The Role of Oxidation During Compaction on the Expansion and Stability of Al Foams Made Via a PM Route

Abstract

The production of Al foams via a powder metallurgy (PM) route is attractive due to the ability to produce near-net-shape components. [1] This method involves mixing Al or Al-alloy powder with a foaming agent, typically TiH 2 , followed by compaction. Heating the compact above its melting point results in expansion, due to gas release from the decomposing foaming agent, leading to a closed-cell foam structure. [2] One major factor for successful foaming via the PM route is to ensure that precursors have a density close to the theoretical density, [3] that is, with little or no residual porosity after compaction. Since the majority of research and commercial activity in PM foams centres around the Al-Si system, where the compressibility of Al-Si alloy powders, or Al with Si added, is low, compaction methods involving the application of heat are employed in order that a reduction in the flow stress of the powder mixture is achieved. These methods, which include hot uniaxial compaction and hot extrusion of isostatically pressed compacts, are expensive, mainly due to higher energy requirements and reduced productivity, and contribute significantly to the prohibitively high cost of the precursor material. Precursors compacted in the optimum temperature range (400-500°C) [2] do, however, result in foams which show good expansions and structures, and it is generally accepted that compaction at elevated temperatures is a prerequisite for good foaming.Cold compaction can, however, be used to produce high density foamable precursors from soft pure Al powder. [3][4][5] By using this system, this study aims to separate the effects of exposure to elevated temperature, through heat treatment of the powders, and the application of pressure, by cold compaction. By understanding what factors in the hot compaction process dominate the foaming behaviour, it may be possible to identify alternative, lower cost processing methods.Results. Foaming behaviour: Figure 1 shows expansion plots for Al precursors that were hot compacted at different temperatures. The expansion behaviour in all cases can be seen to be typical of the foaming process, which involves pore initiation and growth, expansion to a maximum followed by collapse to a steady-state level. As the hot compaction temperature increases from 400 to 450°C, the maximum expansion also increases. When compaction is performed at 500°C, the maximum expansion increases further, to 400 %, and a delay in the rate of foam collapse is observed. Hot compaction at 550°C results in a significantly reduced foam expansion. Figure 2 shows the foaming behaviour for precursors made from either as-received Al powder or Al powder, which was heat treated for 60 min between 400 and 550°C, mixed with as-received TiH 2 and then cold compacted. A progressive increase in the maximum expansion is observed for powders heat treated at temperatures up to 500°C, with the largest maximum expansion of approximately 460 %.

Review: Closed-Cell Metallic Foams Produced via Powder Metallurgy

“…In the case of TiH 2 -and ZrH 2 -based 'zinc' precursors, some researchers used hot pressing at 350 • C [64], while aluminum was hotpressed at 450 • C. Consolidation temperatures of 400-550 • C for aluminum precursors were also reported elsewhere [91]. In this study, the hot pressing of aluminum-TiH 2 precursors at 550 • C was found to lead to poor foam expansion, partly owing to excessive aluminum oxidation [91]. In an interesting study, Jimenez et al [92] compared hot pressing AlSi11 (using heat-treated TiH 2 ) under vacuum to that carried out in air.…”

Effects of Eutectic Modification and Grain Refinement on Microstructure and Properties of PM AlSi7 Metallic Foams

“…This sample series has explicitly been included to rule out a major influence of the oxygen content of Al powders on deviations in expansion characteristics. The parameter as such is known to significantly influence this property, as has, e.g., been shown by Weigand, who determined the effective oxygen content range for pure Al alloys to be between 0.2 and 0.72 wt.% [72], while Asavavisichai reported high expansion for the same material, though in a cold rather than hot compacted state, at oxygen contents 0.24, 0.3 and 0.33 wt.%, with a decrease observed for 0.73 wt.% [73]. The fact that the oxygen content of all powders used falls into this range supports the observation that no significant differences in foamability were found between the two standard sample series without additives and different oxygen content levels.…”

Commentary on ‘Antibiotics for community acquired pneumonia in children’

“…It is indeed found that the oxide content influences stability of both Al7, 262, 263 and Pb179 foams, For less than 0.3% oxide in Al precursors poor stability was noted. More than 0.6% oxide content compromises foam expansion indirectly by its negative impact on powder compaction 7, 263, 264. In general, it is difficult to separate the direct influence of oxides on foam stability, i.e., via stabilization of liquid films, from other effects such as the quality of powder compaction.…”