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Lightweight Design worldwide

, Volume 10, Issue 6, pp 6–11 | Cite as

Lightweight Structures Based on Aluminium Foam Granules

  • Jörg Weise
  • Dirk Lehmhus
  • Joachim Baumeister
Cover Story Metal Foam

Fraunhofer IFAM and University of Bremen scientists combine aluminium foam granules with polymers and polymeric foams to yield lightweight materials. The resulting composites are specially adapted for the production of sandwiches and the flexible filling and reinforcement of hollow structures.

New Material Concepts

Current trends like increasing use of electric driven cars or more flexible and resource-efficient production methods in metalworking industry pose new challenges for lightweight construction. Besides design-based solutions also new material concepts have to make their contribution to address these challenges [1].

Combinations of porous and compact materials are currently used in a large variety of products, e.g. sandwiches with foam core, foam filled profiles or crash absorbing elements. The foams are mainly used in parts of the components where low mechanical loads are expected. Because of their low density, their high mass specific stiffness and their ability to absorb high amounts of deformation (crash) and vibrational energy, they contribute to the overall performance of the components. Besides polymer foams, which are produced in large volumes, also metal foams — especially aluminium foams — have found application in special fields [2]. The main difference between polymer and aluminium foams can be found in the higher thermal stability and conductivity, different thermal expansion behaviour and the generally higher strength and stiffness of the metal foams. The latter have the disadvantage of higher cost of raw materials and a more elaborate production and shaping processes. The higher process temperatures of aluminium in comparison to polymer foams (aluminium: >650 °C) lead to a more complicated process control and to lower productivity, especially when large components or components with complex geometry have to be produced. In the case of hollow structures in-situ filled with aluminium foam, the high process temperatures can lead to degradation of the material of the component.

In comparison to 3-D foam components or foam plates, metal foam granules [3] can be produced in an easy and flexible way and can be combined with polymers, polymeric foams but also inorganic materials resulting in innovative tailor-made hybrid materials. In the present paper the production of aluminium foam granules, techniques for their subsequent processing to larger structures and the basic properties of the resulting hybrid materials will be described. By means of different prototype studies the application potential of the aluminium foam granules and materials derived from them will be illustrated.

These multiphase systems allow a targeted adjustment of the mechanical properties in a wide range.

Production of Aluminium Foam Granules

Starting point for the production of aluminium foam granules is the Foaminal technology [4]: aluminium powder is mixed with a foaming agent and the mixture is subsequently compacted to a “foamable” precursor. Compaction can be done using different processes like e.g. extrusion. The foamable precursor can then be put into moulds and melted. The decomposition of the foaming agent will then cause it to expand and fill the mould. After solidification, the finished foam component can be taken out of the mould. The production of aluminium foam granules differs from this process in the fact that precursor in the shape of wires is used, Figure 1. The wire is cut into small segments which afterwards are melted and expanded in continuous belt furnaces. The internal pressure of the gas, released from the foaming agent, and the surface tension lead to the formation of almost spherical granules. After passing through the different heating zones of the furnace the granules cool down, solidify and can be collected respectively subjected to further treatments, e.g. coatings (for which the residual heat of the spheres is used). Cleaning of the granules’ surfaces is not necessary since the high process temperatures of more than 700 °C lead to decomposition of potential contaminants; furthermore, the main part of the granules’ surface is generated during the expansion phase and is thus inherently free from contaminations. Subsequent treatments like e.g. bonding of the granules to larger structures can be done without problems.
Figure 1

Foamable precursor wire (left) and aluminium foam granules (right) (© Fraunhofer IFAM)

For foaming granules all those aluminium alloys are suitable which exhibit good foaming behaviour and which are available as foamable precursor. Currently, granule production is mainly based on the alloy AlSi10 and titanium hydride as foaming agent. Size and density of the granules are adjusted by the dimensions of the wire segments and the furnace settings (zone temperatures, belt velocity). Typical granule diameters are between 1 and 12 mm, typical true densities between 0.55 and 0.7 g/cm3. If elongated wire segments are used, granules with non-spherical shapes can be produced, too.

Production of Hybrid Materials

The original technology developed for the shaping of hybrid materials based on aluminium foam granules is the Advanced Pore Morphology (APM) process, Figure 2. Here, the granules are coated immediately after the foaming with thermally activated adhesive powder (e.g. polyamide PA12 or epoxy Araldite AT1-1). The pourable coated granules can afterwards be bonded to 3-D components. Alternatively, they can be used for filling hollow parts. Bonding of the granules’ bulk is done at moderate temperatures of 120 to 190 °C. If additional foaming agent is mixed into the polymer powder, the process results in an alternative material — polymer aluminium hybrid foam, Figure 2 (right). The foaming agent releases gas during the hot curing of the polymer, the gas leads to foaming and expansion of the still soft polymer which fills the interspace between the aluminium foam granules. The advantage of this second approach in comparison to the original APM variant is an improved transfer of forces between neighbouring granules when the material is subjected to mechanical loads.
Figure 2

Polymer coated aluminium foam granules (left), APM-filled hollow structure (middle), and epoxy-aluminium foam (right) (© Fraunhofer IFAM)

The process temperatures, which are considerably reduced in comparison to the production of aluminium mono foams (120 to 190 °C as opposed to 700 °C) allow an easy and efficient filling of hollow components and hollow profiles. For most materials, property degradation can be excluded. In case that heating of the component has to be precluded completely — e.g. in the case of thermo-sensitive materials like polymers or heat-treated aluminium, or for very large structures (availability of large furnaces) — a third process variant can be used in the course of which aluminium foam granules are mixed with cold-curing polymers and introduced into the hollow component to be filled. The respective can-time of the polymer system of normally less than 30 min has to be observed. This technology is very similar to the production of polymer concrete [5]. Differences exist mainly in the density and the fracture behaviour of the respective granules.

