Processing and Development of a New High Strength Metal Foam
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Processing and Development of a New High Strength Metal Foam A. Rabiei*, Adrian T. O’Neill and Brian P. Neville Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC, 27695-7910, U.S.A. ABSTRACT The research sited in this paper involves the development of new closed cell metal foam composite materials using powder metallurgy (PM) and gravity casting techniques. The foam is comprised of steel hollow spheres packed into a random dense arrangement, with the interstitial space between spheres occupied with a solid metal matrix. Using a casting technique, an aluminum alloy infiltrates the interstitial spaces between steel spheres. In a powder metallurgy method, steel spheres and iron powder are sintered to form a solid, closed cell structure. The measured densities of the Al-Fe composite foam and iron foam are 2.4 g/cm3 and 3.2 g/cm3, with relative densities of 42% and 41% respectively. The hollow sphere metal foam composite materials developed in this study displayed superior compressive strength as compared to hollow sphere foams currently being produced. The compressive strength of the cast Al-Fe foam averaged 67 MPa over a region of 10 to 50% strain, while the steel PM foam averaged 45 MPa over the same strain region. Densification began at approximately 50% for the cast foam and 55% for the PM foam. INTRODUCTION Metallic foam is a class of materials with very low densities and novel mechanical, thermal, electrical and acoustic properties that are as of yet imperfectly developed and characterized [1-3]. They exhibit a unique combination of these properties, yielding an attractive material for use in the automotive, aerospace, and biomedical industries, among several previously identified applications [4]. However, they have been shown to experience fatigue degradation in both tension and compression. Plastic deformation under cyclic loading occurs within deformation bands, which form preferentially at large cells in the ensemble, until the densification strain has been reached. Such cells develop plastically buckled membranes that experience large strains upon further cycling and will lead to cracking and rapid cyclic straining [5]. As a result, the performance of existing foams has not been promising due to strong variations in their cell structure, mainly because there has never been a process to produce these materials in a uniform manner [5-7]. Various techniques have been used to process these materials including liquid metallurgy, coating techniques and powder metallurgy. One of the obstacles in the production of closed cell metallic foams is the inability to finely control the cell size, shape, and distribution, which makes it difficult to create a consistently reproducible material where the properties are known with predictable failure [1-3,8]. In order to create a uniform closed cell metallic foam, one technique being explored is to use prefabricated spheres of a known size distribution and join them into a solid form. One such material has been developed
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