Functionally Graded Al Foam Fabricated by Sintering and Dissolution Process with Remaining Spacers

Open-cell metallic foam is expected to be used for components in automobiles owing to its superior shock absorbance and light weight1). A space holder method is a promising process for fabricating open-cell metallic foam owing to its control of the pore structures. In this process, a metal/spacer composite is rst fabricated by a casting process2) or by a powder metallurgy process3–5). Thereafter, the spacers are removed from the composite, resulting in the formation of pores in the metal. During the removal process of the spacers, two regions exist. One is the metallic foam region in which spacers have already been removed, and the other is the region with remaining spacers (hereinafter “spacer region”). In the spacer region, it is considered that the metallic foam is reinforced by the spacer particles. Previous studies6) indicated that the compression deformation of a metal/spacer composite, which has both a metallic foam region and a spacer region, rst occurs in the metallic foam layer and then in the spacer layer. This deformation behavior is similar to that of functionally graded (FG) metallic foam, in which the compression properties vary with the position and deformation sequentially occurs from the lower-strength layers to the higher-strength layers7,8). Also, it was shown that the compression properties of a metal/spacer composite change with the percentage of the spacer region in the composite. Therefore, it is expected that FG metallic foam with a gradual change in its mechanical properties, which depend on the removal time of the spacers, can be realized by forming a metal/spacer composite. In this study, an FG aluminum (Al)/sodium chloride (NaCl) composite was fabricated by a sintering and dissolution process (SDP)3), using NaCl as the spacers, so that the removal of spacers is easily achieved by simply immersing the Al/NaCl composite in water. First, the Al/NaCl composite was observed nondestructively by X-ray computed tomography (CT) during the spacer removal process to reveal the changes in the three-dimensional region in which NaCl remained (hereinafter “NaCl region”) to con rm that two layers, i.e., an Al foam layer and a NaCl layer, actually exist in the FG material. Second, a compression test specimen with a remaining NaCl weight ratio of approximately 10% was fabricated. The deformation behavior during compression tests of the specimen was nondestructively observed by X-ray CT and the mechanical properties of the FG Al/NaCl composite were investigated. From these nondestructive observations, the effect of the NaCl region in the Al/NaCl composite on the mechanical properties and deformation behavior of the Al foam was investigated.