Metal Matrix Syntactic Foams Research Articles

Unidirectional fibre reinforcement of syntactic metal foams

Abstract Metal matrix syntactic foams are porous structures where the porosity is formed by introducing a porous filler material into the metal matrix. They have low density and absorb significant energy during compression but cannot withstand tensile stress. The main goal of this study was to apply unidirectional fibre reinforcement to syntactic metal foams. Therefore, a lightweight double-composite structure that can be utilised against tensile load and has advantageous properties during compression was created. Copper-nickel coated carbon fibre bundles were used as unidirectional long fibre reinforcement, lightweight expanded clay aggregate particles as filler material and Al99.5 aluminium as matrix material. The samples were created with vacuum infiltration and formed into cylindrical tensile specimens. Quasi-static tensile tests were carried out on the specimens and fibre trusses, and quasi-static compression was applied on specimens in the parallel and perpendicular direction to the unidirectional fibre truss.

Mechanical Properties of Aluminum Matrix Syntactic Foams Manufactured with Aluminum Smelter Waste

Aluminum smelter waste (ASW) is a big contributor to landfill and its recycling has been of great interest. This study characterizes the ASW powder, manufactures aluminum matrix syntactic foams using ASW, and investigates their mechanical properties. Different particle size groups of the ASW powder contain different impurities and porosities, leading to different strengths. The mechanical behaviors of the ASW syntactic foams are similar to existing metal matrix syntactic foams. The compressive yield strength, plateau strength, and specific energy absorbed vary in the ranges of 111–398, 33–145 MPa, and 18–71 MJ m−3. The flexural strength and flexural modulus vary in the ranges of 8–138 MPa and 13–72 GPa. The peak impact strength and impact energy in drop‐weight impact vary in the ranges of 192–547 MPa and 16–29 J. Particle size and heat treatment of the ASW powder have significant effects on the properties and brittleness of the resultant syntactic foam.

Thermal and mechanical properties of AlSi7Mg matrix syntactic foams reinforced by Al2O3 or SiC particles in matrix

Materials with complex inner structure can be challenging to characterise in an effective way. The effective material parameters significantly depend on the structure, and thus on the interface properties. In the present paper, we focus on metal matrix syntactic foams, and we attempt to determine their effective thermal parameters. We supplement the results with their mechanical properties as well. For thermal characterisation, we use the heat pulse experiment to study their transient thermal response. We observed deviation from Fourier's law in multiple situations, and thus we propose an evaluation procedure for such experimental data using the Guyer–Krumhansl heat equation. We provide an iterative procedure to efficiently find the effective thermal diffusivity values provided by the Guyer–Krumhansl and Fourier equations based on the measured transient response. Furthermore, we studied the effective parameters, the specific heat, mass density, thermal conductivity and thermal diffusivity. We found that the standard estimations significantly overestimate the thermal conductivity. Furthermore, we propose a method for deducing a more reliable thermal diffusivity and thermal conductivity based on the transient temperature history, i.e., we propose to estimate Fourier's thermal diffusivity based on averaging the parameters of the Guyer-Krumhansl equation according to the observed diffusive time scales. We also found that for such metal matrix syntactic foams, even with knowing the constituents, the contact thermal resistance makes more challenging to provide a prior estimate for the effective thermal conductivity, but this difficulty can be resolved using proper effective thermal diffusivity values.

Investigation of the Bending Properties of Ex situ Functional Metal Foams

In each industry, compromises have to be made when choosing materials. Lower-density materials have a significant advantage in the automotive sector, especially due to rising fuel prices, since weight reduction can lower overall fuel consumption. The advantageous properties of metal foams, such as low density, high specific strength, and excellent energy absorption, should be researched and exploited in as many areas and ways as possible. This research aims to perform and evaluate the bending tests of ex situ functional metal foams: aluminum alloy matrix (AlSi7Mg) was used, which was filled with Ø2.5–3.0 mm lightweight expanded clay aggregate particles and surrounded by thin-walled aluminum tubes (AlMgSi0.5) with a wall thickness of 2 mm and an outer diameter of Ø32 mm. Empty tubes, foam-filled tubes (with and without structural epoxy adhesive) and metal matrix syntactic foams were compared based on their flexural strength and energy absorption capacity. Quasi-static three-point bend tests were carried out up to 25 mm deflection. The foam-filled tubes with epoxy adhesive showed an average of 6% increase in flexural strength compared to the foam-filled tubes without adhesive and a 145% increase compared to the metal matrix syntactic foams.

