Porous Metallic Materials Research Articles

Pressure Drop in High-Density Porous Metals via Tomography Datasets

This study combines three-dimensional advanced imaging techniques and computational fluid dynamic modelling and simulation (CFD) to characterise the pressure drop of flowing fluid across high-density porous metals utilising high-resolution X-ray computed tomography data. The modelling approach quantifies the combined effects of pore volume fraction, pore connectivity, pore size and morphology on the flow behaviour of porous metals and to study in more detail the pressure drop behaviour characterised by the sudden change in pore volume by stacking of differential porous samples at the pore-level. The resulting predicted values of the pressure drop as a function of superficial fluid velocity ranging from Darcy to turbulent fluid flow regimes were used to account for the permeability (k0) and Form drag coefficient (C) of these materials. Supportable agreement between CFD modelled data against empirical measurements available in the literature was substantiated. Therefore it is considered that this approach could lead practically to minimizing the number of design iterations required for the processing of novel-attributing porous metallic materials for applications involving fluid flow.

Topology and mechanics of metal rubber via X-ray tomography

This paper describes the structural characteristics of metal rubber (MR) evaluated by X-ray tomography and skeletonisation at microscopic scale. The parameters used to describe the internal structure of the MR were numerically extracted and compared to assess the effect of different MR manufacturing parameters on the mechanical properties of the material. Quasi-static tests were also performed to obtain the stiffness and damping of the metal porous material and to understand their correlation with the topology defining the samples. A simplified helix cell model was used to interpret the mechanical properties of the MR based on the structural parameters identified in this paper. The transverse isotropic stiffness of the MR observed here is partly caused by the distribution of the metal wires, with more helices oriented along the molding direction resulting in smaller equivalent stiffness. The compression, relative density, helix and wire diameters can influence the structural and mechanical characteristics of the MR in a significant manner.

Experimental determination and numerical simulation of material and damage behaviour of 3D printed polyamide 12 under quasi-static loading

In order to characterise the material and damage behaviour of additively manufactured polyamide 12 (PA12) under quasi-static load and to implement it in a numerical model, experiments under quasi-static load as well as microstructural investigations were carried out. Selective laser sintering (SLS) was used as the manufacturing process. For the classification of the material behaviour, quasi-static cyclic tests with holding times as well as tensile tests were performed. X-ray refraction and computed tomography (CT) were used to investigate the damage behaviour. The Chaboche model, which has already been applied for metallic materials under thermomechanical loading, served as the basis for the selection of the numerical material model. The same procedure was used for the selection of the damage model, where the Gurson–Tvergaard–Needleman (GTN) model was chosen, which was already used for porous metallic materials. The Chaboche model shows very good agreement with experimental results. Furthermore, the coupling with the GTN model allows a very good modelling of the damage behaviour. Finally, it could be shown that the selected models are suitable to simulate the material and damage behaviour of 3D printed PA12.

Investigation into Transpiration Cooling of Blades in High-Temperature Gas Turbines

Among the methods for improving the performance of high-temperature gas turbines in gas-turbine engines (GTE) and gas-turbine units (GTU) is the implementation of transpiration cooling of the blades, since the maximum allowable temperature of blades with convective- or film-cooling ranges from 1800 to 1900 K. The basic advantage of this cooling method is in decreasing the required coolant flow due to extended-contact heat-transfer surface. Transpiration cooling systems employ porous metal materials. However, the effect of geometry of porous materials on air flow and heat transfer in them is still not clearly understood. The paper presents the results into investigation of models with a transpiration cooling system made of sintered stainless-steel fibers. The geometric characteristics of the investigated models depended on the material porosity. The thermal tests of the models were performed by the calorimetric method in a liquid-metal thermostat, while the hydraulic tests were carried out by the hot blow method under isothermal conditions. The experimental results can be entered into the database used by heat transfer software packages, thereby decreasing the labor intensity and time for designing a cooling system for gas turbine blades. The implementation of transpiration cooling offers the prospects for increasing the maximum allowable gas turbine temperature up to 2200 K.

