Production of metallic foam under low gravity conditions during parabolic flights

Metallic foams are a recently developed light weight material. They are porous structures consistent of metals like aluminum, tin, zinc, lead etc. or their alloys. The pore sizes are in the range of millimeters and relative densities down to 10% of the original material can be achieved. Since metallic foams combine relative low weight with high stiffness, their applications are mainly for means of light weight structures as used for example in cars. Also other applications like sandwich structures and metallic filters are of interest.     

Fabrication,properties,and applications of open-cell aluminum foams:A review

Open-cell metallic foams or porous metals have a distinctive combination of excellent structural performance and superior functional characteristics,such as their light weight,energy absorption,sound absorption,heat dissipation,and electromagnetic shielding.As a primary representative of metallic foams,aluminum foam has developed into a new engineering material with many unique applications in the fields of aerospace,automotive industry,petrochemical industry,building materials,and etc.This paper summarizes the fabrication methods,properties,and applications of open-cell aluminum foams.The current status and development trends are also introduced.     

Computational modelling of irregular open‐cell foam behaviour under impact loading

Cellular structures represent an important class of engineering materials. Typical representative of such structures are metallic foams, which are being increasingly used in many advanced engineering applications due to their low specific weight, appropriate mechanical properties and excellent energy absorption capacity. For optimal design of cellular structures it is necessary to develop proper computational models for use in computational simulations of their behaviour under impact loading. The paper studies the effects of open‐cell metallic foam irregularity on deformation behaviour and impact energy absorption during impact loading by means of parametric computational simulations, using the lattice‐type modelling of open‐cell material structure. The 3D Voronoi technique is used for the reproduction of real, irregular open‐cell structure of metallic foams. The method uses as a reference a regular mesh structure and utilises an irregularity parameter to reproduce the irregularity of real open‐cell structure. A smoothing technique is introduced to assure proper stability and accuracy of explicit dynamic simulations using the produced lattice models. The effects of the smoothing technique were determined by comparative simulations of smoothed and unsmoothed lattices subjected to dynamic loading.     

Formation of Network and Cellular Structures by Viscoelastic Phase Separation

Network (sponge) and cellular structures are often seen in various types of materials. Materials with such structures are generally characterized by light weight and high mechanical strength. The usefulness of such materials is highlighted, for example, by the remarkable material properties of bone tissue, which often has a highly porous structure. In artificial materials, plastic and metallic foams and breads have such structures. Here, we describe a physical principle for producing network and cellular structures using phase separation, and its potential applications to the morphological control of materials spanning from soft to hard matter.     

Hygrothermal studies on syntactic foams and compressive strength determination

Syntactic foams are commonly used as core materials in composite sandwich structures for weight sensitive applications such as aircraft and spacecraft structures and boat hulls. Moisture absorption is highly undesirable in these applications. The present study evaluates the hygrothermal properties of two types of syntactic foams. Distribution of outer diameter of cenospheres (hollow particles) incorporated in both types of syntactic foams is the same but there is variation in the internal diameter causing difference in the density of syntactic foams. Epoxy resin is used as matrix material and the volume fractions of matrix and cenospheres are kept at 0.35 and 0.65 by volume, respectively. Moisture absorption experiments are conducted at two different temperatures, 25 and 70 °C and in deionized and salt waters. Non-destructive ultrasonic imaging technique is used to find the extent of moisture penetration and damage to the specimens. Syntactic foam samples are tested for compressive strength after moisture absorption and the results are compared with the compression test results of dry syntactic foam samples.     

Adaptation of density distributions for optimising aluminium foam structures

Abstract

Metallic foams show some potential for being produced with controlled spatial variations in their density. This suggests employing them as graded materials in space filling lightweight structures designed in analogy to cortical bone, a natural cellular material, that displays increased density in regions of high loading. In the present study the influence of the mechanical properties of aluminium foams on the results of an optimisation of the foam density distribution with regard to structural strength and stiffness was examined. Regression formulae for the relationships between stiffness and strength of metallic foams on one hand and effective density on the other hand can be fitted to the results of uniaxial compression tests of a certain brand of metallic foam. These results and additional assumptions such as overall isotropy and a yield surface suitable for cellular materials can be implemented into a finite element program adapted for performing stiffness or strength optimisation on the basis of a density adaptation similar to the remodelling of bone. Some applications are presented that show how foams with gradients in the apparent density may be employed to obtain optimal structural behaviour for classical design problems.     

