Closed-cell Aluminum Foam Research Articles

Mimicking the loading adaptation of bone microstructure with aluminum foams

Nature has always been considered as a source of inspiration for scientists and engineers to create or mimic new materials with interesting properties. In this study, we draw inspiration from the loading adaptation of trabecular bone microstructure, which exhibits a graded distribution of porosity and cell size, with axially elongated cells. Using conventional foaming processes, we designed, manufactured and tested two different types of bone-inspired aluminum closed cell foams, aimed at mimicking the loading adaptation feature: the first type, characterized by a directional gradient of pores (i.e. along the length of the specimens) and the second type, characterized by elongated pores by hot rolling process. Micro-Computed Tomography (μCT) is used to compare the morphological properties of aluminum foams and those of bones. Geometry-based Finite Element (FE) models are built from the μCT images and validated by means of experimental data. Using image analysis techniques, virtual models have been created to expand the range of data and get further insight into the structural behavior of the material. This study shows that closed cell aluminum foams can be produced with biomimetic uniaxial mechanical properties in the low strain regime, by controlling their relative density and the axial orientation of cells.

Collapse mechanisms of metal foam matrix composites under static and dynamic loading conditions

The collapse mechanisms of metal foam matrix composites under static and dynamic loading conditions were experimentally and analytically investigated. Closed-cell aluminium foam AlSi10 with 325±10kg/m3 density was used as core material, while stainless-steel-mesh is the faces materials. Prior to characterizing the composite sandwich structure, the stainless steel mesh face material and closed-cell aluminium foam were characterized by tensile testing and compression testing, respectively. Experimental tests were performed on sandwich beams using both High Speed Camera and Digital Image Correlation system for strain distribution. All experimental tests were performed at room temperature with constant crosshead speed of 1.67×10−4m/s for static tests and 2m/s impact loading speed for dynamic tests. Two main deformation behaviours of investigated metal foam matrix composites were observed following post-failure collapse: face failure and core shear. It was showed that the initiation, propagation and interaction of failure modes depend on the type of loading, constituent material properties and geometrical parameters.

Investigation of Sound Absorption Feature of Closed-Cell Aluminum Foams Combined with Porous Materials

Investigation of Sound Absorption Feature of Closed-Cell Aluminum Foams Combined with Porous Materials

Shell-Based Numerical Modelling and Experimental Validation of Closed-Cell Aluminum Foams

This work proposes the creation and the validation of shell-based numerical models of closed-cell aluminum foams, useful for FEM analyses. Foam cylinders were produced and specimens extracted by them were morphologically characterized and mechanically tested. Successively, commercial software packages were used for, firstly, creating and modifying geometrical models and, successively, generating FEM models based on shell elements. The results of FEM fit well with the mechanical tests, in terms of stress-strain response. Moreover, they highlight a results variability, reflecting the original variability associated to random generation of the geometrical models.

Effect of relative density on the dynamic compressive behavior of carbon nanotube reinforced aluminum foam

Closed-cell aluminum foams represent a unique class of solid cellular light metals that are made by deliberately introducing voids or pores during fabrication. This lightweight material is able to undergo large deformation at a nearly constant stress known as Plateau Stress because of which aluminum foams are good energy absorbers under dynamic loads such as an impact. In this investigation, carbon nanotubes (CNT) reinforced closed-cell aluminum foams were fabricated using the liquid metallurgy route through the dissociation of a foaming agent within the liquid metal. Four different relative densities of CNT reinforced Al-foam were used: 0.16, 0.20, 0.26 and 0.30, to study the effect of strain rate on the mechanical properties. The compressive mechanical behavior of CNT reinforced Al-foam has been studied under quasi-static and dynamic loading conditions. The high strain rate compressive response was investigated using a Split Hopkinson Pressure Bar (SHPB) over a range of strain rates up to 2750s−1. Mechanical properties such as peak stress, plateau stress and energy absorption increased with the increase in relative density; however, the densification strain decreased with the increase in relative density. Dynamic compressive properties improved as the strain rate increased indicating that this material is strain rate dependent. Among all the foams, the 0.30 relative density exhibited the highest mechanical properties whereas the 0.20 relative density foam displayed the highest strain rate sensitivity.

