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Aluminium Foam Sandwich

Aluminium Foam Sandwich (AFS) is sandwich panel product which is made of two metallic dense face sheets and a metal foam core made of an aluminium alloy. AFS is an engineering structural material, and has several advantages over other sandwich panel structure. The most notably are its stiffness-to-mass ratio and energy absorption capacity.

Production and Materials
In terms of the bonding between face sheets and foam core the processing of AFS can be categorised into two ways – ex-situ and in-situ bonding.

Ex-situ bonding AFS
Ex-situ bonding is achieved by gluing face sheets with an aluminium foam by using adhesive. Foams used in this method can be either closed-cell or open-cell. If a closed-cell foam is used then it is produced from aluminium alloys either by liquid metallurgy or by powder metallurgy (PM) route. Such closed-cell foams include Alporas©, Cymat© and PM foam. Open-cell foam core can be made of aluminium and other metals as well. Face sheets could be chosen from a variety of aluminium alloy, and even other metal such as steel.

In-situ bonding AFS
For in-situ bonded face sheets the core is essentially closed-cell foam. The main idea of in-situ bonding is to create a metallic bonding between the foam core and face sheets. This can be achieved in three ways. A foamable precursor is expanded between two face sheets with the idea of establishing a metallic bond when the liquid foam comes in contact with the solid face sheets. However this is difficult to realize as the oxidation of both face sheets and foam prevent forming a sound bonding. As a result this is still in an experimental stage.

Another strategy is to rapidly solidify the surface of the molten metal before it can foam into to a dense skin while the interior of the metal evolves to a foam structure. This process yields in an integral foam. Integral foam sandwich can be made of aluminium alloys (AlCu4, AlSi9Cu3) and magnesium alloys (AZ91, AM60). Here the core and face sheet material is the same.

The third way to achieve in-situ bonding is the most challenging one because it requires special processing steps. This process consists of compaction of metal powders together with face sheets. This sandwich-compact assembly goes through several rolling steps to attain desired precursor and face sheet thickness. After which this three-layer composite is heated to transform the core layer into foam. The face sheet material is chosen in such a way so that its melting point is above the melting point of the foamable precursor material. The precursor composition is usually Al-Si, Al-Si-Cu or Al-Si-Mg alloys while the face sheets are 3xxx, 5xxx and 6xxx series aluminium alloys.