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.

Three-point bending behavior of aluminum foam sandwich with steel panel

Static three-point bending tests of aluminum foam sandwiches with glued steel panel were performed. The deformation and failure of sandwich structure with different thicknesses of panel and foam core were investigated. The results indicate that the maximum bending load increases with the thickness of both steel panel and foam core. The failure of sandwich can be ascribed to the crush and shear damage of foam core and the delamination of glued interface at a large bending load, The crack on the foam wall developed in the melting foam procedure is the major factor for the failure of foam core. The sandwich structure with thick foam core and thin steel panel has the optimal specific bending strength. The maximum bending load of that with 8 mm panel and 50 mm foam core is 66.06 kN.

Preparation and characterization of aluminum foams with ZrH_2 as foaming agent

Aluminum foams were fabricated by melt-based route using ZrH2 as a foaming agent. The factors which affected the foaming of aluminum foams during casting process were investigated. The powdered zirconium hydride with content of 0.6%-1.4% （mass fraction） was added to the molten pure aluminum and the foaming condition was controlled in a temperature range from 933 to 1 013 K, Ca amount of 1.5%-3.0% （mass fraction）, stirring time of 0.5-2.5 min and holding time of 1.5-4.0 min to obtain homogeneous aluminum foams. The fabricated aluminum foams were characterized by XRD, SEM and Image-pro plus. The mechanical properties of the aluminum foams with different relative density were tested. The result indicates that the foaming agent （ZrH2） is suitable for the preparation of small aperture aluminum foams with average pore diameter of 1 mm. Inter-metallic compounds and Al2O3 have effect on the melt viscosity. The aluminum foams experience linear elastic, platforms and densification process and had a higher efficiency of energy absorption.

Oxide film on bubble surface of aluminum foams produced by gas injection foaming process

Based on A356 aluminum alloy,aluminum foams were prepared by gas injection foaming process with pure nitrogen,air and some gas mixtures.The oxygen volume fraction of these gas mixtures varied from 0.2%to 8.0%.Optical microscopy,scanning electron microscopy（SEM） and Auger electron spectroscopy（AES） were used to analyze the influence of oxygen content on cell structure,relative density,macro and micro morphology of cell walls,coverage area fraction of oxide film,thickness of oxide film and other aspects.Results indicate that the coverage area fraction of oxide film on bubble surface increases with the increase of oxygen content when the oxygen volume is less than 1.2%.While when the oxygen volume fraction is larger than 1.6%,an oxide film covers the entire bubble surface and aluminum foams with good cell structure can be produced.The thicknesses of oxide films of aluminum foams produced by gas mixtures containing 1.6%-21%oxygen are almost the same.The reasons why the thickness of oxide film nearly does not change with the variation of oxygen content and the amount of oxygen needed to achieve 100%coverage of oxide film are both discussed.In addition,the role of oxide film on bubble surface in foam stability is also analyzed.

Oxidation kinetics of oxide film on bubble surface of aluminum foams produced by gas injection foaming process

In the range of 620？710 °C, air was blown into A356 aluminum alloy melt to produce aluminum foams. In order to study the influence of temperature on the thickness of oxide film on bubble surface, Auger electron spectroscopy （AES） was used. Based on the knowledge of corrosion science and hydrodynamics, two oxidation kinetics models of oxide film on bubble surface were established. The thicknesses of oxide films produced at different temperatures were predicted through those two models. Furthermore, the theoretical values were compared with the experimental values. The results indicate that in the range of 620？710 °C, the theoretical values of the thickness of oxide film predicted by the model including the rising process are higher than the experimental values. While, the theoretical values predicted by the model without the rising process are in good agreement with the experimental values, which shows this model objectively describes the oxidation process of oxide film on bubble surface. This work suggests that the oxidation kinetics of oxide film on bubble surface of aluminum foams produced by gas injection foaming process follows the Arrhenius equation.

Three-point bending performance of a new aluminum foam composite structure

A new composite structure based on aluminum foam sandwich and fiber metal laminate was proposed. A layer of glass fiber was provided at the interface between the metal panel and the aluminum foam core in this composite structure, using adhesive technology to bond the materials together by organic glue in the sequence of metal panel, glass fiber, aluminum foam core, glass fiber and metal panel. The experimental results show that the new composite structure has an improved comprehensive performance compared with the traditional aluminum foam sandwiches. The optimized parameters for the fabrication of the new aluminum foam composite structure with best bending strength were obtained. The epoxy resin and low porosity aluminum foams are preferred, the thickness of aluminum sheets should be at least 1.5 mm, and the type of glass fiber has little effect on the bending strength. The main failure modes of the new composite structures with two types of glues were discussed.