Macro and micro collapse mechanisms of closed-cell aluminium foams during quasi-static compression

The pore collapse mechanisms of closed-cell aluminium foams during quasi-static compression have been investigated. A suite of experiments and numerical simulations were carried out to elucidate the deformation pathways of individual pores during quasi-static compressive loading. X-ray computed tomography was utilized to generate 3D views of the foams before and after deformation. The tomography based foam geometry was imported into the finite element software ABAQUS/Explicit for simulations. The results showed that the simulations accurately reproduced the experimentally observed yielding and post yielding behaviour of the foams. As expected, pores with thin cell-walls were observed to deform at faster rates both experimentally and in simulations. The simulations aided to reveal the complex deformation evolution of cell-walls and junctions (plateau borders) during compression. While the cell-walls experienced bending, buckling and rotation by forming hinges; the plateau borders experienced considerably less deformation. The thickness/strength of cell-walls and topological foams' heterogeneities are observed as the governing factors for collapse. Significant lateral strain is observed at the cell level, although the bulk lateral strain was negligible. Finally, it was also observed that the micro-pores and cracks present within the cell-walls contribute to their deformation.

Accelerated Formation of an Ultrafine-Grained Microstructure in Closed-Cell Aluminum Foam after Extrusion and Differential Speed Rolling

Accelerated Formation of an Ultrafine-Grained Microstructure in Closed-Cell Aluminum Foam after Extrusion and Differential Speed Rolling

The Effect of Shot-Peening Treatment on Microstructure and Corrosion Behavior of Closed-Cell Aluminum Foam

Closed-cell aluminum foam was shot peened at different processing time (0 s, 5 s, 10 s, and 20 s), the intensity was the 0.12 mmA. The X-ray diffraction results showed that the reflections became weakened obviously with the shot peened time increased. Combined with Popa model and lognormal distribute model, the surface microstructure of closed-cell aluminum foam was inves-tigated by using the Rietveld whole pattern fitting analysis method. The results revealed that domain size and microstrain fluctuated along different reflection directions after shot peened, which attributed to the random and anisotropic deformation direction. With the shot peened processing time prolonged, a decrease in domain size and an increase in microstrain were also observed. Moreover, the corrosion behavior of closed-cell aluminum foam was studied by weight-loss test. The results indicated that corrosion properties of specimen subjected to shot peened processing was better than the unpeened specimens.

Dynamic High Temperature Compression of Carbon Nanotubes Reinforced Aluminum Foams

The dynamic mechanical behavior of closed cell aluminum foam reinforced with carbon nanotubes (CNT) has been investigated at high temperatures and varying strain rates using the split Hopkinson pressure bar (SHPB). CNT concentration was varied from 0 to 3 wt%. Experimental results of closed cell Al-foam composites show that the dynamic mechanical properties in terms of peak stress and plateau stress increase as the strain rate increases under increasing temperatures. Compared to unreinforced Al-foam, the CNT reinforced foam shows an increase in peak and plateau stress of over 50–100% depending on the CNT concentration and temperature. Among the different CNT concentrations investigated, the 2 wt% CNT Al-foam shows the highest peak stress, plateau stress and energy absorption under the temperature range studied.

Energy Absorption of Foam Filled Aluminum Tubes under Dynamic Bending

In this article, energy absorption of foam filled and empty aluminum alloy tube under dynamic bending were investigated numerically and experimentally. The material model for aluminum foam was based on the implementation of crushable foam with isotropic hardening combined with strain rate dependence in ABAQUS. Model parameters were determined from quasi-static (ὲ=1x10-3 s-1) and dynamic compression tests on closed cell aluminum foam. The Johnson cook material model was used for tubes. Energy absorption capacity of foam filled aluminum alloy AA7075 tubes was higher than the empty tubes. The peak loads and energy absorption values between experimental results and simulation were found to be in good agreement. At high strain rates, an increase in the energy absorption capacity was observed, which is useful for crashworthy applications.

Thermal properties of closed-cell aluminum foams prepared by melt foaming technology

Closed-cell aluminum foam has incomparable advantages over other traditional materials for thermal insulation and heat preservation because of small thermal conductivity coefficient. Spherical bubble three-dimensional model of aluminum foam is built to deduce the relationship among pore wall thickness, porosity and average pore size. Non-uniform closed-cell foam aluminum model with different structural parameters and random pore distribution is established based on the relationship via C programming language. And the temperature distribution is analyzed with ANSYS software. Results indicate that thermal conductivity increases with the reducing of porosity. For the aluminum foam with the same porosity, different pore distributions result in different thermal conductivities. The temperature distribution in aluminum foam is non-uniform, which is closely related with the pore size and distribution. The pores which extend or distribute along the direction perpendicular to heat flow strengthen obstructive capability for heat flow. When pores connect along the direction perpendicular to heat flow, a “wall of high thermal resistance” appears to decline the thermal conductivity rapidly, which shows that only porosity cannot completely determine effective thermal conductivity of closed-cell aluminum foam.

