Plastic deformation and compressive mechanical properties of hollow sphere aluminum foams produced by space holder technique

In this study, experimental procedure and numerical methods were utilized to evaluate the effect of regular and irregular pore distribution as well as loading direction on compressive properties and deformation mechanism of hollow sphere aluminum foams. In order to study scaling laws, different volume fractions of the regular samples were produced and loaded in horizontal and vertical directions to address the loading conditions effects. For this purpose, expanded polystyrene (EPS) grains were expanded to a designed diameter size and positioned in different configurations. Compression test results showed higher elastic properties for irregular sample due to the thicker cell walls while energy absorption capability at high strains was found to be reduced due to the non-uniform deformation in comparison with regular foams. In regular samples, a nonlinear behavior in the elastic regime was observed since the imperfections during casting procedure. Furthermore, similar deformation mechanisms were found for the set of samples with similar pore configurations indicating the feasibility of controlling deformation mechanism by manipulating morphological characteristics. Finite element results well predicted deformation mechanism of structures and plastic properties of regular hollow sphere samples especially for plateau stress with less than 12% relative error.     

Manufacturing of open-cell Mg foams by replication process and mechanical properties

Open-cell pure Mg foams were produced by replication casting process. The preform used was manufactured with spherical particles of NaCl with sizes ranging from (A) 1 mm to (D) 2 mm. It was found that increasing the pore size, the relative density decreased, while the porosity increased, registering a minimum relative density of 0.22 and a maximum percentage porosity of 78% for sample (D) 2 mm. The mechanical properties and energy absorption characteristics were investigated by means of compression test. Under the present experiment conditions, the sample (A) 1 mm with the smaller pore size and the lower percent porosity 67%, showed the highest mechanical properties; Young’s modulus, yield stress, and the high energy absorption capacity. The mechanical properties obtained and the large plateau region could be favorable for scaffold and energy absorbing applications.     

Influence of cell aspect ratio on architecture and compressive strength of titanium foams

Titanium foams have been of interest in dental and orthopedic implants over the past few decades on account of their excellent mechanical properties, chemical stability, and biocompatibility. A powerful tool, X-ray computed microtomography was used to measure quantitatively the effect of pore morphology on foam architecture. Mechanical properties of titanium foams with varying pore structure were investigated. Aspect ratio of the pores was quantitatively demonstrated to affect strength, degree of anisotropy and strain-rate sensitivity of the produced titanium foams. Needle-like pored foams showed 30-55% lower strength when compared to the foams having lower aspect ratio pores. Lower aspect ratio pored foams were 3-11%, higher aspect ratio pored foams were 17-34% weaker in the direction parallel to the compaction direction when compared to the perpendicular one. High aspect ratio pores also resulted in more pronounced strain-rate sensitivity.     

The effects of manufacturing parameters on geometrical and mechanical properties of copper foams produced by space holder technique

We describe a powder metallurgical space holder method to produce open-cell metallic foams. By changing the values of the main manufacturing parameters such as volume percentage and the particle size of the space holder agent, we produce different copper foam samples which cover a wide range of solid fraction, pore size and cell wall thickness. All the specimens were synthesized based on a series of designed experiments. We demonstrate how the foams’ density, cell size and specific surface area can be accurately controlled using two easily adjustable manufacturing parameters. The three-dimensional structure of these foams was investigated using X-ray micro tomography. The image quality is sufficient to measure local structure and connectivity of the foamed material, and the field of view large enough to calculate material properties. By combining the finite element method with the tomographic images, we calculate the mechanical response of the foams. We show that the foams’ bulk and shear moduli are strongly correlated to their cell size, cell wall thickness and specific surface area. These parameters can be easily controlled during manufacturing.     