The mechanical behaviour of aluminium foam structures in different loading conditions

The use of foam has the potential for energy absorption enhancement. Many types of materials can be produced in the form of foams, including metals and polymers. Of the metallic based foams, aluminium based are among the most advanced. Aluminium foams couple good specific mechanical properties with high thermal stability. Among the various aspects still to be investigated regarding their mechanical behaviour is the influence of a hydrostatic state of stress on yield strength. Unlike metals, the hydrostatic component affects yields. Therefore, different loading conditions have to be considered to fully identify the material behaviour. Another important issue in foam structure design is the analysis of composite structures. The mechanical behaviour of an aluminium foam has been examined. The foam was subjected to uniaxial, hydrostatic stress, pure deviatoric stress, and combinations thereof. Results obtained will be presented as quasi-static and dynamic uniaxial compression and quasi-static bending and shear loading. Moreover, composite structures were made by assembling the foam into aluminium cold extruded closed section 6060 aluminium tubes. The results show that the energy absorption capability of the composite structures is much greater than the sum of the energy absorbed by the two components, the foam and the tube.     

Editorial

In this study the Taguchi method is used to find the optimal process parameters for aluminium foam manufacturing. Porous metals are the unique materials used for light weight structural components, for filters and electrodes and for shock or sound absorbing products. Recently, interesting foaming technology developments have proposed metallic foams as a valid commercial chance. Metallic foam manufacturing techniques include solid state powder methods, gas blowing processes, metal deposition onto a polymer precursor and liquid state processing. The aluminium foams presented in this study are produced by the powder metallurgy route starting from aluminium powders with titanium hydride as the foaming agent. During the experimental work, many samples are made by utilizing the combination of process parameters based on Taguchi orthogonal design. Three manufacturing parameters are studied: the silicon carbide content in powder mixture, the compaction pressure and the foaming temperature. The Taguchi method is applied to design an orthogonal experimental array and a multi-objective optimization approach is then proposed by simultaneously minimizing the relative density and maximizing the absorbed energy. Verification test is also performed to prove the effectiveness of the presented technique.     

Water immersion effect on swelling and compression properties of Eco-Core,PVC foam and balsa wood

Syntactic foam, balsa wood and PVC foams are commonly used as core materials in sandwich structures for weight critical applications such as aircraft and ship structures. Water absorption is highly undesirable in these applications. The present study evaluates the effect of water immersion on three types of core materials, namely, Eco-Core, balsa wood and PVC foam. Eco-Core is a new fire resistant core material under development that utilizes about 83% by weight of fly ash. Designers of naval ships and aircraft commonly specify balsa wood and PVC foam as core materials for sandwich structures. These three core materials were subjected to water immersion to determine the relative resistance to property change. Both tap water as well as seawater was used. Core samples were studied for dimensional change, weight gain and compression properties after water immersion and the results were compared with the test results of dry core samples. Time periods included in the study ranged from 4 h to 500 days. The results showed that Eco-Core is as good as PVC foam in resisting swelling, water absorption and changes in compression properties due to water immersion. Where as balsa wood showed a significant swelling, water absorption and deterioration of compression properties.     

Effects of statistics of cell’s size and shape irregularity on mechanical properties of 2D and 3D Voronoi foams

In the study of the mechanical properties of metallic foam, the relative density (or porosity) and the average size of cells are two key parameters of the meso-geometry, but it is also well known experimentally that the two parameters alone are not enough since the mechanical properties of metallic foams are different even with the same initial relative density and average cell size. In this paper, we have classified the irregularity of cells into two types to describe polygonal and polyhedral cells in 2D and 3D metallic foams, which are called size irregularity and shape irregularity, respectively. The former reflects the deviation of the size of a cell from the average cell size in the foam and the latter reflects the deviation of the shape of a cell from a circle with the same area (2D) or a globe with the same volume (3D). With the two kinds of definitions, effects of the irregularity of cells in aluminum foam on mechanical properties are investigated using the Voronoi tessellation technique and the finite element method. The well- designed 2D and 3D Voronoi models are constructed, of which the statistic distributions of size and shape irregularity are presented. The compression simulations of Voronoi-based models indicate that the yield stress of metallic foam is seldom affected by the size irregularity, but significantly affected by the shape irregularity. The more regular the foams, the higher will be their yield plateau at constant overall relative density and average cell size.     

