Fabrication of skin layer on aluminum foam surface by friction stir incremental forming and its mechanical properties

Porous metals with a nonporous skin surface layer (sandwich structure) have a potential to improve their mechanical properties. Friction stir incremental forming process for sheet metals is applied to form the surface of a closed-cell type aluminum foam. In this process, the cell walls near the aluminum foam surface are plastically deformed and stirred by the rotation of a forming tool at a very high rate, and the nonporous skin layer is fabricated on the surface of the aluminum foam. Nonporous aluminum skin layer with a thickness less than 400 μm is fabricated at the surface without internal fracture of the aluminum foam under the following forming conditions; a tool rotation rate of 8000 rpm, a tool feed rate of 60 mm/min, and a total forming depth of 7 mm. To investigate the mechanism of formation of the skin layer, the skin layers fabricated with friction stir incremental forming and incremental hammering are compared. The compressive deformation behavior of aluminum foam with a skin surface layer is investigated by performing uniaxial compression test. The specific compressive strength of aluminum foam with a nonporous skin surface layer is improved by approximately 20-50%.