Effective Thermal Conductivity Calculation and Measurement of Litz Wire Based on the Porous Metal Materials Structure

Litz wires are employed in high-frequency electrical machines due to their advantages of reducing the ac losses, including minimizing the skin effect and the proximity effect. In order to improve the reliability of such machines, and enable accurate thermal predictions at the design stage, accurate calculation of the thermal conductivity of litz wire is important. In this paper, a calculation method based on the Gasar porous metal materials model is put forward. In this method, a cell model is extracted from the litz wire, and a thermal resistance network is used to calculate the equivalent thermal conductivity (ETC). Following this, two finite-element analysis (FEA) models for the same litz wire are built, one with actual thermal conductivities for the different constituent materials and another with the calculated equivalent thermal conductivity for an equivalent material, with the two models showing similar thermal characteristics. Finally, an experimental setup is built for measuring the steady-state ETC of litz wire. The apparatus structure and characteristics are described in detail, and the experiment uncertainty and measurement errors are analyzed. Three types of litz wire are measured in the experimental, and the results from experiment and calculation are consistent.

Comparing Initial Wound Healing and Osteogenesis of Porous Tantalum Trabecular Metal and Titanium Alloy Materials.

Porous tantalum trabecular metal (PTTM) has long been used in orthopedics to enhance neovascularization, wound healing, and osteogenesis; recently, it has been incorporated into titanium alloy dental implants. However, little is known about the biological responses to PTTM in the human oral cavity. We have hypothesized that, compared with conventional titanium alloy, PTTM has a greater expression of genes specific to neovascularization, wound healing, and osteogenesis during the initial healing period. Twelve subjects requiring at least 4 implants in the mandible were enrolled. Four 3 × 5mm devices, including 2 titanium alloy tapered screws and 2 PTTM cylinders, were placed in the edentulous mandibular areas using a split-mouth design. One device in each group was trephined for analysis at 2 and 4 weeks after placement. RNA microarray analysis and ingenuity pathway analysis were used to analyze osteogenesis gene expression and relevant signaling pathways. Compared to titanium alloy, PTTM samples exhibited significantly higher expressions of genes specific to cell neovascularization, wound healing, and osteogenesis. Several genes-including bone morphogenic proteins, collagens, and growth factors-were upregulated in the PTTM group compared to the titanium alloy control. PTTM materials may enhance the initial healing of dental implants by modifying gene expression profiles.

Experimental measurements and numerical computation of nanofluid and microencapsulated phase change material in porous material

The electronic industry is increasingly investigating different approaches for the cooling of electronic equipment. The use of bulk phase change materials is also a promising approach for energy storage. The introduction of microencapsulated phase change materials combined with nanofluids can be beneficial. The combined use of a nanofluid and a metallic porous material can be used to mitigate problems resulting from small thermal conductivity. This study investigated a ternary mixture of water with a nanofluid and a microencapsulated phase change material in a porous medium. The model was previously validated with experimental data using a 0.5%vol concentration nanofluid in water. The results revealed that heat storage capability can be achieved as long as the microencapsulated phase change materials, which consists of encapsulated eicosane, is at a concentration of 20%. Because the melting temperature of microencapsulated phase change materials is approximately 36°C, energy storage at a low flow rate and heat flux is recommended.

Redox-active metal–organic frameworks for energy conversion and storage

We review progress in the study of electroactive MOFs with redox activity for energy conversion and storage. Recent advances in mixed-valence MOFs are highlighted, which have led to record conductivities towards metallic porous materials.

Experimental Study of Acoustic Performance of Porous Metals at High Temperatures

Based on an improved two-microphone transfer function method, a testing system for the exploration of acoustic performance of porous materials under high temperature conditions was developed with porous foam copper as one of research objects. The acoustic performances of some porous metallic materials were studied in the temperature range of 300°C to 700°C under the premise of ensuring the temperature stability that makes the measurement uncertainty be ± 6°C at high temperatures. The sound absorption coefficient and the acoustic impedance ratio of porous coppers at different ambient temperatures were acquired accordingly. And then the influence of the variation of temperature fields on the acoustic properties of porous metals was analyzed. The experimental results are in good agreement with the theoretical analysis, which proves the rationality of design of the device and provides important references and specific guidance for future study of the acoustic properties of porous metal materials.

