Anne Jung

Study of the magnetic flux density distribution of nickel coated aluminum foams

Open cell aluminum foams are metal cellular structures with a large volume fraction of pores. Due to their high stiffness to weight ratio, they are commonly used in applications for energy absorption and mechanical damping. The stiffness of the aluminum foam was increased by a nanocrystalline nickel coating via an electrodeposition process. The deposition process and thus the coating thickness strongly depend on mass transport limitations. To visualize the coating thickness distribution of the foam, we measured the magnetic flux density distribution by scanning the surface of cuts of coated foams with a commercial Hall probe. By measuring the magnetic flux density distribution, deposition parameters as the current density and flow conditions could be optimized with regard to a more homogeneous coating thickness distribution. Furthermore, a model of the mass transport limitation at a complex three dimensional foam electrode could be evaluated from the magnetic flux density distribution of the nickel coated foam cuts.

Dual-energy X-ray micro-CT imaging of hybrid Ni/Al open-cell foam

In this paper, we employ dual-energy X-ray microfocus tomography (DECT) measurement to develop high-resolution finite element (FE) models that can be used for the numerical assessment of the deformation behaviour of hybrid Ni/Al foam subjected to both quasi-static and dynamic compressive loading. Cubic samples of hybrid Ni/Al open-cell foam with an edge length of [15]mm were investigated by the DECT measurement. The material was prepared using AlSi7Mg0.3 aluminium foam with a mean pore size of [0.85]mm, coated with nanocrystalline nickel (crystallite size of approx. [50]nm) to form a surface layer with a theoretical thickness of [0.075]mm. CT imaging was carried out using state-of-the-art DSCT/DECT X-ray scanner developed at Centre of Excellence Telc. The device consists of a modular orthogonal assembly of two tube-detector imaging pairs, with an independent geometry setting and shared rotational stage mounted on a complex 16-axis CNC positioning system to enable unprecedented measurement variability for highly-detailed tomographical measurements. A sample of the metal foam was simultaneously irradiated using an XWT-240-SE reflection type X-ray tube and an XWT-160-TCHR transmission type X-ray tube. An enhanced dual-source sampling strategy was used for data acquisition. X-ray images were taken using XRD1622 large area GOS scintillator flat panel detectors with an active area of [410 × 410]mm and resolution [2048 × 2048]pixels. Tomographic scanning was performed in 1,200 projections with a 0.3 degree angular step to improve the accuracy of the generated models due to the very complex microstructure and high attenuation of the investigated material. Reconstructed data was processed using a dual-energy algorithm, and was used for the development of a 3D model and voxel model of the foam. The selected parameters of the models were compared with nominal parameters of the actual foam and showed good correlation.

Identification of strain fields in pure Al and hybrid Ni/Al metal foams using X-ray micro-tomography under loading

Hybrid foams are materials formed by a core from a standard open cell metal foam that is during the process of electrodeposition coated by a thin layer of different nanocrystalline metals. The material properties of the base metal foam are in this way modified resulting in higher plateau stress and, more importantly, by introduction of strain-rate dependence to its deformation response. In this paper, we used time-lapse X-ray micro-tomography for the mechanical characterization of Ni/Al hybrid foams (aluminium open cell foams with nickel coating layer). To fully understand the effects of the coating layer on the materials effective properties, we compared the compressive response of the base uncoated foam to the response of the material with coating thickness of 50 and 75 μm. Digital volume correlation (DVC) was applied to obtain volumetric strain fields of the deforming micro-structure up to the densification region of the deforming cellular structure. The analysis was performed as a compressive mechanical test with simultaneous observation using X-ray radiography and tomography. A custom design experimental device was used for compression of the foam specimens in several deformation states directly in the X-ray setup. Planar X-ray images were taken during the loading phases and a X-ray tomography was performed at the end of each loading phase (up to engineering strain 22%). The samples were irradiated using micro-focus reflection type X-ray tube and images were taken using a large area flat panel detector. Tomography reconstructions were used for an identification of a strain distribution in the foam using digital volumetric correlation. A comparison of the deformation response of the coated and the uncoated foam in uniaxial quasi-static compression is summarized in the paper.

