Solidification of metal foams

Expansion and contraction phenomena during solidification of liquid metal foams were studied. Such foams were processed by mixing metal powders with TiH2 powder and compacting the resulting blends, after which the compacted powders were melted. The subsequent foaming process was monitored in situ by X-ray radioscopy. An intermediate expansion stage during solidification was observed. This solidification expansion (SE) could be linked to phase transformations in the alloy. SE was found to depend mainly on the time spent at the foaming temperature before cooling (holding time), the cooling rate and the alloy composition. The interplay between gas shrinkage, solidification shrinkage, gas production by the blowing agent and gas losses due to out-diffusion was identified as the main reason for SE. While the blowing agent had a major influence on SE, gas dissolved in the metal also played a role, since some SE was observed in foams blown without TiH2 by pure pressure manipulation.