Experimental and numerical analysis of the compression behavior of aluminum syntactic foams reinforced with alumina hollow particles

In this study, analytical, numerical, and experimental methods are used to investigate the compressive behavior of alumina hollow particles reinforced aluminum matrix syntactic foams (AMSF). The representative volume element (RVE) models are generated for different volume fractions to analyze the elastic and elastio-plastic behavior of AMSFs using the FE solver ABAQUS. The geometric feature of the hollow particles is selected close to the actual physical dimensions. The effective elastic properties are estimated using linear homogenization models. For example, the elastic properties predicted using the Mori-Tanaka model are close to the FE-based RVE model data. Later, the macroscopic response (stress-strain data) is determined using the volume-averaging method proposed in the literature. The stress-strain response of the FE model with a 30% volume fraction has shown close agreement with the experimental data. Further, a parametric study is conducted for different particle wall thicknesses. These FE models have great potential in studying the ceramic hollow particles reinforced metal matrix syntactic foams and developing new porous composites under compression loading using RVE-based models.

Application of Substructure Techniques to Syntactic Metal Foams in a Finite Element Environment

The presented work focuses on the development of a novel method that can numerically describe the properties of metal matrix syntactic foam (MMSF) with low memory requirements and short computational times without losing the properties of the interior structure. In this paper, we propose a novel method that avoids using the homogenization technique and instead rearranges stiffness matrices and constructs specific substructures to perform the overall construction. The two-dimensional cases are discussed in order to focus on the methodology itself. First, the reductions and structural design with solid mesh structures were performed, and then the model was applied on structures filled with iron hollow spheres. So far, the method has been used to evaluate small deformations to see how suitable the subspace technique is for describing metal foams. Aluminum was used as the matrix material, as it is one of the most common materials for MMSFs. The optimal parameters were searched that resulted in the shortest running time for the given construction. Since in the proposed substructure technique only the displacement values at the boundary points are computed, a back-calculation step for each selected substructure was performed to see the interior deformations in the vicinity of an iron hollow sphere.

A novel approach for in-situ producing Al2O3/aluminum matrix syntactic foam with high specific strength

Low-density foams with high strength are eagerly desired to use as energy absorption and shock isolation systems during transportation. In this work, an Al2O3/aluminum matrix syntactic foam with a density of 1.41 g/cm3 and a compressive strength of 238 MPa was prepared by a novel approach, which specific strength outperforms the most advanced aluminum matrix syntactic foams as reported in the literature. Especially, the hollow Al2O3 spheres were produced by an in-situ reaction of the hollow glass beads in this foam. Moreover, an electric current was first utilized in this new approach, which proved to have a crucial effect on the in-situ reaction, making the hollow glass beads rapidly and completely transform into Al2O3 spheres to improve interface conditions. The approach is believed to provide a favorable strategy to design different metal matrix syntactic foams with improved performance.

Characteristic compressive properties of AlSi7Mg matrix syntactic foams reinforced by Al2O3 or SiC particles in the matrix

Metal matrix syntactic foams (MMSFs) were produced by liquid-state low-pressure infiltration. AlSi7Mg alloy was applied as matrix material, while the filler was a set of ceramic hollow spheres (CHSs) or light expanded clay agglomerate particles (LECAPs). The matrix was reinforced by Al2O3 (0.6 mm or 1.2 mm nominal size) or SiC (0.4 mm nominal size) particles. The produced samples were investigated structurally (microstructure) and mechanically (standardized compressive tests). According to the microscopic investigation, liquid-state low-pressure infiltration was found to be a suitable method to produce MMSFs with a reinforced matrix. The shape of the engineering stress – strain curves was mainly influenced by the filler. CHS filled MMSFs showed a high stress peak, while LECAP filling ensured a smoother transition from the linear elastic part to the plateau region. The compressive strength and the structural stiffness of the MMSFs were significantly increased by the reinforcing particles in the matrix. Linear connections were found in the compressive strength – nominal size of the Al2O3 particles and in the structural stiffness – nominal size of the Al2O3 particles relationships. The plateau strength of the MMSFs and thus the absorbed mechanical energy was decreased by the presence of the reinforcement. The decrement was caused by the stress-concentrating particles and could only be equalized by the stronger SiC particles. The failure modes of the MMSFs were dependent on the filler material. Stronger CHS fillers resulted in fracture by cleavage, while the weaker LECAP fillers caused plastic collapse in the samples.

