Lightweight Structures Based on Aluminium Foam Granules
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 .
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 . 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  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
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 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 . 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.
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.
▸ 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.
▸ aluminium foam granules: alloy, density, size distribution, volume content (packing density)
▸ polymer foam: polymer type, density, curing parameters
▸ surface treatments.
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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