Response of closed-cell aluminum foams under static and impact loading: Experimental and mesoscopic numerical analysis

This paper investigates the mechanical behavior and mesoscopic deformation of closed-cell aluminum foam under static and impact loading. Two groups of closed-cell aluminum foam specimens are prepared for static and dynamic tests, respectively. The configurations of the specimen (such as porosity and base material) are considered. The mechanical response and the energy absorption capability are studied. In order to reveal the mesoscopic deformation mechanism, a mesoscopic model is developed considering the randomness of pores in size and distribution. The dynamic response under low and high impact velocity is investigated based on the mesoscopic model. Insights into the deformation mode are conducted. The collapse and the fracture of cell-walls under different impact velocity are discussed. It reveals that the mesoscopic deformation is closely related to the impact velocity. Porosity is also an important aspect on the dynamic property. At low impact velocity, the plastic deformation can be found in the middle domain of the specimen. At high impact loading, a dynamic mode is formed, in which the collapse and fracture of cell-walls is mainly located near the impact end. At a very high impact velocity, a shock mode can be found, forming a strain localization at the impact end for the metallic foam. It leads to significant improvements on the dynamic plateau stress and the energy absorption capability. This originates from the inertia effects.

Yield properties of closed-cell aluminum foam under triaxial loadings by a 3D Voronoi model

Considering the lack of abundant experimental data on metal foams under tensile triaxial loadings, characterization of the yield surface of metallic foam is controversial. In this paper, we explore a numerical method to study the yield properties of closed-cell aluminum (Al) foam under triaxial loadings using a three-dimensional (3D) Voronoi model. The finite element model was verified by a uniaxial compressive experiment with optimized modeling and computing parameters. Three normal stresses (including tension and compression) were proportionally applied on the cubic Voronoi foam to conduct numerical simulation experiments, in which each stress proportion of the von Mises stress over the mean stress includes at least three different combinations of triaxial loadings. Results indicated that the yield surface, covering the entire spectrum of stress paths from hydrostatic compression to hydrostatic tension in the plane of the von Mises stress and the mean stress, could be approximately depicted by a parabola or ellipse, but it is not symmetric about the zero mean stress. Owing to the asymmetry of the yield surface in the compressive and tensile states, the yield surfaces should be normalized by compressive and tensile uniaxial yield stresses, respectively, which leads to the conclusion that normalized yield surfaces are almost independent of both relative density and thickness distribution of the cell wall. Moreover, the scatter in the same proportion of the von Mises stress over the mean stress, but different triaxial stress combinations, shows that it might not be sufficient to depict the yield surface only by the von Mises stress and the mean stress.

Compressive behaviour of unconstrained and constrained integral-skin closed-cell aluminium foam

The aim of this paper is to evaluate the quasi-static and dynamic compressive crush performance of integral-skin closed-cell aluminium alloy foam with and without radial constraints. The foam specimens were prepared by the powder compact foaming method. The behaviour under different loading conditions (loading velocity and radial constraints) has been determined by an extensive experimental program. The results show a significant increase in the collapse stress of the integral-skin closed-cell aluminium foam under quasi-static loading when radial constraints are applied. The radial constraint induces a significant strain hardening of the foam, where the densification occurs at lower strains, consequently enhancing the energy absorption per unit volume of the deformed foam. The strain hardening is also sensitive to the foam density, increasing with the density.

Effect of solidification rate on pore connectivity of aluminium foams and its consequences on mechanical properties

This study evaluates the influence of solidification rate on the generation and control of pore connectivity of closed-cell aluminium foams. Additionally, it gives the experimental support to evaluate and model the effect of this pore connectivity on the mechanical properties. A collection of AlSi10 foams produced via powder metallurgy route, with porosities between 0.65 and 0.85, were examined. During production, applied heating conditions were the same in all cases but the cooling conditions were varied in order to promote different solidification rates in a wide range (from −1 to −15K/s). Structural characterisation was performed by gas pycnometry and X-ray microtomography while the mechanical properties were evaluated by microhardness measurements and uniaxial compression tests. Results showed a clear reduction of pore connectivity when increasing the solidification rate. The consequence is a prominent improvement of the foam strength over the one expected from just the matrix refinement. Further analysis on this relationship between the pore connectivity and the mechanical properties, has allowed to propose a correction to the theoretical model for collapse strength in closed cell foams to consider such contribution and predict more accurate results.