Effects of cell wall property on compressive performance of aluminum foams

The effects of cell wall property on the compressive performance of high porosity, closed-cell aluminum foams prepared by gas injection method were investigated. The research was conducted both experimentally and numerically. Foam specimens prepared from conditioned melt were tested under uniaxial compressive loading condition. The cell wall microstructure and fracture were observed through optical microscope（OM） and scanning electron microscope（SEM）, which indicates that the cell wall property is impaired by the defects in cell walls and oxide films on the cell wall surface. Subsequently, finite element（FE） models based on three-dimensional thin shell Kelvin tetrakaidecahedron were developed based on the mechanical properties of the raw material and solid material that are determined by using experimental measurements. The simulation results show that the plateau stress of the nominal stress-strain curve exhibits a linear relationship with the yield strength of the cell wall material. The simulation plateau stress is higher than the experimental data, partly owing to the substitution of solid material for cell wall material in the process of the establishment of FE models.

Mechanical properties and energy absorption properties of aluminum foam-filled square tubes

Longitudinal and transverse mechanical properties and energy absorption properties of foam-filled square tubes under quasi-static loading conditions were studied.The foam-filled thin-walled square tube was fabricated with aluminum tube as its shell and closed-cell Al-Mg alloy foam as its core.The results indicated that the plateau region of the load-displacement curve exhibited a marked fluctuant serration which was clearly related to the formation of folds.The longitudinal deforming mode of foam-filled square tube was the same as that of the empty tube,but the fold number of foam-filled square tube was more than that of the empty tube.The longitudinal compression load and energy absorption value of foam-filled square tube were higher than the sum of that of aluminum foam (alone) and empty tube (alone) due to the interaction between tube and filler.In transverse direction,the compression load and energy absorption ability of foam-filled square tubes were significantly lower than those in longitudinal direction.

Effects of foaming parameters on microstructure and compressive properties of aluminum foams produced by powder metallurgy method

Semi open-cell aluminum foams having channels between individual cells were produced using low cost CaCO3foamingagent and applying the powder compact melting process.To this end,the aluminum and CaCO3powder mixtures were coldcompacted into dense cylindrical precursors for foaming at specific temperatures under air atmosphere.The effects of severalparameters including precursor compaction pressure,foaming agent content as well as temperature and time of the foaming processon the cell microstructure,linear expansion,relative density and compressive properties were investigated.A uniform distribution ofcells with sizes less than100μm,which form semi open-cell structures with relative densities in the range of55.4%-84.4%,wasobtained.The elevation of compaction pressure between127-318MPa and blowing agent up to15%(mass fraction)led to anincrease in the linear expansion,compressive strength and densification strain.By varying the foaming temperature from800to1000°C,all of the investigated parameters increased except compressive strength and relative density.The results indicated theoptimal foaming temperature and time as900°C and10-25min,respectively.

Damage and penetration behavior of aluminum foam at various impacts

In this work, the damage and penetration behavior of aluminum foam at various types of impact were examined through experiments. The impact energy of a striker was applied on the fixed aluminum foam having a thickness of 25 mm while increasing its impact by 2 J at each strike from 6 J to 16 J. The results show that the impact energies from 6 J to 12 J could not penetrate aluminum foam. However, the aluminum foam applied with the impact energy of 12 J incurred severe damages on its lower part. Finally, the aluminum foam applied with the impact energy of 14 J was penetrated. The striker having the impact energy of 6 J could penetrate aluminum foam around 10 mm. At this moment, aluminum foam could absorb the impact energy of around 9 J. When the impact energy of 14 J was applied on the aluminum foam, the aluminum foam was penetrated and it absorbed the impact energy of around 17.2 J. It is possible to create the safer structure against impact using the results of this work. The simulation results for the verification of the experimental results imply that the results for all the experiments in this work are reliable. It is possible to predict the structural safety of the aluminum foam for an impact if the impact behavior of aluminum foam performed in this work is utilized.

Preparation of open-celled aluminum foams by counter-gravity infiltration casting

A novel counter-gravity infiltration casting device and corresponding fabricating process for producing open-celled aluminum foams were presented. The experimental results show that defects such as insufficient or excessive infiltrating can hardly be found in the foam samples prepared by counter-gravity infiltration casting. The foam materials exhibit excellent mechanical properties. The void content strongly affects the mechanical properties of aluminum foams. The yield stress and plateau stress significantly increase with the decrease of void content. Raising pre-heating temperature and increasing packing pressure are effective to lower the void content in aluminum foams.

Novel foaming agent used in preparation process of aluminum foams

The performances of a novel foaming agent used in the preparation process of aluminum foams were investigated, and the effects of some factors, such as addition of the foaming agent, foaming temperature on the porosity, and appearance of aluminum foams were also discussed. Experimental results show that the novel foaming agent has a wide decomposition temperature range and a mild decomposed rate; the foaming agent has the ability to enhance the viscosity of aluminum melt, as a result, an extra viscosifier such as Ca or SiCp is unnecessary while using this foaming agent; the bubble-free zone in material decreases and the foaming effi- ciency increases with the increase of foaming agent; the bubble-free zone disappears and the foaming efficiency is near 100% when the addition of foaming agent is more than 1.4wt%; the porosity of the aluminum foam increases with the increase of foaming agent when the addition of foaming agent is less than 2.2wt%.

