Towards Reaching the Theoretical Limit of Porosity in Solid State Metal Foams: Intraparticle Expansion as A Primary and Additive Means to Create Porosity

Abstract

Metallic foams and porous metal structures are valuable for their unique characteristics such as high specific strength, energy absorption at constant crushing load, efficient heat transfer and acoustic properties, all of which can be tailored by controlling the porosity. [1][2][3] Many techniques for generating metal foams exist, but the vast majority of metal foam production is through liquid state processes such as the melt processing of aluminum by gas injection or decomposition of a dispersed foaming agent. [4] Aluminum has dominated the metal foam industry due to its low melting temperature and relative stability in air. [5] Reactive metals and those with higher melting temperatures require special processing, usually through solid state techniques. [3,[6][7][8][9][10][11] The solid state foaming of metals by gas entrapment [12,13] typically uses a two-step process: (i) entrap gas within the interparticle voids during powder consolidation and (ii) heat to expand the entrapped gas such that the internal pressure exceeds the yield strength and enables plasticity or creep to increase porosity. [14] While some powder metallurgy (PM) processes can exceed 85% porosity (e.g. using a polymer foam as a fugitive template [7] ), it is more common for PM processes to produce porosity levels between 20 and 40%. [5,14] In fact, Elzey and Wadley [14] calculated that the limit of the solid state foaming process by gas entrapment is %65% porosity, even under ideal, superplastic conditions. Experimentally, this method has produced foams with porosity only as high as 53%, [15] but porosity has typically been limited to %40%. These relatively low porosity levels (compared to liquid state processes) constrain the applications of metal foams produced via solid state foaming. To extend the capabilities of solid state foaming, we have developed an additive means of creating porosity by intraparticle expansion.The current limitation of solid-state expansion by gas entrapment is dictated by voids formed between solid particles during consolidation. In this model, the initial gas pressure and foaming temperature determine the resulting porosity. However, if the expanding gas is not limited to just that which is trapped between particles, but is also located within particles, solid state foaming may assume a character more akin to expandable polymers which foam from the constituent pellets. This concept is a paradigm shift in terms of the solid state foaming process and the achievable levels of porosity. On the basis of this simple, powder feedstock expansion, there is universal application to powder metallurgy methods for foaming. In this work, we examine the microstructure and morphology of a Cu-Sb alloy that expands to porosities of close to 40% within individual particles, resulting in absolute porosity of 69% in sintered samples.The Cu-Sb alloy powder was formed by mechanically alloying Cu and Sb (Alfa Aesar, 99.9 and 99.5%, respectively) at cryogenic temperature (À196°C) for 4 h using a modified SPEX 8000M Mixer/Mill. The elemen...

Solid State Foaming of Nickel, Monel, and Copper by the Reduction and Expansion of NiO and CuO Dispersions

“…The grain size of these samples was examined using ion channeling contrast (ICC), and the results are shown in Figure 4. As found in earlier work, [33,34] the grain size is small, with the majority of grains being less than 1 mm in Ni ( Figure 4A) and Monel ( Figure 4C), but Cu is comprised of areas of small, sub-micron grains mixed with others between 1 and 2 mm ( Figure 4E). Since grain size determines the matrix strength, and thereby resistance to pore expansion, a quantitative assessment is valuable to understand the foaming mechanisms.…”

“…This may be useful for applications where processing before foaming (e.g., hot compaction, extrusion, sintering, etc.) requires temperatures above 600 C (i.e., the foaming temperature with maximum porosity in previous work [33,34] ). In all instances, micron-scale pores and sub-micron grains were created that may be uniquely useful to thermal and electrical applications.…”

“…All FIB sections were taken on the edge of compacted and annealed samples. Pore size and shape were determined by processing images in ImageJ software as described elsewhere, [34] and at least 200 pores were included for each material. The pores were analyzed for area, A, equivalent circular diameter, D (D ¼ ffiffiffiffiffiffiffiffiffiffi ffi 4A=π p ), perimeter, P, circularity (4πA=P 2 ), roundness (4A=πa 2 , where a is the length of the major axis), and solidity (A/A convex , where A convex is the area of a convex hull enclosing the pore).…”

“…This method, referred to as intraparticle expansion or Additive Expansion by the Reduction of Oxides (AERO), has only been demonstrated in copper, where copper oxides are distributed within the matrix by mechanical alloying/mixing and are subsequently reduced under hydrogen to form pores throughout the metal. [31][32][33][34] Cu and CuO have beneficial properties for this process, including the low strength and high ductility of the Cu matrix at elevated temperature and the modest reduction temperature of CuO in a hydrogen atmosphere. In addition to the utility of Ni alloys, the Ni-Cu system provides a useful test case for metal/oxide pairings.…”

Surpassing the Theoretical Limit of Porosity in Conventional Solid-State Foaming: Microstructure Characterization of Length Scales in a Copper Metal Foam

“…46 The relatively simple technique involves only 2 steps: milling the powder and then annealing it in a reducing atmosphere. The working hypothesis is that oxides within the particles are reduced during annealing, creating steam that expands into microscale voids.…”