Solidification microstructure and grain refinement of closed-cell aluminum foams

The solidification microstructure of aluminum foams is evidently different from that of bulk aluminum alloys. Generally, a single α-Al grain runs through the entire thickness of a cell-wall, and the grain morphology is irregular. However, the formation mechanism of the special microstructure in aluminum foams is unclear now, and few studies focused on optimizing the microstructure. In this study, grain refinement of aluminum foams by increasing cooling rate and inoculation were carried out, and thermal analysis was used to measure and judge solidification of aluminum foams. Experimental results show that grain refinement can be achieved by the above two methods, and the latter is more effective. The nucleation undercooling of aluminum foams decreases with the increase of Al–5Ti–1B addition. Grains can be well refined even in thin cell walls after inoculation, and the grain morphology changes from irregular to equiaxed. Under the same conditions, α-Al grains of Al foams are smaller than those of the bulk Al alloy, either with or without inoculation. An anisometric growth model of α-Al grains in foamed aluminum melt restricted by surrounding bubbles is proposed. The model shows that: (1) the size of the inhibited nucleation zone (INZ) around growing grains decreases due to the high constitutional undercooling caused by solute enrichment in cell walls, thus grains in cell walls are smaller than those of bulk Al alloy; (2) under normal solidification conditions, the shape of the INZ is irregular. Grains in aluminum foams will replicate contours of cell walls or Plateau borders, presenting irregular shapes. Increasing cooling rates and/or inoculation can reduce the size of INZ and change its shape. The grain morphology will change from irregular to equiaxed under proper conditions. Graphical abstract