Eutectic high entropy alloy syntactic foam

High-strength metallic foams have a wide range of applications in engineering as lightweight structural and energy-absorbing materials. However, it is challenging to obtain metallic foam with both good energy absorption performance and high strength. Here, we developed a novel metal matrix syntactic foam fabricated with AlCoCrFeNi2.1 eutectic high entropy alloy and alumina cenospheres that exhibits a remarkable combination of high strength and energy absorption performance under both quasi-static and dynamic compression. The porous structure of syntactic foam fully exploits the properties of the AlCoCrFeNi2.1 alloy matrix with a unique FCC/B2 dual-phase eutectic microstructure and thus yields exceptional performance. We discovered that this dual-phase microstructure not only provides high strength but also allows the pores to collapse in a progressive and diffusive way, which enables the formation of a high and smooth energy absorption platform. It is found that the heterogeneity between the two phases in the matrix can provide back stress strengthening, and it also induces multiple micro shear bands and microcracks as additional energy dissipation modes as the deformation proceeds. This unique mechanism ensures the strength of microstructures and makes them fracture promptly, which causes the balance of strengthening and softening on the macro scale. This work opens the avenue for developing advanced high-strength lightweight structural and energy-absorbing materials.

Enhanced energy absorption and microstructural studies on hollow glass microsphere filled closed cell aluminum matrix syntactic foam

The research on lightweight materials for advanced engineering applications attracted the development of metal matrix syntactic foams. The automobile sector has started using Al-based alloys in structural components such as crash-box, underride-guard, fenders, dampers, A, B, and C Pillars. The present study explores the energy absorption behavior; microstructural characterization such as SEM, EDS, and XRD analysis of hollow glass microsphere (HGM) filled aluminum matrix syntactic foam. A380 aluminum alloy reinforced with different volume fractions 10%, 20%, 30%, and 35% of hollow glass microspheres were used in the fabrication of syntactic foam using the stir casting technique. The quasi-static compression test conducted, evaluated the plateau strength, which improved from 284.14 to 341.69 MPa, and energy absorption capacity was observed in the range 139.25–187.92 MJ/m3. The plateau strength and energy absorption capacity were improved by 16.82% and 25.89% for the 35 vol.% HGM sample as compared with 10 vol.% HGM filled aluminum matrix syntactic foam. The addition of a calcium thickening agent in the casting process improved the bonding between aluminum and HGM particle and also the homogeneous distribution of HGM. The XRD analysis revealed the chemical reaction that occurred between aluminum and SiO2 that produced the AlSiO2 and Al2SiO5 interfacial compounds. This reaction tends to collapse the HGM cell wall and fills it with matrix material.

Compression Properties and Fabrication of Closed-Cell Metal Matrix Syntactic Foams Al2O3hs/AZ91D.

Closed-cell metal syntactic foam is a new material consisting of hollow spheres embedded in metal matrix syntactic foams. These foams have good physical and mechanical properties and are increasingly used worldwide in industrial and high-tech fields. Magnesium matrix syntactic foams containing hollow Al2O3 spheres ((Al2O3hs)/AZ91D) were successfully fabricated by hot press sintering at different temperatures. The fabrication of Al2O3hs/AZ91D and the effect of sintering temperature on the microstructure and properties are reported in this paper. Additionally, sandwiched magnesium matrix syntactic foams were prepared by placing magnesium plates on both sides of the syntactic foam. Some Al2O3hs particles became filled with matrix particles during preparation. Thus, the actual density was greater than the theoretically calculated value and increases with increasing sintering temperature. Above 723 K, a brittle phase MgAl2O4 formed in Al2O3hs/AZ91D. The quasistatic and dynamic compressive strengths of Al2O3hs/AZ91D first increased and then decreased with increasing sintering temperature, and the maximums were 162 MPa and 167.87 MPa, respectively. Thus, this paper reports a new strategy for the controlled preparation of metal matrix syntactic foams with predetermined porosity. The results show that this strategy improved the performance of lightweight and high-strength syntactic foam materials and shows potential for further research.

New Aluminum Syntactic Foam: Synthesis and Mechanical Characterization.

