Closed-cell Al alloy composite foams: Production and characterization

Foamy Al alloy SiCp composites of different densities ranging from 0.4 to 0.7 g/cm³ were manufactured by melt-foaming process, which consisted of direct CaCO₃ addition into the molten A356 aluminum bath. Mechanical properties and morphological observations indicated that the three-stage deformation mechanism of typical cellular foams is dominant in the produced A356 aluminum foams. Middle-stage stress plateau shrinkage plus compressive strength and bending stress enhancements were observed in denser foams. With the same Al/SiCp ratio, energy absorption ability and plastic collapse strength of the closed-cell foams were increased with the foam density. Doubling cell-face bending effects resulted in larger compressive than bending strengths in the closed-cell foams; while stiffness lowering was due to the cell-face stretching conditions.     

Prediction of compressive properties of closed-cell aluminum foam using artificial neural network

The design of artificial neural network (ANN) is motivated by analogy of highly complex, non-linear and parallel computing power of the brain. Once a neural network is significantly trained it can predict the output results in the same knowledge domain. In the present work, ANN models are developed for the simulation of compressive properties of closed-cell aluminum foam: plateau stress, Young’s modulus and energy absorption capacity. The input variables for these models are relative density, average pore diameter and cell anisotropy ratio. Database of these properties are the results of the compression tests carried out on aluminum foams at a constant strain rate of 1 × 10⁻³ s⁻¹. The prediction accuracy of all the three models is found to be satisfactory. This work has shown the excellent capability of artificial neural network approach for the simulation of the compressive properties of closed-cell aluminum foam.     

Anisotropic damping behavior of closed-cell aluminum foam

Closed-cell aluminum foams were prepared by molten body transitional foaming process. The anisotropic damping property of closed-cell aluminum foam was measured in two directions using the forced vibration method. The measured results show that the loss factors of the TD specimens are higher than that of the LD specimens. The loss factors ratio β_L/β_T depends linearly on the shape-anisotropy ratio R. Anisotropic damping behavior is due to the variation of Young’s modulus resulting from anisotropic cell morphology.     

Fabrication of ferrous metallic foams by reduction of ceramic foam precursors

A process has been developed for obtaining closed cell metallic foams using a ceramic foam precursor. In the present study, the major constituent of the ceramic foam precursor was iron oxide (Fe₂O₃), which was mixed with various foaming/setting additives. The mixture set rapidly at room temperature, stabilizing the foam generated by hydrogen release. The oxide foam was then reduced by annealing at 1240^∘C in a non-flammable hydrogen/inert gas mixture to obtain a metallic foam with a relative density of 0.23 ± 0.017, and an average cell diameter of 1.32 ± 0.32 mm. The iron foams were tested in compression and yielded an average compressive strength of 29 ± 7 MPa. The compressive stress-strain curves obtained were typical of cellular metals. The normalized strengths of the metal foams obtained in the present study compare favorably with those of steel foams produced by other techniques.     

Effect of hollow sphere size and size distribution on the quasi-static and high strain rate compressive properties of Al-A380–Al2O3 syntactic foams

Metal matrix syntactic foams are promising materials for energy absorption; however, few studies have examined the effects of hollow sphere dimensions and foam microstructure on the quasi-static and high strain rate properties of the resulting foam. Aluminum alloy A380 syntactic foams containing Al₂O₃ hollow spheres sorted by size and size range were synthesized by a sub-atmospheric pressure infiltration technique. The resulting samples were tested in compression at strain rates ranging from 10^?3 s^?1 using a conventional load frame to 1720 s^?1 using a Split Hopkinson Pressure-bar test apparatus. It is shown that the quasi-static compressive stress–strain curves exhibit distinct deformation events corresponding to initial failure of the foam at the critical resolved shear stress and subsequent failures and densification events until the foam is deformed to full density. The peak strength, plateau strength, and toughness of the foam increases with increasing hollow sphere wall thickness to diameter (t/D) ratio. Since t/D was found to increase with decreasing hollow sphere diameter, the foams produced with smaller spheres showed improved performance. The compressive properties did not show measurable strain rate dependence.     

