The limits of solid state foaming

Ultralight metal foams can be produced by gas expansion in either the molten or solid state by the release of H-2 during the decomposition of TiH2 particles. An alternative approach uses a powder metallurgical route to deliberately trap a low solubility gas within interparticle spaces during consolidation. This is subsequently used to plastically expand the voided solid during a post-consolidation heat treatment. Porosities of about 40% have been reported. However this is only about half that of melt-foamed materials and there is much interest in designing processes that increase it. Micromechanical models for the plastic expansion process are developed and used to identify the practical porosity limits in this entrapped gas expansion approach. It is shown that the porosity is limited by the reduction in pore pressure as voids expand, and ultimately by the loss of gas accompanying void coalescence. Increasing the initial pore pressure is shown to also lead to the formation of face sheet delaminations in stiffened, porous core sandwich panels. Its dependence on the process methodology is considered. Achievable porosities during solid state foaming are shown to be limited to less than 50%; much less than that of metals foamed in the liquid state. A simple extension of the analysis to semi-solid stare expansion shows that much higher porosities could he achievable under these conditions because void coalescence can be avoided. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.