Powder Manufacture

Chips produced by the multihead cutter are relatively coarse and must be ground or comminuted to fine particle sizes before consolidation into a homogeneous, fine-grained billet. Before discussing grinding and comminution, however, it is important to recognize the effects that the close-packed hexagonal structure and chemical reactivity have upon the powder-making processes and the physical and mechanical properties of the final product.

During the late 1960s and early 1970s, mechanical attritioning and ball milling were used almost exclusively to reduce machining chips to powder. Mechanical attritioning is a rotary grinding procedure similar to the grinding of corn in a grist mill. Chips enter an area between a stationary and rotating beryllium plate, where they are ground. The particles then exit the mill at its periphery.

This process results in the occurrence of basal plane cleavage. Therefore, the powder particles produced by attritioning tend to be flakes, with large, flat surfaces corresponding to {0002} basal planes. A scanning electron photomicrograph of attritioned powder is shown in Fig. 1. Ball-milled powders have a similar shape. As a result of their flat particle shape and lack of a truly random crystallographic texture, consolidated billets exhibit anisotropy of both physical and mechanical properties. During vibratory die loading for consolidation, the flat faces of many particles align, forming extended volumes with a common crystallographic feature. The basal plane corresponds to the flat faces.

Fig. 1 Scanning electron micrograph of flakelike attritioned powder. 270x

During consolidation--usually uniaxial vacuum hot pressing--the imposed stress causes these extended areas to rotate so that the c-axis poles align parallel to the stress axis. In billets made with attritioned powder, c-axis poles exist with two to three times the frequency in the longitudinal as compared to the transverse direction. The net result is substantial anisotropy of mechanical and physical properties. For example, ductility is substantially reduced in the longitudinal direction.

Impact grinding is used to minimize anisotropic effects. Figure 2 schematically illustrates the impact grinding process. Chips and powder are introduced into a high-pressure gas stream and are impacted against a beryllium target. The high speed of this pulverization process, coupled with relatively low temperature due to adiabatic gas cooling, activates the secondary and tertiary (nonbasal) cleavage systems.

Fig. 2 Impact grinding system

Resultant particles are blocky, rather than flakelike (Fig. 3). Consequently, there is a substantially reduced correlation between crystallographic basal planes and flat powder particle faces, leading to reduced anisotropy and an improvement in longitudinal tensile elongation (Ref 5). These effects are readily seen in Table 1, which illustrates the relationship between the method of powder comminution, {0002} pole density, and ductility.

Table 1 Comparison of billets using impact ground and attritioned powder

Powder lot(a) Ductility and

crystallographic orientation effects

Test

Tensile

Elongation

{0002} pole density

direction

elongation, %

T/L ratio

T/L ratio

Attritioned powder

Lot A Transverse

4.4

2.93 to 1

2.62 to 1

Longitudinal

1.5

Lot B Transverse

5.0

3.3 to 1

2.5 to 1

Longitudinal

1.6

Impact ground powder

Lot A Transverse

5.0

1.7 to 1

1.48 to 1

Longitudinal

2.9

Lot B Transverse

5.4

1.5 to 1

1.45 to 1

Powder lots A and B are produced from separate melts.

Powder lots A and B are produced from separate melts.

Fig. 3 Scanning electron micrograph of blocky impact ground powder. 270x

The transverse-to-longitudinal (T/L) basal pole density ratio and ductility T/L ratio for attrition milled powder are about 3 to 1. For impact-ground powder, these values are approximately 1.6 to 1. The isotropy of the impact-ground powder billet is improved, as is the minimum elongation value in any direction. This demonstrated correlation between crystal texture and ductility is causing attritioning to be gradually replaced by impact grinding.

The relationship of powder particle size to oxide content and grain size also affects billet properties. Beryllium obeys a Hall-Petch relationship, as shown in Fig. 4. As the size of the powder particles decreases, the oxygen content of the powder increases due to the increase in specific surface area. The mean free path between oxide particles also is reduced. Because oxides are a primary grain-boundary pinning agent, fine particle size enhances higher oxide content, finer grain size, and higher strength. The strength imparted by fine grain size and the potential embrittling effect of excessively high oxide content must be balanced against each other. Characteristics of powder used to produce commercial beryllium billets are given in Table 2.

Table 2 Characteristics of powder used for commercial beryllium billets

Grade

SP-65B

SP-200F

IP-70

IP-220

Composition, %

Beryllium assay, min

99.0

98.5

99.0

98.0

Beryllium oxide, max

1.0

1.5

0.7

2.2

Aluminum, ppm max

600

1000

700

1000

Carbon

1000

1500

700

1500

Iron

800

1300

1000

1500

Magnesium

600

800

700

800

Silicon

600

600

700

800

Boron

2

Cadmium

2

Calcium

100

Chromium

100

Cobalt

10

Copper

150

Lead

20

Lithium

3

Manganese

120

Molybdenum

20

Nickel

300

Nitrogen

300

Silver

10

Other metallic impurities(a), ppm max

200

400

400

400

Particle size

98% -325 mesh

98% -325 mesh

98% -325 mesh

98% -325 mesh

Each; determined by normal spectrographs methods

Each; determined by normal spectrographs methods

Fig. 4 Hall-Petch diagram of strength versus grain size of vacuum hot pressed beryllium with intermediate purity. Dashed line is yield strength values from Ref 6.

Gas Atomization. Inert gas atomization has been used to produce beryllium powder in semi-production quantities (Ref 8). The major advantage of this process over conventional impact ground powder is the increased isotropy of the consolidated product. This can be important in optical systems, in which isotropy is a primary factor in component performance, particularly in cryogenic systems wherein thermal contractions can cause distortion in anisotropic components. Economic benefits may also be realized if this technique is brought to production scale. As yet, the use of inert gas atomization has been confined to the advanced development stage, and no specification has been written for this material. The chemistry of a typical heat of inert gas atomized beryllium powder compares as follows with S-200F:

Impurity

Spherical powder, wt%

S-200 F, wt%

Beryllium oxide

0.32

1.2

Carbon

0.07

0.15

Iron

0.09

0.10

Aluminum

0.03

0.10

Silicon

0.02

0.06

Magnesium

0.01

0.06

0 0

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