Stainless Steel Foam Research Articles

Synthesis and characterization of 316L stainless steel foam made through two different removal process of space holder method

Stainless steel foams were fabricated by evaporative and leachable space holder method. The results demonstrate that the cell walls are strong and interconnected; twin’s grains are visible in the cell wall surface in the evaporative process foam specimens. The cell structures and porosities have a significant effect on the mechanical properties. Calculated yield stress and energy absorption of the evaporative and leachable process are in the range of 43.2 to 6.92 & 29.1 to 3.96 MPa and 32.6 to 8.98 & 20.97 to 6.35 MJ/m3. The mechanical properties are higher in foams made by evaporative as compared to the leachable process.

In situ shear behavior of open-cell austenitic 316L steel foams

Foam cores in crash boxes are subjected to large shear forces during crash situations. Here, the failure behavior of 316L open-cell stainless steel foams under shearing conditions was investigated. A custom-made grip system was used to conduct in situ experiments with a scanning electron microscope and a digital light microscope. A foam model was created based on μ-CT data and imported into FE software for simulation of the shearing test. Furthermore, the shear-induced strain fields were evaluated using digital image correlation. On the mesoscale, the foam cells mainly experienced torsion, while on the microscale, the foam struts failed due to tensile and bending loading. Twisted cells became almost parallel to the loading direction, leading to the tensile loading of the foam struts. Many slip lines developed on the strut walls, revealing that the steel struts underwent ductile fracture. In conclusion, the failure behavior of the foam struts and cells was greatly influenced by the loading conditions of the foams.

Fabrication and Mechanical Properties of Open‐Cell Austenitic Stainless Steel Foam by Electrostatic Powder Spraying Process

An open‐cell metallic foam is newly manufactured using the electrostatic powder spraying (EPS) process, and its room temperature compressive and energy absorption properties are investigated. EPS is a process that uses electrostatic forces. The powder is negatively charged by applying direct current high voltage and undergoes electrostatic spraying on a positively charged polyurethane pre‐form. To control relative density (ρ*/ρ), three different spray amounts are applied (2.5, 3.5, and 4.5 kg m−2). Also, the relative density is measured to increase constantly with the spray amount. The room temperature compressive curves of the EPS foams represent the typical compressive stages (linear elastic, plastic collapse, and densification) of metallic foam. The plateau strengths of the EPS foams measure 0.48 MPa (spraying amount: 2.5 kg m−2), 0.76 MPa (spraying amount: 3.5 kg m−2), and 1.23 MPa (spraying amount: 4.5 kg m−2), respectively. Also, EPS 316L foam with the smallest spray amount has a lower energy absorption amount compared with other types of foam. This study also discusses the compressive and energy absorption behaviors of an open‐cell metallic foam manufactured using the EPS process in relation to its structural and microstructural characteristics.

Sound Absorption Performance of Highly Porous Stainless Steel Foam with Reticular Structure

There have been some previous studies on the acoustic properties of metal foams, with the main relevant works focused on the aluminum foam, but quite fewer on the metal foams of non-aluminum species. Such research, especially, has not yet been found on the stainless steel foam, of which the pore-structure related parameters and the morphology are quite different from the aluminum foam. The present work provides the investigations on the sound absorption performance of the 304 stainless steel foam and its composite structure. A sort of three-dimensional reticular stainless steel foam was successfully prepared, with the average pore size of about 1.8 mm and the porosity of about 93.7%. The sound absorption performance was investigated at 200–6300 Hz for this foam and at 2500–4000 Hz for its composite structures. The results show the whole absorption performance of the sample can be significantly improved by introducing an air gap, and further improved with introduction of both the gap and a perforated plate. It was found that when the thickness of the gap increased (from 3.5 to 17.5 mm), the resultant change of the resonance frequency of the sample could lead to a decrease of the absorption coefficient at a certain frequency range, and the total absorption efficiency could be significantly increased.

Fabrication method for stainless steel foam block in directed energy deposition

Using powder-based directed energy deposition (DED), composite materials can easily be produced by mixing powders. In this study, a foam stainless block is fabricated through DED by combining SUS316L and titanium-hydride (TiH2) powders together. Although TiH2 successfully foams the melt pool and leaves many pores inside the deposit, it is difficult to enhance the porosity rate because the porous structure is destroyed when the upper layer is deposited. This study demonstrates a laser power control and deposition trajectory that can be used to create a low-density porous structure and measures the compression strength of the fabricated block.

