Porous Metal Foam Research Articles

A Novel Manufacturing Route for Fabrication of Topologically‐Ordered Porous Magnesium Scaffolds

AbstractThe use of porous metal foams as biomaterial scaffolds has been widely adopted; however, many of these porous structures are manufactured with pore architectures that are inherently random. This makes structural optimization for a specific purpose challenging. Scaffolds containing ordered pore architectures can be fabricated to meet design criteria, such as porosity, stiffness, and volume fraction. Mg and its alloys offer potential as a new class of degradable metallic orthopedic biomaterials. In comparison with current metallic orthopedic implant materials, Mg offers advantages such as, closer‐to‐bone stiffness and biodegradability, thereby eliminating the need for a second surgery to remove hardware. Currently there are few methods described in literature to manufacture ordered porous Mg. The aim of this study was to determine the resolution of a novel indirect solid free‐form fabrication (SFF) process for producing topologically ordered Mg (TOPM) structures. The multi‐step method involved the printing of an SFF mould, NaCl infiltration, and liquid Mg casting techniques. Using a range of characterization methods, we demonstrated that the selected structures were manufactured with a high level of accuracy. Differences in dimensions from CAD models to Mg scaffolds ranged from 2.5% to a maximum of 8.33%. Similarly, there was a maximum of 6.1% reduction in porosity in Mg scaffolds compared with initial design. Meanwhile, with surface roughness of 10.17 ± 1.75 µm, there was an average of 70% increase in surface area. This study demonstrates a simple, reliable, and safe route to manufacture TOPM scaffolds for potential application in medical device design.

Performance of Magnetorheological Fluids Flowing Through Metal Foams

If magnetorheological (MR) fluids are stored in porous materials, when excited by the external magnetic field, MR fluid will be drawn out and produce MR effect, which could be used to solve the following problems of the MR damper, such as the seal, volume and the cost of MR fluid damper. In this paper, the effect of structure of metal foams on the performance of MR fluid is investigated; the relationship between the penetrability and the porosity of the metal foams is measured, the change of MR fluid performance flowing though the metal foams is obtained. It shows that, after flowing through metal foams, the change of performance of MR fluid is about 2.5%. Compared to the sponge, the porous metal foams have the obvious advantages in high porosity and rigidity, which provide a convenient and low-cost way to design the MR damper.

Failure by buckling mode of the pore-strut for isotropic three-dimensional reticulated porous metal foams under different compressive loads

Compression is one of the most basic loading modes for engineering materials, and the failure of lost stability is possibly resulted from buckling for the bar under compression. The pore-strut within porous metal foams under compression may be similar to the compression bar, so the strut is possible to buckle when the porous body is under compressive loads. With the analytical property model of the simplified structure, the pore-strut buckling behavior is analyzed for isotropic three-dimensional reticulated porous metal foams, and the failure modes resulting from this buckling are investigated for these materials under compressive loadings. These loading modes cover all of three loading conditions, including uniaxial compression, biaxial compression and triaxial compression. The treating ways of the pore-strut are relative to three slenderness-ratios, including three conditions of thin-long bar, middle-long bar and stocky bar. Based on these works, the mathematical relationships between nominal main stresses and porosity are found for this buckling failure of these materials under compression. Through the relevant expression, the relevant strength criterion and the relevant loading condition resulting in the strut buckling are further achieved for these porous metal foams under compression.

