Bubble Size Distribution in Foaming of Liquid Aluminum and the Role of Coarsening and Coalescence

Evolution of bubble size distribution in liquid aluminum foam prepared by dissociation of TiH₂ particles is studied by rapidly cooling samples during foam expansion and carrying out metallographic analysis of the solidified foam. Further, diameters of bubbles growing directly on TiH₂ particles are calculated to arrive at volume expansion and bubble size distribution. The cells in the solidified foam are orders of magnitude coarser and fewer than those predicted by the “pure growth” based model at all stages of foam expansion. If bubble coarsening was accounted for, the calculated bubble sizes are less than, but the same order of magnitude as the experimentally observed cell sizes. The remaining discrepancy of cell size (and population) can be explained based on bubble coalescence, strongly supported by observations on early stages of foaming.