Compressive properties of aluminum foam produced by powder-Carbamide spacer route

Effects of cell shape and size, and relative density of aluminum foam on its compressive behavior have been investigated. Aluminum foams were produced via aluminum powder-Carbamide spacer route. The results show that angular cells significantly reduce mechanical properties of the foam. They also indicate that compressive properties of the foams, including plateau stress (σ_pl), densification strain (ε_D), and energy absorption, increase by cell size and relative density of the foams. Experimental results were compared with theoretical predictions; they were fairly corresponded to theoretical conceptions; this arises from near-ideal architecture of the foams with almost spherical cells, in this study. Constant values of C, n and α in theoretical modulus and densification strain equations wear calculated as 1.22, 2.09 and 0.95, respectively. The values indicate compressive behavior approaches to ideal morphology foam via employing spherical space holder.     

Polypropylene foam behaviour under dynamic loadings: Strain rate,density and microstructure effects

Expanded polypropylene foams (EPP) can be used to absorb shock energy. The performance of these foams has to be studied as a function of several parameters such as density, microstructure and also the strain rate imposed during dynamic loading. The compressive stress–strain behaviour of these foams has been investigated over a wide range of engineering strain rates from 0.01 to 1500 s⁻¹ in order to demonstrate the effects of foam density and strain rate on the initial collapse stress and the hardening modulus in the post-yield plateau region. A flywheel apparatus has been used for intermediate strain rates of about 200 s⁻¹ and higher strain rate compression tests were performed using a viscoelastic Split Hopkinson Pressure Bar apparatus (SHPB), with nylon bars, at strain rates around 1500 s⁻¹ EPP foams of various densities from 34 to 150 kg m⁻³ were considered and microstructural aspects were examined using two particular foams. Finally, in order to assess the contribution of the gas trapped in the closed cells of the foams, compression tests in a fluid chamber at quasi-static and dynamic loading velocities were performed.     

Effect of cell shape anisotropy on the compressive behavior of closed-cell aluminum foams

The closed-cell Al–Si foams have been prepared by molten body transitional foaming process using TiH₂ foaming agent. The cell shape anisotropy ratio of specimens with various relative densities was measured. The quasi-static compressive behavior of Al–Si foams in both longitudinal and transverse directions were investigated. The results show that Al–Si foam loaded in the transverse direction exhibits a lower stress drop ratio. The relationship between plastic collapse stress ratio and cell shape anisotropy is in accordance with Gibson and Ashby model. The plastic collapse stress and the energy absorption property of Al–Si foams increase following power law relationship with relative density. Moreover, Al–Si foams exhibit higher plastic collapse stress and the energy absorption property in the longitudinal direction than that in the transverse direction.     

SiC-particulate aluminum composite foams produced from powder compacts: foaming and compression behavior

The foaming behavior of SiC-particulate (SiC_p) aluminum composite powder compacts containing titanium hydride blowing agent was investigated by heating to 750°C in a pre-heated furnace. Aluminum powder compacts were also prepared and foamed using similar compaction and foaming parameters in order to determine the effect of SiC_p-addition on the foaming and compression behavior. The SiC_p-addition (10 wt%) was found to increase the linear expansion of the Al powder compacts presumably by increasing the surface as well as the bulk viscosities. The compression tests conducted on Al and 10 and 20% SiC_p foams further showed a more brittle compression behavior of SiC_p/Al foams as compared with Al foams. The collapse stresses of Al and 10% SiC_p/Al foams were also predicted using the equations developed for the open and closed cell foams. Predictions have shown that Al foam samples behaved similar to open cell foams, while 10% SiC_p/Al foam collapse stress values were found between those of open and closed cell foams, biasing towards those of the open cell foams.     

An investigation on the dynamic response of polymeric,metallic, and biomaterial foams

Mechanical characterization of foams at varying strain rates is indispensable for the selection of foam as core material for the proficient sandwich structure design at dynamic loading application. Both servo-hydraulically controlled Material Testing System (MTS) and Instron machines are generally considered for quasi-static testing at strain rates on the order of 10⁻³ s⁻¹. Split Hopkinson pressure bar (SHPB) with steel bars is typically utilized for characterizing metallic foams at high strain rates, however modified SHPB with polycarbonate or soft martial bars are used for characterizing polymeric and biomaterial foams at high strain rates on the order of 10³ s⁻¹ for impedance match between the foam specimens and bars. This paper reviews the effect of strain rate of loading, density, environmental temperature, and microstructure on compressive strength and energy absorption capacity of various closed-cell polymeric, metallic, and biomaterial foams. Compressive strength and energy absorption capacity increase with the increase in both strain rate of loading and density of foams, but decrease with the increase in surrounding temperature. Foams of same density can have different strength and can absorb unequal amount of energy at the same strain rate of loading due to the variation of microstructure.     