Effect of Annealing on the Structure and Properties of Titanium Alloy with Cellular Architecture for Medical Applications

The methods of optical metallography, SEM, x-ray fluorescence analysis, microindentation and compression testing were used to study the effect of annealing of Ti – 6%Al – 4%V alloy produced by direct laser sintering of the powder on structure, phase composition and mechanical properties of the samples with cellular architecture intended for use as bone replacement implants. Annealing of the cellular samples having a relative density of 45% was conducted in the temperature range of 720 – 1000°C for 1 h under high vacuum. Mechanical properties were determined according to ISO 13314 for compression testing of porous metallic materials.

Tensegrity cell mechanical metamaterial with metal rubber

We present here a design of the unit cell of a mechanical metamaterial based on the use of a tensegrity structural configuration with a metal rubber. Tensegrity combines the use of compression and tension-only elements, and allows the creation of structures with high rigidity per unit mass. Metal rubber is a multiscale porous metal material with high energy absorption and vibration damping capabilities under compressive load. The combination of the two structural and material concepts gives rise to a mechanical metamaterial with increased energy absorption and tuneable nonlinearity under quasi-static, vibration, and impact loading. We develop prototypes, models, and perform tests under static and dynamic loading conditions to assess the performance of this mechanical metamaterial.

Melting process in porous media around two hot cylinders: Numerical study using the lattice Boltzmann method

Instantaneous melting of ice is accelerated inside a horizontal rectangular cavity with two vertically arranged cylinders using metallic porous matrix made of Ni-Steel alloys. The enthalpy-based lattice Boltzmann method (LBM) with a double distribution function (DDF) is employed at the representative elementary volume (REV) scale. Single-phase model is used and there is a local thermal equilibrium between porous structure and ice. Inserting metallic porous material into the base PCM results in the enhancement of the heat conduction and weakening of the natural convection flow. Concentric pattern of the solid–liquid interface persists in porous samples comparing to the pure PCM melting. Reducing porosity causes decrease of the full melting time and thermal storage capacity of the system. Thermal conductivity ratio has to be enlarged in porous samples with higher porosity from energy saving viewpoint.

Effect of track spacing on porosity of metallic foam fabricated by laser melting deposition of Ti6Al4V/TiH2 powder mixture

We fabricated porous metallic materials using laser melting deposition, wherein a mixture of a metal powder and a foaming agent is sprayed while a high-energy laser is irradiated on it to melt and deposit it, resulting in foaming action. To fabricate the porous materials, we used a powder of Ti6Al4V alloy. TiH2, a foaming agent, was employed to create pores within the deposited material. The track spacing was varied to examine its effect on the foaming characteristics and the porosity and pore size and distribution of the deposited material. We also investigated the effects of representative process parameters, including the laser power, foaming agent content, and powder feed rate. The results showed that, when the track spacing was smaller than the width of the deposited beads, pores were generated by the foaming agent. In the opposite case, however, open-cell pores were created between the tracks. This was because the fluidity of the molten metal varied with the track spacing, confirming that the porosity can be adjusted by changing the track spacing. Moreover, as the laser power was decreased or the powder feed rate increased, the porosity increased. Hence, the porosity is related to the cooling rate of the molten metal.