Effect of cell-size on the energy absorption features of closed-cell aluminium foams

The effect of cell-size on the compressive response and energy absorption features of closed-cell aluminium (Al) foam were investigated by finite element method. Micromechanical models were constructed with a repeating unit-cell (RUC) which was sectioned from tetrakaidecahedra structure. Using this RUC, three Al foam models with different cell-sizes (large, medium and small) and all of same density, were built. These three different cell-size pieces of foam occupy the same volume and their domains contained 8, 27 and 64 RUCs respectively. However, the smaller cell-size foam has larger surface area to volume ratio compared to other two. Mechanical behaviour was modelled under uniaxial loading. All three aggregates (3D arrays of RUCs) of different cell-sizes showed an elastic region at the initial stage, then followed by a plateau, and finally, a densification region. The smaller cell size foam exhibited a higher peak-stress and a greater densification strain comparing other two cell-sizes investigated. It was demonstrated that energy absorption capabilities of smaller cell-size foams was higher compared to the larger cell-sizes examined.

Computational Modelling of Closed-Cell Aluminium Foams to Investigate Structural Deformation under Quasi-Static Loading

Computational modelling has been performed to understand the deformation mechanisms of closed-cell aluminium foams under quasi-static compression. A micro-computed tomography-based 3D foam geometry was developed that described the interior of the foam. The FE software ABAQUS/Explicit has been used for the present meso-scale modelling. A good correlation of load-strain response was found between the simulations and experimental results. A sectional view was observed during deformation to explore the internal deformation mechanisms. It was revealed that localized cell collapse occurs due to complex movements of the cell-walls with thin/weaker cells being deformed faster than thick/stronger cells.

Filling of surface pores of aluminum foam with polyamide by selective laser melting for improvement in mechanical properties

To improve mechanical properties of porous structured materials, selective laser melting (SLM) was used to fill the surface pores of a commercial closed-cell type aluminum foam with polyamide 12 (nylon 12). It was found that polyamide powder was continuously melted and solidified under laser irradiation at an average energy density of 10J/mm2. The mechanical properties of the aluminum foam with fabricated polyamide surface layer (composite sandwich structure) was investigated using a uniaxial compression test. The specific compressive strength of the foam with surface layer increased 2.5 times in maximum higher than that of the foam without surface layer. The improvement was not only due to the high specific strength of the polyamide but also due to the formation of the sandwich structure.

Investigation on dynamic penetration of closed-cell aluminium foam using in situ deceleration measurement

Abstract In this work, the experimental study on the resistance of closed-cell aluminium foam to penetration is undertaken by using a novel in situ deceleration measurement unit. Most previous experimental studies in the literature only reported final penetration depth and impact velocity and in situ measurements of the impact deceleration history are hardly available. For this reason, an instrumented penetrator is designed with an on-board data recorder and accelerometer. This penetrator measures in situ rigid-body deceleration and provides a measure for net resistance on the penetrator during the penetration process. Closed-cell aluminium foam specimens that were prepared in this work are grouped into two groups with different densities and gauge length. The specimens were subjected to impact velocities ranging from 40 to 68 m/s. The effects of the impact velocity, nominal strain rate, the specimen’s density and strain hardening on the obtained deceleration-time histories are studied in detail. The obtained deceleration-time histories in this work are interpreted with the quasi-static data and the strength enhancement mechanisms of the metal foams. Two deceleration profiles are observed: namely plateau deceleration and increasing deceleration. It is found that at impact velocity above 50 m/s, the effect of strain hardening of the specimen imposes on the penetrator to obtain an increasing deceleration profile. It is found that the inertia effect has a larger influence on the rate sensitivity of the lower density foams compared with the higher density foams. Lastly, this work also presents the effect of the densification of cells on the penetrator’s deceleration response.

Modelling and characterization of cell collapse in aluminium foams during dynamic loading

Plate-impact experiments have been conducted to investigate the elastic–plastic behaviour of shock wave propagation and pore collapse mechanisms of closed-cell aluminium foams. FE modelling using a meso-scale approach has been carried out with the FE software ABAQUS/Explicit. A micro-computed tomography-based foam geometry has been developed and microstructural changes with time have been investigated to explore the effects of wave propagation. Special attention has been given to the pore collapse mechanism. The effect of velocity variations on deformation has been elucidated with three different impact conditions using the plate-impact method. Free surface velocity (ufs) was measured on the rear of the sample to understand the evolution of the compaction. At low impact velocities, the free-surface velocity increased gradually, whereas an abrupt rise of free-surface velocity was found at an impact velocity of 845 m/s with a copper flyer-plate which correlates with the appearance of shock. A good correlation was found between experimental results and FE predictions.