Cellular metals based on iron have been intensively investigated during the last two decades. Because of the significant decrease in of the structural density of iron based cellular structures, numerous technologies have been developed for their manufacturing. Besides the tremendous weight reduction a combination with other properties like energy and noise absorption, heat insulation and mechanical damping can be achieved. This contribution will give an overview about the latest state in iron based cellular materials, including technologies in manufacturing, properties and potential applications.
Magnesium alloys offer excellent properties with regard to application as degradable implant. For bone implants, it is often desirable to use porous materials. However, the preparation of high-porosity magnesium implants has been difficult so far. The present study uses melt extracted magnesium fibers as the starting material for the sintering of highly porous magnesium bodies, i.e., from alloys MgY4 (W4) and MgY2Zn1CaMn (WZ21). Single short fibers of these alloys with an equivalent diameter between 100 and 250 mm and a length of 4-8 mm are manufactured by melt extraction. Thermodynamic calculations are used to determine the best conditions for liquid phase sintering of these Mg alloys. As no organic or other substances are needed in the process, it is possible to obtain high-purity, high-porosity (up to 75%) bodies with exclusively open porosity. Metallographic studies as well as mechanical and corrosion testing experiments complete this work.
Porous bodies that are resistant to corrosion at high temperatures and thermal shock may be produced from metallic fibers. In order to accomplish reasonable homogeneity and high porosity, the cross-sectional area of the fibers and the width of distribution thereof need to be small. This article studies two techniques for making fibers. Melt extraction out of a crucible yields filaments with a typical diameter ranging from 50 to 200 lm, which is too thick. Also patented for a long time is the extraction from a pendant drop. Even though relatively fine fibers can be manufactured with this method, it never exceeded crucible extraction with respect to industrial importance owing to the low productivity of the process. The present article addresses the drawbacks of both variants of melt extraction of metallic filaments. Because metallic melts are electrically conducting, the use of magnetic fields allows for contactless process optimization. It is well believed that increasing the extraction speed diminishes the fiber diameter. Being not always true, at least in the case of crucible melt extraction, as indicated by the present findings, however, undesired fluid flow, i.e., turbulence, imposes an upper limit on the rotation rate of the extraction wheel. Application of a static magnetic field leads to both higher wheel speed and thinner filaments. The low productivity of extraction from the molten tip of a rod suffers from the fact that only one melt drawing edge can be used. As the bare rod is problematic with respect to heating its tip in contact with the extraction wheel, it is challenging to melt the entire edge of a sheet. A special design of the induction-heating magnetic field is also proposed to solve also this task.
Powder-based techniques are gaining increasing interest for the fabrication of microstructures on planar substrates. A typical approach comprises the filling of a mold pattern with micron-sized particles of the desired material, and their fixation there. Commonly powder-loaded pastes or inks are filled into the molds. To meet the smallest dimensions and highest filling factors, the utilization of dry powder as the raw material is more beneficial. However, an appropriate automated technique for filling a micro mold pattern with dry micron-sized particles is missing up to now. This paper presents a corresponding approach based on the superimposition of high- and low-frequency oscillations for particle mobilization. Rubber balls are utilized to achieve dense packing. For verification, micromagnets are created from 5 µm NdFeB powder on 8” Si substrates, using the novel automated mold filling technique, as well as an existing manual one. Subsequent atomic layer deposition is utilized to agglomerate the loose NdFeB particles into rigid microstructures. The magnetic properties and inner structure of the NdFeB micromagnets are investigated. It is shown that the novel automated technique outperforms the manual one in major terms.
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