Aluminum Alloy Foam Research Articles

Experimental analysis of deformation mechanisms in a closed-cell aluminum alloy foam

The evolution of plastic deformation in a cellular Al alloy upon axial compression is monitored through a digital image correlation procedure. Three stages in the deformation response have been identified. The first involves localized plastic straining at cell nodes. It occurs uniformly and leads to a nominal loading modulus appreciably lower than the stiffness. The second comprises discrete bands of concentrated strain containing cell membranes that experience plastic buckling, elastically constrained by surrounding cells. In this phase, as the loading increases, previously formed bands harden, giving rise to new bands in neighboring regions. The localized bands exhibit a long-range correlation with neighboring bands separated by 3–4 cells along the loading direction. This length scale characterizes the continuum limit. Thirdly, coincident with a stress peak, σ o, one of the bands exhibits complete plastic collapse. As the strain increases, this process repeats, subject to small stress oscillations around σ o.

Optimized acoustic properties of cellular solids

The optimized cell size and shape, cell size variations, sample thickness, and air cavity depth behind the sample for best sound absorption performance of air-filled porous materials having simple cell morphologies are studied in this paper. The focus is on cellular foams that are rigidly framed, e.g., aluminum alloy foams and honeycombs. The governing equations of wave propagation are solved by using the point-matching method, and the predictions are compared with known analytical solutions. The effects of cell size variations are studied for Voronoi polygons. A domain-matching method is introduced to obtain the optimal combination of cell size and shape, sample thickness, and cavity depth for selected ranges of frequency. At given porosity, the effect of cell shape on sound absorption is small. The optimized cell size for best sound absorbers is on the order of ∼0.1 mm for practical combinations of sample thickness, cavity depth, and porosity. A random distribution of cell sizes tends to tighten the region where combinations of sample thickness and cavity depth achieve high sound absorption coefficient.

Fatigue failure of an open cell and a closed cell aluminium alloy foam

The tension–tension and compression–compression cyclic properties are measured for an open cell “Duocel” foam of composition Al 6101–T6, and a closed cell “Alporas” foam of composition Al–5Ca–3Ti (wt%). The Duocel foam has a relatively uniform microstructure, and undergoes homogeneous straining in both monotonic and fatigue tests. In contrast, the Alporas foam is more irregular in microstructure, and exhibits crush-band formation at random locations under uniaxial compression; in compression–compression fatigue, a single crush band forms and broadens with additional fatigue cycles. Progressive shortening of the specimen in compression–compression fatigue, and progressive lengthening in tension–tension fatigue are due to a combination of low cycle fatigue failure and cyclic ratchetting. S– N fatigue curves are presented for the onset of progressive shortening in the compression tests, and material separation in the tension tests; it is envisaged that such curves will be of practical use in design.

Sound absorption in metallic foams

The sound absorption capacity of one type of aluminum alloy foams—trade name Alporas—is studied experimentally. The foam in its as-received cast form contains closed porosities, and hence does not absorb sound well. To make the foam more transparent to air motion, techniques based on either rolling or hole drilling are used. Under rolling, the faces of some of the cells break to form small sharp-edged cracks as observed from a scanning electronic microscope. These cracks become passage ways for the in-and-out movement of air particles, resulting in sound absorption improvement. The best performance is nevertheless achieved via hole drilling where nearly all of the sound can be absorbed at selected frequencies. Combining rolling with hole drilling does not appear to lend additional benefits for sound absorption. Image analysis is carried out to characterize the changes in cell morphologies due to rolling/compression, and the drop in elastic modulus due to the formation of cracks is recorded. The effects of varying the relative foam density and panel thickness on sound absorption are measured, and optimal relative density and thickness of the panel are identified. Analytical models are used to explain the measured increase in sound absorption due to rolling and/or drilling. Sound absorbed by viscous flow across small cracks appears to dominate over that dissipated via other mechanisms.

Uniaxial stress–strain behaviour of aluminium alloy foams

The tensile and compressive stress–strain behaviour of closed cell aluminium alloy foams (trade name “Alulight”) has been measured and interpreted in terms of its microstructure. It is found that the foams are anisotropic, markedly inhomogeneous and have properties close to those expected of an open cell foam. The unloading modulus and the tensile and compressive yield strengths increase non-linearly with relative density. The deformation mechanisms were analysed using image analysis software and a d.c. potential drop technique. The scatter in results is attributed to imperfections within the foam. These include non-uniform density, weak oxide interfaces, and cell faces containing voids and cracks.

Thermal transport and fire retardance properties of cellular aluminium alloys

Closed-cell aluminium alloy foams exhibit exceptional resistance to fire. It is unclear why this happens, although the protection imparted by oxide Al 2O 3 layers has been suggested. This work attempts to uncover the thermal transport processes in metallic foams. The apparent thermal conductivities of two-dimensional foams having a variety of cellular microstructures are first calculated. These include regular honeycombs, Voronoi structures and Johnson–Mehl models. The effects of several types of geometric imperfection—Plateau borders, cell-edge misalignments, fractured cell edges, missing cells, inclusions and cell size variations—are studied by using analytical as well as finite element methods. The focus is on metallic foams where the transport of heat is dominated by solid conduction and thermal radiation; contributions from gaseous conduction and convection are neglected. The coupling of solid conduction with thermal radiation is dealt with by using the method of finite elements. These results are then applied to solve the transient temperature field of a cellular metal plate subjected to a sudden introduction of a high-temperature source of heat such as fire. The factors which dictate the thermal and structural fire retardance of cellular metallic foams are identified.

Tensile strength of a closed-cell al foam in the presence of notches and holes

School of Mechanical & Production Engineering, Nanyang Technological University,Singapore-639798(Received December 18, 1998)(Accepted in revised form January 26, 1999)IntroductionMetallic foams have many applications such as packaging of delicate electronic components, cores oflight-weight structural sandwich panels, and sound and energy absorption applications. The requirementof improved structural efficiency with relatively low cost has created interest in cellular materialsrecently [1]. The properties of foams can be made to vary significantly, depending on the choice ofcell-wall material, the volume fraction of the solid and the geometry of the structure [2]. Sugimuraetal. [3] and Grenestedt [4] assessed the roles of cell morphology and of imperfections in governing thebasic properties such as stiffness, yield strength and fracture resistance. In view of emerging applica-tions, detailed characterization of mechanical behavior of metal foams is an important task in order toassess their performance. In some applications of foams, it may be necessary to introduce holes ornotches for fastening and these can cause a severe drop in load bearing capability of the component. Anunderstanding of the behavior of foams in the presence of notches will be helpful in the design. Theobjective of this study is to develop such an understanding. In particular, our objective is to examineif the notch sensitivity characteristics of metallic foams are similar to those of fully dense metals. In thiswork, an experimental investigation is carried out on a closed cell Al foam and the preliminaryexperimental observations are presented here.ExperimentalA closed cell aluminum alloy foam material (supplied by Shinko Wire Company Ltd, Japan) with thetrade name ALPORAS was used in this study. It is processed by adding about 5 wt.% Ca (to enhancethe viscosity) and 3 wt.% TiH