Consolidation

Vacuum hot pressing is the traditional method by which beryllium components have been fabricated. Other methods, such as cold pressing and sintering and vacuum sintering, have also been used successfully to consolidate beryllium powder.

Vacuum hot pressing (VHP) typically is carried out at 1000 to 1200 °C (1830 to 3630 °F) and at a compacting pressure of approximately 8.3 MPa (1200 psi) or less. This process yields a product that is essentially 100% dense. Billets up to 183 cm (72 in.) in diameter have been manufactured by this technique. Properties of commercial vacuum hot pressed block are given in Table 3.

Table 3 Properties of beryllium hot pressed block

Property

Grade

S-200F

S-65B

I-70A

I-220B

Chemical composition, %

Beryllium assay (min)

9S.5

9S.5

99.0

9S.0

Beryllium oxide (max)

1.5

1.0

0.7

2.2

Aluminum (max)

0.1000

0.06

0.07

0.10

Carbon (max)

0.15

0.12

0.07

0.15

Iron (max)

0.1S

0.12

0.10

0.15

Magnesium (max)

0.06

0.0S

0.07

0.0S

Silicon (max)

0.06

0.06

0.07

0.0S

Other metal impurities (max), %

0.04

0.04

0.04

0.4

Powder type

Impact ground

Impact ground

Impact ground

Impact ground

Minimum density, g/cm

1.S4

1.S4

1.S4

1.S4

Theoretical density, %

99

99

993

99

Average grain size (max), -'-'m

25

15

20

25

Ultimate tensile strength (min), MPa (ksi)

S24 (47)

2S9.6 (42)

2413 (S5)

S79 (55)

Yield strength, 0.2% offset (min), MPa (ksi)

241 (S5)

206.S (SO)

1723 (25)

275 (40)

Elongation (min) in 25 mm (1 in.), %

2

S

2

2

Note: Pressing (billet) sizes can range from 18 to 183 cm (7 to 27 in.) in diameter and 15 to 168 cm (6 to 66 in.) in length, depending on grade and composition. (a) Special grades available with 9.000 permissible exposure limit (min) or 1.878 g/cm3 (min)

Hot isostatic pressing (HIP) is the consolidation method used for an increasing proportion of beryllium products and now approaches vacuum hot pressing in terms of the volume of material processed.

In the HIP process, the powder is loaded into a mild steel canister, degassed to remove air and water vapor, and the canister is sealed. Hot isostatic pressing takes place at a pressure of 103 MPa (15 ksi) with temperature in the range of 850 to 1000 °C (1560 to 1830 °F), depending upon the desired properties. A processed density in excess of 99.75% of theoretical is achieved in production. As a result of the greater latitude in consolidation temperature allowed by the use of HIP process, mechanical property improvements over conventionally vacuum hot pressed materials can be achieved. The specification mechanical properties of three grades of HIPed beryllium are compared to their vacuum hot pressed counterparts in Table 4.

Table 4 Comparison of beryllium specification mechanical properties for VHP and HIP

Property

Grade

S-200F

I-70

I-220

HIP

VHP

HIP

VHP

HIP

VHP

0.2% yield strength, MPa (ksi)

296 (43)

241 (35)

207 (30)

172 (25)

345 (50)

276 (40)

Ultimate tensile strength, MPa (ksi)

414 (60)

323 (47)

345 (50)

241 (35)

448 (65)

379 (55)

Elongation, %

3

2

2

2

2

2

A cold pressing/sintering/coining technique has been used successfully for specific applications, such as the production of aircraft brakes, heatsinks, rotors, and stators. The relationship of green density to pressure depends strongly on the specific powder used; a typical curve is shown in Fig. 5. After cold pressing to obtain a high green density, sintering is carried out in vacuum. An argon sintering atmosphere severely restricts final density, as shown in Fig. 6.

1.60

1.40

1.00

Pressure, tsi

30 50

Pressure, tsi

30 50

1.40

1.00

1

y

• M

ngle va L'ltiple v

ues alues

! /

300 600 900 1200 1500

300 600 900 1200 1500

Pressure, MPa

Fig. 5 Green density versus compacting pressure in uniaxially cold pressed beryllium powder. Source: Ref 7

Compacting pressure, tsi

2C

40 60 30

°""05 y - 120C

m vacuum

/

S A

s s y y jgon 1200

aC, 3 h

if

Compacting pressure, M Pa

0 300 600 900 1200

Compacting pressure, M Pa

Fig. 6 Effects of compacting pressure on density after sintering in argon and in vacuum. Source: Ref 7

0 0

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