Microstructure, mechanical and EMI shielding performance in open cell austenitic stainless steel foam made through PU foam template

Open cell austenitic stainless steel foams having different porosities were made by impregnation of polyurethane foam followed by sintering. The austenitic stainless steel foams were characterized with respect to various techniques such as XRD, FESEM, EDX and compressive behaviour. The results show that the yield stress, elastic modulus and energy absorption are in the range of 5.2–10.5 MPa, 2.01–7.31 GPa and 1.2–3.5 MJ/m3 respectively. The yield strength, elastic modulus, average plateau stress and energy absorption increased with an increase in relative density and densification strain deceased with increase in relative density. The experimentally determined plateau stress, elastic modulus and densification strain were compared with Gibson-Ashby's relation and are found to be in close agreement. The electromagnetic interference shielding of austenitic stainless steel were measured in X-band frequency region (8.2–12.4 GHz) and found that electromagnetic interference shielding increases with relative density in all cases. The austenitic stainless steel foam has maximum electromagnetic interference shielding of −31.5 dB with absorption dominating mechanism at relative density of 0.17. This has been explained from the microstructural characteristics of austenitic stainless steel foam.

Comparative analysis of characteristics of stainless steel cellular material prepared through powder metallurgy using accicular and crushed urea as spaceholder

Stainless steel has an excellent mechanical property as well as high corrosion resistance. Stainless steel foams, therefore, seemed like an attractive material for impact energy absorption applications where damping capability is required such as in vehicles and buildings. Also when stainless steel foam is produced as stainless steel foam, the material density will be reduced thus the resulting foam will be a combination of light weight and high strength that can also be used in high strength applications. In our analysis, we tried to produce stainless steel foam through powder metallurgy in order to control mechanical properties in a better manner compared to the casting method. Also, we try to compare the pore morphology in foams on changing the space holder from accicular urea to crushed urea using FE-SEM. The properties of stainless steel foam, to a large extent, are found to depend on the arrangement of the pores which is decided by the space holder utilized during its synthesis using powder metallurgy route. The stainless steel obtained using acicular carbamide as space holder is found to possess acicular or irregular pores whereas those produced with crushed urea as space holder possesses nearly circular holes. Also, the previous foams are found to have better mechanical properties contributing towards more useful metallic foam.

Composition and Type of a Binder Effects on the Stainless Steel Foam Microstructure Prepared by Sponge Replication Method

Biomaterials with a porous structure are beneficial for a wide range of medical engineering applications such as filtration, bone replacement and implant development. Stainless Steel 316L (SS316L) foam has been fabricated by foam replication method at different SS316L powder composition which is 60 wt%, 65 wt%, and 70 wt%. The binders used were Polyethylene Glycol (PEG), Carboxymethyl Cellulose (CMC) and Polyvinyl Alcohol (PVA), while distilled water was used as a solvent. The effects of using different composition of SS36L powder, binders and sintering time to the SS316L foam properties was studied. The materials were mixed by using a mechanical stirrer at 250 rpm for 1 hour. Polyurethane foam (PU) which was used as a sacrificial template was dipped into the SS316L slurry until fully coated. The coated samples were then dried in a drying oven within 24 hours before being sintered in an argon gas environment at 1200°C. The samples were characterized to observe the microstructure of the SS316L foam produced. As expected, the viscosity of the SS316L slurry was increased as the SS316L composition increases. The viscosity of SS316L slurry prepared by using PVA as a binder is higher than the SS316L slurry prepared with CMC and PEG as binder. The SS316L foam produced consisted of a large volume of open and interconnected pores especially at higher SS316L composition.

Single steel strut mechanical testing: challenges and future research directions

ABSTRACTOpen-cell stainless steel foams, composed of hollow struts, are excellent candidates for energy absorption and thermo-mechanical applications. The basic mechanical element responsible for the function of these foams is the single strut. However, testing and characterisation of single foam struts to predict the foam strength have stirred up a new debate about approaches to micro-tensile testing of such elements. In this paper, we present a protocol for in-situ micro-tensile testing of hollow steel struts using a custom-made grip system. The adapted grips make it possible to perform analysis of the deformation of multiple sintered struts. Here we present and discuss challenges encountered during such micro-tensile testing of hollow steel struts.

