Aluminum Alloy Foam 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.

Size effects in metallic foam core sandwich beams

Foams exhibit size effects: if the specimen dimensions are of the same order as the cell size, the moduli and strength depend on specimen size. Metallic foams have particularly large cells (typically 2–20 mm), potentially giving rise to size effects when they are used in sandwich beams. Previous studies have shown that the shear strength of metallic foams bonded to rigid plates increases by over 50% if the specimen thickness is equal to the cell size. In this study, sandwich beams with aluminum alloy faces and aluminum alloy foam cores were tested in three-point bending to characterize the effect of the beam depth on the limit load. Beams of constant ratio of core thickness to span length but different absolute values of core thickness were tested, and the measured limit loads were compared to analytical values. The analysis gives a good description of the measured limit load when the shear size effect is accounted for.

Strain rate effects on the compressive property and the energy-absorbing capacity of aluminum alloy foams

The strain rate sensitivity of various relative densities, open-cell aluminum alloy foams fabricated by a powder metallurgical method is investigated under compression loading. Their response to strain rate has been tested over a wide range of strain rates, from 10 −3 to 2600 s −1 at room temperature. Within this range, the experimental results show that the yield strength and the energy absorbed increase with an increase of strain rate. However, the yield strength of higher relative density foams increases bilinearly with the logarithm of strain rate, and the yield strength of lower relative density foams shows only a linear increase. The compaction strain slightly decreases with an increase of strain rate. The higher relative density aluminum alloy foams are more sensitive to strain rate than the lower relative density foams.

Size effects in the constrained deformation of metallic foams

The constrained deformation of an aluminium alloy foam sandwiched between steel substrates has been investigated. The sandwich plates are subjected to through-thickness shear and normal loading, and it is found that the face sheets constrain the foam against plastic deformation and result in a size effect: the yield strength increases with diminishing thickness of foam layer. The strain distribution across the foam core has been measured by a visual strain mapping technique, and a boundary layer of reduced straining was observed adjacent to the face sheets. The deformation response of the aluminium foam layer was modelled by the elastic–plastic finite element analysis of regular and irregular two dimensional honeycombs, bonded to rigid face sheets; in the simulations, the rotation of the boundary nodes of the cell-wall beam elements was set to zero to simulate full constraint from the rigid face sheets. It is found that the regular honeycomb under-estimates the size effect whereas the irregular honeycomb provides a faithful representation of both the observed size effect and the observed strain profile through the foam layer. Additionally, a compressible version of the Fleck–Hutchinson strain gradient theory was used to predict the size effect; by identifying the cell edge length as the relevant microstructural length scale the strain gradient model is able to reproduce the observed strain profiles across the layer and the thickness dependence of strength.

The fatigue strength of sandwich beams with an aluminium alloy foam core

Sandwich beams with aluminium face sheets and an aluminium alloy foam core are tested in cyclic four point bend, and S–N fatigue curves are determined for the failure modes of face fatigue, core shear and core indentation. The operative failure mode is dictated by the relative fatigue strength of face sheets to core, and upon the geometry of the sandwich beams. Simple analytical models are developed to predict the fatigue strength for each of the competing failure modes, and a design map is produced to display the fatigue strength and mode of failure as a function of sandwich beam geometry.

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.

The plastic collapse of sandwich beams with a metallic foam core

Plastic collapse modes of sandwich beams have been investigated experimentally and theoretically for the case of an aluminium alloy foam with cold-worked aluminium face sheets. Plastic collapse is by three competing mechanisms: face yield, indentation and core shear, with the active mechanism depending upon the choice of geometry and material properties. The collapse loads, as predicted by simple upper bound solutions for a rigid, ideally plastic beam, and by more refined finite element calculations are generally in good agreement with the measured strengths. However, a thickness effect of the foam core on the collapse strength is observed for collapse by core shear: the shear strength of the core increases with diminishing core thickness in relation to the cell size. Limit load solutions are used to construct collapse maps, with the beam geometrical parameters as axes. Upon displaying the collapse load for each collapse mechanism, the regimes of dominance of each mechanism and the associate mass of the beam are determined. The map is then used in optimal design by minimising the beam weight for a given structural load index.

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®).

Characterization of close-celled cellular aluminum alloys

The deformation behaviour of two different types of aluminium alloy foam are studied under tension, compression, shear and hydrostatic pressure. Foams having closed cells are processed via batch casting, whereas foams with semi-open cells are processed by negative pressure infiltration. The influence of relative foam density, cell structure and cell orientation on the stiffness and strength of foams is studied; the deformation mechanisms are analysed by using video imaging and SEM (scanning electronic microscope). The measured dependence of stiffness and strength upon relative foam density are compared with analytical predictions. The measured stress versus strain curves along different loading paths are compared with predictions from a phenomenological constitutive model. It is found that the deformations of both types of foams are dominated by cell wall bending, attributed to various process induced imperfections in the cellualr structure. The closed cell foam is found to be isotropic, whereas the semi-open cell foam shows strong anisotropy.

