Thermal transport and fire retardance properties of cellular aluminium alloys

Closed-cell aluminium alloy foams exhibit exceptional resistance to fire. It is unclear why this happens, although the protection imparted by oxide Al2O3 layers has been suggested. This work attempts to uncover the thermal transport processes in metallic foams. The apparent thermal conductivities of two-dimensional foams having a variety of cellular microstructures are first calculated. These include regular honeycombs, Voronoi structures and Johnson-Mehl models. The effects of several types of geometric imperfection-Plateau borders, cell-edge misalignments, fractured cell edges, missing cells, inclusions and cell size variations-are studied by using analytical as well as finite element methods. The focus is on metallic foams where the transport of heat is dominated by solid conduction and thermal radiation; contributions from gaseous conduction and convection are neglected. The coupling of solid conduction with thermal radiation is dealt with by using the method of finite elements. These results are then applied to solve the transient temperature field of a cellular metal plate subjected to a sudden introduction of a high-temperature source of heat such as fire. The factors which dictate the thermal and structural fire retardance of cellular metallic foams are identified. (C) 1999 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. All rights reserved.