Open-cell Aluminum Alloy Foams Research Articles

Continuum modeling of crushing of low density foams

Under compression deformation in low-density foams localizes into narrow bands of crushed cells. Crushing spreads at nearly constant stress with crushed and relatively undeformed material coexisting. The material returns to homogeneous deformation with increasing stress when the crushing has spread over the whole specimen. This paper presents a first attempt at representing the inhomogeneous behavior exhibited by open-cell aluminum alloy foams with relative density of 8%. A plasticity model is presented with a compressible yield function calibrated to a set of multiaxial foam crushing tests coupled with a non-associated flow rule. An essential component of the modeling effort is the introduction of a softening branch to the material stress-strain response. A cubical finite element model with an irregular mesh of solid elements is used to simulate a set of numerical crushing tests on micromechanically accurate foam models performed in a true triaxial apparatus in Yang and Kyriakides (2019). Small geometric imperfections are used to trigger localized deformation in the form of planar bands of high strain normal to the loading directions of compression. The bands broaden with the stresses tracing plateaus that mimic the random foam test results. The predicted mean stress–change in volume responses perform equally well up to a volume reduction of about 50%. At higher compressions, the stresses exhibit the gradual increase characteristic of foams in the densification regime. These parts of the stress–displacement trajectories, and mean stress–change in volume responses under-predict to some degree those recorded in the random foams. The material hardening in the densification regime appears to be deformation-dependent causing this difference.

On the crushing response of random open-cell foams

This study examines the effect of randomness of the cellular microstructure on the calculated compressive response of a class of open-cell aluminum alloy foams. The foams are modeled using realistic random soap froth with N3 cells generated using the Surface Evolver software. The ligaments are made straight but with non-uniform cross sectional area distributions that mimic those of the physical foams. The models are also assigned the density and anisotropy measured. The ligaments are modeled as shear deformable beams with the elasto-plastic material behavior of the Al-alloy. The microstructure is discretized with finite elements using LS-DYNA, which allows for beam-to-beam contact on the outer surface of the ligaments. 103 cell domains compressed between rigid planes are shown to reproduce the measured compressive responses in both the rise and transverse directions. This includes the complete response from the initial elastic regime, through “yielding,” the extended stress plateau, to densification. More importantly, localized bands of crushed cells that develop and gradually spread throughout the domain resemble closely experimental observations made using X-ray tomography. This is a major improvement over previous models that idealized monodispersed foams as periodic Kelvin cells, and should allow modeling of polydisperse foams. The contact algorithm, friction between ligaments, and generally the discretization play crucial roles in the accuracy of the calculation as well as their numerical stability.

Microstructural Modification of Laser-Bent Open-Cell Aluminum Foams

Laser forming tests have been performed on open-cell aluminum alloy foams with different pore size. Laser power was fixed at 150 W, a total of 150 laser scans led to a bending angle up to 60°, depending on the laser scan rate. At the end of the laser bending, the foams were left to cool and samples were extracted for analysis by means of an optic microscope. The alloy microstructure was investigated in different points of the samples and correlated with the processing conditions. Image analysis was also carried out to extract the percentage of melted area due to laser heating.

Creep-rupturing of open-cell foams

A repeating element consisting of four straight and uniform-thickness cell struts in a pentagonal dodecahedron model is employed to analyze theoretically the creep-rupturing of open-cell foams. In the repeating element, the solid making up cell struts is assumed to follow power-law creep and the Monkman–Grant relationship. Consequently, the theoretical expressions for describing the steady-state creep strain rate and creep-rupturing time of open-cell foams are obtained. It is shown that the creep-rupturing of open-cell foams can also be described by the Monkman–Grant relationship. Moreover, the Monkman–Grant parameters m ∗ and B ∗ of open-cell foams depend on their cell structure and those of solid cell struts. The Monkman–Grant parameters determined from the existing experimental results on the creep-rupturing of open-cell aluminum alloy foams are compared to those calculated theoretically from the proposed pentagonal dodecahedron model. The difference between theoretically calculated and empirically determined B ∗ is attributed to some pre-existing cell structural imperfections in open-cell aluminum alloy foams.

Effect of Heat Treatment on Compressive Properties of Open Cell Aluminum Foams

This paper presents a study of heat treatment on the quasi-static and dynamic compressive properties of the open cell aluminum alloy foams in as-fabricated (F), age-hardened (A) and T6-strengthened (T6) conditions. Although the strain rate and heat treatment of foams are different, all exhibit similar deformation behavior in the subsequent deformation. The yield stress of foams at different strain rates are improved by heat treatment, all exhibit some strain rate sensitivity. However, the densification strain of foams is not sensitive to heat treatment.

Effect of Cell Size on the Dynamic Compressive Properties of Aluminum Alloy Foams

The static and dynamic compressive behaviors of open-cell aluminum alloy foams with virtually the same relative density of 0.4 were investigated. The foams have different cell sizes (0.5mm, 1.5mm, 2.5mm) but similar cell morphology and microstructure. The yield strength of these foams was characterized as a function of strain rate and cell morphology. The experimental results indicated that the mechanical responses of foams are sensitive to strain rate, and dependent of the cell size. The present results are compared in details with recent findings obtained from the aluminum foams.

Dynamic Compressive Strength of Aluminum Alloy Foams

The dynamic compressive behavior of open-cell aluminum alloy foams with different length of specimens was investigated using the split Hopkinson pressure bar technique. Plastic strength was measured for aluminum alloy foam specimens having the three cell sizes but similar cell microstructure. Longer specimens exhibited lower mean strength and broader scattering of the strength values than the shorter ones. It can be observed that mechanical response of aluminum alloy foams appear to be dependent of the cell size for both the shorter and longer specimens.

Uniform and graded chemical milling of aluminum foams

Commercial open-cell aluminum alloy foams are subjected to chemical dissolution to reduce their density. Dissolution rates are measured for various pH, temperature and alloy heat-treatment conditions, and the resulting foam structures and surface conditions are evaluated by microscopy and X-ray microcomputed tomography to identify conditions which minimize corrosive damage. The effect of uniform dissolution on foam compressive mechanical properties is interpreted in terms of these observations. A method for production of density-graded foam samples from uniform-density precursors is also demonstrated.

Effects of heat treatment on dynamic compressive properties and energy absorption characteristics of open-cell aluminum alloy foams

Abstract The effects of heat treatment on the dynamic compressive properties and energy absorption characteristics of open cell aluminum alloy foams (Al-Mg-Si alloy foam and Al-Cu-Mg alloy foam) produced by infiltrating process were studied. Two kinds of heat treatment were exploited: age-hardening and solution heat treating plus age-hardening (T6). The split Hopkinson pressure bar (SHPB) was used for high strain rate compression test. The results show that both age-hardened and T6-strengthened foams exhibit improved compression strength and shortened plateau region compared with that of foams in as-fabricated state under high strain rate compression, and the energy absorption capacity is also influenced significantly by heat treatment. It is worthy to note that omitting the solution treating can also improve the strength and energy absorbed much.

Studies on the dynamic compressive properties of open-cell aluminum alloy foams

Abstract The compressive deformation behavior of open-cell aluminum foams with different densities and morphologies is assessed under quasi-static and dynamic loading conditions. The experimental results show that density, in contrast with cell size, is the primary variable characterizing the elastic modulus and yield strength of foams. Yield strength and energy absorption of foams are almost insensitive to strain rate, over a wide range of strain rates.

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.