Metal matrix syntactic foams (MMSF) are advanced cellular materials constituted by a system of a minimum of two phases, in which a dispersion of hollow particles is embedded by a continuous metal matrix. The incorporation of porous fillers favors the development of low-density materials with exceptional behavior for damping vibrations, impacts, and blast effects, shielding acoustic, thermal, and electromagnetic energies. There are three main techniques to produce them: infiltration casting technique (ICT), stir casting technique (SCT), and powder metallurgy technique (P/M). The first two techniques are used for embedding filler into lower melting point metallic matrices than fillers, in contrast to P/M. The present study demonstrates the feasibility of producing MMSF with components of similar melting points by ICT. The fillers were synthesized in-situ with aluminum and a natural foaming agent from wastes of Spanish white marble quarries. These novel aluminum syntactic foams (ASF) were mechanically characterized following the ISO-13314 and exhibited a porosity, plateau stress, and energy absorption capacity of 41%, 37.65 MPa, 8.62 MJ/m3 (at 35% of densification), respectively. These properties are slightly superior to equal porosity LECA ASF, making these novel ASF suitable for the same applications as LECA-ASF.

Mechanical and Physical Characteristics of Bubble Alumina Reinforced Aluminum Syntactic Foams Made Through Recyclable Pressure Infiltration Technique

Metal matrix syntactic foams (MMSFs) are advance hybrid materials which reflect synergetic combination of particle reinforced composites and close cell metal foams. Recently, MMSFs have become considerably popular for several industrial areas because of their low density, high compression strength, good ductility and perfect energy absorption ability. In this paper, Al 7075/bubble alumina syntactic foams were fabricated by recyclable pressure infiltration casting method. Macroscopic and microscopic investigations showed that almost flawless infiltration was obtained between 2 – 4 mm hollow bubble alumina spheres and total porosity values of fabricated foams varied between % 43.2 and % 45.6. As for mechanical analyses, all foam samples were subjected to quasi-static compression test (1 mm/min deformation rate) and their crucial properties such as compression strength, plateau strength, densification strain and energy absorption capacity were determined. Besides, aging heat treatment (T6) was applied to certain samples in order to explore probable effects of heat treatment on the mechanical responses. The outcomes indicated that there was a positive relationship between the aging treatment and compressive features. Under the compressive loading, even though T6 treated foams showed a tendency to brittle fracture, their as-cast variants exhibited ductile behavior.

Stir Casting Routes for Processing Metal Matrix Syntactic Foams: A Scoping Review

Metal matrix syntactic foams (MMSFs) are advanced lightweight materials constituted by a metallic matrix and a dispersion of hollow/porous fillers. Physical and mechanical properties can be fitted regarding matrix and filler properties and processing parameters. Their properties make them potential materials for sectors where density is a limiting parameter, such as transport, marine, defense, aerospace, and engineering applications. MMSFs are mainly manufactured by powder metallurgy, infiltration, and stir casting techniques. This study focuses on the current stir casting approaches and on the advances and deficiencies, providing processing parameters and comparative analyses on porosity and mechanical properties. PRISMA approaches were followed to favor traceability and reproducibility of the study. Stir casting techniques are low-cost, industrially scalable approaches, but they exhibit critical limitations: buoyancy of fillers, corrosion of processing equipment, premature solidification of molten metal during mixing, cracking of fillers, heterogeneous distribution, and limited incorporation of fillers. Six different approaches were identified; four focus on limiting buoyancy, cracking, heterogeneous distribution of fillers, and excessive oxidation of sensitive matrix alloys to oxygen. These improvements favor reaching the maximum porosity of 54%, increasing the fillers’ size from a few microns to 4–5 mm, reducing residual porosity by ±4%, synthesizing bimodal MMSFs, and reaching maximum incorporation of 74 vol%.

Effects of particle size, bimodality and heat treatment on mechanical properties of pumice reinforced aluminum syntactic foams produced by cold chamber die casting

In recent years, metal matrix syntactic foams (MMSFs) have become highly attractive owing to their unique physical, microstructural and mechanical features. Due to their promising potential for different industrial areas like automotive, aviation, and defense, these advanced engineering materials can also be evaluated as serious alternatives to particle reinforced metallic composites and conventional metallic foams. Differently from previously reported laboratory scaled techniques in the literature, this experimental effort focuses on the feasibility of MMSF manufacturing via a fully automated and industrial-based cold chamber die casting technique. Accordingly, 1–2 mm, 2–4 mm, and bimodal (50vol.%) natural-based pumice filled aluminum syntactic foams were manufactured utilizing a purpose-made casting machine. Physical, macroscopic, and microscopic examinations show that all of the fabricated samples display perfect matrix/filler harmony. Average density levels of fabricated syntactic foams range between 1.50 and 1.80 g·cm−3 depending upon the pumice particles size interval. To assess mechanical responses, quasi-static compression tests were performed. Furthermore, half of the foam samples were subjected to heat treatment to explore possible influences of aging on the compressive features and damage modes. Results indicate that although the heat treatment enhances the compressive strength, plateau stress, and energy absorption properties of the fabricated foams, it changes damage mode of the samples by causing brittle dominant deformation.

