Powder Compaction Methods

Powders are compacted under high pressure by various methods (Table 1) where the application of pressure packs the powders and reduces porosity. Unlike shaping methods, compaction techniques cause particle deformation. Many compaction methods are used, but the most prevalent method for P/M parts production is uniaxial compaction in a rigid die. This method is cost effective with relatively straightforward tooling.

However, compaction methods inherently produce parts with density variations that result in dimensional variations of the final sintered part. This occurs because the transmittal of stress through a mass of powder particles is not uniform, even in isostatic pressing. In the case of compaction in a rigid die, the uniformity of stress transmitted through the powder mass and the density distribution in the green compacts produced in rigid dies are much less uniform than in isostatic compacting. This can be understood by considering flow around corners. When a liquid is subjected to hydrostatic pressure inside a confined, rigid die, the stress transmitted by the liquid upon the interior surfaces of the die is uniform, regardless of whether the liquid must flow around corners. This is not so when a powder is pressed; it flows only in the direction of the applied pressure and not around corners. This phenomenon is illustrated in Fig. 3, which schematically shows the compaction of powder in a die with a sidearm. If pressure is applied only to the top punch, the powder is compacted only in the vertical section of the die, but remains loose in the horizontal section. Compaction in the sidearm is obtained when pressure is applied to both the top punch and the side punch.

Fig. 3 Density distribution of compacts pressed in dies with a sidearm Uniaxial Die Compaction

In uniaxial compaction of metal powder, pressure is applied to the powder only with punches that move in the vertical direction. Compacting metal powder parts in rigid dies, where the punches move only in the vertical direction, imposes limitations on the shape of the parts that can be readily produced. Parts with re-entrant angles and with holes at an angle to the vertical direction generally are not produced by compacting.

Another consequence of powder flowing only in the direction of the applied pressure during compaction is that, when parts with different levels of thickness in the direction of pressing are compacted with only a single lower punch, they develop different green densities in the different levels. Individual punches for each level are necessary for a more uniform density. These punches must travel in such a way that the ratio of the height of the loose powder to that in the green compact (the compression ratio) is the same for all levels. This type of punch arrangement is illustrated in Fig. 4. Pressing from both top and bottom also reduces density variations (Fig. 5).

Fig. 4 Density distribution in a two-level compact

Si H

Pressed from top and bottom

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