Multi length-scale characterisation of microstructure/geometry interactions for tailoring properties of open-cell Al alloy foams

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Metal foams are promising candidates for energy absorption and light weight construction applications. On the μm- to the cm-length-scale, phases, struts and cells are hierarchical structural elements. Thus, the base material and its microstructure, e.g. the shape and amount of second phases, as well as geometry, topology and size of struts and cells, determine the mechanical properties. Here, size effects play an important role, both regarding solidification and mechanical properties. Control of these variables is the basic condition for the development of a metal foam with well defined quasi-static and dynamic properties We aim at modifying the hierarchical structural elements and their interactions under consideration of metallurgical and geometrical aspects. Our scientific goal is the understanding of the mechanisms acting on the different length-scales, how these ineract between the hierarchical structural elements, and their influence on the mechanical properties, on the example of open porous Al-alloy foams. The microstructure of the mm-sized struts will be adjusted by adaptation of the alloy composition, the content of modifying elements and through heat treatments. Here, we will consider size and geometry effects both of precipitates and of the dendrite arm distance. The geometry, length and thickness of the struts will be varied to change the stiffness of the struts, and the cell topology of the foams will be modified by introduction of closed cell walls or additional struts. The metal foams will mainly be produced by the block moulding casting process, on the basis of foamed polyurethane or additive-manufactured wax models. Additionally, metallic open-cell foams, produced in-house by additive manufacturing, as well as commercially available sintered open-cell and closed-cell foams, will be included in the investigations. The microstructure and the development of mechanically induced damage will be observed on the length-scales of the foams, cells and struts, by a variety of high resolution 2D and 3D imaging methods, such as SR-μCT, SEM or TEM. The results will help us better understand whether and to what extent metallurgical and mechanistic knowledge of bulk Al-alloys can be transferred to the mm-sized struts and the foams themselves. The results will be summarised in a model describing the microstructure/property-relationship of open-cell metal foams for an application-adapted modulation of the deformation behaviour.