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 heatpreservation because of small thermal conductivity coefficient. Spherical bubble three-dimensional model of aluminum foam is builtto deduce the relationship among pore wall thickness, porosity and average pore size. Non-uniform closed-cell foam aluminummodel with different structural parameters and random pore distribution is established based on the relationship via C programminglanguage. And the temperature distribution is analyzed with ANSYS software. Results indicate that thermal conductivity increaseswith the reducing of porosity. For the aluminum foam with the same porosity, different pore distributions result in different thermalconductivities. The temperature distribution in aluminum foam is non-uniform, which is closely related with the pore size anddistribution. The pores which extend or distribute along the direction perpendicular to heat flow strengthen obstructive capability forheat flow. When pores connect along the direction perpendicular to heat flow, a “wall of high thermal resistance” appears to declinethe thermal conductivity rapidly, which shows that only porosity cannot completely determine effective thermal conductivity ofclosed-cell aluminum foam.

Biaxial mechanical behavior of closed-cell aluminum foam under combined shear-compression loading

Combined shear-compression tests and simulations were performed on a closed-cell aluminum foam over a wide range of loading angles in order to probe their yield behaviors under biaxial loading conditions.Combined shear-compression tests were carried out by using a pair of cylindrical bars with beveled ends.The yield surfaces were experimentally measured and compared with various theoretical yield surface models.The cellular structures of closed-cell aluminum foams were modeled as tetrakaidecahedrons and their biaxial crushing behaviors were simulated by the finite element method.The results show that,yield initiates from the stress-concentrated corners in the specimens under combined shear-compression loading and the stress distribution is no longer uniform at the specimen/bar interfaces.In the range of cell sizes studied,the larger the foam cell size is,the higher the yield stress is.Aluminum foam density is found to be the dominant factor on its mechanical properties compared with the cell size and is much more significant in engineering practice.

Tensile property of Al-Si closed-cell aluminum foam

Al-Si closed-cell aluminum foams of different densities were prepared by molten body transitional foaming process.The tensile behavior of Al-Si closed-cell aluminum foam was studied and the influence of relative densities on the tensile strength and elastic modulus was also researched.The results show that the fracture surfaces of Al-Si closed-cell aluminum foam display quasi-cleavage fracture consisting of brittle cleavages and ductile dimples.The tensile strength and elastic modulus are strictly affected by the relative density of Al-Si closed-cell aluminum foam.With increasing relative density,the tensile strength increases and the strain at which the peak strength is measured also increases;in addition,the elastic modulus increases with increasing relative density.

Shape formation of closed-cell aluminum foam in solid–liquid–gas coexisting state

The mold pressing process was applied to investigate the formability of closed-cell aluminum foam in solid–liquid–gas coexisting state.Results show that the shape formation of closed-cell aluminum foam in the solid–liquid–gas coexisting state was realized through cell wall deformation and cell movement caused by primary α-Al grains that slid,rotated,deformed,and ripened within cell walls.During formation,characteristic parameters of closed-cell aluminum foam were almost unchanged.Under proper forming conditions,shaped products of closed-cell aluminum foam could be fabricated through mold pressing.

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.

Cell-structure and mechanical properties of closed-cell aluminum foam

The density, cell size and structure of closed-cell aluminum foam were measured by optical microscopy and image analysis. The properties and the mechanism of compressive deformation that occur in closed-cell aluminum foam were measured and discussed. The results show that the cell size of foam with density of 0.37 mg/m^3 is distributed in the range of 0.5 4.0 mm. The cell size of foam with density of 0.33 mg/m^3 is distributed in the range of 0.55.0 mm. The cell wall thickness of both types is 0.10.3 mm. The closed-cell aluminum foam almost belongs to isotropic one, with a variation of ±15% in elastic modulus and yield strength in longitudinal and transverse direction. Under compressive loading, foam materials show inhomogeneous macroscopic deformation. The site of the onset of local plastic deformation depends on the cell structure. The shape of cell is more important than size in determining the yielding susceptibility of the cells. At early stage of deformation,the deformation is localized in narrow bands having width of one cells diameter, and outside the bands the cell still remains the original shape. The cells within bands experience large permanent deformation. The band normals are usually within 20° of the loading axis.

