Correlation between foam structure and mechanical performance of aluminium foam sandwich panels
Introduction
Metal foams are cellular materials consisting of a solid phase and a gaseous phase dispersed therein. As a result of their structure, they exhibit a combination of properties such as low density, favourable stiffness-to-mass ratio and good energy absorption properties. A sandwich design based on dense face sheets can yield compression, tension, torsion or bending properties beyond those of a metal foam alone. Face sheets protect the foam core from surface damage and corrosion, and allow the structure to bear tensile loads, where the bare metal foam performs poorly. An optimisation like this can also be carried out for other materials and yields structures such as honeycomb panels, stringer-stiffened structures or waffle plates. However, such even stiffer structures are prone to earlier failure under certain shear stresses due to the engineered anisotropic nature of these materials. Metals foams have a more isotropic behaviour due to their more arbitrary structure [[1], [2], [3]].
Most of these structural materials are made for lightweight construction. The base material is often from the group of lightweight metals and most commonly an aluminium alloy. This combination makes it interesting for applications in transportation. The focus of this paper lies on aluminium foam sandwich (AFS®) panels. For the different production routes and the corresponding properties of metal foams see Ref. [4]. They have been proposed for use as crash absorbers for cars or trams, battery cases for electric cars, supports of working platforms on a mobile crane vehicle, train front structures or floors of a wagons [5,6]. Additionally, due to their excellent heat diffusion and conduction properties [7] it is also possible to use AFS as a thermal conductor for cooling systems [8].
Since a product rarely consists of only one part joints are an essential component and are usually subjected to larger loads and different stress modes. Joining of metal foams has been described in detail elsewhere [9]. Joining of a sandwich structure is facilitated by the face sheets, which can be joined by traditional sheet joining methods [1]. Due to the deformability of the AFS, the edge areas can be compressed to protect the foam inside from corrosion or mechanical penetration by external bodies and to allow for connections by, for example, screwing.
Reliable processing of AFS panels into products should be aided by this work. We show correlations of the structure observed by X-ray inspection and the mechanical properties measured by tension, compression and 4-point bending tests. It is demonstrated that X-ray inspection of the panels is a viable option as quality control in production. We also provide mechanical data of AFS measured on in total 120 samples.
Section snippets
Methods
All the samples were prepared from AFS panels produced by Pohltec Metalfoam GmbH (Cologne, Germany) through a patented process [10]. Precursor production consists of mixing metal and blowing agent powders, filling a container with the mixture, and compacting and rolling the container. The precursor can then be cut to size and foamed e.g. in an infrared furnace. The process is described in more detail elsewhere [11]. After foaming, however, the sandwich material is not completely flat due to
Levelling
Levelling of foamed AFS panels is needed for most applications, because the AFS plate after foaming is not flat enough due to non-uniform expansion of the foam core. For this, the already solidified foam is mechanically levelled at temperatures close to its solidus temperature, where plastic flow is more likely than at ‘room temperature’. Curved or kinked cell walls are repeatedly found in levelled specimens (compare Fig. 2a and b). In panels which have been pressed down more severely, areas
Levelling
Levelling equalises differences in thickness. Depending on which level thickness differences must be compensated, a considerable amount of material movement might be necessary. Since levelling is carried out at elevated temperatures, the cell walls do not necessarily break but can deform at least partially plastically. As shown in Fig. 2c, bands of deformed or compressed pores can form. This has already been reported in the literature [13,14], where bands have been associated with locally
Conclusions
Large aluminium foam sandwich panels (AFS) were foamed, levelled and in some cases partially densified. The foam structure was analysed with X-ray radiography and tomography and correlated to the mechanical behaviour for a large number of samples. We find:
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Depending on the thickness of the AFS panels, the location of the pores flattened during compression or levelling changes: For thin panels almost directly next to the face sheets, for thicker panels mostly in the interior of the foam core.
Data availability
The experimental data are available from T.R.N. or F.G-M. upon reasonable request.
CRediT authorship contribution statement
Tillmann Robert Neu: Investigation, Formal analysis, Visualization, Writing - original draft, Writing - review & editing. Paul Hans Kamm: Software, Formal analysis, Visualization. Nadine von der Eltz: Investigation. Hans-Wolfgang Seeliger: Conceptualization, Resources. John Banhart: Project administration, Resources, Writing - review & editing. Francisco García-Moreno: Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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