How Do Particle Size and Size Distribution Influence the Compaction Process in Powder Metallurgy?

May 16, 2019

Scientific precision shapes Powder metallurgy processing. While a post about compaction basics could get away with describing the process in a few simple lines of text, these rudimentary descriptions would fail to capture the complexities that arise as a powder pressing operation proceeds. Right now, with a precision-based approach foremost, we’re going to delve deep into particle size mechanisms to see how those particle proportions affect the results of P/M procedures.

Particle Size Differences Cause Compressibility Changes

As the powdered metal compacts, the pressing phase commences. There’s a relationship linking the two P/M forming factors during the compaction. As the average size of the millions of metal particles shrinks, there’s a subsequent reduction in compaction flowability. Similarly, the compressibility of the Green Compact also drops. Think of it, the spheroidal grain walls are moving closer together, so there’s less space between those metal bits. With less volume to compress, there’s less empty space to squeeze out of the mixture. Moreover, the powdered metal becomes a less flowable, less fluidic construct because there’s no room for the individual grains to pass each other.

Evaluating The Compression-Resistant Upshots

Less flowable and even less compressible, the smaller particle size seems to be causing fluid-dynamic challenges. However, as the less mobile particles anchor themselves, the space-starved grains become stronger. The densified compaction mix exhibits greater material durability and green strength. As a further size shrinking perk, the finer particle blend also gains more Transverse Rupture Strength (TRS). In short, although the morphology and mixability of the semi-fluidic mass drop as the particles shrink, the green strength and other application-relevant physical characteristics increases.

The Effects Of Size Distribution Changes

In-depth studies show a marked increase in intergranular friction when the particle distribution pattern runs without any process management. Particle distribution equations, as used by the processing engineers, seem to imply a direct correlation between particle averageness and pressing consistency. The larger particles press and amalgamate while the smaller grains fill interstitial voids. By leaning towards a grain distribution curve that favours larger particles, a compacted mix loses structural integrity. Going in the opposite direction, smaller grains densify and won’t flow. On equalizing the size distribution spread, smaller and larger grains support one another while reinforcing a stronger network of capillary voids.

Those voids perform as lubricant-bearing pores. Therefore, when higher green strengths are ordered, smaller metal grains are produced by the metal atomization stage. If a more porous, not quite as strong metal structure is required, well, the opposite approach is ordered. This time, with larger grains pressing and mixing, a more porous metal is produced after the compaction operation has concluded.

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