Experimental investigation of the compaction process of powder materials

Conclusions

• 1. An improved technique is proposed for determining, by means of resistance strain gauges, coefficients of lateral pressure and the external friction of powder against the die walls during pressing.

• 2. It is shown that, for ductile metals, the coefficient of lateral pressure (ξ) is not a constant magnitude. With rise in compaction pressure, it increases, although less rapidly than would be expected from direct proportionality to the relative density of the compact. For low-ductility metals (W) and brittle materials (WC), ξ increases slightly at low pressureş, but above ∼2 tons/cm² it becomes virtually constant.

• 3. The coefficient of lateral pressure for a fine-grained, oxidized powder of a given metal is slightly less than that for a coarser, unoxidized powder, because of increased interparticle friction.

• 4. The use of an inactive lubricant, whether applied to the die set walls or, in the form ofa thinlayer, to particles with a well-developed, uneven surface, has no effect on the value of ξ. This is evidence that interlocking constitutes the main mode of interaction between particles of such powders.

• 5. The coefficient of external friction (μ) diminishes with rise in pressure at low compaction pressures, but remains constant at high pressures (above 1–2 tons/cm² for low-ductility and brittle materials).

• 6. The product of coefficients ξ·μ remains approximately constant (with deviations of up to ∼ 5% for powders of ductile metals and up to: ±3% for powders of low-ductility and brittle materials) over a wide range of pressure. In view of this, the equation In N-In N_h=4ξ·μ·h/D, which characterizes the drop in compaction pressure over the compact height, must be regarded as approximate within the limits indicated, over an extensive pressure range.

The equations δN_fr=p·ξ·gm·π·D·H and δN_fr/N=4ξμH/D, which have been proposed in the literature for describing the drop in compaction pressure over the compact height, are inaccurate in that they employ the high value of pressure p on the top compact face instead of the lower, true value P_m (mean pressure over the compact height).