Prealloyed Bronzes

Sintered bronze alloys are rather uncommon in powdered metal usage. This is primarily due to their relatively high cost compared to low-alloy steels. However, sintered bronze properties can be advantageous for non-magnetic applications that require very good corrosion resistance, good mechanical strength, and excellent ductility.

Prealloyed 80Cu-9Sn-2Zn bronze powders with a select lubricant are intended for the fabrication of high-density P/M structural components. Unlike many elemental copper-tin premixes, the sintering of prealloyed bronze results in the attainment of high sintered densities (85 to 90% of theoretical ), that provide correspondingly high strengths and hardnesses.

Sintering properties. Prealloyed bronze powders are relatively easy to work due to the excellent ductility obtained on sintering. For example, an 89Cu-9Sn-2Zn bronze pressed at 414 MPa (30 tsi) to 85% of theoretical density, exhibits >30% elongation thus allowing for a substantial degree of cold working. Compared with sintered brass, bronze powders reach higher yield strength and hardness levels when compacted under similar conditions. For example, pressing at 550 MPa (40 tsi) results in hardness of 45 HRB for an 89Cu-9Sn-2Zn-2Fe bronze and of 90 HR for a 70Cu-30Zn brass, respectively (after 30 min sintering at 840 °C (1550 °F) under dissociated ammonia).

The addition of 2% of a select grade iron in he composition improves the sintered structure, which has a more uniform grain size and results in comparatively higher yield strength and hardness. Figure 34 shows the microstructure of a prealloyed bronze. Sintering temperature and atmospheres of prealloyed bronzes are similar to those of bronze premix. Figure 35 shows the dimensional change and transverse rupture strength of prealloyed bronzes as a function of green density.

Fig. 34 Typical microstructure of sintered prealloyed 89Cu-9Sn-2Zn-2Fe bronze, etched in K2Cr2O7 solution. 350x

Fig. 35 Properties of prealloyed bronzes. Sintered 30 min at 840 °C (1550 °F), under DA
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