Compressive fracture features of syntactic foams-microscopic examination

Syntactic foams made by mechanical mixing of polymeric binder and hollow spherical particles are used as core materials in sandwich structured materials. Low density of such materials makes them suitable for weight sensitive applications. The present study correlates various postcompression microscopic observations in syntactic foams to the localized events leading the material to fracture. Depending upon local stress conditions the fracture features of syntactic foam are identified for various modes of fracture such as compressive, shear and tensile. Microscopic observations were also taken at sandwich structures containing syntactic foam as core materials and also at reinforced syntactic foam containing glass fibers. These observations provide conclusive evidences for the fracture features generated under different failure modes. All the microscopic observations were taken using scanning electron microscope in secondary electron mode.     

A preliminary study on laser assisted aluminum foaming

Closed cell cellular solids defined by randomly distributed air pores in metallic matrix are a special class of light weight porous material that exhibit several unique characteristic applications in various domains of industries. Among these, in particular aluminum foam has shown a popular interest in recent years and is potentially used for many applications due to its light weight structure. In this paper laser assisted foaming experiments have been performed using CO₂/Nd-YAG lasers. The results show a unidirectional and localized expansion of foam. Closed cell porous aluminum structures with different relative densities (0.39–0.33) and porosity of (61–67%) are fabricated. It is found that the beam interaction time (t _int. = spot size/processing speed), the so called dwell time, plays a significant role in the evolution of the cell morphology and the expansion mechanism of foam. Preliminary results suggest that a pore size gradient and a density gradient exist in the structure as the processing condition changes. The foam has large pores and lower density for slow processing speed, in contrast to the fast processing speed with small pore size but higher density. Additionally, a few examples on laser assisted cutting of aluminum foam are also well demonstrated.     

Cellular Metals Manufacturing

Open cell, stochastic nickel foams are widely used for the electrodes and current collectors of metal – metal hydride batteries. Closed cell, periodic aluminum honeycomb is extensively used for the cores of light, stiff sandwich panel structures. Interest is now growing in other cell topologies and potential applications are expanding. For example cellular metals are being evaluated for impact energy absorption, for noise and vibration damping and for novel approaches to thermal management. Numerous methods for manufacturing cellular metals are being developed. As a basic understanding of the relationships between cell topology and the performance of cellular metals in each application area begins to emerge, interest is growing in processes that enable an optimized topology to be reproducibly created. For some applications, such as acoustic attenuation, stochastic metal foams are likely to be preferred over their periodically structured counterparts. Nonetheless, the average cell s ize, the cell size standard deviation, the relative density and the microstructure of the ligaments are all important to control. The invention of more stable processes and improved methods for on‐line control of the cellular structure via in‐situ sensing and more sophisticated control algorithms are likely to lead to significant improvements in foam topology. For load supporting applications, sandwich panels containing honeycomb cores are much superior to those utilizing stochastic foams, but they are more costly than stochastic foam core materials. Recently, processes have begun to emerge for making open cell periodic cell materials with triangular or pyramidal truss topologies. These have been shown to match the stiffness and strength of honeycomb in sandwich panels. New cellular metals manufacturing processes that use metal textiles and deformed sheet metal are being explored as potentially low cost manufacturing processes for these applications. These topologically optimized systems are opening up new multifunctional applications for cellular metals.     