Compressive characteristics of closed-cell aluminum foams with different percentages of Er element

In the present study, closed-cell aluminum foams with different percentages of erbium （Er） element were successfully prepared. The distribution and existence form of erbium （Er） element and its effect on the compressive properties of the foams were investigated. Results show that Er uniformly distributes in the cell walls in the forms of Al3Er intermetallic compound and AI-Er solid solutions. Compared with commercially pure aluminum foam, Er-containing foams possess higher micro-hardness, compressive strength and energy absorption capacity due to solid solution strengthening and second phase strengthening effects. Additionally, the amount of Er element should be controlled in the range of 0.10wt.%-0.50wt.% in order to obtain a good combination of compressive strength and energy absorption properties.

Sound absorption and insulation property of closed-cell aluminum foam

The closed-cell aluminum foams (specimenρ=0.31 g/cm3, diameter of 100 mm, and thickness of 20 mm for sound absorption testing; specimenρ=0.51 g/cm3, length of 1 240 mm, width of 1 100 mm, and thickness of 30 mm for sound insulation testing) were prepared by the method of molten body transitional foaming process. Its sound absorption property under frequency of 160-2 000 Hz and the sound insulation property under frequency of 100-4 000 Hz were tested. The sound absorption results show that the sound absorption property is much better under middle frequencies than that under low and high frequencies. The sound absorption coefficient climbs when frequency increases from 160 Hz to 800 Hz and then drops when frequency is increased from 800 Hz to 2 000 Hz. The function of the sound absorption mainly depends on the Helmholtz resonator, the microphone as well as cracks of closed-cell aluminum foam. The sound insulation experiments show that the sound reduction index (R) is small under low frequencies, and large under high frequencies; the weighted sound reduction index (Rw) and the highest sound reduction index (R) can reach around 30.8 dB and 43 dB, respectively.

Sound insulation property of Al-Si closed-cell aluminum foam bare board material

Al-Si closed-cell aluminum foam bare boards of 1 240 mm×1 100 mm with different densities and thicknesses wereprepared by molten body transitional foaming process.The sound reduction index(R)of Al-Si closed-cell aluminum foam bareboards was investigated experimentally under different frequencies(100-4 000 Hz).It is found that sound reduction index(R)issmall under low frequencies,large under high frequencies and is controlled by different mechanisms.The sound insulation propertybasically conforms with the monolayer board sound insulation theory.The sound reduction index(R)increases with the even growthof thickness and density,but its rising trend is tempered.The single number sound reduction indexes(RW)of specimen with thicknessof 20 cm and density of 0.51 g/cm3are 30.8 dB and 33 dB respectively,which demonstrates good sound insulation property forlightmass materials.

Effect of TiH_2 on preparation of closed-cell aluminum foam and its compressive behavior

The vesicant problem during the process of preparing closed-cell aluminum foam by molten body transitional foaming process was discussed and the effect of granularity and addition of TiH2on porosity of closed-cell aluminum foam was investigated. The static compressive behavior of closed-cell aluminum foam and the influence of porosity on static compressive property of closed-cell aluminum foam were researched as well. The results show that with increasing granularity of TiH2,the porosity of closed-cell aluminum foam firstly increases and then decreases gradually, the granularity should be controlled in the range of 38-74μm which can result in higher porosity. The porosity of closed-cell aluminum foam increases with the increasing addition of TiH2, and the addition of TiH2 should be controlled from 1.5% to 2.5% which can result in homogeneous cell and moderate strength of closed-cell aluminum foam. The compressive process of closed-cell aluminum foam obviously displays linear elastic phase, plastic collapse phase, and densification phase, and the compressive strength grows with decreasing porosity.

Study of the Reconstruction of Fractal Structure of Closed-cell Aluminum Foam and its Thermal Conductivity

Based on the characteristics of the internal structure of closed-cell aluminum foam, this paper attempts to illus- trate the process of reconstructing the internal structures of closed-cell aluminum foam in Monte-Carlo method and the fractal characteristics of the reconstructed model. Furthermore, Binary Array Method is proposed by analyzing the reconstructed model and the thermal conductivity model of closed-cell aluminum foam is established. At the same time, the thermal conductivity of the foam materials with different porosity is calculated by Binary Array Method, and the calculated value coincides with the experimental results in the reference, which proves the correctness of these methods.

