For in-situ bonded face sheets the core is closed-cell foam. The goal of in-situ bonding is to create a metallic bonding between the foam core and face sheets. This is achieved in three ways. A foamable precursor is expanded between two face sheets.  When the liquid foam comes in contact with the solid face sheets a metallic bond is established. This is difficult to realize as the oxidation of both aluminium face sheets and foam prevent forming a sound bonding. There is also a risk of melting the face sheets. This procedure is successful when steel is used as face sheets instead of aluminium, while the foam core is aluminium.
Another strategy is to rapidly solidify the surface of a foamable molten metal before it can foam into a dense skin while the interior of the metal evolves to a foam structure. This process yields in an integral-type foam structure. Integral foam sandwich is made of aluminium alloys (AlCu4, AlSi9Cu3) and magnesium alloys (AZ91, AM60). In this process the material for the core and face sheet is the same.
The third way to achieve in-situ bonding 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 melting point of the face sheet material 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.