The limits of solid state foaming

Ultralight metal foams can be produced by gas expansion in either the molte n or solid state by the release of H-2 during the decomposition of TiH2 par ticles. An alternative approach uses a powder metallurgical route to delibe rately trap a low solubility gas within interparticle spaces during consoli dation. This is subsequently used to plastically expand the voided solid du ring a post-consolidation heat treatment. Porosities of about 40% have been reported. However this is only about half that of melt-foamed materials an d there is much interest in designing processes that increase it. Micromech anical models for the plastic expansion process are developed and used to i dentify the practical porosity limits in this entrapped gas expansion appro ach. It is shown that the porosity is limited by the reduction in pore pres sure as voids expand, and ultimately by the loss of gas accompanying void c oalescence. Increasing the initial pore pressure is shown to also lead to t he formation of face sheet delaminations in stiffened, porous core sandwich panels. Its dependence on the process methodology is considered. Achievabl e porosities during solid state foaming are shown to be limited to less tha n 50%; much less than that of metals foamed in the liquid state. A simple e xtension of the analysis to semi-solid stare expansion shows that much high er porosities could he achievable under these conditions because void coale scence can be avoided. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.