Alloy development and studies about the gas nucleation phase for metal foaming

The motivation to focus on aluminium alloy compositions for metallic foams made from powder metallurgical precursors seems to be obvious: The alloy chosen strongly influences the properties of the foam, like tension strength, ductility or corrosion resistance. But more interesting is the fact that the foaming behavior and the quality of the foam structure differs from alloy to alloy. In the past, a number of well-known technical alloys, sometimes slightly modified, turned out promising and dominated the research and industrial activities.

As there were still certain remaining problems, especially as the Aluminium Foam Sandwich (AFS), an important semi-finished metal foam product, sets some additional requirements. Therefore a number of established and experimental alloys was screened for there foaming behavior and surprisingly alloys in a region of the AlMgSi(Cu)-System that had no technological usage before turned out promising.

More detailed research on a group of this new and some established alloys than focused on the mechanisms leading to the differences in foaming behavior. The focus was the characterization of the alloying elements influences at the beginning of the foaming process, when the pore structure evolves. It turned out that for the in detail observed alloys the foaming process can be described by characteristic phases, each phase with one dominating influence parameter, that allowed us to understand the effect of the alloys composition on the foaming process.

As the powder metallurgical precursor material is in fact not a homogeneous alloy but a more or less dense bulk of single particles, the precursor compaction step was also included in the work summarized in the field of alloys. Therefore a study of the impact of the compaction conditions on the expansion capability was observed for one alloy with excellent foaming behavior. This study lead on the one hand to a better knowledge about the interaction of the different technological process parameters and on the other to the discovery of the previous unknown and counter-intuitive effect of overcompaction.

Last but not least, one of the alloys developed at HZB in corporation with the former company ALM GmbH (now part of Alulight International) became alloy of the choice for AFS foam cores, resulting in a corporate patent application.