Compressive behaviour of open cell Al–Al2O3 composite foams fabricated by sintering and dissolution process

Abstract

The sintering and dissolution process (SDP) was used to produce the fine open cell Al–Al₂O₃ composite and pure Al foams with the relative density of 0·25–0·40 and the pore size of 112–400 μm. The composite foam exhibited much higher yield strength and Young's modulus than the pure Al foam, and thus had an elevated plateau stress. Moreover, the composite foam showed a unique dependence of the compression stress on the pore size, i.e. it increased with increasing pore size, which was quite different from that for the common metal foams.     

The mechanical behavior of foamed aluminum

Experiments have been carried out to investigate the mechanical behavior of foamed aluminum with different matrixes and states. It is found that the matrix composition has a significant influence over the deformation, failure and fracture of foamed aluminum. Like other cellular solid materials, Al foam shows a smooth compression stress–strain curve with three regions characteristic of plastic foams: linear elastic, plastic collapse and densification. AlMg10 foam has a serrated plateau and no densification, characteristic of brittle foams. AlMg10 foam has higher compressive and tensile strength but lower ductility than Al foam. The difference in the mechanical properties between Al foam and AlMg10 foam decreases as the relative density decreases, and when it is lower than roughly 0.15, no difference can be discerned. The mechanical properties in compression are clearly higher than those in tension, which can be explained in terms of dislocation theory and stress concentration behavior.     

The mechanical response of Rohacell foams at different length scales

The quasi-static mechanical response of polymethacrylimide (PMI) foams of density ranging from 50 to 200 kg m⁻³ is investigated in order to provide experimental data to inspire and validate numerical constitutive models for the response of polymer foams. The macroscopic mechanical response is characterised by conducting quasi-static compression, tension, shear and indentation experiments, whereas microscopic deformation mechanisms are identified by conducting in situ SEM observations during static compression and tension tests; it is shown that foams of low density collapse by cell wall buckling while foams of high density undergo plastic cell-wall bending. As a result, both the elastic and plastic macroscopic response of the foam display a tension/compression asymmetry.     

Fabrication and investigation of influence of CaCO3 as foaming agent on Al-SiCp foam

Closed cell aluminum foams have been used in various disciplines of engineering. Aluminum foams provide high strength with the advantage of low weight. In the current research, CaCO₃ is used as a foaming agent for producing closed-cell aluminum foams. For the fabrication of homogenous foam, optimization of process parameters was done. The effect of SiC as a thickening agent on structural property of foams viz. density and porosity have been inspected. Foams with density 0.40–0.86 g/cm³ were produced. The produced foams were studied under axial compression tests for evaluating mechanical properties. It can be inferred from the results that by adding 3 wt.% CaCO₃, the uniform viscosity of melt was achieved and a homogeneous foam structure is achieved with optimum porosity. Also, 5 wt.% addition of CaCO₃ in melt and stirring speed at 1400 rpm tend to increase porosity and decrease cell wall thickness. The optimum values for thickening agent SiC, foaming agent CaCO₃ at stirring speed 1400 rpm were found out to be 15 wt.%, 3 wt.%. The effect of relative density, the addition of thickening and foaming agent is studied.     

Preparation of polybenzoxazine foam and its transformation to carbon foam

An organic foam derived from a new type of phenolic resin, namely polybenzoxazine, was successfully prepared with a noncomplex and economical foaming method by using azodicarbonamide (AZD) as a foaming agent. The influence of foam density on the physical and mechanical properties of the foams was studied. All resulting polybenzoxazine foams and carbon foams exhibit a tailorable uniform microstructure. Polybenzoxazine foams showed a density in the range of 273–407 kg/m³, and a compressive strength and a compressive modulus in the range of 5.2–12.4 MPa and 268–681 MPa, respectively. The foam density not only affects the physical and mechanical properties, but also affects the deformation response of the foam. In addition, the polybenzoxazine foam was further transformed into carbon foam by carbonization at 800 °C under an inert atmosphere, and its properties were examined.     

