Volume 49, Issue 11

Open-porous metal foams are cellular structures that can be defined, in simplified terms, as interwoven networks of metal ligaments that are surrounded by a fluid (liquid or gaseous). They are characterized by a very low relative density of usually 4 to 12% of the density of a solid composed of an identical base material. As such they have a small volume along with a very large surface area, providing for numerous interesting physical properties with resulting practical applications. A detailed description of the physical properties of these materials essentially requires a characterization of their geometric structure. Within the scope of this work, microsections of these foam structures are captured in a defined depth distance of s z ≤0,5 mm. In the course of the experiment, these microsections are software-converted to binary images. They are optimized in a way that artifact characteristics, which would have a negative effect on the analysis, converge to a minimum. Based hereon, it is now possible to generate realistic 3D models of individual microsections, allowing for a three-dimensional measurement of the material. Furthermore, binary format microsections allow for determining miscellaneous statistical characteristic values of the examined foam structures. Based on these results, it is consequently very easy to draw conclusions on the geometry of large-dimensional foam structures. When varying individual characteristic parameters, such a database also provides the possibility to generate differing geometric structures, which, in turn, provide for a sound basis in order to create scalable simulation models.

A detailed microstructural study was carried out using a field emission-scanning electron microscope with the aim of analysing the effect of surface work-hardening which occurs when milling the cobalt based hard facing alloy ”Stellite 12“. The strengthening was determined to be caused by the formation of two-dimensional structural defects in the cobalt solid-solution matrix. The assumption that a stress induced martensitic transformation of the crystalline areas of the cubic cobalt phase into thin hexagonal cobalt platelets occurs was confirmed by X-ray diffraction phase analysis. In addition, it was assumed that stacking faults would be formed. For the microscopic investigation, the channeling effect achieved by imaging using a conventional ring-shaped back scattered electron detector proved most useful. Because of needed high quality of polished sections, an optimised method of preparation was developed.

Tests have been performed on hot-dip galvanized four point bend specimens to investigate how different levels of compressive stresses and tensile loads under simultaneous thermal activation below the melting temperature of the zinc coatings affect liquid metal assisted cracking (LMAC). While the material volumes under tensile stress developed intergranular cracking, areas under compressive stress – even using a tin and lead alloyed zinc melt – did show no signs of damage. The flat specimens tested without superimposed loading did only show intergranular edge defects in the order of several grain layers when ageing during several hours in differently alloyed zinc melts. Moreover, in the context of the tests, the dominating effect of the alloying elements lead and tin could be confirmed for the intergranular crack formation. Furthermore, the particular importance of the material condition for the occurrence of the damage by LMAC could be shown by means of the progression of the crack formation in the microstructure.