The use of dolomite as foaming agent and its effect on the microstructure of aluminium metal foams—Comparison to titanium hydride

Abstract

Abstract:In this paper dolomite MgCa(CO 3 ) 2 a naturally occurring mineral was demonstrated to be an effective foaming and stabilizing agent for aluminium with several notable advantages relative to the currently used titanium hydride foaming agent. Characteristic cell structures and microstructural features of foams produced with a dolomite foaming agent are examined and the properties of dolomite based foams produced in a one step process are compared with those produced using titanium hydride based process. The most notable structural feature of dolomite based foams is a smaller cell size and thinner cell faces. Foaming with MgCa(CO 3 ) 2 also gives rise to a marked increase in the stability of molten foams with a large range of foaming temperatures possible, and an almost complete absence of melt drainage even with extended foaming times. Many of these properties are attributed to the cell surfaces being covered by a thin oxide film formed during the foaming process.

Morphology of Aluminum Alloy Foams Produced with Dolomite via Partial Sintering of Precursors

“…This fact is not surprising considering that, for a given melt viscosity, the amount of gas that can be efficiently retained and properly dispersed during foaming is usually the same. More gas released also increases the probability that the gas bubbles will coalesce and create a less uniform porous structure [29], a fact clearly observed in Figure 7b for Al–10Ni foams with 10 wt.% dolomite. These findings are in agreement with the study conducted by Barnhart et al [17,54], in which it was demonstrated that there is an optimal content of foaming agent above which the maximum expansion of the foams becomes independent.…”

“…A potential blowing agent for aluminum foam production that is both safer and inexpensive is dolomite. In addition to the attractive price ($20–30/ton) and abundance, dolomite, which is a double carbonate of calcium and magnesium (CaMg(CO 3 ) 2 ), can act simultaneously as a foaming agent and as a foam stabilizer [29]. It can act as a foaming agent owing to the release of CO 2 gas over a wide temperature range (600–800 °C), and as a stabilizer due to the oxides formed in situ (MgO, CaO) and/or undecomposed CaCO 3 .…”

“…Among the published data, several investigations are prominent. One of the first studies to examine dolomite as a blowing agent was carried out by Papadopoulos et al [29,33]. The aluminum foams produced from melts presented small (<1 mm), irregular pores, and morphological defects such as big voids and collapsed cells.…”

“…Despite these encouraging findings which report similar/improved microstructures and properties of dolomite-based Al foams when compared to their TiH 2 -based counterparts [5], dolomite is still being used only to a limited extent. It appears that the general agreed upon culprit remains the mismatch between dolomite decomposition temperature and aluminum melting point [29]. Notable efforts [39] were explored recently in order to reduce the temperature interval between these two events.…”

Method of Preventing Shrinkage of Aluminum Foam Using Carbonates

“…Thus, aluminum foam produced using carbonates has a finer cell structure than that produced using TiH 2 . The effect of oxide layer on foam stabilization has been reported from other researchers [3][4][5]. On the other hand, foam by TiH 2 are stabilized by solid particles, like SiO 2 [6].…”