Manufacturing, Heat Treatment and Investigation of Foam-Filled Tubes

Abstract Composite metal foams are hybrid structures with the main advantages of high specific strength and mechanical energy absorption associated with low density. In the course of our research, we successfully manufactured functional metal foams of EN AC-44200 matrix filled with lightweight expanded clay aggregate particles (LECAPs) in EN AW-6060 alloy tubes with a diameter of 50 mm and a wall thickness of 5 mm. Manufacturing was performed by low-pressure infiltration directly into the aluminium tube. Six different types of samples were examined: metal matrix syntactic foam, in-situ metal foam, ex-situ metal foam, and their heat-treated pairs. In the compression tests, the heat treatment provided a visible improvement in the results of the ex-situ metal foams.

Compressive properties of metal matrix syntactic foams in uni- and triaxial compression

Metal matrix syntactic foams (MMSFs) with Al alloy matrix (Al99.5 and AlSi12) and pure Fe metallic hollow spheres (MHSs, 64 vol%) were produced by pressure infiltration process. The density of the produced MMSFs was between 1.18 and 1.54 g cm−3. Cylindrical samples were investigated by compressive tests in free (between rigid plates) and radially constrained (in a thick wall steel sleeve) condition. The compressive curves were generalized, and the characteristic properties were identified (structural stiffness, yield strength, plateau strength, energy absorption). The general compressive curves altered in their plateau region: in free condition the plateau was flat, in constrained condition it was increasing. The characteristic properties of the MMSFs could be described by scaling laws and were influenced by the matrix properties and by the relative density of the samples. For the implementation of MMSFs into finite element systems, their constitutive law was formalized by the implementation of the Shima-Oyane model.

Mechanical investigation of in-situ produced aluminium matrix syntactic foam-filled tubes

This research focuses on the mechanical investigations of in-situ produced AlSi12 aluminium alloy matrix syntactic foam-filled tubes. The samples were manufactured by low-pressure infiltration in a tube made from AlMgSi0.5. As second phase 3.5–4 mm average diameter lightweight expanded clay aggregate particles were used. The mechanical analysis extends to compressive tests of three different type of samples: empty tubes, metal matrix syntactic foams, and foam-filled tubes. The results show the best specific mechanical energy absorbing capacity in the case of foam-filled tubes, since the matrix creates a durable boundary layer with the tube wall.

Ultra-high energy absorption high-entropy alloy syntactic foam

Metal matrix syntactic foams (MMSFs) are a class of fascinating particulate composites consisting of metal matrix and hollow spheres, which are promising in the light-weight structural and energy absorbing applications. However, the energy absorption capacity of most MMSFs is suffocated inevitably by relatively low strength of traditional metal matrix alloys, such as widely used aluminum and magnesium alloys. Here, we developed a novel MMSF with CoCrFeMnNi high entropy alloy matrix and alumina cenosphere fillers by pressure infiltration, which exhibits an ultra-high energy absorption capacity of 242.8 ± 20.6 MJ m−3, among the highest reported values of MMSFs. We show that this exceptional energy absorption capacity stems from excellent combination of high strength of alumina cenospheres and super-large ductility and fracture toughness of CoCrFeMnNi high entropy alloy. These results shed new light into design of advanced light-weight and energy absorbing materials.

Optimization and characterization of novel injection molding process for metal matrix syntactic foams

Metal matrix syntactic foams are particulate composites comprised of hollow or porous particles embedded in a metal matrix. These composites are difficult to manufacture due primarily to the lightweight, relatively fragile filler material. In this work, an injection molding process was developed for metal matrix syntactic foams. First, an aqueous binder was optimized for low-pressure injection molding. A mixture model was used to optimize the composition of the binder to achieve the highest relative density. The model predicted the maximum relative density was at a binder composition (in vol.%) of 7% agar, 4% glycerin, and 89% water. Second, this binder was used to manufacture copper matrix syntactic foams with 0, 5, 10, and 15 vol.% porous silica as the filler material. The solids loading for these compositions decreased with increasing filler material from 55 to 44 vol.%, likely due to binder filling the pores in the porous silica particles. Finally, the sample quality after injection molding was characterized. Only 0.11 ± 0.06 vol.% carbon remained in the samples. Silica particles were well-dispersed in the samples after sintering, and they did not appear to be fractured. The specific strength of the copper matrix material increased with increasing porous silica additions.