Magnetic field-assisted electroforming of complex geometries

A new magnetic field-enhanced electroforming process for complex 3D structures was presented. The procedure was optimized for the electroforming of thick-walled nickel tools used in automotive or aerospace manufacturing. It was shown that the variation of the current density and also the use of leveling agents are not suitable for the filling of notches or for the homogeneous growth of undercuts. The problem was solved by the superposition of the electrochemical deposition process with a magnetic field. The authors developed a procedure with permanent magnets positioned in the backing of the cathode. It could be shown that the local magnetic field enhances the local metal deposition rate. This work demonstrates the homogeneous notch filling in dependence of the magnetic field strength, the position of the magnetic field sources, and the material of cathode backing. Beside the influence on the deposition rate, a superposed magnetic field also enhances the material properties of deposited metals. Investigations on grain size and hardness were performed in relation to applied magnetic field.

Feuerfeste MgO-C Komposite mit zellularer Kohlenstoffstruktur, Teil 1: Experimentelle Charakterisierung

ZusammenfassungEs wurden MgO-C-Feuerfeststeine mit einer zellularen Kohlenstoffstruktur hergestellt und experimentell untersucht. Zum Schutz des Kohlenstoffs vor Oxidation wurde dieser mit einer Beschichtung aus Yttrium stabilisiertem Zirkonoxid (YSZ) oder Siliciumcarbid (SiC) versehen und anschließend mit einer Magnesiumoxidsuspension infiltriert. Aus den experimentellen Ergebnissen wurden die finalen Risslängen und Rissdichten nach einem Thermoschock berechnet. Die hohe Porosität der Proben mit zellularer Kohlenstoffstruktur beeinflusst deren Eigenschaften und verringert ihre Thermoschockbeständigkeit im Vergleich zu einer Referenzprobe mit pechgebundenem Kohlenstoff.AbstractExperimental results of cellular carbon foams coated with yttria-stabilized zirconia (YSZ) or silicon carbide (SiC) and infiltrated with magnesia suspension are presented. The calculated parameters for final crack length and final crack density allow conclusions about the thermal shock behavior.

Feuerfeste MgO-C Komposite mit zellularer Kohlenstoffstruktur, Teil 2: Mikrostrukturoptimierung durch thermo-mechanische Simulation

KurzfassungFeuerfestmaterialien sind in ihrer Anwendung sowohl quasistatischen als auch dynamischen thermischen Belastungszuständen ausgesetzt, welche zu Materialschädigungen bis hin zum Materialversagen führen. Zelluläre MgO-C-Komposite, bestehend aus einem Kohlenstoffschaum und mit Periklas (MgO) gefüllten Poren, konnten als vielversprechendes Material für Feuerfestmaterialen identifiziert werden. In dieser Arbeit wurde deren Thermoschockbeständigkeit in Abhängigkeit mikrostruktureller Parameter, wie Porengröße, Stegdicke und offene Porosität mit Hilfe der Finiten-Elemente-Methode (FEM) untersucht und optimiert. Durch Anwendung der empirisch motivierten Hasselman-Beziehung konnte Rissinitiation für unterschiedliche Thermoschockraten numerisch identifiziert werden.AbstractIn their applications, refractory materials are subjected to both quasi-static and dynamic thermal loading conditions, leading to material damage up to material failure. Cellular MgO-C composites consisting of a carbon foam, filled with periclase (MgO) are identified as potential refractory materials. In this contribution, the thermal shock resistivity of MgO-C cellular refractories as function of microstructural parameters like pore size, strut thickness and open porosity was investigated and optimized using the finite element method (FEM). Applying the empiric motivated Hasselman relationship, crack initiation could be numerically identified for different thermal shock rates.

Improved Mechanical Properties of Nano-nickel Strengthened Open Cell Metal Foams

Metal foams are low impedance materials which are often used as light weight construction elements, energy absorber and for structural damping. We have coated open cell aluminum foams by a nanocrystalline nickel coating via an electrodeposition process and in this way we could improve the stiffness, energy absorption capacity and the damping behavior of the foams. The mechanical behavior of the coated foams could be tuned by the crystallite size and the thickness of the coating. The foams were characterized in quasi-static compression tests as well as in dynamic compression tests, using a Split Hopkinson pressure bar (SHPB). At the optimized coating thickness of 150 µm Ni, there was an enhancement effect of the energy absorption capability of 800% for 10 ppi foams. Ballistic tests showed the applicability of the foams as splinter shield. A big opportunity of open cell metal foams is that they can be filled for example with elastomeric materials and build a composite structure.