Compressive and tensile behaviour of aluminum foams

The uniaxial compressive and tensile modulus and strength of several aluminum foams are compared with models for cellular solids. The open cell foam is well described by the model. The closed cell foams have moduli and strengths that fall well below the expected values. The reduced values are the result of defects in the cellular microstructure which cause bending rather than stretching of the cell walls. Measurement and modelling of the curvature and corrugations in the cell walls suggests that these two features account for most of the reduction in properties in closed cell foams.     

Experimental Investigation on Steel Foams Fabricated by Sintering-Dissolution Process

This article describes a new process to manufacture open-cell steel foams. Calcium chloride anhydrous is used as a space holder. By changing the values of the main manufacturing parameters such as volume percentage, and the size and shape of the space holder, we produce different steel foam samples which cover a wide range of solid fraction, pore size, and shape. The effects of space-holder content and sintering condition such as temperature and time on the porosity of steel foam samples are discussed. The microstructure and composition of steel foam samples are observed and analyzed by scanning electron microscope and X-ray diffraction. The compressive curves of steel foams are measured by a universal testing machine. The experiment results show the compressive strength of steel foam samples with porosities between 65% and 85% is in the range of 66.4 ～ 12.9 MPa. The compressive strength depends mainly on the porosity and pore shape. The absorbed energy per unit volume (W) of steel foams with porosities between 85% and 65% is in range of 6.8 ～ 31.2 MJ/m³. Under the condition of identical porosity, the absorbed energy per unit volume (W) of steel foam is about three times of aluminum foam. In compression, steel foam specimens show heterogeneous macroscopic deformation.     

Performance improvements of aluminum foam with different solder compositions via liquid diffusion welding

Aluminum foam joints were fabricated via liquid diffusion welding with zinc-aluminum alloy solder aided with ultrasonic vibration at 520 °C. Zinc-aluminum alloys with different compositions (10 % aluminum, 20 % aluminum, 30 % aluminum) were used as solder material. The control group was fabricated under the same conditions but without ultrasonic assistance. The microstructure of aluminum foam joints was analyzed by optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Among the different soldering alloys, those with the zinc-20aluminum solder had the widest diffusion area and the most continuous interface. The tensile strength of metallurgic alloy joined aluminum foams was tested. Zn20Al samples had the best performance among all samples, including the low-density substrate aluminum foam (0.3 g/cm³ and 0.4 g/cm³), but it still showed a lower performance than the high-density substrate aluminum foam (0.6 g/cm³). Therefore, ultrasonic vibration remarkably improved the tensile strength and impact toughness of joints. Samples with ultrasonic assistance had better tensile strength and impact toughness than high-density substrate aluminum.     

Characterization of fracture toughness (<Emphasis Type="Italic">G</Emphasis><Subscript>c</Subscript>) of PVC and PES foams

The fracture behavior of polyvinyl chloride (PVC) and polyethersulfone (PES) foams has been examined using the single-edge notch bend and the double cantilever beam (DCB) tests. PVC foam densities ranging from 45 to 100 kg/m³ and PES foam densities ranging from 60 to 130 kg/m³ were examined. The PVC foams failed in a linear elastic brittle manner, whereas the PES foams displayed much more ductility and substantially larger toughness at a comparable foam density. The cell wall thickness of the PES foams was almost twice the thickness of the PVC foams which may have contributed to the high fracture toughness here defined as critical energy release rate (G _c). The PES foam, further displayed low initiation toughness, due to the sharp artificial crack tip and large toughness corresponding to propagation from a natural crack. The results show that the ductile PES foams have toughness close to its solid counterpart whereas the toughness of the PVC foams falls substantially below its solid counterpart.     

Fracture of open- and closed-cell metal foams

Two closed cell aluminium foams and one open cell nickel-chromium foam were subjected to microstructural characterization, in situ fracture tests and fractography. The failure process of the open cell foam was observed to be rather ductile, while that of the closed cell foams was found to be brittle. The ductility was related to the purity of the nickel chromium alloy, resulting in necking to be the dominant source of energy dissipation during failure. The brittleness of the closed cell foams was related to the presence of precipitates and particles in the cell wall microstructure, limiting the amount of plastic dissipation. The embrittling phases were traced back to the alloy composition, viscosity enhancing additions and foaming agent.