Mesoporous Metal Electrocatalysts for the Oxygen Evolution Reaction

Introduction Development of anode electrocatalys for the PEM type water electrolyzer is challenging since carbon supports cannot be used in order to disperse small catalyst particles under the high potential condition over 1.5 V. Without carbon supports, electrocatalysts result in relatively low active surface area and low activity. Our group has been working on mesoporous materials for PEFC and has found that mesoporous materials can keep their porous structure even after being built into the device. Based on the similar idea used in our PEFC studies, we propose development of carbon-free mesoporous metal electrocatalysts with high active surface area to increase the efficiency of water electrolyzer cells. In this study, mesoporous platinum and iridium materials were particularly developed, and their oxygen evolution reaction (OER) activity was evaluated. Experimental method Mesoporous platinum and mesoporous iridium materials were synthesized using Pt(acac)2 or Ir(acac)3 as a metal precursor and Pluronic F127 as a surfactant. After mixing the surfactant and metal precursor, the resulting mixture was heated up under N2 or Air atmosphere to decompose Pluronic F127 and make the mesoporous structure. Their electrochemical measurements were performed using a common half-cell setup in the solution, in order to evaluate OER activity. The durability against high potential like 1.8 V was also analyzed. Results and discussion The resulting platinum and iridium materials were confirmed to have the randomly oriented porous structure constructed by aggregation of Pt and Ir/IrOx nanoparticles, respectively. The pore diameter was found mostly smaller than 100 nm. With these porous metal materials, OER activity was evaluated. Regarding platinum based materials, the initial activity resulted in the same as a standard Pt/KB electrocatalyst, TEC10E50E, even though ECSA was six times higher for Pt/KB. However, for their durability, as expected, Pt/KB lost most of OER activity owing to carbon corrosion. Consequently, carbon-free porous Pt electrocatalyst with high activity and durability was successfully prepared. Similarly, a porous iridium electrocatalyst has been prepared, and its OER activity will be discussed.

Experimental Study on Tunable Electromagnetic Shielding by Microlattice Materials with Organized Microstructures

Porous metal materials are widely used for preventing electromagnetic interference, control of heat conduction and impact absorbing. However, a limitation associated with porous metals is that their irregular and non‐uniformal microstructures make their functions non‐desirably tunable. In this report, the authors propose a microlattice material with precisely controlled microstructures for electromagnetic (EM) shielding. An effective medium model taking into account of the EM stimulated surface current of the metal wires is developed to characterize the EM shielding effect of the microlattice metal. In addition, a compact and integrated measuring device is constructed by using a 3D printing method for experimental verification. The theoretical predictions agree well with the experimental measurement. The influence of the geometrical parameters of the microlattice metals on EM shielding effect is also studied. The theoretical model and the compact measuring system provide robust and efficient tools for the design of EM shielding materials.

Energy Demand Reduction in Nearly Zero-Energy Buildings by Highly Efficient Aluminium Foam Heat Exchangers

The capability periodically to store and release the latent heat of phase transition during melting and solidification of Phase Change Materials (PCMs) has been currently the main subject of interest with regard to cost reduction efforts for cooling, heating of interiors and Domestic Hot Water (DHW) necessary for the operation and maintenance of adequate thermal comfort in new modern as well as old renovated residential buildings. The main principle of PCMs facilities to reduce significantly the energy consumption in the building industry of the future is based on the ability of thermo-active heat exchangers to absorb and later to dissipate into the surroundings excessive heat which can be easily obtained from renewable sources (e.g. solar energy, geothermal heat, etc.) directly in a building or in its immediate vicinity. Smart interior tiling and furnishing systems can provide high energy efficiency by stabilizing the room temperature at a level ensuring sufficient thermal comfort basically governed by the thermal conductivity and heat exchange area between ceiling (respectively also wall and floor if necessary) heat exchangers (radiators) and the heat storage medium in the form of PCMs. Unfortunately, most conventional building materials, e.g. aerated concrete, bricks, gypsum, ceramic tiles, etc. are particularly characterized by very low thermal conductivity, which disadvantages them to be used for these purposes. However, highly porous metallic material such as aluminium foam prepared by powder metallurgy [10, 11] is on the contrary excellently heat conductive, which predisposes it to be used for light-weight design of supporting structure of very energy efficient indoor as well as outdoor thermo-active heat exchangers for building industry of the future. This contribution points to the possibility to apply aluminium foam for both the novel innovative roofing system to cover pitched roofs and the interior ceiling panels, with the minimum energy demands for maintaining the sufficient thermal comfort in future nearly Zero-Energy Buildings (nZEBs).

