Bronzes

Prealloyed atomized bronze compositions are not used extensively as base powders for compacted parts fabrication because of their nodular particle form and high apparent density, both of which contribute to poor compacted green strength. Common prealloyed compositions are 90%Cu-10%Sn and 85%Cu-15%Sn, prepared in the same manner as brass powder, except that high-purity elemental copper and tin are used. A scanning electron micrograph of an 89%Cu-9%Sn-2%Zn alloy powder is shown in Fig. 3. Typical physical properties of a bronze alloy composition are given in Table 1.

Fig. 3 Scanning electron micrograph of prealloyed, air-atomized bronze (89%Cu-9%Sn-2%Zn). 165x

Prealloyed bronze powders are also made commercially by water atomizing. Application is more extensive in Europe, where 90/10 prealloyed bronze powders are incorporated in bronze premixes for bearing manufacture. Low green strength due to high apparent density (3.2 to 3.6 g/cm3) is overcome by incorporating lower apparent density copper powders and choice of lubricants that have a less deleterious effect on green strength. Physical properties are similar to air-atomized powders, but particle morphology is different (Fig. 4). Powders contain 0.1 to 0.2% P to aid sintering.

Fig. 4 Scanning electron micrograph of a typical prealloyed water-atomized bronze powder (90%Cu-10%Sn); apparent density 3.4 g/cm3. 200x

Spherical 89/11 bronze powders are used to make filters. These are made by horizontal air atomizing and dry collection. The spherical shape is achieved by addition of small amounts of phosphorus, 0.2 to 0.45% (in the form of a Cu/15% P alloy) to the molten bronze prior to atomizing. During air atomizing, surface oxidation of atomized molten particles of bronze and brass, which cause them to solidify in an irregular shape (see Fig. 3) is prevented. The oxygen in the air preferentially reacts with phosphorus to form phosphorus pentoxide (P2O5), which is volatile at atomizing temperatures.

The spherical powders are screened to produce a number of grades, each with a narrow particle size range. An assortment of filters made from bronze powders and properties of four grades of filters are shown in Fig. 5 and Table 2.

Table 2 Properties of four grades of filter materials produced by loose powder sintering spherical powders

Particle size of spherical powder particles

Tensile strength

Recommended minimum filter thickness

Largest dimensions of particles retained, Pra

Viscous permeability coefficient, m2

Mesh range

Range in m

MPa

ksi

mm

in.

20-30

850-600

20-22

2.9-3.2

3.2

0.125

50-250

2.5 x 10-4

30-40

600-425

25-28

3.6-4.1

2.4

0.095

25-50

1 x 10-4

40-60

425-250

33-35

4.8-5.1

1.6

0.063

12-25

2.7 x 10-5

80-120

180-125

33-35

4.8-5.1

1.6

0.063

2.5-12

Source: Ref 1

Fig. 5 Assorted filters made from P/M bronze. Courtesy of Arrow Pneumatics, Inc.

Microbearings. A more recent development has been prealloyed bronze powders for microbearings. These are very small bearings, often weighing less than 1 g, used in electronic equipment such as computers, audiocassette players, and videocassette recorders. Most powders used in this application are made by diffusion alloying tin into copper powder to produce a substantially alloyed powder with particles of uniform composition and a particle shape that gives high green strength (apparent density of 2.3 to 2.7 g/cm3). As previously stated, water-atomized bronze powders have relatively high apparent density (3.2 to 3.6 g/cm3) and are usually limited to application in high-density structural parts. Figure 6 shows the particle morphology of a typical diffusion-alloyed bronze powder.

Fig. 6 Scanning electron micrograph of a typical diffusion-alloyed bronze powder (90%Cu-10%Sn); apparent

density 2.6 g/cm3. 200x

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