Green Strength Theories

The two mechanisms used to explain the origin of green strength are cold welding and mechanical interlocking of particles. Cold welding refers to the formation of metallic contacts between neighboring particles during pressing; mechanical interlocking refers to their entangling in the manner of a three-dimensional jigsaw puzzle.

Because both of these mechanisms respond similarly to many powder properties, it is difficult to provide conclusive experimental evidence for either theory. For instance, the important influence of a low apparent density of a powder and the accompanying increase of shearing and heat development during compaction is beneficial to the formation of metallic bonds, as well as to improved keying and interlocking of particles. Nevertheless, some of the features and relationships believed to be specific to each of the two mechanisms are summarized below.

Proponents of the cold welding theory argue that the well-known fact that clean metal surfaces form strong attractive junctions when joined (Ref 11) also applies to the pressing of parts from metal powders. In fact, experimental evidence of such interatomic or metallic bonding forces between powder particles has been presented (Ref 12).

Large and similar reductions in both green strength and electrical conductivity of copper compacts pressed from pure and oleic acid-coated copper powders (Fig. 17) were interpreted to indicate that green strength, like electrical conductivity, depends on the number and size of metallic microcontacts formed between particles. Thin oxide films on copper powder particles gave similar results (Ref 13).

Fig. 17 Effect of thin film of oleic acid on green strength and electrical conductivity of copper parts

High green strength powders pressed and sintered to the same density as low green strength powders of the same chemical composition produce significantly stronger parts. Because the strength of sintered parts is explainable in terms of metallic bonding and microstructural features, such as grain size and pore structure, and because proportionality exists between green and sintered strength, it is reasonable to assume that bonding forces responsible for strength are identical for both green and sintered parts.

Proponents of the interlocking theory of green strength argue that scanning electron micrographs of fractured surfaces of green parts do not show any evidence of metallic contacts and that parts pressed from spherical powders have no measurable green strength, in spite of substantial deformation and nominal contact areas between particles. Furthermore, roll-compacted green strip has good mechanical strength, yet is quite flexible (Ref 12). Some evidence shows that interlocking may be prevalent at low densities and that cold welding may be dominant at high densities (Ref 13, 14).

The theories on friction welding and interlocking of particles explain the mechanisms responsible for the strength of a green part, but these theories do not permit prediction of green strength from basic powder characteristics. They do, however, provide principles for the development of powders with specific green strength characteristics. The large body of empirical green strength data that is available may be conveniently separated into intrinsic, surface-related, and geometric powder properties (Ref 14).

Intrinsic Powder Properties. Figure 12 shows green strength compacting pressure curves of several metal powders. These curves are typically linear or concave upward (with respect to the pressure axis) and illustrate that green strength increases rapidly with increasing compacting pressure. These curves may be converted into green strength/green density curves by means of the corresponding compressibility (green density/compacting pressure) curves of the powders.

At high compacting pressures or with high-speed compaction, green strength may be lower than expected due to air entrapment during compaction. Such parts may have lamination defects with an appearance similar to those produced by differential expansion during ejection from the die.

In general, green strength increases with the increasing intrinsic softness, or plasticity, of a powder. This relationship is, however, often masked by the influence of geometric powder characteristics. Annealing of a work-hardened powder raises its green strength compacting pressure curve; it may lower its green strength/green density curve, because the annealed powder requires a lower compacting pressure to achieve a given density.

If compaction exceeds the work-hardening capacity of a powder, particle fracture may occur and may cause green strength irregularities.

Surface-Related Factors and Powder Mixtures. The most important commercial examples in this category include the use of mixtures of powders (such as copper and tin or the addition of copper and nickel to iron), the addition of graphite and lubricants, and the presence of oxide films due to tarnishing. The green strength of mixtures of similar metal powders may be estimated from the green strength of its components by applying the rule of mixtures.

The addition of graphite and most lubricants reduces green strength to a much greater extent than indicated by the rule of mixtures, however. The effect of a 1% addition of zinc stearate on the green strength of a water-atomized low-alloy steel powder is shown in Fig. 15. The detrimental effect of the lubricant is more pronounced at high compacting pressures, because it causes the shape of the curve to change from concave to convex.

Because lubricants affect many other powder properties, most notably compressibility and powder flow, selection of amount and type of lubricant addition is made on the basis of the best compromise for a given application. Typically, lubricants that are less detrimental to green strength reduce both powder flow and compressibility.

Most metal powders possess thin oxide films--either directly from manufacturing, as in the case of water-atomized powders, or from subsequent tarnishing in air. The effect of these oxide films on green strength appears to depend on whether these films break down during compaction. For both iron and copper powders, tarnishing or surface oxidation produces significant losses in green strength. Figure 18 illustrates the extent of such losses for copper powder and also shows that oxide films on water-atomized stainless steel powders do not impair green strength. These oxide films were produced by low-temperature oxidation with air in a fluid bed.

Fig. 18 Effect of surface oxide films on green strength of copper and type 316L stainless steel powders

Geometric powder properties include particle shape, particle size, and particle porosity. Spherically shaped solid particles (typical of many gas-atomized powders) have such low green strength that they cannot be used in conventional die compaction. Many water-atomized powders have an irregular particle shape with a solid bulk structure. Such powders typically provide intermediate green strength. Oxide-reduced powders are of irregular particle shape and possess internal particle porosity. Such powders give maximum control over green strength. Green strength increases with increasing particle porosity and with decreasing particle pore size.

Other geometric factors that are beneficial to green strength include decreasing particle size and increasing specific surface area of a powder. Figure 19 shows the effect of particle size for isostatically compacted iron powders. The true effect of particle size is sometimes masked by other factors. Figure 20, for example, shows a maximum for the green strength of a water-atomized steel powder of intermediate particle size. In this instance, particle shape becomes more regular with decreasing particle size; oxygen content also varies with particle size. This is typical of many water-atomized powders. In such cases, the apparent density of a powder is frequently a useful measure of particle shape and green strength. All geometric factors that are beneficial to green strength are detrimental to compressibility.

Fig. 19 Effect of particle size on green strength of isostatically pressed electrolytic iron powder. Fine: 100% -325 mesh, 90% 10 to 44 Medium: 22% -325 mesh, 78% -65 + 325 mesh. Coarse: 100% -42 + 100 mesh. Source: Ref 16

Fig. 20 Green strength, green density, and apparent density of water-atomized steel powder. Source: Ref 17

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Responses

  • alexander
    What type of powder particle by gas atomization is expected to provide higher green strength?
    2 years ago

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