Acoustic properties of closed-cell aluminum foams with different macrostructures

As structural materials, closed-cell aluminum foams possess obvious advantages in product dimension, strength and process economics compared with open cell aluminum foams. However, as a kind of structure-function integration materials, the application of closed-cell aluminum foams has been restricted greatly in acoustic fields due to the difficulty of sound wave penetration. It was reported that closed-cell foams with macrostructures have important effect on the propagation of sound waves. To date, the relationship between macrostructures and acoustic properties of commercially pure closedcell aluminum foams is ambiguous. In this work, different perforation and air gap types were designed for changing the macrostructures of the foam. Meanwhile, the effect of macrostructures on the sound absorption coefficient and sound reduction index were investigated. The results showed that the foams with half-hole exhibited excellent sound absorption and sound insulation behaviors in high frequency range（〉2500 Hz）. In addition, specimens with air gaps showed good sound absorption properties in low frequency compared with the foams without air gaps. Based on the experiment results, propagation structural models of sound waves in commercially pure closed-cell aluminum foams with different macrostructures were built and the influence of macrostructures on acoustic properties was discussed.

Low strain rate compressive behavior of high porosity closed-cell aluminum foams

The impact of a rigid body(protected structure) together with cushion material(cellular metal foam) on hard ground from a fixed height was investigated.An analytical one-degree-of-freedom colliding model(ODF-CM) was established to analyze the protection ability and energy absorption by the foam under low velocity impact conditions.For validation,drop hammer experiments were carried out for high porosity closed-cell aluminum foam specimens subjected to low velocity impact loading.The dynamic deformation behavior of the specimen was observed and the velocity attenuation of the drop hammer was measured.The results demonstrated that the aluminum foam had excellent energy absorption capabilities,with its dynamic compressive behavior similar to that obtained under quasi-static loading conditions.Finite element method(FEM) was subsequently employed to obtain stress distributions in the foam specimen.As the propagating period of stress in the specimen was far less than the duration of attenuation,the evolution of the stress was similar to that under quasi-static loading conditions and no obvious stress wave effect was observed,which agreed with the experimental observation.Finally,the predicted velocity attenuation by the ODF-CM was compared with both the experimental measurements and FEM simulation,and good agreements were achieved when the stress distribution was considered to be uniform and the "quasi-static" compressive properties are employed.

Pyrimidine derivative as eco-friendly corrosion inhibitor for nickel−aluminum bronze in seawater

A pyrimidine derivative,6-phenyl-2-thiouracil(PT),was synthesized for developing a corrosion inhibitor(CI)applied in the protection of the nickel−aluminum bronze(NAB)in seawater.The anti-corrosion effect of PT was evaluated by the mass loss experiment,electrochemical tests and surface analysis.The results show that PT exhibits excellent inhibition performance and the maximum inhibition efficiency of PT reaches 99.6%.The interaction mechanism was investigated through X-ray photoelectron spectroscopy(XPS)and molecule dynamics simulation based on the density functional theory(DFT).The S-Cu,Al-N and Cu-N bonds are formed by the chemical interactions,leading to the adsorption of PT on the NAB surface.The diffusion of corrosive species is hindered considerably by the protective PT film with composition of(PT-Cu)_(ads)and(PT-Al)_(ads)on the PT/NAB interface.The degree of suppression is increased with the addition of more PT molecules.

Anomalies in mid-high-temperature linear thermal expansion coefficient of the closed-cell aluminum foam

Low-density closed-cell aluminum foam is promising to be used as load-bearing and thermal insulation components. It is necessary to systematically study its thermal expansion performance. In this work, linear thermal expansion coefficient(LTEC) of the closed-cell aluminum foam of different density was measured in the temperature range of 100–500 °C. X-ray fluorescence was used to analyze elemental composition of the cell wall material. Phase transition characteristics were analyzed with X-ray diffraction and differential scanning calorimetry. LTEC of the closed-cell aluminum foam was found to be dominated by its cell wall property and independent of its density. Particularly, two anomalies were found and experimentally analyzed. Due to the release of the residual tensile stress, the LTEC declined and even exhibited negative values. After several thermal cycles, the residual stress vanished. With temperature higher than 300 °C,instantaneous LTEC showed hysteresis, which should result from the redistribution of some residual hydrogen in the Ti2Al20 Ca lattice.

