Manufacturing of metallic lightweight materials with cellular structures from particulate starting materials

Hertha Firnberg Position T 52 Lightweigt cellular metallic materials Susanne STROBL 29.06.1999 The project aims at manufacturing porous metallic lightweight materials with cellular structure wich are increasingly interesting for e.g. the transportation industry. Current approaches either start from metal powders and consolidate them to form porous structuresor start from compacts, heating them above the melting point and forming foams by internally setting free gaseous components. In the present project, formation of cellular bodies is to be attained by first manufacturing hollow powder particles of define size, shape and surface rugosity which are then consolidated to form materials with favourable strength-to-weight ratio - which can be adjusted by varying the wall thickness/diameter ratio - and probably attractive damping properties. Manufacturing of hollow powder particles will be done following a chemical process that has been in principle developed for Cu base materials within FWF project P1 1014 but seems to be applicable for numerous other materials. Characterisaztion of the materials obtained will result in well defined property profiles wich then will be used for assessing possible technical applications, in particular in the automotive and aerospace industries.

Powder metallurgical techniques including a heat treatment in protective atmosphere (sintering) of metal powders to obtain consolidated parts can be used for the production of metallic cellular materials. Typical for these lightweight materials are their low density, high porosity (ranging from 40 to 98 vol%) and large specific surface. In this work two variants of highly porous metals have been produced, their manufacturing was optimised and properties such as density, structure, and energy absorption characteristics were determined and the fracture surfaces were analysed. In the first approach, tin bronze hollow spheres have been produced via chemical reactions. It was shown that e.g. hollow Cu particles can be obtained simply by cementation of Cu on Fe particles. The hollow spheres were tin coated, filled into a mould, and then sintered. This process is called loose powder sintering or gravity sintering. The properties of this bronze bodies were determined and described. The relative densities - the quotient of measured density and solid density - range between 0.18 - 0.34, mainly as a function of the sintering temperature. The structures of these cellular materials, examined by light- and scanning electron microscopy, was shown to contain uniform spherical closed pores inside the cells and an interconnected pore network between the sintered bronze spheres. Static compression tests were performed to obtain information about the energy absorbing characteristics. The particular microstructure of the materials results in a high degree of energy absorption, in particular at higher density levels. Through fractographic analysis the ductile character of this material was confirmed. This technique also yielded information about the quality of the sintering contacts, which depends on the sintering conditions and is important for the strength of a material. As a second manufacturing route, sponge-like materials were used as a temporary support structure (precursor) for producing highly porous open-celled metallic foams. A polymer sponge structure was soaked with a slurry containing the metal powder and the slurry vehicle which was an organic liquid. In an alternative process the metal powder and the polymer precursor were mixed and afterwards foamed together. To remove the polymeric precursor, the sponge is heated to a temperature sufficiently high to decompose and evaporate the organic material. The metallic residue is further heated in a reducing atmosphere at a still higher temperature to obtain a sintering effect. The relative densities of iron, copper, bronze or nickel foams produced range between 0.03 and 0.12, depending on the amount of metal powder and the sintering temperature. The structures of this high porosity metallic foams were investigated by light and scanning electron microscopy and can be described as open celled with porosity from 88 to 97%. The cells were of varying size, depending on the polymer sponge used as precursor. The possible applications include insulating, energy and sound absorbing structures for automotive and aerospace applications, dust and fluid filters, catalyst supporters etc.