Commercial Production

Powder rolling was described in a number of early patents (Ref 1), but the first in-depth analysis of the process was undertaken by Naeser and Zirm (Ref 2), who documented their work on rolling RZ iron powder into strip. Powder rolling of copper powder is described in Ref 3 and 4.

From 1958 to 1968, powder rolling processes received considerable attention in Great Britain, the United States, Canada, the Soviet Union, and Japan. Principal interest centered around combining low-cost processes for producing metal powders, such as copper, nickel, cobalt, iron, and aluminum, with the powder rolling process to develop methods of fabricating thin sheet or strip more economically.

Nickel Powder Strip. The Sherritt Gordon process of rolling nickel powders into strip and sheet was one result of these efforts (Ref 5). This strip was used to make coinage blanks for the Canadian Mint. Typical mechanical properties of roll compacted nickel strip at various stages of manufacturing are given in Table 1.

Table 1 Typical mechanical properties of hydrometallurgical powder rolled nickel strip

Strip

Strip

thickness

Strip

Ultimate tensile strength

Yield

strength

Elongation,

mm

in.

density, %

MPa

ksi

MPa

ksi

%

Green strip

4.0

0.158

79

4

0.6

4

0.6

0

Sintered strip

4.1

0.161

79

138

20

136

19

0

Hot rolled strip

2.1

0.084

100

358

52

165

24

38

Cold rolled strip

1.3

0.052

100

579

84

572

83

5

Annealed strip

1.3

0.052

100

362

53

83

12

48

After nickel powder is compacted into a green strip, the strip is sintered between 1000 and 1200 °C (1830 and 2190 °F) in a muffled furnace. Hydrogen, dissociated ammonia, or even less reducing gases provide a suitable atmosphere. Strip is conveyed through the furnace on a mesh belt or rollers.

Densification of the roll-compacted nickel strip can be achieved by hot or cold rolling. Hot rolling of nickel strip is performed above 800 °C (1470 °F), with a reduction in thickness of approximately 50%. Because of its porous nature, the sintered strip should be protected from oxidation when heating for hot rolling. This can be accomplished by using an inert or exothermic atmosphere.

Cold rolling and annealing cycles also may be used to densify the sintered strip. To be successful, sintering must be performed at 1100 °C (2010 °F) or higher. Limited reduction is achieved on the first cold mill pass. The strip must then be fed directly to a furnace without coiling, where it is annealed and cold rolled to full density. Reductions in excess of 35% are necessary to achieve full density.

Several advantages are gained from producing high-purity nickel strip by roll compacting. Lower electrical resistivity is possible (73 to 79 x 10"6 Q m, or 44 to 48 Q cir mil/ft). Stability can be maintained at ±2% throughout the coil and from heat to heat. Wrought nickel offers an erratic ±6% tolerance. Work hardening rates for roll-compacted nickel strip are 25% less than for wrought nickel strip.

The lower softening (annealing) temperature coupled with high purity makes this nickel useful in clad metal combinations. In these applications, low and closely controlled annealing temperatures are required to minimize interdiffusion and to prevent incipient melting reactions.

Finished nickel strip produced from powder is virtually indistinguishable from strip produced from an ingot. Differences in physical properties are the result of compositional variances, rather than the method of fabrication. Typical physical properties for strip made from nickel powder are:

Density

8.90 g/cm3

Coefficient of thermal expansion at 20-100 °C

14 .'-m/m ■ °C

Coefficient of thermal expansion at 20-500 °C

15 .■■m/m ■ °C

Thermal conductivity

86.23 W/m ■ K (0.206 cal/cm ■ s ■ °C)

Cold working capacity

Good

Hot forming capacity

Good

Hot work temperature

800-900 °C (1470-1650 °F)

Annealing temperature

700-900 °C (1300-1650 °F)

Magnetic properties

Curie temperature

353 °C (667 °F)

Initial permeability

130

Maximum permeability

1240

Saturation induction

6.05 T

Remanance

3.25 T

Coercivity

23.87 A/m

Magnetostriction (soft) 1590 A/m

0.000032 mm/mm

Cobalt Powder Strip. Other metal powders can be roll compacted using methods similar to that of nickel powder. Powder characteristics and material properties alter processing parameters and conditions, however. For example, cobalt has a close-packed hexagonal crystal structure, which changes the surface morphology of the cobalt powder. Roll-compacted cobalt powder produces a green strip that is stronger and denser than the comparable green nickel strip. Typical mechanical properties of powder rolled cobalt strip at each stage of the production process are given in Table 2.

