Closed-cell Aluminum Alloy Foams Research Articles

Electrical Conductivity of Aluminium Alloy Foams

Closed-cell aluminium alloy foams were produced using the powdermetallurgical technique. The effect of porosity and cell diameter onthe electrical conductivity of foams was investigated and the resultswere compared with a number of models. It was found that thepercolation theory can be successfully applied to describe thedependence of the electrical conductivity of aluminium alloy foams onthe relative density. The cell diameter has a negligible effect on theelectrical conductivity of foams.

The microstructure and electrical conductivity of aluminum alloy foams

Closed-cell aluminum alloy foams were produced by applying the powder metallurgical technique, i.e. by mixing metal powders and foaming agent (titanium hydride powder) and compacted them into a dense foamable precursor and the foamable precursor material expanded by heating it up to above its melting point. The important geometric parameter that affects the properties of foams, such as cell diameter, edge thickness and edge length, has been examined. The effect of porosity and cell diameter on the electrical conductivity of foams was investigated and the results were compared with a number of models. It was found that the percolation theory can be successfully applied to describe the dependence of the electrical conductivity of aluminum alloy foams on the relative density. The cell diameter has a negligible effect on the electrical conductivity of foams.

Inertia effects in uniaxial dynamic compression of a closed cell aluminium alloy foam

The dynamic compressive characteristics of a closed cell aluminium alloy foam (manufactured by Hydro Aluminium AS, Norway) have been studied experimentally by using a direct impact technique for a range of velocities up to 210 m s–1. Experimental data on the dynamic initial crushing and plateau stresses are compared for two average cell sizes of approximately 4 and 14 mm. The data reveal significant dynamic enhancements of the initial crushing strengths throughout the range of velocities used. The dynamic plateau stresses are insensitive to impact velocity below the values of 50 and 100 m s–1 for the large and small cell foams respectively. Beyond a critical velocity value of ~ 100 m s–1, the crushing wave front propagates through the foam with shock like characteristics. The inertia effects associated with the dynamic localisation of crushing and the microinertia of the cell wall/edge material on the dynamic strength enhancement are discussed. A one-dimensional shock model based on a rate independent, rigid, perfectly plastic locking idealisation of the nominal stress–strain curve for foams is employed to provide a first order understanding of the various parameters involved in the crushing process. The results of the analyses are seen to predict well the dynamic strength enhancements that are measured experimentally. The sources of discrepancies are highlighted and discussed, as are the limitations and shortcomings of the shock model.

The investigation of morphometric parameters of aluminium foams using micro-computed tomography

The development of different manufacturing techniques for foams, most especially metallic foams have opened a new market of opportunities for multifunctional cellular metallic materials. However, it is yet unclear how to quantitatively characterise the microstructure and internal architecture of foams precisely. In this paper, the potential of using micro-computed tomography in characterising the morphometric parameters of foams are explored. 3D morphometric parameters of different types of closed cell aluminium alloy foam have been characterised, and the effects of the measurement resolution, and the integration time on the measured morphometric parameters have been studied. The model based and model independent morphometric parameters are comparable with measurements using other characterisation techniques. We conclude that the new generations of micro-computed tomography equipment have versatile capability for a non-invasive and -destructive characterisation of foams.

Fatigue crack propagation in aluminium alloy foams

The mode I fatigue crack propagation (FCP) response of the closed-cell aluminium alloy foams Alulight and Alporas have been measured for a relative density in the range 0.1 to 0.4. The validity of linear elastic fracture mechanics (LEFM) to characterise the fatigue crack propagation (FCP) response is demonstrated, and K-increasing and K-decreasing tests are used to determine the full shape of the FCP response. The classical sigmoidal variation of log d a/d N with log Δ K is evident, with a Paris-law exponent m=20 for Alulight and m=25 for Alporas. The effects of relative density, mean stress and a single peak overload on the FCP response are investigated. The study concludes by analysing the mechanism of fatigue crack growth; it is suggested that the fatigue crack growth rate is controlled by the progressive degradation of crack bridging by fatigue failure of the cell edges behind the crack tip.

Development of a Closed Cell Aluminum Alloy Foam with Enhancement of the Compressive Strength

There is a great demand for more weight reduction in aluminum foams for fuel economy in the automotive and aerospace fields. The enhancement of compressive strength in a close-celled aluminum can be achieved by the selection of matrix material or modification of the cellular structure without increasing relative density. A commercial closed-cell aluminum (ALPORAS®) is made by adding foaming agent into a molten alloy. In this study, the matrix aluminum was strengthened by adding 5%Zn and 1%Mg without an increase in weight. The compressive strength of the present foam is found to be approximately twice as high as that of the conventional foam (ALPORAS®).

The stress–life fatigue behaviour of aluminium alloy foams

The tension–tension and compression–compression nominal stress versus fatigue life responses of Alulight closed cell aluminium alloy foams have been measured for the compositions Al–1Mg–0.6Si and Al–1Mg–10Si (wt %), and for relative densities in the range 0.1–0.4. The fatigue strength of each foam increases with the relative density and with the mean applied stress, and is greater for the transverse orientation than for the longitudinal orientation. Under both tension–tension and compression–compression loading the dominant cyclic deformation mode appears to be material ratchetting; consequently, the fatigue life is highly sensitive to the magnitude of the applied stress. A micromechanical model is given to predict the dependence of life upon stress level and relative density. Panels containing a central hole were found to be notch insensitive for both tension–tension and compression–compression fatigue loading: the net‐section strength equals the unnotched strength.

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.

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.

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.

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

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.