MC-Sandwich | Realization of functionally integrated, highly complex metal-CFRP sandwich structures

Production of filigree core structures by using additive manufacturing

By using additive manufacturing to produce the filigree and complex metallic core structures, existing gaps in the performance spectrum of sandwich structures will be closed. Incorporating functional elements, which allow, for example, a mechanical connection to connected assemblies or electrical conductivity, is supposed to be ensured.

Optimization of the bonding of metallic core to CFRP face sheets

The thin and stiff cover layers made of carbon-fiber-reinforced plastic (CFRP) can also be optimized by fiber orientation. The top layer is applied to the core directly in the automated fiber laying process so that no further bonding steps are necessary.

In addition, depending on the material, the usual post-curing heat treatment of additively manufactured metal structures is to be carried out together with the curing process of CFRP so that synergies are also exploited here to shorten the process chain.

In addition to the processing technologies, the surface pretreatment of the metal structure will also be considered to ensure optimum bonding between the core and surface layers and thus achieve the targeted load transfer.

In the project, thin-walled honeycomb structures made of the aluminum alloy AlSi10Mg and the copper alloy CuCr1Zr have been realized by laser beam melting (LBM) to create the core structure. A sample geometry was used to investigate how different exposure strategies and orientations of the components in the build space affect buildability and wall thicknesses. Subsequently, the project identified possible directions of the components in the build space of the LBM systems as well as rules for component placement. By performing a stabilized compression test, the correlation of process parameters, heat treatment conditions, and core properties was investigated.

To produce even more filigree core structures, experiments on a process with modulated laser power are currently underway. Due to lower energy input, thinner structures can be achieved than with the conventional, continuous operation of the laser. After initial tests have been successfully carried out with the nickel-based alloy IN718, which is easy to process, the transfer to the titanium alloy Ti6Al4V, which is interesting for lightweight applications, is pending.

To optimize the bond between the metallic core and the top layer of CFRP, investigations were first carried out into the wettability of the metallic surface with epoxy resin. Contact angle measurements were made on polished aluminum test specimens with different chemical coatings and later on test specimens with other mechanical surface treatments. Based on the results, Double-Lap Shear (DLS) specimens were developed, which were used for initial tensile shear tests. Surface modification with organosilanes and a DLC (Diamond-like-Carbon) coating showed that the bonding could be significantly increased.

One focus for further increasing the bonding and thus the sandwich performance is increasing the bonding area of the core structures. Particular attention is being paid here to the ability to parameterize the tops of webs in honeycomb structures to control the area available for laminate adhesion in line with requirements. The buildability through additive manufacturing is ensured by considering specified parameters in the workflow of a digital development environment.