Dynamic compressive behavior of aluminum alloy foams

Dynamic compressive behavior of aluminum alloy foams

Foaming characteristics control during production of aluminum alloy foam

In recent years, there is a high demand for the development of light-weight metals foams. In this paper, the factors which affect the foaming in a foamed aluminum casting process were investigated. The powdered titanium hydride as a foaming agent was added to the molten aluminum alloy in the amount of 0.5 to 2.5% to adjust a proper amount of the foaming agent, and foaming condition was controlled in the temperature range, from 893 to 933 K, to determine the optimum foaming process to obtain a homogeneous aluminum foam. It was found that a properly controlled holding temperature and titanium hydride content of the melt were good for the acquirement of foamed aluminum which contains uniform cell structure of high porosity. The optimum foamed aluminum with air bubbles of 2 to 6 mm diameter and the uniform distribution with 86% porosity was obtained by the addition of 1.0% titanium hydride at 913 K.

Deformation and fracture of aluminium foams

The tensile and compressive properties and the fracture resistance of two aluminium alloy foams have been measured. The yield strength, unloading modulus and toughness increase with relative density in such a manner that the closed cell foams of this study behave as open cell foams. These relationships can be described adequately by power law fits. Experimental results, when compared with theoretical models based on idealised foam structures, reveal unexpected discrepancies. We conclude that they are caused by morphological defects in the microstructures of the foams, the effects of which were not included in the models. Tests on samples with deep sharp notches show that the tensile and compressive strengths are notch-insensitive. Fracture toughness measurements show an R-curve behaviour. This is analysed in terms of the underlying microstructure — the major cause of the R-curve was observed to be the development of crack bridging ligaments behind the crack tip. The compact tension specimens employed were sufficiently small for the uncracked ligaments to suffer plastic yielding during the fracture tests. The crack bridging response was quantified in terms of the normal traction versus plastic displacement curve; the area under this curve for a deep double edge-notched specimen is approximately equal to the measured steady state toughness. The accuracy of an existing micromechanical model for the fracture toughness of brittle open cell foams is assessed, and a new toughness model for ductile foams is derived.

Effect of inclusions and holes on the stiffness and strength of honeycombs

A finite element study has been performed on the effects of holes and rigid inclusions on the elastic modulus and yield strength of regular honeycombs under biaxial loading. The focus is on honeycombs that have already been weakened by a small degree of geometrical imperfection, such as a random distribution of fractured cell walls, as these imperfect honeycombs resemble commercially available metallic foams. Hashin–Shtrikman lower and upper bounds and self-consistent estimates of elastic moduli are derived to provide reference solutions to the finite element calculations. It is found that the strength of an imperfect honeycomb is relatively insensitive to the presence of holes and inclusions, consistent with recent experimental observations on commercial aluminium alloy foams.

Isotropic constitutive models for metallic foams

The yield behaviour of two aluminium alloy foams (Alporas and Duocel) has been investigated for a range of axisymmetric compressive stress states. The initial yield surface has been measured, and the evolution of the yield surface has been explored for uniaxial and hydrostatic stress paths. It is found that the hydrostatic yield strength is of similar magnitude to the uniaxial yield strength. The yield surfaces are of quadratic shape in the stress space of mean stress versus effective stress, and evolve without corner formation. Two phenomenological isotropic constitutive models for the plastic behaviour are proposed. The first is based on a geometrically self-similar yield surface while the second is more complex and allows for a change in shape of the yield surface due to differential hardening along the hydrostatic and deviatoric axes. Good agreement is observed between the experimentally measured stress versus strain responses and the predictions of the models.

Flow and Heat Transfer Correlations for Porous Fin in a Plate-Fin Heat Exchanger

The present experimental study investigates the impact of porous fins on the pressure drop and heat transfer characteristics in plate-fin heat exchangers. Systematic experiments have been carried out in a simplified model of a plate-porous fin heat exchanger at a controlled test environment. The porous fins are made of 6101 aluminum-alloy foam materials with different permeabilities and porosities. Comparison of performance between the porous fins and the conventional louvered fins has been made. The experimental results indicate that friction and heat transfer rate are significantly affected by permeability as well as porosity of the porous fin. The porous fins used in the present study show similar thermal performance to the conventional louvered fin. However, the louvered fin shows a little better performance in terms of pressure drop. For compactness of the heat exchanger, the porous fins with high pore density and low porosity are preferable. Useful correlations for the friction factor and the modified j-factor are also given for the design of a plate-porous fin heat exchanger. [S0022-1481(00)01103-8]

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.

High strain rate compressive behaviour of aluminium alloy foams

The high strain rate compressive behaviour of two cellular aluminium alloys (Alulight and Duocel) has been investigated using the split Hopkinson pressure bar and direct impact tests. It is found that the dynamic behaviour of these foams is very similar to their quasi-static behaviour. The plateau stress is almost insensitive to strain rate, for strain rates ε ̇ up to 5000 s −1 . Deformation is localised in weak bands in the Alulight foam but is spatially uniform for the Duocel foam, over the full range of strain rates 10 −3 s −1⩽ ε ̇ ⩽5000 s −1 .