Mechanical relations for porous metal foams under several typical loads of shearing, torsion and bending

Porous metal foams with isotropic three-dimensional reticulated structure are excellent engineering materials with dual attributes of structural and functional characteristics, and they may bear shearing or torsional or bending loads when they are used as engineering components in various cases. With the octahedral model, the mechanical behavior is investigated for the component of these materials under shearing loads, the loading state of struts within the porous body is analyzed for these materials under torsion, and the mechanical performance is further discussed for these materials under bending moment based on their properties of uniaxial tension and compression. Using the simple manner of segregating analysis, the relevant mechanical relationships are respectively derived from the corresponding mathematical deduction for these materials under those loads. The results show that, when the porous body endures the fracture resulting from shearing loads or from torsion, the maximum nominal shearing stress or the nominal torque for the porous component can be described with the porosity and the inherent characteristic parameters of the metal foam itself. Also, the applicable maximum nominal bending moment can be expressed as a function of the porosity for the metal foams under bending moment. This mathematical relationship may be regarded as the extension of the expression about the relationship between the maximum nominal stress and the porosity for these materials under uniaxial tensile and compressive loads. From these mathematical relations, the loading criterions may be directly and conveniently achieved for these materials under the corresponding loads, respectively.

A new method for calculating the specific surface area of porous metal foams

Porosity and pore diameter are two elementary parameters that are easy to be measured for porous materials. A new method is proposed for calculating the specific surface area of porous metal foams based on a mathematical relationship between this area and the porosity and pore diameter. Using this method, the specific surface areas of metal foams prepared by different methods, namely electrodeposition and high-pressure permeation casting, are successfully calculated. The results indicate that the present model can accurately describe experimental data.

Heat flux dispersion in natural convection in porous media

Analytical solutions derived in this paper confirm the experimental and numerical results revealing a widespread dispersion of heat flux data in natural convection in porous media. The weak non-linear method of solution is used to evaluate the heat flux in a porous layer heated from below and subject to weak boundary and domain imperfections. Little attention has been paid so far to the effect that the lower branch of the imperfect bifurcation has on the average heat flux. The results presented in this paper demonstrate the latter effect and explain the reason behind the dispersion of data. The comparison of the results with existing experimental and numerical data confirms the findings. In addition the latter effect is shown to be essential in ones ability to control heat transfer enhancement via natural convection in porous metal foams, for example.

Numerical Analysis of Convective Heat Transfer From an Elliptic Pin Fin Heat Sink With and Without Metal Foam Insert

A numerical analysis of forced convective heat transfer from an elliptical pin fin heat sink with and without metal foam inserts is conducted using three-dimensional conjugate heat transfer model. The pin fin heat sink model consists of six elliptical pin rows with 3 mm major diameter, 2 mm minor diameter, and 20 mm height. The Darcy–Brinkman–Forchheimer and classical Navier–Stokes equations, together with corresponding energy equations are used in the numerical analysis of flow field and heat transfer in the heat sink with and without metal foam inserts, respectively. A finite volume code with point implicit Gauss–Seidel solver in conjunction with algebraic multigrid method is used to solve the governing equations. The code is validated by comparing the numerical results with available experimental results for a pin fin heat sink without porous metal foam insert. Different metallic foams with various porosities and permeabilities are used in the numerical analysis. The effects of air flow Reynolds number and metal foam porosity and permeability on the overall Nusselt number, pressure drop, and the efficiency of heat sink are investigated. The results indicate that structural properties of metal foam insert can significantly influence on both flow and heat transfer in a pin fin heat sink. The Nusselt number is shown to increase more than 400% in some cases with a decrease in porosity and an increase in Reynolds number. However, the pressure drop increases with decreasing permeability and increasing Reynolds number.

Mechanical relation for porous metal foams under complex loads of triaxial tension and compression

The mechanical relation of isotropic three-dimensional reticulated porous metal foams with stochastic pores is investigated under complex loads of triaxial tension and compression. From the simplified structural model, the mathematical relationship between three nominal main stresses and porosity has been derived for this class of porous materials at failure under the above-mentioned complex loads, covering three loading conditions of biaxial tension with monoaxial compression, of biaxial compression with monoaxial tension, and of triaxial compression. Through the relevant expression from the deduction, the criterion of strength design can be further obtained for these porous materials under these multiaxial complex loadings.