Investment Casting and Mechanical Properties of Open‐Cell Steel Foams

This study presents a novel manufacturing process for open‐cell stainless steel foams using a modified investment casting process and a novel approach to identify transitions between elastic‐plastic, plateau, and densification region in the stress–strain history of compressed foams, based on its strain and structural hardening behavior. The influence of microstructure on the mechanical properties under quasi‐static compression loading (plateau stress, energy absorption, and strain hardening) of austenitic (AISI 304, 316L) and super duplex (AISI F55) stainless steels is investigated. Microstructure is characterized prior and subsequent to mechanical testing using light microscopy and SEM. The manufacturing process yields open‐cell foams with relative densities in the range of 14–20%, solid struts being circular in shape and defect‐free surfaces. This morphology leads to improved yield strengths compared to open‐cell steel foams produced by the powder metallurgical route. Among the manufactured open‐cell steel foams, F55 foams with fine‐grained duplex microstructures show highest yield strength, strain hardening, and energy absorption with a sufficient ductility. Although a martensitic transformation is present in highly deformed struts of soft austenitic stainless steel foams, strain hardening in the plateau region is lower compared to duplex foams.

Microstructure and compressive deformation behavior of SS foam made through evaporation of urea as space holder

Abstract Open cell 316L stainless steel foam (SSF) of varying porosities have been developed through powder metallurgy route using evaporative spherical urea particles (UP) as a space holder. Stainless steel powders (SSP) were cold compacted under 500 MPa pressure and sintered at 1200 °C for 1 h in a high vacuum atmosphere (10−4 mbar). Detailed Energy dispersive X-ray spectroscopy (EDS) analysis and X-ray diffraction pattern (XRD) conformed that no residue of space holder (urea) in sintered samples. The compressive deformation behavior of sintered foam samples with varying relative densities (ρrd) was conducted at 0.01s−1 strain rate. The yield strength, elastic modulus (Ef), plastic modulus, average plateau stress (σpl) and energy absorption (Eab) of the foam increases with increases in the relative density and these follows power law relationship with relative density. On the other hand, densification strain (ɛd) decreases with increases relative density. This has been discussed with the deformation mechanism of the stainless steel foam (SSF). Deformation of SSF is associated with cell wall (CW) bending, CW collapse due to bulking followed by shearing and facture of cell wall layer by layer.

Effect of hardening heat treatment on the mechanical properties of a 17-4PH stainless steel foam

AbstractThis paper presents the results of a study on the impact of a precipitation hardening treatment on the mechanical properties of 17-4PH stainless steel open-cell foams produced using a powder-metallurgy-based process patented by the National Research Council Canada (NRC). Pre-alloyed powder was used to manufacture stainless steel (SS) foams with either medium or high porosity by changing the nature of the organic binder used to process the porous materials. Some of these were kept in the as-sintered state, while others were submitted to the H900 precipitation hardening treatment frequently prescribed for 17-4PH stainless steels.Metallurgical and physical characterization was carried out on the resulting materials, along with mechanical testing at the micro (indentation testing) and macro (compressive testing) scales. It was found that the Medium-Porosity Foams (MPF) and High-Porosity Foams (HPF) had very different morphologies, the HPFs having a delicate porous structure featuring thin sintered walls with many openings (a.k.a. windows) between the main cells, while the MPFs exhibited much thicker walls with few windows connecting the larger pores. As expected from these foam morphologies, the mechanical properties of MPFs were much higher than those of the more porous and delicate HPF materials. For both foam types, the average mechanical properties were improved by the H900 treatment. A comparison with compressive properties of 17-4PH foams taken from the literature resulted in reasonable agreement. However, the large scatter observed on the average compressive properties of the NRC foams and the slightly different structure/composition of the literature materials mean that any comparison between these porous alloys must be interpreted with caution.

Compressive behavior of double-layered functionally graded 316L stainless steel foam

Abstract In this study, double-layer structured 316L stainless steel foam is fabricated through a layer by layer space holder method. The fabricated foams contain uniform pore distribution and can be categorized as body centered cubic crystal structures. Each sample is made of two layers in the axial direction. To induce 51 vol.% and 62 vol.% porosity in the first and the second layers, 1.7 to 2.0 mm and 2.0 to 2.4 mm spherical carbamide particles are used, respectively. In this study, the effect of the height of each layer on the compressive behavior of the fabricated foams is investigated in detail. The results indicate that compressive deformation of the double-layered foams starts from the high porosity (62 vol.%) layer and then shifts to the low porosity (51 vol.%) layer. Deformation of the double-layered 316L foams demonstrates two plateau regions, whose lengths depend on the heights of the low and high porosity layers.

Microstructure and Properties of Porous High-N Ni-Free Austenitic Stainless Steel Fabricated by Powder Metallurgical Route.

