Powder Compaction Effect on Foaming Behavior of Uni‐Axial Pressed PM Precursors

Abstract

A study on foaming behaviour of metal powder precursors compacted by cold uni‐axial compression is reported. PM precursors have been produced by pressing the aluminum powders in a cylindrical die, under single or double compaction. Billet densification was characterized by testing the green sample cross‐sections by micro‐hardness measurements. The so obtained hardness maps were correlated to the foamed sample. The results demonstrated the fundamental role played on the powder densification by wall‐die frictions during the compaction operations. Best results were obtained from billets compacted by the double pressing action in a lubricated die. Finally, double compacted pellets were prepared from aluminum powders milled for different times and the pore morphology of the resulting foamed samples was evaluated in terms of cell circularity. The powder granulometry resulted to be an other important parameter influencing the green densification and the final foam quality.

Review: Closed-Cell Metallic Foams Produced via Powder Metallurgy

“…It is accepted that above a 92% relative density, closed pores become more dominant [83]; however, open (interconnected) porosity is still present in decreasing amounts until a few percent beyond that point, when they are totally eliminated. Interestingly, Bonaccorsi and Proverbio [84] produced aluminum foams of final densities of 0.78 and 0.75 g/cm 3 from foamable precursors that were pressed to 93.5% (lubricated singleaction compaction) and 94.1% (lubricated double-action compaction) relative densities, respectively. The authors also pointed to a relevant finding, which is that the density distribution within the foamable precursor can give rise to uneven foaming within the precursor.…”

“…As such, double-action compaction (where the top and bottom punches independently move to compress the powders in a rigid die) leads to more homogenous density distributions than single-action compaction (where only one of the punches moves to compress the powders) [83]. Another reported effect was at the powder compact/die boundaries, in which the least powder densification was observed, leading to poor foaming in these locations [84]. With the data so far, it seems that a logical step is to standardize cold isostatic pressing (CIP) (where the powders are placed and sealed in an elastomeric mold and hydrostatically pressurized with surrounding water medium) as a preferred cold compaction method, which will guarantee uniform density distributions with the foamable precursor, due to the lack of friction between powders and the die.…”

“…A number of studies investigated the PCMFP of pure aluminum [84,105]; however, stronger foams and improved foam expansions can be realized through alloying. As for aluminum alloy foams, silicon, copper, tin, zinc, and magnesium have been investigated as alloying elements.…”

“…As mentioned, in addition to pure aluminum [84,105], a large number of Al alloys have been investigated in PCMFP, including AlSi6 [109], AlSi7 [64], AlSi9 [43], AlSi10 [90], AlSi12 [111], AlSi9.6Mg1 [112], Al6061 (0.4-0.8 Si, 0.8-1.2 Mg, and <0.7 Fe) [82], AlSi11Cu4Zn2 [113] (a quaternary alloy). The ternary alloy AlSi8Mg4 was found to result in good expansion and fine/uniform pore sizes and structures, as seen in Figure 7 [109].…”

Critical Review on Engineering Mechanical Quality of Green Compacts using Powder Properties

“…Compressibility, which is based on the relative density of powder as described above, is still widely used as a straightforward descriptor of mechanical behavior of powder in industry as well as researches. For example, Bonaccorsi and Proverbio (2006) measured relative density (termed as green density) as an indicator of compaction quality for metallic powder compaction. PerezGandarillas et al (2015)'s study used relative density change to explain tensile strength gain of compacts for different granulated pharmaceutical formula.…”