Anisotropy and strain localization in steel foam
Introduction
Metallic foams exhibit unusual mechanical and thermal properties, including energy absorption, vibrational and acoustic damping, and thermal insulation [1], [2]. Because of these unique properties, metallic foams may find applications in impact absorbers, ultralight sandwich structures, compact heat exchangers, and heat dissipation media. However, before these applications can be realized, two things are required, a mature and robust process technology for manufacturing metallic foams with consistent properties, and a knowledge amongst design engineers about the mechanical characteristics of metallic foams, their durability, and how to use them intelligently in structural applications. These requirements have not been met, although various molten metal and powder metallurgical processing methods have been reported [3], [4], [5], [6], [7], [8], and the mechanical behavior of foams has been reviewed in the treatise by Gibson and Ashby [9].
The basic mechanical properties of metallic foams are not unlike polymeric foams, and both generally conform to the theory of cellular materials [9]. However, there are notable deviations from theoretical predictions. For example, the elastic modulus and compressive strength of closed-cell aluminum foams is reportedly lower than predicted values [9], [10]. Insight into this discrepancy is provided by microstructural observations, which indicate that defects present in the cell structure may degrade mechanical properties [11]. Simple defects include cell wall curvature and non-equiaxed cell shapes. The latter defect can contribute to anisotropic mechanical properties, while the former can lead to localized deformation [12].
The present work focuses on mechanical anisotropy in steel foams synthesized by a powder metallurgical process. In this process, steel powder is blended with a granular foaming agent, compacted, then expanded to foam by heating the compact briefly above the melting point [13], [14]. Results from compression tests performed parallel and perpendicular to the foaming direction are compared with model predictions, and deformation mechanisms are observed and related to the measured response.
Section snippets
Experimental
A closed-cell steel foam material was synthesized by a P/M route and used for experimental testing [13]. Commercially available steel powder (Fe-2.5C blend) was blended with 0.2 wt.% of a granular foaming agent (MgCO3 or SrCO3). Blended powders were compacted by uniaxial cold pressing, yielding virtually non-porous, semi-finished steel samples. The resulting semi-finished samples were subsequently melted in an open graphite mold held in an air furnace at 1330°C to effect foam expansion. The
Results
Expanded foams were elongated in the foaming (vertical) direction, resulting in roughly ellipsoidal cells (Fig. 2(a)). During foam expansion, ellipsoidal cells evolve because of the microstructure of the pressed compact. During powder compaction by uniaxial cold-pressing, some flattening of the steel powder particles occurs, resulting in ellipsoidal shapes [12], [15]. As a result, prior particle boundaries (PPBs) tend to extend normal to the pressing direction, and foaming agent granules tend
Conclusion
Steel foams fabricated by a P/M route show distinct anisotropy when subjected to uniaxial compressive loading. The foams are ∼3× stronger in the transverse direction than in the longitudinal direction, and exhibit a more pronounced yield point drop. The anisotropic response is directly related to the foam structure, which is characterized by ellipsoidal cells with thick walls extending transverse to the foaming direction. In both orientations, behavior is described by a power-law with a similar
Acknowledgements
Financial support from the TRW Foundation and from the M.C. Gill Corporation is gratefully acknowledged.
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