Ex-situ bonding is achieved by gluing face sheets with an aluminium foam by adhesive bonding, brazing or diffusion bonding. Foams used in this method are either closed-cell or open-cell. When a closed-cell foam is used then it is produced from aluminium alloys either by liquid metal route (e.g. Alporas, Cymat) or by powder metallurgy route. Open-cell foam core is made of aluminium and other metals as well. Face sheets are chosen from a variety of aluminium alloy, and other metals such as steel.