Processing,microstructure and mechanical properties of micro-SiC particles reinforced magnesium matrix composites fabricated by stir casting assisted by ultrasonic treatment processing

The novel stir casting assisted by ultrasonic treatment processing was studied. Unlike traditional stir casting, short semi-solid stir time was needed for addition and pre-dispersion of the particles in the novel processing. For ultrasonic treatment, there existed an optimal time. Both too short and too long time for the treatment resulted in nonhomogeneous particle distribution. Furthermore, the liquid stirring after ultrasonic treatment was proved to be necessary to further improve particle distribution. The mechanical properties of the composites fabricated by different parameters indicated that ultrasonic treatment evidently improved the mechanical properties compared with traditional stir casting. 5–20% SiCp/AZ91 composites were fabricated by the novel processing. The particle distribution was uniform in these composites. The grains were refined by addition of SiC particles. Grain sizes of composites decreased with the increases of particle contents. The ultimate tensile strength, yield strength and elastic modulus were enhanced as the particle contents increased.     

Production of aluminum foam by spherical carbamide space holder technique-processing parameters

Spherical carbamide has been employed to produce aluminum foams by space holder technique via powder metallurgy route. The effect of different processing parameters such as applied pressure, dissolving time of spacer, sintering temperature and time, metallic additives, on compression properties of the resultant foams has been evaluated. Aluminum foam samples with 40–85 vol.% porosity were successfully produced. Addition of 1 wt.% Sn and Mg to aluminum powder increased strength of the sintered foams. The results indicate that the appropriate compressive properties of foams are achieved under 330 MPa compacting pressure, sintering temperature and time of 640 °C and 2 h, respectively.     

Mechanical properties and microstructure of pure polycrystalline magnesium rolled by different routes

Mechanical and microstructural characterizations were performed on pure magnesium samples processed by different rolling routes and annealing conditions to investigate the effects of thickness reduction strain and annealing temperature and duration on mechanical behaviors. The results indicate that (a) rolling increases yield strength significantly by about 100% to 130% and decreases ductility by about 55% to 66%; (b) a rolling route with a lower thickness reduction strain per rolling pass corresponds to a higher strength increase; and (c) annealing does not affect the magnitudes of yield strength and ductility, but largely increases ultimate tensile strength and strain hardening rate compared with the as-rolled material without experiencing annealing.     

AZ91C magnesium alloy modified by Cd

In the present work, the effect of Cd on the microstructure, mechanical properties and general corrosion behaviour of AZ91C alloys was investigated. Addition of Cd was found not to be efficient in modifying/refining the microstructure or β-phase. A morphology change in β-phase from fine continuous precipitates to discontinuous β-phase upon the addition of Cd was observed. A marginal increment in mechanical properties was observed. General corrosion behaviour was followed with weight loss measurements, potentiostatic polarisation studies and surface studies in 3.5% sodium chloride solution and 3.5% sodium chloride with 2% potassium dichromate solution. Cd addition deteriorated the corrosion behaviour of AZ91C. This behaviour was attributed to the formation of chunks of β-phase upon the addition of Cd. AZ91C with refined β-phase distribution, performed rather better in the NaCl solutions.     

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.     

A comparison of composite metal foam's properties and other comparable metal foams

New closed cell composite metal foams are processed using casting and powder metallurgy (PM) techniques. The foam is comprised of steel hollow spheres packed into a random loose arrangement, with the interstitial spaces between spheres occupied with a solid metallic matrix. The characterization of composite metal foams was carried out using monotonic compression, compression-compression fatigue, loading-unloading compression, micro-hardness and nano-hardness testing. The microstructure of the composite metal foams was studied using optical, scanning electron microscopy imaging and electron dispersive spectroscopy. The composite metal foams displayed superior (5-20 times higher) compressive strengths, reported as 105 MPa for cast foams and 127 MPa for PM foams, and much higher energy absorbing capability as compared to other metal foams being produced with similar materials through other technologies.     

Improvement of formability and mechanical properties of magnesium alloys via pre-twinning: A review

Twinning can generate the change of texture and a large of twin boundaries, which can greatly influence the mechanical properties of magnesium alloys. Thus, pre-twinning can be considered to be a simple and feasible method to improve the mechanical properties of magnesium alloys. Recently, some studies have confirmed that pre-twinning can be an effective way to enhance the strength, formability and mechanical anisotropy of magnesium alloys. Based on these results, some aspects of the present research on the improvement of mechanical properties via pre-twinning are reviewed. The relevant mechanisms have been summarized. Finally, for this research field, a few critical scientific problems are also proposed.     

