Investigation and comparison of aluminium foams manufactured by different techniques

Cellular metals offer a large potential for industrial application. Nevertheless, besides the costs, there are a lot of technical improvements necessary in order to gain more widespread use. The reasons for the lack of applications, since now, are multiple and they depend from the physical properties of foams that are still not good enough and not completely experienced, and from the insufficient spreading of research results to designers. To fulfill parts of these requirements, this work investigates in detail three foam production processes, studies the effect of modifications to standard manufacturing methods, clarifies the influence of process parameters on the structure of foams, characterises relevant properties, and finally discusses the difference between powder or melt routes. During experimentation hundreds of samples were realised to reach these objectives. Moreover mechanical, physical, and microstructural properties of the produced foams have been studied using various techniques including compression testing, scanning electron microscopy, visual inspection, and software measurement tool. The three methods for manufacturing aluminium foams, applied in this investigation, are named TiH2, SDP and MGI. Two of them (TiH2 and SDP) start from powdered aluminium as row material, while MGI starts from melt aluminium. Moreover TiH2 and MGI methods let the production of closed cells foam while SDP of open cells foam named, more correctly, metal sponge. The term ‘‘foam’’ was firstly reserved for a dispersion of gas bubbles in a liquid. The morphology of this type of foams can be preserved by letting the liquid solidify, thus obtaining what is called a ‘‘solid foam’’. When speaking of ‘‘metallic foams’’ one generally means a solid foam. The liquid metallic foam is a stage that occurs during the fabrication of the material. For metallic systems it is possible to define the following classification according with Babcsan et al. (2003): • cellular metals are materials with a high volume fraction of voids made up of an interconnected network of walls and membranes; • porous metals have isolated spherical pores and a porosity level of usually less than about 70%; • metal foams are a soubgroup of cellular metals usually having a polyhedral cells with closed or open cells (even if there are no membranes across the faces and the voids are inteconnected, the better definition is metal sponges).