Aluminum foam as buffer layer used in soft rock tunnel with large deformation

The squeezing deformation of surrounding rock is an important factor restricting the safe construction and long-term operation of tunnels when a tunnel passes through soft strata with high ground stress.Under such soft rock geological conditions,the large deformation of the surrounding rock can easily lead to the failure of supporting structures,including shotcrete cracks,spalling,and steel arch distortion.To improve the lining support performance during the large deformation of squeezed surrounding rock,this work selects aluminum foam with densities of 0.25 g/cm3,0.42 g/cm3 and 0.61 g/cm3 as the buffer layer material and carries out uniaxial confined compression tests.Through the evaluation and analysis of energy absorption and the comparison of the yield pressure of aluminum foam with those of other cushioning materials and yield pressure support systems,the strength,deformation and energy absorption of aluminum foam with a density of 0.25 g/cm3 meet the yield pressure performance requirements.The numerical model of the buffer layer yielding support system is then established via the finite element analysis software ABAQUS,and the influence of the buffer layer setting on the lining support is analyzed.Compared with the conventional support scheme,the addition of an aluminum foam buffer layer can reduce the stress and deformation of the primary support and secondary lining.The maximum and minimum principal stresses of the primary support are reduced by 13%and 15%,respectively.The maximum and minimum principal stresses of the secondary lining are reduced by 15%and 12%,respectively,and the displacement deformation of the secondary lining position is reduced by 15%.In summary,the application of aluminum foam buffer layer can reduce the stress and deformation of the primary support and secondary lining,improve the stress safety of the support and reduce the deformation of the support.

Compressive and energy absorption properties of closed-cell magnesium foams

The quasi-static compressive mechanical behavior and deformation mechanism of closed-cell magnesium foams were studied, and the ef- fects of the density of magnesium foams on the compressive and energy absorption properties were also discussed. The results show that the compressive process of closed-cell magnesium foams is characterized by three deformation stages： linear elastic stage, collapsing stage and densification stage. At the linear elastic stage, the peak compressive strength （t70） and Young＇s modulus （E0） increase as the density increases Magnesium foams can absorb energy at the collapsing stage. In a certain strain range, the energy absorption capacity also increases as the density of magnesium foams increases.

Numerical and experimental analysis of the closed-cell aluminium foam under low velocity impact using computerized tomography technique

In the present work, the response of closed-cell aluminum foams under low-velocity impact has been studied numerically and experimentally. Computerized tomography is employed to access three-dimensional (3D) microstructure of the closed-cell aluminum foam. Effective parameters including foam density and the velocity of impactor on foam dynamic behavior are investigated. In order to show the validity and accuracy of results, some static experiments and low-velocity impact tests have been conducted. Results in dicate a remarkable agree me nt between the simulation and experimental data. Moreover, the results show that by increasing the density of foam samples, the highest difference between numerical and experimenidi results for peak stress and absorbed energy are 35.9% and 6.9%, respectively, which is related to the highest density. For impact velocities ranging from 3.1 to 4.2 m/s, the maximum discrepancy in peak stress and absorbed energy occur at an inipact velocity of 3.1 m/s in which corresponding errors are 33.3% and 6.6%, respectively. For the impact velocity of 40 m/s, the highest increase in peak stress and absorbed energy are 667.9% and 370.3% associated with the density of 0.5 and 0.3 g/cm^3, respectively.

Fatigue analysis of closed-cell aluminium foam using different material models

The fatigue analyses of AlSi7 closed-cell aluminium foam were performed using a real porous model and three different homogenised material models:crushable foam model,isotropic hardening model and kinematic hardening model.The numerical analysis using all three homogenised material models is based on the available experimental results previously determined from fatigue tests under oscillating tensile loading with the stress ratio R=0.1.The obtained computational results have shown that both isotropic and kinematic hardening models are suitable to analyse the fatigue behaviour of closed-cell aluminium foam.Besides,the kinematic hardening material model has demonstrated significantly shorter simulation time if compared to the isotropic hardening material model.On the other hand,the crushable foam model is recognized as an inappropriate approach for the fatigue analyses under tension loading conditions.

High corrosion and wear resistant electroless Ni–P gradient coatings on aviation aluminum alloy parts

A Ni–P alloy gradient coating consisting of multiple electroless Ni–P layers with various phosphorus contents was prepared on the aviation aluminum alloy. Several characterization and electrochemical techniques were used to characterize the different Ni–P coatings’ morphologies, phase structures, elemental compositions, and corrosion protection. The gradient coating showed good adhesion and high corrosion and wear resistance, enabling the application of aluminum alloy in harsh environments. The results showed that the double zinc immersion was vital in obtaining excellent adhesion (81.2 N). The optimal coating was not peeled and shredded even after bending tests with angles higher than 90°and was not corroded visually after 500 h of neutral salt spray test at 35℃. The high corrosion resistance was attributed to the misaligning of these micro defects in the three different nickel alloy layers and the amorphous structure of the high P content in the outer layer. These findings guide the exploration of functional gradient coatings that meet the high application requirement of aluminum alloy parts in complicated and harsh aviation environments.