铸态AZ81镁合金ECAP态组织与性能研究

采用自制的90°模具,经Bc路径在温度为300℃下研究对比了铸态及不同道次的等通道挤压(ECAP)态AZ81镁合金微观组织和力学性能.结果表明ECAP随着挤压道次的增加,AZ81镁合金显微组织和力学性能发生显著变化.当挤压到4道次,平均晶粒尺寸由原来铸态的145um细化为9.6um,拉伸断口韧窝明显增多;抗拉强度从180 MPa提高到306 MPa,延伸率和硬度分别达到15.8%和142HL.分析表明,AZ81镁合金在高温挤压过程中Mg17Al12相粒子被破碎,并部分溶入基体,$-Mg基体与%-Mg17Al12相互相阻碍其晶粒长大,获得细小晶粒组织.     

Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part II: Physical properties

The present work is the continuation of the previous one published in the same issue of this journal, but now focuses on some selected physical properties of tannin-based rigid foams and derived glasslike carbon foams. Such materials are new, lightweight, cellular solids, prepared from 95% natural precursors: bark extracts and furfuryl alcohol, as detailed in the companion paper. After a few structural characteristics are briefly recalled, physical properties like compressive strength, permeability to fluids, solvent absorption, and electrical conductivity are measured, discussed and modelled. The effects of changing a few experimental parameters that have been varied in the synthesis of the foam: amounts of blowing agent, strengthener and nanofillers, shape of the moulds and restricted foaming are discussed in relation with the pore structure observed in the companion paper. Slightly anisotropic properties are evidenced, in agreement with the orientation of the cells, as expected for foams grown vertically in cylindrical moulds.     

Enhancing tensile/compressive response of magnesium alloy AZ31 by integrating with Al2O3 nanoparticles

AZ31 nanocomposite containing Al₂O₃ nanoparticle reinforcement was fabricated using solidification processing followed by hot extrusion. The Al₂O₃ nanoparticle reinforcement was isolated prior to melting by wrapping in Al foil of minimal weight (<0.50 wt% with respect to AZ31 matrix weight). The AZ31 nanocomposite exhibited slightly smaller grain and intermetallic particle sizes than monolithic AZ31, reasonable Al₂O₃ nanoparticle distribution, non-dominant (0 0 0 2) texture in the longitudinal direction unlike monolithic AZ31, and 30% higher hardness than monolithic AZ31. Compared to monolithic AZ31, the AZ31 nanocomposite exhibited higher 0.2%TYS, UTS, failure strain and work of fracture (WOF) (+19%, +21%, +113% and +162%, respectively). Also, compared to monolithic AZ31, the AZ31 nanocomposite exhibited higher 0.2%CYS and UCS, similar failure strain, and higher WOF (+5%, +5%, −4% and +11%, respectively). Inclusive of crystallographic texture changes, the effect of Al₂O₃ nanoparticle integration on the enhancement of tensile and compressive properties of AZ31 is investigated in this paper.     

Thermomechanical Processing and Superplasticity of AZ91 Magnesium Alloy

The effect of extrusion on grain refinement has been studied in the AZ91 cast ingots. It is found that grain sizesmaller than 10μm can be obtained by the extrusion processing. Vickers hardness measurements were also carriedout to evaluate the effect of these processes on the room temperature mechanical properties. The experimentalresults of high temperature tensile tests revealed that the stress was inversely proportional to the square of the grainsize and that the activation energy for superplastic flow was higher than that for grain boundary diffusion.     

Thermal conductivity improvement of composite carbon foams based on tannin-based disordered carbon matrix and graphite fillers

Carbonaceous porous matrices were prepared from a tannin-based resin by physical foaming, having improved thermal properties by addition of various kinds and various amounts of graphite fillers. The resultant composite carbon foams presented much higher thermal conductivity, making them suitable for hosting phase-change materials with the aim of using them in seasonal storage applications. These materials were investigated in terms of porous structure, thermal and mechanical properties. It was shown that, unlike what was a priori expected, smaller particles were far more suitable for getting conductive, strong and porous matrices. The smaller were the particles, the better were the results. These findings were explained and justified, making such biomass-based composite carbon foams interesting and cheap candidates for thermal storage applications.     