Metal foam granules can be produced in an easy and flexible way.

Again, if the cold curing polymer is a foaming system, polymer aluminium hybrid foam can be generated, Figure 3. When contrasted to polymer mono foams, cold-curing hybrid foams exhibit the following advantages: reduced fire load density, thermal expansion coefficients which are generally better adjusted to the material of the component to be filled, well-defined filling of large volumes, reduced release of curing heat and reduction of shrinkage effects.
Figure 3

Cross-section of cold-cured polymer aluminium hybrid foam (left) and sandwich structure with hybrid foam core (right) (© Fraunhofer IFAM)

For very small aluminium foam granules another subsequent treatment is possible, i.e. the integration into injection moulded polymer components. To this end, polymers and granules are mixed, compounded, granulated and injected into moulds. Test components made of Polymethyl Methacrylate (PMMA) with different volume contents of granules are shown in Figure 4.
Figure 4

Small aluminium foam granules (left) and injection moulded PMMA specimens with different filling degrees of small aluminium foam granules (right) (© Universität Kassel | Fraunhofer IFAM)

One disadvantage of the combination of aluminium foam granules with polymeric organic materials is the reduced thermal stability in comparison to pure aluminium foams. Remedy can be found by the application of heat resistant binders, e.g. based on silicones.

Properties

The above-mentioned materials are characterised by high volume contents of the foam granules. APM structures and hybrid foams therefore exhibit the basic properties typical of porous metals and polymers, like
  • ▸ low density

  • ▸ high specific stiffness

  • ▸ reduced strength in comparison to massive materials

  • ▸ good absorption capability for deformation energy

  • ▸ good acoustic and vibration damping

  • ▸ reduced (electric, thermal) conductivity.

The fact that these materials represent multiphase systems allows a targeted adjustment of the mechanical properties in a wide range. This is illustrated in Figure 5 using the example of cold-curing epoxy aluminium hybrid foams. Compressive and tensile strength can be varied by the factors of 5 respectively 2 for materials with the same basic components by adjusting the density level. In the case of high-strength polymer foams the hybrid foams can reach the same strength level as pure aluminium foams. Furthermore, rather unusual mechanical properties can be obtained. For example, hybrid foam made from elastomer foam combined with aluminium foam granules exhibit a reversible flexible behaviour in the early stages of compression which changes to plastic deformation at higher levels of compressive strain. This latter stage is dominated by the compression behaviour of the aluminium foam. Parameters for adjusting the material properties are for the example of hybrid foams:
  • ▸ aluminium foam granules: alloy, density, size distribution, volume content (packing density)

  • ▸ polymer foam: polymer type, density, curing parameters

  • ▸ surface treatments.

Figure 5

Compressive strength and tensile strength of cold-cured epoxy aluminium hybrid foams with different compositions in dependency on the integral density (© Fraunhofer IFAM)

Prototype Studies

Loose bulks of aluminium foam granules are currently used in niche applications like “bean bags“ for out-door-photography. Apart from that, the application potential of light weight structures based on the combination of polymers and aluminium foam granules has been demonstrated in several prototype studies. Examples are components of machine tools [6], damping elements [7] or battery housings [8], Figure 6. In the restoration of metal sculptures, cold-curing epoxy aluminium hybrid foam has also been used [9], Figure 7: motivated by the reduced curing heat release and better adapted thermal properties, hybrid foam pads have been applied for the transmission of weight loads between the sculpture and the inner support structure, made from stainless steel pipes.
Figure 6

Cold-bonded battery module with hybrid foam sandwiches (© Fraunhofer IFAM)

Figure 7

Inner support structure with hybrid foam pads, restoration of the male figure of the casket of Duke Heinrich of Saxonia-Merseburg (© Fachhochschule Potsdam)

Summary

Aluminium foam granules can be combined with various organic polymers and polymeric foams, but also heat-resistant inorganic binders to yield large structures with low density. The resulting materials can be assigned to the material class of foams. They exhibit typical foam properties but offer several advantages. The main reason for this is that features of polymeric and metal foams can be combined, and the dominance of either type of behaviour easily tailored in accordance with the requirements of a specific application. First examples of practical usage as well as several prototype studies demonstrate the potential of this new material.

Compression and tensile strength can be varied by the factors of 5 and 2 respectively.

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Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2017

Authors and Affiliations

  • Jörg Weise
    • 1
  • Dirk Lehmhus
    • 2
  • Joachim Baumeister
    • 1
  1. 1.Fraunhofer Institut für Fertigungstechnik und Angewandte Materialforschung IFAMBremenGermany
  2. 2.ISIS Sensorial Materials Scientific CentreUniversity of BremenBremenGermany

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