigated. Single-lap and double-lap samples were tested, single-lap-specimen shown in figure 1. For all film thicknesses, only superficial structural damage to the PEEK due to the heat input from the aluminium melt could be determined. The damage manifests itself through increased pore formation (presumably outgassing). The C-fibres never came into contact with the molten aluminium and therefore remained undamaged.

Figure 1. Single-lap-specimen.

In the case of the double-lap samples, good connection qualities with form-fitting interlocking between aluminium and CFRP organic sheets could be determined. Within the single-lap specimen a good material connection could be realized during the casting process. In tension tests on these samples, adhesive fractures, mixed fractures and delamination in the CFRP could be observed. Due to the knowledge gained continuously from the basic tests on the single lap samples, an application-related hybrid holding element (bracket) from aircraft construction was developed and implemented during the project. Further experiments with the bracket are planned.

Due to the price structure of PEEK, it is mainly suitable for niche applications with high material requirements in use, such as aerospace, chemical or medical technology. [2]

For broad applications with higher volumes, there is a need to use cheaper matrix materials for the CFRP (e.g. epoxy matrices). This requires the use of a thermal insulation material to realize the joint within the Al HPDC process with CFRP materials that have a much lower temperature resistance than the PEEK used so far. The current work deals with investigations regarding the selection and characterization of different insulating layer concepts on the CFRP, as well as an evaluation of the suitability of the insulating layers for realizing the joining technology described above.

In the recent work different lacquer-systems as well as different foil-systems (PEEK and PEI) and matrix-systems (PA6, Epoxy and PEI) where tested. The application of coating systems in suitable layer thicknesses proves to be challenging. Polymer layers, whether in the form of subsequently applied films or layers applied in a matrix, are found to be worthwhile for further processing. These layers exhibit higher strength and result in lower damage to carbon fiber-reinforced plastics (CFRP). Additionally, increasing layer thickness enhances insulation effects.

In the course of testing, a ceramic protective coating, for instance, through thermal spraying, is being examined. A promising alternative involves utilizing die casting tools with reduced wall thicknesses, closer to realistic applications. This leads to lower heat input and impact, consequently yielding higher mechanical strengths.

References

[1] L. Kroll and D. Nestler, Verbundwerkstoffe und Werkstoffverbunde, in Technologiefusion für multifunktionale Leichtbaustrukturen, L. Kroll, Ed. Berlin: Springer Vieweg, 2019, pp. 11–22.

[2] https://readingplastic.com/peek-plastic-price-per-pound/