THE STRUCTURE CONTROL OF ALUMINUM FOAMS PRODUCED BY POWDER COMPACTED FOAMING PROCESS

A new technique, powder compact foaming process for the production of aluminumfoams has been studied in this article. According to this method, the aluminum pow-der is mixed with a powder foaming agent (TiH_2). Subsequent to mixing, the powderblend is hot compacted to obtain a dense semi--finished product. Upon heating to tem-peratures within the range of the melting point, the foaming agent decomposes to evolvegas and the semi--finished product expands into a porous cellular aluminum. Foamingprocess is the key in this method. Based on experiments, the foaming characteris-tics were mainly analyzed and discussed. Experiments show that the aluminum--foamwith closed pores and a uniform cell structure of high porosity can be obtained usingthis method by adjusting the foaming parameters: the content of foaming agent andfoaming temperature.

Impact damage behavior of sandwich composite with aluminum foam core

Impact property of the sandwich composite with aluminum foam core was investigated by experiment and simulation analysis. Impact energies of 50, 70 and 100 J were applied to the specimens in impact tests. The results show that the striker penetrates the upper face sheet, causing the core to be damaged at 50 J test but the lower face sheet remains intact with no damage. At 70 J test, the striker penetrates the upper face sheet and the core,and causes the lower face sheet to be damaged. Finally at 100 J test, the striker penetrates both the upper face sheet and the core, and even the lower face sheet. The experimental and simulation results agree with each other. By the confirmation with the experimental results, all these simulation results can be applied on structure study of real sandwich composite with aluminum foam core effectively.

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.

Thermal properties of open-celled aluminum foams prepared by two infiltration casting methods

Contrastive research was carried out to study the thermal properties of open-celled aluminum foams prepared by counter-gravity infiltration casting system and the traditional process respectively.The experimental results show that the thermal conductivity coefficients of aluminum foams prepared by two different infiltration methods have similar increasing trend with the increase of particle size;along with the reducing porosity,the thermal conductivity coefficients will be enhanced oppositely.However,with the same particle size,the open-celled aluminum foam prepared by the former method has a higher thermal conductivity coefficient obviously.It is largely because that the sample prepared by counter-gravity infiltration casting has a lower void content and better dense crystallization of metal-matrix after the constant pressure process.

Preparation of Aluminum Foam Sandwich by Rolling-bonding/Powder Metallurgy Foaming Technology

Aluminum foam sandwich was prepared by rolling-bonding/powder metallurgical foaming technology, and the effects of rolling on bond strength of face sheet/powders and powder density were studied. Moreover, the foaming agent, TiH2, was heat treated and a certain amount of Mg was added into powder in an attempt to understand how the stability and uniformity of foam was improved. The experimental results show that the foaming precursors with ideal quality were obtained by rolling-bonding process. When rolling reduction is 67%, the consistency of powders reach to 99.87%. Throughout consideration of the bonding of face sheet/ core layer powders and deformation characteristic of powders, the optimum rolling reduction is 60%-70%. Cracks and drainage during foaming were inhibited by heat treatment of foaming agent TiH2 and the addition of a certain amount of Mg. The optimum heat treatment way of TiH2 is that heat preserving 1 hour at 450 ℃; the amount of adding Mg is 1wt%.

Compressive properties of aluminum foams by gas injection method

The compressive properties of aluminum foams by gas injection method are investigated under both quasi-static and dynamic compressive loads in this paper. The experimental results indicate that the deformation of the aluminum foams goes through three stages: elastic deforming, plastic deforming and densification stage, during both the quasi-static and dynamic compressions. The aluminum foams with small average cell size or low porosity have high yield strength. An increase in strain rate can lead to an increase of yield strength. The yield strength of the aluminum foams under the dynamic loading condition is much greater than that under the quasi-static loading condition. Dynamic compressive tests show that a higher strain rate can give rise to a higher energy absorption capacity, which demonstrates that the aluminum foams have remarkable strain rate sensitivity on the loading rate.

Peculiarities of forming diffusion bimetallic joints of aluminum foam with a monolithic magnesium alloy

The work is carried out to determine an optimal method to obtain the welded bimetallic joints of monolithic Mg-alloy with porous Al-alloy using gallium as chemical activator and heating up to 300 ℃ by two different methods:long-term in vacuum oven and short-term without vacuum by passing of low voltage current.There is no microstructure change in Al-foam but indentation test records the negligible reduction of the mechanical properties.SEM showed the crystallization of two types of Mg_(5)Ga_(2) and Mg_(2)Ga inter-metallic phases in the wavy uneven diffusion zone on Mg-alloy side with significant increase of micro-hardness and Young’s modulus.The narrow depth of the diffusion zone takes place in joints by short-term heating,so this method is more applicable for welding of monolithic and porous alloys at chemical activation using gallium.