Mechanical model of porous metal foams under multiaxial tensile loads

Mathematical expressions for the mechanical properties of porous metal foams with an isotropic three-dimensional reticulated structure under multiaxial tensile loads are developed. From the simplified structural model, a relation between three nominal main stresses and the porosity is derived for these porous materials at failure under multiaxial tension. In addition, the relevant strength criterion for foamed metals in this loading case is obtained.

Experimental and Numerical Investigation of the Thermal Performance of the Gas-Cooled Divertor Plate Concept

The helium-cooled plate-type divertor concept proposed by Malang was designed to accommodate a surface heat load of ˜10 MW/m2. This design can potentially reduce the number of modules needed for the divertor by over two orders of magnitude compared with other concepts, thereby significantly reducing coolant delivery system complexity and manufacturing costs. While previous analyses have predicted that the plate design can accommodate heat fluxes of 10 MW/m2, no experimental data have been published to date to validate such analyses. Experiments have therefore been conducted using air as the coolant at Reynolds numbers similar to those proposed for the actual helium-coolant operating conditions on an instrumented test module with cross-sectional geometry identical to the prototypical plate-type divertor. A second test module where the planar jet exiting the inlet manifold is replaced by a two-dimensional hexagonal array of circular jets over the entire top surface of the inlet manifold has also been tested. The thermal performance of both test modules with and without a porous metallic foam layer in the gap between the outer surface of the inlet manifold and the cooled surfaces was directly compared to test the numerical simulations of Sharafat which predict that the metallic foam significantly enhances heat transfer.

Uniaxial tensile stress–strain behavior of entangled steel wire material

Entangled steel wire material with total porosities of 36.3–61.8% and pore sizes of 15–825 μm has been investigated in terms of uniaxial tensile behavior and mechanical properties. The entangled wire materials after sintering processing exhibit higher specific strength and higher toughness than other porous metals or metallic foams. The experimental observation shows that the dominant deformation and failure mechanisms for the materials are twisted wire stretching/orienting, joints breaking and the entangled wire structure loosing. Under tensile loading the sintered entangled steel wire materials with 36.3% porosity exhibit average 15.0 MPa yielding strength, 112.7 MPa ultimate strength, and 7.5 GPa Young's modulus. With 61.8% porosity, the sintered materials have 7.5 MPa yield strength, 40.1 MPa ultimate strength, and 2.7 GPa Young's modulus.

Porous Metals and Metallic Foams: Current Status and Recent Developments

AbstractPorous metals and metallic foams are presently the focus of very active research and development activities. There are currently around 150 institutions working on metallic foams worldwide, most of them focussing on their manufacture and characterisation. Various companies are developing and producing these materials which are now being used in numerous industrial applications such as lightweight structures, biomedical implants, filters, electrodes, catalysts, and heat exchangers. This review summarizes recent developments on these materials, with particular emphasis on research presented at the latest International Conference on Porous Metals and Metallic Foams (MetFoam 2007).

Cover Picture: Metallic Foams (Adv. Eng. Mater. 9/2008)

AbstractPorous metals and metallic foams are presently the focus of very active research and development activities. There are currently around 150 institutions working on metallic foams worldwide, most of them focussing on their manufacture and characterisation. Various companies are developing and producing these materials which are now being used in numerous industrial applications such as lightweight structures, biomedical implants, filters, electrodes, catalysts, and heat exchangers. This review summarizes recent developments on these materials, with particular emphasis on research presented at the latest International Conference on Porous Metals and Metallic Foams (MetFoam 2007).

Analytical Modelling of the Radiative Properties of Metallic Foams: Contribution of X‐Ray Tomography

Two metallic foams exhibiting a similar porosity but different cell sizes have been characterized using X-ray tomography. The images have been processed and analysed to retrieve the morphological properties required for the calculation of the radiative properties such as the extinction coefficient. The multiple possibilities of using the X-ray tomography method rather than conventional optical methods like SEM have been quantified. The extinction coefficient has then been determined from two approaches. First, the resulting morphological properties have been used as the input data of the conventional independent scattering theory. A special emphasis is put on the determination of morphological properties and their influence on the results. In the second approach, an original method is also proposed in order to determine the extinction coefficient of highly porous open cell metal foams, from the tomographic images and without any calculation or hypothesis. Results show a good agreement with the extinction coefficient obtained from experimental measurements. Our novel method enables to reduce uncertainties considerably.