Table 2 Typical mechanical properties of hydrometallurgical powder rolled cobalt strip

Strip

Strip

thickness

Strip

Ultimate tensile strength

Yield

strength

Elongation,

mm

in.

density, %

MPa

ksi

MPa

ksi

%

Green strip

2.1

0.084

86

22

3

22

3

0

Sintered strip

2.1

0.084

86

201

29

195

28

5

Hot rolled strip

1.2

0.048

100

758

110

413

60

15

Cold rolled strip

0.9

0.036

100

1103

160

1100

159

1

Annealed strip

0.9

0.036

100

793

115

345

50

20

Note: Compacting roll diameter, 254 mm (10 in.). Roll speed, 6.0 rpm. Roll gap (green strip), 1.5 mm (0.06 in.)

After sintering at 1100 to 1150 °C (2010 to 2100 °F), cobalt may be densified by hot or cold rolling. Cold rolling of pure cobalt is inhibited by the close-packed hexagonal crystal structure, which characteristically produces rapid work hardening and restricts cold reduction to approximately 25% between anneals.

The use of high-purity cobalt powders for the feed to a powder rolling mill results in a relatively ductile strip. Use of controlled conditions during annealing results in increased amounts of cubic phase (usually associated with cobalt above 445 °C, or 835 °F), which is retained at room temperature. The retained phase is stable and shows little tendency to transform to the hexagonal structure at room temperature.

The rapid work hardening characteristics of cobalt make it an ideal candidate for powder rolling. The initial strip can be rolled to a thickness very close to the required final gage. Because initial strip thickness is thin (1.5 to 2.0 mm, or 0.06 to 0.08 in.), cold rolling is minimized. Typical physical properties of powder rolled cobalt strip are:

Density

8.85 g/cm3

Coefficient of thermal expansion at 20-100 °C

13 .'■"m/m ■ °C

Coefficient of thermal expansion at 20-500 °C

14 -'-'m/m ■ °C

Thermal conductivity at 70 °C (160 °F)

115.9 W/m ■ K (0.277 cal/cm ■ s ■ °C)

Cold working capacity

Poor

Hot forming capacity

Good

Hot work temperature

600-800 °C (1110-1470 °F)

Annealing temperature

800-1000 °C (1470-1830 °F)

Magnetic properties

(Unworked high-purity powder rolled cobalt strip)

Curie temperature

1121 °C (2049 °F)

Initial permeability

11.6

Maximum permeability

29.2

Saturation induction

1.9 T

Saturation field strength

310,000 A/m

Remanance

0.3 T

Coercivity

3,600 A/m

Coefficient of friction at 70 °C (160 °F) cobalt/cobalt

0.3

Production of heterogeneous alloys is possible with roll compacting processes. In cobalt-iron materials, for example, the presence of the iron promotes a suppression of the cubic to hexagonal phase transformation in cobalt. The ductility of the cobalt-iron mixture increases with increased iron content. The effects of adding iron powder to cobalt powder on the properties of cobalt strip are shown in Fig. 10.

S3 c

130 120 110 100 90 80 70 60 60 40 30 20 10 0

Nje

nsile

stren

3th

Hi

irdne

;s

Eli

>ngat

on

Spr

ingbc

,ck

Cobalt 99 98 97 96 95 94 93 92 91 90 Iron 0 1 2 3 4 5 6 7 8 9 10

ffl E

130 120 110 100

iz SO Oi

70 M

Cobalt 99 98 97 96 95 94 93 92 91 90 Iron 0 1 2 3 4 5 6 7 8 9 10

Composition, wt%

40 ZJ

Fig. 10 Properties of cobalt-iron alloys. Manufactured by powder rolling of blended cobalt and iron powders

0 0

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