Formation of skin surface layer on aluminum foam by friction stir powder incremental forming

To improve strength–mass relationship of a closed-cell type aluminum foam, friction stir powder incremental forming (FSPIF) process was applied to form nonporous skin surface layer on the foam surface from aluminum powder. In this process, the powder was supplied to the surface pores of the foam, and then, the powder and the cell wall near the foam surface were incrementally pressed by a rod-shaped tool without rotation (incremental hammering (IH)) and incrementally stirred by the tool at a very high rotation rate (friction stir incremental forming (FSIF)). The skin layer with a maximum thickness of 1.4 mm was formed on the foam surface without deforming the porous structure of the inside of the foam under the forming conditions, a tool rotation rate of 8000 rpm, a tool feed rate of 60 mm/min, and a total forming depth of 10 mm. The formation mechanism of the skin layer from the powder was investigated by microscope observation. Due to combination of sandwich structure and increase of bulk density, the specific plateau stress and the plateau end strain of the foam specimen with the skin layers were improved up to 1.2–1.6 times greater in a uniaxial compression test.