John Banhart

Deformation characteristics of metal foams

The deformation behaviour of a series of aluminium and zinc foams was investigated by uniaxial testing. Because the deformation behaviour of metal foams is expected to be anisotropic owing to the existence of a closed outer skin and with respect to the foaming direction, a series of measurements was carried out where the orientation of the outer skin and the foaming direction were varied. Stress–strain diagrams and corresponding compression strengths were determined for aluminium- and zinc-based foams. The influence of an age-hardening heat treatment was investigated. Finally, the axial deformation behaviour of aluminium tubes filled with aluminium foam was tested under uniaxial loading conditions. The results of the measurements are discussed in the context of possible applications of metal foams as energy absorbers.

Investigation of water evolution and transport in fuel cells with high resolution synchrotron x-ray radiography

The authors report on in situ investigations of liquid water evolution and transport in an undisturbed operating fuel cell at the microscopic level. Synchrotron x-ray radiography enhances the spatial resolution by two orders of magnitude compared to the state-of-the-art techniques in this field. The primary spots of liquid water formation, their growth, and transport inside the porous gas diffusion material were analyzed; correlations between operating conditions and the dynamics of droplet formation are described. Previous findings from modeling and simulation approaches are confirmed and the applicability for the description of in situ processes of a recently proposed model has been proven.

A study of aluminium foam formation : Kinetics and microstructure

Abstract Aluminium foams were produced by applying the powder compact melting method, i.e. by mixing metal powders and powdered gas-releasing blowing agents and pressing them to a foamable precursor material after this. The resulting precursor was then foamed by heating it up to above its melting point inside an “expandometer”, which allowed for the volume and temperature to be measured throughout the entire process. The present studies comprise the effects of the aluminium alloy composition (AlSi7 and 6061), some of the pressing parameters of the foamable precursor material, the foaming temperature and the heating rate during foaming on the expansion behaviour of the foam. Moreover, the morphological and microstructural evolution of metal foams is investigated.

Aluminium foams for transport industry

Abstract Foamed materials are widespread in transportation industry applications. While polymeric foams have been applied for many years foamed metals are now beginning to move into the focus of interest. A powder metallurgical method which allows the production of aluminium foams with porosity levels up to 90% is described. The foams typically have closed pores and densities ranging from 0.4 to 1 g cm −3 , so that this foamed metals float on water. The unique mechanical properties of metal foams are described. The density dependence of metal foam properties is shown with the Youngs modulus, flexural strength and compression strength as examples. A non-linear dependency of these properties on the density is found and discussed. The discussion then focuses on the energy absorption properties of aluminium foams and tools to select appropriate foams for a given energy absorption task.

Industrialization of Powder Compact Toaming Process

The potential for applying metal foams in lightweight construction is mainly based on the increased stiffness of two flat or curved sheets that are separated by a foam layer as compared to a single sheet of the same weight. By using sandwiches with an aluminum-foam core, it is possible to obtain a higher stiffness and rigidity, maintaining stability against buckling and additionally making use of the high energy dissipation capability of the foams. A recent highlight in lightweight construction is the use of aluminum-foam sandwiches (AFSs) in space-frame constructions.

Cross-sectional insight in the water evolution and transport in polymer electrolyte fuel cells

The evolution of liquid water and its transport through the porous gas diffusion media in an operating fuel cell were investigated applying an experimental setup for high spatial resolution of 3μm. Fundamental aspects of cluster formation in hydrophobic/hydrophilic porous materials as well as processes of multiphase flow are addressed. The obtained water distributions provide a detailed insight in the membrane electrode assembly and the porous electrode with regard on the existence and transport of liquid water. In addition, the results approve transport theories used within the framework of percolation theory and demonstrate the need for adapted modeling approaches.

Advances in neutron radiography and tomography

Neutron imaging can provide two- or three-dimensional, spatially resolved images of the internal structure of bulk samples that are not accessible by other techniques, making it a unique tool with many potential applications. The method is now well established and is available at neutron sources worldwide. This review will give a survey of the technique of neutron imaging with a special focus on neutron tomography; the basics of the method as well as the technology of instrumentation will be outlined, and the techniques will be illustrated by representative applications. While the first part of the paper focuses on conventional attenuation contrast imaging, the second part reviews and critically assesses recent methodical developments.

On the Road Again: Metal Foams Find Favor

Lightweight yet stiff, metal foams are experiencing a resurgence of interest for applications ranging from automobiles to dental implants.

Aluminium foams for lighter vehicles

Metallic foams have become an attractive research field both from the scientific viewpoint and the prospect of industrial applications. Various methods for making such foams are available. Some techniques start from specially prepared molten metals with adjusted viscosities. Such melts can be foamed by injecting gases or by adding gas-releasing blowing agents which decompose in-situ, causing the formation of bubbles. A further way is to start from solid precursors containing a blowing agent. These can be prepared by mixing metal powders with a blowing agent, compacting the mix and then foaming the compact by melting. Alternatively, casting routes can be used to make such precursors. The unique properties of foams promise a variety of applications in vehicle design ranging from light-weight construction, impact energy absorption to various types of acoustic damping and thermal insulation. Four applications are discussed, including a lifting arm on a lorry, an automobile and a train crash box, and a motor bracket.

Desorption of hydrogen from blowing agents used for foaming metals

Abstract Hydrogen desorption from TiH2, ZrH2, and MgH2 was studied by thermal desorption spectroscopy (TDS), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Loose hydride powders as well as powder compacts of zinc and various hydrides were studied. It was found that during the powder compaction process free surfaces on the hydride powder particles were created. As a consequence, the desorption temperature of the hydride in the precursor was lowered in comparison to loose powder exposed to air. Foam expansion of zinc was highest for TiH2 which also exhibits the highest desorption rate at the melting point of zinc, followed by ZrH2 and MgH2 which decompose at lower temperatures and are therefore less effective for foaming. The desorption kinetics of Al and AlSi7 compacts containing TiH2 were also studied for matters of comparison. The much lower foam expansions compared to Zn foams could be explained by higher hydrogen losses at temperatures below the melting point of Al and AlSi7.