Simulative Determination of Effective Mechanical Properties for Digitally Generated Foam Geometries

Metal foams constitute a promising and emerging material class in the context of lightweight construction. There exists a variety of different foam topologies, on which resulting mechanical properties depend. To maximize the potential of foams in material use under mechanical load, the present work addresses the question how different geometrical parameters influence the material behaviour. Therefore, an algorithm for digital generation and design of open pore foam structures is presented, that allows to regulate the geometry precisely. A method for retrieving effective mechanical properties from numerical simulations of compression tests in the elastic regime is introduced. Additionally, the representativeness of foam volumes considered for simulations is investigated. This yields a fully digital workflow, which enables the investigation of geometry influence on mechanical properties. This approach is used to conduct simulation studies on generated foam structures with a systematic variation of geometrical parameters. Herein, a range of effective Young's moduli varying by up to a factor of 1.3 for different foam structures at the same porosity is found. This shows a significant impact of the foam geometry on the elastic properties of metal foams. The presented methodology yields insights, which can guide design and optimization of materials for specific applications.     

Fatigue Behavior,Strength, and Failure of Aluminum Foam

Foaming processes and the quality of the foamed material have improved considerably during the recent years. It now worth to evaluate the application of aluminum foams for helicopter components to reduce production costs and weight. Since the operating conditions of helicopters expose materials to repeated or fluctuating strains the fatigue behavior of foam materials is of crucial importance for their evaluation as potential materials for helicopter components.     

Novel polymer–metal based method for open cell metal foams production

Abstract

Metal foams have acquired popular interest in recent years and are potentially useful for many applications due to their light weight, high specific stiffness, high surface to volume ratio, and adjustable cell structure. Here, current methods of producing metal foams are briefly reviewed. The requirements for high porosity metal foams with open cells are discussed. A novel powder metallurgy route involving a polymeric vehicle is introduced that can readily generate open cell foams with porosity greater than 90%. Coarse Ti powder and fine carbonyl iron powder were tested. Although the resulting polymer metal foam was closed cell, particles were not retained in the windows. Upon pyrolysis to remove the resin, the windows opened and the final sintered metal foam was highly reticulated. Such foams offer a fine reticulated structure with cell diameters in the region of 100–200 µm, and may find applications in the areas of catalysis, biomaterials, and composites.     

Enhancing the strength of pre-made foams for foam concrete applications

Chemical and mechanical foaming techniques are commonly used in foam concrete technology for developing lightweight construction materials. The characteristics of the foam before the lightweight structure sets and maintains its shape has a great impact on the properties of foamed concretes. The tendency of the foams to coalesce and collapse during the preparation process brings some challenges in controlling the properties of cellular structures. Consequently, it is critical to improve the stability of fresh foams in order to produce high quality cellular structures using a predictable and reliable approach. Aggregating the liquid film around bubbles is known to be effective in improving the stability of foams, but the impact of this stabilizing method has not been investigated for foam concrete applications. In this paper, Xanthan gum (with a thickening capacity) has been utilized as the foam stabilizer to aggregate the liquid film. This stabilizing method is shown to significantly enhance the pore size distribution of foam concretes. The resulting pre-made foams are remarkably more stable than the control foam, and the mechanical properties of the final cellular structure are considerably improved (about 34% in mechanical foaming and 20% in the chemical foaming technique).     

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.     

Ultrasonic Torsion Welding of Sheet Metals to Cellular Metallic Materials

One of the most important requirements for finding new applications for cellular metals is to integrate them in complex technical structures. The metal foams have to be joined to each other, or to sheet materials, by suitable joining techniques. The main topics of this paper are the ultrasonic torsion welding of cellular metallic materials to sheet metals and the investigation of the mechanical properties of the joints. The basic materials of foams and sheet metals were different aluminum and iron alloys. Depending on the materials used, weldings with tensile shear strengths of up to 25 MPa were realized. Using aluminum foam sandwich (AFS) and sheet metals, successful weldings were performed before and after the foaming process. Furthermore, it was possible to perform a successful foaming process with the unfoamed AFS/sheet metal joints. Microscopic investigations showed that the ultrasonic welding technique allows the joining of the metal foams with sheet metals without significant deformation of the joining partners. The temperatures during the welding process in the interface were below the melting point of the foams and the sheet metals.