Parametric optimization and analysis of metallic porous material used for discharge system of hermetic compressor

Gas pulsation is an important noise source in household refrigerators. The pulsation is generated in the compressor discharge chamber and it is transferred to the cabinet through the condenser, which is directly connected to the back of the system. The discharge muffler makes a considerable contribution to refrigerator noise. The noise can be controlled through the use of metallic porous material in the expansion chamber of the muffler. The macro-acoustic parameters of porous medium (airflow resistivity, porosity, tortuosity, viscous and thermal length) were characterized experimentally to validate numerical and analytical results. The porous material was modeled, using Johnson-Champoux-Allard (JCA) approach, as an equivalent fluid. Parametric optimization was employed to maximize the transmission loss of the dissipative muffler. The optimized frequency range was 500 Hz to 5 kHz. Prototypes were developed with different distributions of porous material in the chamber and they showed improved performance compared with the reactive muffler.

A review on metallic porous materials: pore formation, mechanical properties, and their applications

This paper presents a comprehensive review on the mechanism of pore formation, mechanical properties, and applications of metallic porous materials. The different manufacturing techniques of metallic porous materials using various pore-forming agents (e.g., sodium chloride, polymethyl methacrylate, magnesium, and cenosphere) are highlighted in the first part of this review. Subsequently, the pore formation mechanism and pore morphology in final products as well as corresponding pore-forming agent removal techniques (e.g., sintering-dissolution process, thermally stimulated decomposition, thermally melted elimination, and embedding cenosphere technique) are specifically discussed. Then, some major influential factors on the mechanism of pore formation, including pore size, shape, distribution, and porosity, are analyzed in detail. Meanwhile, the primary mechanical properties such as compressive strength, elastic modulus, fatigue properties, and flexural strength of metallic porous materials depending on pore morphology and porosity are explored in detail. Furthermore, their applications in structural and functional aspects according to their pore morphology and mechanical properties are emphatically summarized. Finally, this review article highlights some important factors for advanced wear-resistant tool and biomedical implant applications of porous metallic materials.

Quantitative analysis for acoustic characteristics of porous metal materials by improved Kolmogorov’s turbulence theory

This paper investigates the sound absorbing property of porous metal materials quantitatively using turbulence analogy model. Firstly, the improved Kolmogorov’s turbulence theory is obtained by catastrophe theory for the first time, to our knowledge. Secondly, a quantitative turbulence analogy model is proposed though the improved Kolmogorov’s turbulence theory. Finally, this model is adopted to analyze the wave propagation inside the porous metal materials. The quantitative relationship of energy spectrum density is fully obtained, which is also related to the average pore diameter and porosity. With the increase of excitation frequency, the energy spectrum density decreases with certain power law. And the energy increases with the increase of the average pore diameter. Our theoretical results are consistent with the experimental results. Our study can provide a feasible theoretical guidance for controlling the vibration and noise of porous metal materials.

Effects of process parameters on properties of porous foams formed by laser-assisted melting of steel powder (AISI P21)/foaming agent (ZrH2) mixture

This paper describes the fabrication of lightweight metal foams using the directed energy deposition (DED) method. DED is a highly flexible additive manufacturing process wherein a metal powder mixed with a foaming agent is sprayed while a high-power laser is used to simultaneously melt the powder mixture into layered metal foams. In this study, a mixture of a carbon steel material (P21 powder) and a widely used foaming agent, ZrH2, is used to fabricate metal foams. The effects of various process parameters, such as the laser power, powder feed rate, powder gas flow rate, and scanning speed, on the deposition characteristics (porosity, pore size, and pore distribution) are investigated. The synthesized metal foams exhibit porosities of 10% or lower, and a mean pore area of 7×105μm2. It is observed that the degree of foaming increases in proportion to the laser power to a certain extent. The results also show that the powder feed rate has the most pronounced effect on the porosity of the metal foams, while the powder gas flow rate is the most suitable parameter for adjusting the size of the pores formed within the foams. Further, the scanning speed, which determines the amounts of energy and powder delivered, has a significant effect on the height of the deposits as well as on the properties of the foams. Thus, during the DED process for fabricating metal foams, the pore size and distribution and hence the foam porosity can be tailored by varying the individual process parameters. These findings should be useful as reference data for the design of processes for fabricating porous metallic materials that meet the specific requirements for specialized parts.