Joining strategies for open-porous metallic foams

Abstract Within the Collaborative Research Centre 561 “Thermally highly loaded, porous and cooled multilayer systems for combined cycle power plants” open porous and high-temperature stable Ni-based structures are being developed for the requirements of effusion cooling. A two-dimensional cooling strategy for the walls of combustion chambers, that allows the outflow of the cooling medium over the complete wall area of the combustion chamber, could be realised by an open porous metallic foam structure. The open porous metallic foam is produced by the “Slip Reaction Foam Sintering” process, a powder metallurgical process. For joining several foams to assemble structural elements laser beam welding and capacitor discharge welding have been used. Different joining strategies have been examined for both processes to find out the most suitable method for joining these foams. In this paper the process setups, settings of the different strategies and results of trials (seam geometry and strength tests) are discussed. The need for graded structures to combine the essential permeability and adequate weldability is also shown.

Cover Picture: Joining Strategies for Open Porous Metallic Foams on Iron and Nickel Base Materials (Adv. Eng. Mater. 8/2007)

The cover shows a cavity in a metal foam which developed due to large heat effects in the foam during welding. More about joining strategies of open porous metallic foams can be found in the paper by S. Angel, W. Bleck et al. on page 670.

Joining Strategies for Open Porous Metallic Foams on Iron and Nickel Base Materials

Joining Strategies for Open Porous Metallic Foams on Iron and Nickel Base Materials

Application of Thermal Barrier Coatings on Open Porous Metallic Foams

An increase in the efficiency of gas turbines results in higher thermal loads. Open porous metallic foams coated with a thermal barrier coating (TBC) and a dense array of laser drilled cooling holes are very promising for the realization of effusion cooling. Application of TBCs on such substrates has not been reported so far. Here, the application of TBCs using air plasma spraying (APS) and high velocity oxygen fuel (HVOF) on such substrates is researched. A TBC consisting of a metallic bond coat and a ceramic top coat is deposited. The results show that the application of TBCs on open porous foams is possible. However, a uniform coating growth is primarily linked to the pore sizes. In the case of large pores, the required coating thickness for a closed surface is leading to coating delamination caused by extensive tensile residual stresses.

Porous current collectors for passive direct methanol fuel cells

A passive direct methanol fuel cell (DMFC) with its cathode current collector made of porous metal foam was investigated experimentally. The measured polarization curves, constant-current discharging behavior and EIS spectra showed that the passive DMFC having the porous current collector yielded much higher and much more stable performance than did the cell having the conventional perforated-plate current collector with high methanol concentration operation. It was demonstrated that the improved performance for the porous current collector was attributed to: (i) the enhanced oxygen transport on the cathode as a result of a larger specific transport area, (ii) the increased operating temperature as a result of the lower effective thermal conductivity of the porous structure, and (iii) the faster water removal as a result of the capillary action in the porous structure, The experimental results also revealed that the porous current collector with a smaller pore size yielded higher performance as a result of the lower cell resistance.

Solid/vapour sorption heat transformer: Design and performance

A high temperature high lift solid sorption based heat transformer has been successfully designed and tested. The sorption reactor concept is based on a tube-fin heat exchanger where the heat exchanging fluids can flow through the hollow fins. The plates were brazed together with porous metal foam that was impregnated with either of the sorbents, LiCl and MgCl 2. The adsorbate is ammonia. The batch system was tested as to the power delivered at high temperatures, 150–200 °C. Peak power at 200 °C was about 0.8 kW, the average power about 0.4 kW. The thermal efficiency, COP, was calculated from the experimental results to be 0.11. This is only 40% of the expected theoretical value and can largely be attributed to the thermal mass of the reactor.