Density-Graded Cellular Aluminum

Conventional metallic foam processing seeks to maximize uniformity in pore size, relative density, and other aspects of foam structure in order to minimize property variation associated with the statistical nature of foams, and thereby increase their reliability in service. However, property uniformity is in many cases an inefficient approach to meeting overall design criteria, as demonstrated by the nonuniform structures of many naturally-occurring porous materials (e.g., bone and wood), as well as by advances from the field of functionally-graded materials (FGM). Recently, the potential of density-graded foam structures was demonstrated explicitly by Daxner et al., who showed that spatially-varying relative densities led to improved mass efficiency even in fairly simple load-bearing foam components. Several methods have been developed over the last decade for processing of functionally-graded composite materials, some of which include as intermediate steps the production of density-graded ceramic foams, which are later infiltrated by metals to form graded interpenetrating composites (IPC). Other methods have been developed for processing of graded porous ceramics directly, without any metallic matrix. These methods generally, however, include processing steps that are not easily extended to metals, and literature pertaining to density-graded metallic foams is therefore comparatively sparse. Though several general methods exist for density-graded porous metals, only a few are suited specifically to metallic foams (i.e., with porosity in excess of 50–60 vol.%); the latter have been demonstrated for Cu, Ni, Mg, and Al-based metallic foams. However, a general feature of these methods is the production of stepwise, discontinuous density gradients, which are likely to be accompanied by higher flaw densities and/or property incompatibilities in the interfacial regions separating adjacent uniform-density layers. In this work, we describe a new method for production of metallic foams having density gradients which are both controllable and continuous. The method, based on replication of density-graded polymer foams through investment casting, shares an initial step with the approach introduced by Cichocki et al. for production of graded porous ceramics, but varies substantially in its later steps and in the porosity and structure of the final products. Though the method is only demonstrated here using simple graded aluminum strucR ES EA R C H N EW S