Modeling failure modes of isotropic three-dimensional reticulated porous metal foams under several typical loads

Three dimensional reticulated porous metal foams are widely used engineering materials. A failure model with the simplified structure of these porous materials has been established, and the failure modes have been analyzed for the corresponding porous components under several typical loads, which include torsion, shearing and bending. The failure modes cover the tensile fracture, the shearing and the buckling of the strut, which may lead to the final destruction of the whole porous structure. The mathematical relationships, which characterize different failure modes, have been derived for the strut failure resulting from loading for these porous components under the above loading conditions. The results also show that the failure mode is related with the material species for these materials under the above loads. The tensile fracture of the strut will occur for the porous body with metallic materials in most cases, and the shearing fracture of the strut may occur for that in a relatively little cases. Moreover, the elastic buckling, the elastic–plastic buckling and the edge yielding may also occur on the strut of porous bodies when certain conditions are met.     

Microstructures and mechanical properties of ultrafine-grained Ti/AZ31 magnesium matrix composite prepared by powder metallurgy

The microstructure of ultrafine grain for magnesium alloys can result in drastic enhancement in their room temperature strength, but the issue of low strength at elevated temperature becomes more serious as well due to grain boundary slide. Here ultrafine-grained Ti/AZ31 magnesium matrix composites with high strength at both room and elevated temperature were prepared by vacuum hot pressing and subsequent hot extrusion. The microstructure of the composite samples before and after consolidation processing was characterized, and the mechanical properties of the as-consolidated bulk samples were measured at room and elevated temperatures. The results indicate that after extrusion ultrafine-grained magnesium alloys were obtained and Ti particulates with particulate size of ～310?nm disperse in Mg matrix. The magnesium grain of AZ31-15at.%Ti grows from 66?nm to 800?nm. Meanwhile, the relative densities of Ti/AZ31 composites are higher than 99%. The yield strength (YS) of extruded AZ31-15at.%Ti composite at room temperature is 341?MPa, being 2.4 times higher than original AZ31 alloy. Theoretical estimation shows that remarkably enhanced room-temperature mechanical strength attributes to grain boundary strengthening with the contribution ratio of 74%. In addition, the peak stress of extruded AZ31-15at.%Ti composite at 573?K is 82?MPa and ultrafine Ti dispersions are responsible for the enhanced strength.     

Strain-energy based homogenisation of two- and three-dimensional hyperelastic solid foams

A possible classification of cellular solids can be made based on the dimension into honeycombs and foams. In numerical simulations 2D models that are employed primarily to study honeycombs can also be used to model open-cell foams. Thereby, a loss of information regarding the 3D connectivity of the microstructure is involved. To answer the question how the missing third dimension in 2D models affects the overall properties, spatially periodic 2D and 3D model foams are adopted. From the point of homogenisation, a strain-energy based scheme is used for adequately determining the effective mechanical properties at large strains. The key idea behind this method is to use directly the equivalence condition between the meso-strain energy and the macro-strain energy. In a first step a representative volume element with the given microstructure and a corresponding volume element containing the effective medium are subjected to equivalent states of deformation. Subsequently, the macroscopic stress-strain relationships are determined by volume-averaging of the stored strain energy. The results of some fundamental loading cases indicate that both model foams represent the deformation characteristics of hyperelastic solid foams like localized bending and elastic buckling. In addition, the development of anisotropy due to microstructural changes at large strains can be traced with both model foams. Nevertheless, the different cell morphology affects the stress-strain curves in a quantitative manner.     

Gradient microstructure and enhanced mechanical performance of magnesium alloy by severe impact loading

Subjecting a workpiece to a surface treatment with severe impact loading is a novel severe plastic deformation procedure to fabricate gradient microstructures through the thickness and longitudinal direction.Mechanical performance is a function of twin density and the newly-formed grain size gradients.{1012}tensile twins created from processing without excessive grain refinement lead to strength enhancement with retained ductility.Creation of residual strain by a single impact results in a significant reduction in time and cost of the process.This paper investigates the effect of applying severe impact loading on mechanical and microstructural properties of magnesium for various impact velocities.