Nonferrous Materials

Many lubricants frequently used in iron-base powder systems have not been effective in all copper-base prealloyed systems. Without lubricant system development, primarily lithium stearate, it is unlikely that brass and nickel-silver P/M parts would hold the strong market position that they do today (Ref 3). The use of lithium stearate overcame many early sintering problems and provided consistently high properties in brass P/M parts.

Lubricants such as zinc stearate, lithium stearate, stearic acid, and Acrawax have been used successfully with conventional premixed bronze materials (90%Cu-10%Sn). A recent study (Ref 4) comparing the two stearates and Acrawax revealed slightly better compressibility and lower ejection pressures with zinc stearate. Furthermore, green strength increased with decreased amounts of lubricant, and Acrawax provided superior green strength zinc and lithium stearate.

Bi-lubricant systems, such as lithium-zinc stearate, provide the advantages of high physical properties and relatively clean surface appearance. Lithium stearate and lithium-zinc stearate lubricant additions in the range of 0.5 to 0.75% are used most often in brass and nickel-silver parts. Although waxes are excellent for maintaining green strength, their use results in lower apparent densities, higher briquetting pressures, and lower sintered strengths in brass parts.

Frequently, a bi-lubricant system of zinc stearate and stearic acid at 0.75 wt% is employed for conventional bronze premix blends; these two lubricants complement each other. Stearic acid has excellent lubricating characteristics, but its spherical morphology tends to promote segregation, therefore, the irregular shape of the zinc stearate particles provides a supportive interlocking effect. Acrawax often is used alone to increase green strength, although it can lower the apparent density and retard the flow of bronze premixes.

Lubricants play a unique role in the production of copper P/M parts for high electrical conductivity applications (Ref 5). Lithium stearate allows the highest electrical conductivity among lubricated compacts, as illustrated in Fig. 14. Compacts pressed at 275 MPa (20 tsi), using a synthetic wax lubricant, have conductivities of about 74% IACS; similar compacts produced with lithium stearate exhibit conductivities of about 86%. The effect of the quantity of lubricant on electrical conductivity of copper P/M parts is shown in Fig. 15. The electrical conductivity reaches a maximum with approximately 0.6% lithium stearate in compacts pressed at 275 MPa (20 tsi) and with approximately 0.5% at 415 MPa (30 tsi).

Fig. 14 Effect of lubricant on electrical conductivity of copper P/M parts. Compacting pressure: (a) 276 MPa (20 tsi); (b) 414 MPa (30 tsi)
Fig. 15 Effect of lubricant quantity on the electrical conductivity of copper P/M parts

Synthetic organic waxes, which have low moisture and ash content, are employed in the production of aluminum P/M premixes (Ref 6). The addition of 1.5% lubricant (nominally) permits the consistent production of high-density aluminum P/M parts with fine surface quality and eliminates tool problems with seizing and cold welding. High sintered properties are achieved because these lubricants leave virtually no ash to interfere with particle bonding during sintering.

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