Porous high-N Ni-free austenitic stainless steel was fabricated by a powder metallurgical route. The microstructure and properties of the prepared porous austenitic stainless steel were studied. Results reveal that the duplex stainless steel transforms into austenitic stainless steel after nitridation sintering for 2 h. The prepared high-N stainless steel consists of γ-Fe matrix and FCC structured CrN. Worm-shaped and granular-shaped CrN precipitates were observed in the prepared materials. The orientation relationship between CrN and austenite matrix is [011]CrN//[011]γ and (-1-11)CrN//(1-11)γ. Results show that the as-fabricated porous high-nitrogen austenitic stainless steel features a higher mechanical property than common stainless steel foam. Both compressive strength and Young’s modulus decrease with an increase in porosity. The 3D morphology of the prepared porous materials presents good pore connectivity. The prepared porous high-N Ni-free austenitic stainless steel has superior pore connectivity, a good combination of compressive strength and ductility, and low elastic modulus, which makes this porous high-N Ni-free austenitic stainless steel very attractive for metal foam applications.

Strain hardening reduces energy absorption efficiency of austenitic stainless steel foams while porosity does not

Strain hardening significantly affects the mechanical function of steel foams. We compare hardening and failure of two commercially available austenitic stainless steel foams (316L and 310) spanning strut porosities of 9.4 to 14.4%. Damage is correlated with strut microstructure and geometry, combining in-situ quasi-static compression testing in the SEM, 3D-evaluation by synchrotron μCT and bending simulations. We provide an analytical model for the experimentally observed strain hardening. Upon compression, 316L steel foams exhibit a plateau regime of continuously increasing stress due to the hardening effect, whereas 310 steel foams show almost constant plateau stress. This is explained by the much less ductile behaviour of the 310 steel foam struts as compared to 316L steel foam struts. Finite element modelling suggests that significant stress concentrations develop around microporosities in the 310 struts. Due to its finer and less porous microstructure, the 316L foam exhibits a larger energy absorption capacity than the 310 foams. This results in distinctly different efficiency-strain curves. However, up to about 25% strain, the efficiency values are surprisingly similar. Thus, modification of microstructure and/or pore micro-geometry can be used to optimise the stress-strain response to achieve the desired energy absorption property of steel foams.

Effect of the Operating Conditions on the Growth of Carbonaceous Nanomaterials over Stainless Steel Foams. Kinetic and Characterization Studies

Abstract This work is an advance on the development of structured catalytic reactors. Here, we present the results of the effect of the main operational variables (reaction temperature, % H2 and % C2H6) on the kinetics of carbonaceous nanomaterials (CNMs) formation by catalytic decomposition of ethane over stainless steel foams. Some of the main drawback problems that occur during the operation of chemical structured reactors are related to the preparation of long term stable coatings. The washcoating is the most used technique to deposit the catalytic layer over the substrate. The application of this procedure is quite complex in the case of geometries such as foams or cloths. In the case of the deposition of layers of carbonaceous nanomaterials, an alternative route, avoiding the washcoating, is their direct growth by catalytic decomposition of light hydrocarbons over the surface of the metallic substrate. In the case of structured steel foams, the substrate already contains the catalytic active phases for this reaction, like Fe and Ni, among of the minor components (Cr, Mn, Mo) that can act as promotors/stabilizers. The nanomaterials obtained after reaction were characterized by Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The characterization results indicate that there is a maximum, obtained at ca. 900 °C, 33.3 % of C2H6 and 1.7 % of H2, in the quality of the carbonaceous nanomaterials grown. Under these conditions, the CNMs consist mainly of few layer graphene (FLG) and graphite nanolayers (GNL) encapsulating the metallic nanoparticles. In addition, the kinetic results indicate the existence of another optimum, at ca. 800 °C, 33.3 % of C2H6 and 1.7 % of H2, in the productivity to the carbonaceous nanomaterials. The existence of these optimums is due to the driving force for the diffusion of the carbon atoms through the Fe-Ni nanoparticles (NPs) obtained at high temperatures (e. g. above 800 °C) caused by the competence between two opposite phenomena: the increase of the rate of carbon diffusion through the metallic nanoparticles of Fe-Ni and the deactivation of these nanoparticles. The deactivation is the consequence of the encapsulation and reconstruction of the nanoparticles during the formation of the several types of CNMs. The evolution of the carbon mass during the reaction time was analyzed using a phenomenological kinetic model that takes into account the main stages involved during the formation of carbonaceous nanomaterials: hydrocarbon decomposition, carburization, diffusion, precipitation and deactivation. The results obtained from the kinetic model, along with the characterization results, enable quantify the influence of the operating variables on each stage of the carbonaceous nanomaterial formation and therefore open the way to optimize the process.

