Variables Affecting Green Strength

Many theories have been proposed to explain the precise mechanics of green strength. Because the strength of green compacts results mainly from mechanical interlocking of particle surface irregularities, particle shape is the most important factor contributing to green strength. Powders with irregularly shaped particles produce compacts of higher green strength than powders with spherical particle shape. Spherical particles provide the lowest degree of mechanical strength because of low initial surface contact between adjacent particles and a low surface-to-volume ratio.

Generally, green strength is increased by increasing the powder surface area. This can be achieved by increasing the particle surface roughness and/or reducing the average particle size, thus providing more sites for mechanical interlocking. These characteristics also result in decreased apparent density. Figure 13 illustrates the dependence of green strength on apparent density for various iron powders.

ta CL

7500

S500

5500

4500

2500

1500

Green density, % theoretical 30 90

7500

S500

5500

4500

2500

1500

T V

Y^PPi

irent den

sity

V

Grec

densltj

f \

sV

10 20 30 40 Apparent density, % theoretical

Fig. 13 Effect of apparent density and green strength on green strength of various iron powders. Source: Ref 9

Green strength also is increased when oxidation and contamination of particle surfaces are reduced. Optimum mechanical interlocking is impeded when particles are covered with heavy oxide films and adsorbed gases.

Green strength also is affected by other variables such as green density and compressibility. Increasing green density or compacting pressure promotes increased particle movement and deformation, which are the bases for mechanical interlocking. Particle porosity also increases green strength (Fig. 14a), although this also reduces green density (Fig. 14b).

Fig. 14 Effect of internal powder porosity on (a) green strength and (b) green density. Solid and porous iron powders pressed at 414 MPa (30 tsi) using die wall lubrication. Figures in parentheses signify BET-specific surface areas and average intraparticle pore sizes of powders.

Effects of Lubricants. Increasing the amount of additives also decreases green strength. The addition of some alloying elements, such as adding graphite to iron, or the addition of lubricants can reduce the green strength of a compact. Compacts from mixtures of base metal powders and a lubricant generally have considerably lower green strength than those from the powder alone, since the lubricant, which has been added to facilitate ejection from the die wall, does not act as a binder. Instead, the lubricant interferes with particle-to-particle binding. Therefore, green strength tests should be performed on the powder-lubricant mixture as well as on the powder alone.

Table 3 illustrates the effect of a lubricant on green strength and green density of iron and copper compacts. Using a different lubricant, such as an amide wax rather than a metal stearate, can also improve green strength values. Die wall lubrication also improves green strength over that of mixed lubrication (Fig. 15).

Table 3 Effect of lubricant on density and strength of iron and copper compacts pressed at 550 MPa (40 tsi)

Material

Metal

Green

Green

stearate

density,

strength

lubricant, %

g/cm3

MPa

psi

Reduced iron

None

6.47

32

4600

1

6.57

23

3300

Electrolytic copper

None

7.97

67

9700

1

8.11

35

O 10

Compacting pressure, tsi 10 20 30 40

O 10

1

1

l 1

" 1

Lubrici

ited die/

f

/

-

L

200 400 600 B00 Compacting pressure, MPa

200 400 600 B00 Compacting pressure, MPa

Fig. 15 Effect of admixed lubricant on green strength of water-atomized 4600 low-alloy steel powder

The effect of lubricants on green strength is complex and depends on various factors such as production speed, part size, and lubricant type. Figure 16 and Table 4 illustrate the differences for five different kinds of lubricants. These data are meant to show that different lubricants may help if there are difficulties in the production of green parts.

Table 4 Comparison of lubricants in Fig. 16

Lubricant type

Particle size

Melting point

Ash content,

°C

°F

%

Ethylenebisstearmide atomized

0.1% +325 mesh

143

289

0.05

Ethylenebisstearmide ground

1% max 100 mesh

143

289

0.05

Ethylenebisstearmide with zinc stearate and stearic acid

0.1% +325 mesh

137

279

1.0

Ethylenebisstearmide

0.5% +325 mesh

140

284

2000

1000

2000

1000

410 ss

316 SS

304 SS

BCD Lubricant Lype (see Table 4)

3000

2000

1000

2000

1000

316 SS

n ^ï^ **

304 SS -

Vi c

Lubricant type

Fig. 16 Green strength and ejection pressures for various stainless steels and lubricants listed in Table 4. Source: Ref 9

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