Porous metals and hybrid materials

Current research topics

Investigation of the influence of reinforcing fibres on the mechanical characteristics of aluminium foam (PM route).

Analysis of the influence of ball mill parameters on the foaming behaviour of aluminium.

Porous metals

Porous metallic materials today cover all size scales. Thus, nanoporous materials and coatings up to open or closed porous materials with cells or pores of several millimetres in size are produced. Materials or semi-finished products made of metal fibres are also counted among the porous materials today. In order to further optimize the properties of metal foams, especially the mechanical properties, for future fields of application, among other things the alloy systems have to be analyzed with regard to their composition and the foaming process, i.e. a systematic optimization of the manufacturing process has to be carried out with regard to powder composition, powder particle size, degree of compaction, pre- and post-treatment of the powders and temperature regime.

Aluminium foams can be produced in different ways. The classification of the manufacturing methods results from the aggregate state of the starting material. The foams can be produced from steam or gas, from molten metal, from powders or from ionic solutions. Despite the different methods, there are only two fundamentally different processes which form the basis of production /1/: - the molten metal production and - the powder metallurgical production.

The processes can further be differentiated according to the intrinsic formation of the pores and prefabricated pores. According to the first principle, the pores form themselves by the formation of gas bubbles based on different physical principles. The formation of pores is subject to statistical laws. The pores have a structure with a random character, which depends on the manufacturing conditions. This leads to a comparatively high scattering of the material properties. Furthermore, the parameters relative density and average pore size are interdependent and cannot be adjusted separately. /1/

Books

/1/   Hipke, T.; Lange, G.; Poss, R.: "Taschenbuch für Aluminiumschäume";
        Aluminium-Verlag (jetzt Beuth-Verlag); 2007; ISBN 978-3-87017-285-5
/2/   M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson and H.N.G.
       Wadley: "Metal Foams: A Design"; Butterworth-Heinemann;
       Boston; 2000; ISBN 0-7506-7219-6
/3/   H.-P. Degischer: "Metallschäume";
       WILEY.VCH Verlag GmbH & Co. KGaA; Weinheim; 2001;
       ISBN-10: 3-527-30280-8
/4/   H.-P. Degischer, B. Kriszt: "Handbook of Cellular Metals";
       WILEY.VCH Verlag GmbH & Co. KGaA; Weinheim; 2002;
       ISBN 3-527-30339-1
/5/   Gibson, L.J.; Ashby, M.F.: "Cellular solids: Structure and properties -
       Second edition"; 2001; ISBN 0-521-49560-1
/6/   Körner, C.: "Integral Foam Molding of Light Metals";
       Springer-Verlag Berlin Heidelberg; 2008; ISBN: 978-3-540-68838-9
/7/   Nihad, D. (Herausgeber): "Metal Foams: Fundamentals and Applications";
       DEStech Publications, Inc; 1. Auflage; 2012; ISBN-10: 1605950149
/8/   P. Stevenson: "Foam Engineering: Fundamentals and Applications";
       John Wiley & Sons; 2012; ISBN 978-0-470-66080-5

Composite materials

According to Ondracek, composites are... "...multi-phase, (heterogeneous) at least two-phase and macroscopically homogeneous (microscopically quasi-homogeneous). Their components - or phases - do not usually belong to a single main group of materials. They may be combinations of metallic and ceramic, ceramic and polymeric (non-metallic), polymeric (non-metallic) and metallic components (phases) or may also contain metallic and ceramic and non-metallic components (phases). Macroscopically inhomogeneous composites of components or phases which belong to different material groups are therefore not composite materials..., but material composites."

(Ondracek, G.: Verbundwerkstoffe: Phasenverbindung und mechanische Eigenschaften, Deutsche Gesellschaft für Metallkunde, 1984)

                       ... the question is not "are composite materials good?
                                                              ... but...
                                      "Are we intelligent enough to use them?"

Nowadays, the term "hybrid" is increasingly used, also as "hybrid composite" or "hybrid fabric". Zeilinger drew up a first definition for this in 1985:

Hybrid materials:
   a. are compounds of several components which belong to different
       material groups;
   b. are a combination of metallic and ceramic, ceramic and polymer or polymer and
       metallic components; 
   c. show a structure as a layered composite with at least two materials of different
       main groups, which is macroscopically homogeneous, but microscopically
       quasi-homogeneous or heterogeneous.

(Zeilinger, H.: Verbund- und Hybridwerkstoffe - Definition, Zusammensetzung, Anwendung, In: Konstruieren mit Verbund- und Hybridwerkstoffen, Tagungsband, VDI Verlag, 1985)

Books

/1/    Kainer, K.U. (Herausgeber): "Metallische Verbundwerkstoffe";
        WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; 2003; ISBN 3-527-30532-7
/2/    Nestler, D.: "Verbundwerkstoffe - Werkstoffverbunde: "Status quo und
        Forschungsansätze" Habilitation; TU Chemnitz; 2014;
        ISBN 978-3-9446400-12-9
/3/    Ehrenstein, G.W.: "Faserverbund-Kunststoffe: Werkstoffe, Verarbeitung,    
        Eigenschaften"; 2. Auflage; Hanser Verlag; München; 2006; ISBN 3446227164
/4/    Krenkel, W.: "Keramische Verbundwerkstoffe"; WILEY-VCH; 2003;
        ISBN 3-527-30529-7
/5/    Taha, M.A.; El-Sabbagh, A.M.; Taha, I.M.: "Trends in composite materials and
        their design: Special topic volume with invited peer reviewed papers only";
        Key engineering materials; Trans. Tech. Publ.; Stafa-Zürich; 45 (2010)