The Impact of Composition and Sintering Temperature for Stainless Steel Foams (SS316L) Fabricated by Space Holder Method with Urea as Space Holder

Metal foams are a cellular structure that has a solid matrix made of metal and has pores in their structure. Metal foams offer excellent combination of properties which led researchers interested in investigation in recent years. Closed-cell stainless steel (SS316L) foams for biomedical application were prepared by space holder method and the physical and morphological properties of SS316L foams were studied. Stainless steel (SS316L) powders as metallic material, polyethylene glycol (PEG) as a binder and Urea as a space holder material were mixed homogenously to avoid the particle wrecked. This mixture was compacted using uniaxial pressing machine and pressurized to 8 tons to formed the green body. By using tube furnace, the SS316L foams was two-stage sintered, the first phase at 600°C for 2 hours to decompose the urea, and the second phase at 1000°C, 1100°C, and 1200°C respectively to sinter the steel. The porosity and density test was carried out by applying Archimedean principles, while morphological observation was done by using Field Emission Scanning Electron (FESEM). The samples with 40wt.% SS316L composition and sintered at temperature of 1100°C, leads to porosities of about 44.539% and show the potential as the best metal foams.

A novel process for manufacturing porous 316 L stainless steel with uniform pore distribution

In the present work, a new technique for creating uniform distributed space holder is used for producing porous 316L stainless steel foam. Uniform pore distribution with Face-Centered Cubic (FCC) packing is created by a novel method using spherical carbamide as a space holder. The specimens have maximum porosity about 71.5% after sintering at 1200°C for 60min in N2 atmosphere. The microstructure of the metal foams, cell size and shape was studied using optical microscopy and Clemex Vision PE commercial image-analyzer software. The compression test was used to evaluate the mechanical properties of the foam samples. The results showed that uniform pore distribution can improve elastic properties and compressive strength.

Experimental investigation on pressure drop and heat transfer in metal foam filled tubes under convective boundary condition

Heat transfer under convective boundary condition is common in heat exchangers. This paper presented the experimental results of air flow and heat transfer through three stainless steel foam filled tubes under convection boundary condition. The air flow velocity inside the tube is relatively high, which varied from 7.0 to 26.0m/s. The stainless steel foam filled tubes, manufactured using high-temperature metallic sintering technique, are of different pore densities (10, 30 and 70 PPI) but have the same porosity of 0.93.The air pressure drop through the stainless steel foam filled tubes was measured. It was found that the inertial drag is the dominant part of the pressure drop at higher velocity. The pressure drop experimental data under high velocity were compared to the predictions by the correlations obtained under relatively low velocity and great discrepancies have been found. A new correlation for the pressure drop through metal foams under high velocity was presented. The effect of the boundary condition on the heat transfer performance was addressed by the comparison of Nusselt number obtained in the present study with that obtained under constant heat flux boundary condition in the published investigation. It was found that the Nusselt number obtained under convective boundary condition is much lower than that obtained under constant heat flux boundary condition. A new correlation for Nusselt number under convective boundary condition was developed.

Growth of carbonaceous nanomaterials over stainless steel foams. Effect of activation temperature

Some of the problems that occur during the operation of chemical reactors based of structured catalytic substrates, as monoliths, foams, membranes, cloths, fibres and other systems, are related to the preparation of long term stable coatings. Frequently, the deposition of the catalytic layer is carried out by washcoating, requiring this step a cautious attention, especially in the case of complex geometries, like of that of foams or cloths. In the case of the deposition of layers of carbonaceous materials (CNMs), an alternative route, avoiding the washcoating, it is their direct growth by catalytic decomposition light hydrocarbons (also called CCVD), over the surface of the metallic substrate. In this case, if the metallic substrate is of stainless steel, it already contains the catalytic active phases like Fe and Ni.In order to optimize the process of CNMs growth over structured metallic substrates, we are studying the effect of the main operational variables of the ethane decomposition reaction on stainless steel foams. In this contribution we present a study of the influence of the temperature of the activation (oxidation and reduction) stage on the type and morphology of the carbonaceous materials formed. The results obtained allow us to determine the optimal operating conditions to maximize the amount and the selectivity of the process to obtain a given type of CNM.