Stabilization of Powder Mixtures

Because of the wide use of powder mixtures and their sensitivity to demixing, much work has been done towards the stabilization of powder mixtures.

Stabilizers. A powder mixture is optimally mixed if its components approach a random (statistical) distribution free of agglomeration (Fig. 1). The quality of mixing can be measured by the number of particle contacts (in green compacts) between identical or different powder components as illustrated in Fig. 2 for iron-copper mixtures, or by the chemical analysis of appropriate samples taken from the powder mixture.

Fig. 1 Schematic representation of particle patterns in a powder mixture. (a) Ordered. (b) Agglomerated. (c) Statistical (random) distribution. (d) Demixed or segregated iri c o

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20 40 00 SO 100 Copper, vol%

Fig. 2 Effect of stabilizer on iron-iron contact formation in binary iron-copper system. Lower curve represents theoretical random mixture.

Blenders and mixers that rely mainly on gravity (tumblers) are suitable for powders that mix readily. More intense mixing is accomplished with low-shear agitated-type blenders that use ribbons, slow-speed paddles, screw-type augers, or other means of motion. Figure 3 illustrates how spherical powders mix quite readily, but also are subject to demixing or overblending. Consequently, mixing should be stopped once a near-random distribution has been achieved. The variability coefficient in Fig. 3 represents the standard deviation of the measured degree of mixing divided by the average value of the measured property. The quality of the mixture improves with decreasing variability coefficient.

0 4 8 12 16 20 24 28 32 Mixing time, rrtin

Fig. 3 Effect of particle size and shape of components of 90%Fe-10%Cu mixtures on degree of blending. Quality of blending improves as variability coefficient decreases. Particle size and shape for components: (a) Cu, 200 to 300 /£m; Fe, <63 /Jm of spherical particle shape, (b) Cu 200 to 315 /Jm; Fe, 100 to 200 /Jm of

20 40 00 SO 100 Copper, vol%

spherical particle shape, (c) Cu, 200 to 315 ^m; Fe, <63 /Jm of irregular particle shape, (d) Cu, 200 to 315 t1 m; Fe, 100 to 200 /'m of irregular particle shape

Demixing is often caused by the accumulation of electric charges, which frequently can be dissipated by the addition of a small amount of water. Surfactants and stabilizers are sometimes used to improve the flow of materials. They consist of wetting liquids and oils, which have no negative effects on the sintering process. Figure 4 shows the beneficial effect of oleic acid (dissolved in benzene) for 90%Fe-10%Al mixtures. In this example, the best mixture results from the addition of the optimized amount of a stabilizer after eight minutes of mixing, when the mixture has approached a random distribution (curved).

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cu s

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Fig. 4 Effect of stabilizer on degree of blending of 90%Fe-10%AI mixtures. Particle size for both components: 100 to 200 f m. Stabilizer: benzeneloleic acid, (a) Without stabilizer, (b) With 1.0% stabilizer, (c) 1.0% stabilizer added after 8 min of blending. (d) 3% stabilizer added after 8 min of blending

Diffusion-Bonded Powders. In a diffusion-bonded powder, a powder mixture is heated in a reducing atmosphere to yield a sintered powder cake, which after grinding, produces a powder that consists of agglomerates of the components of the powder. The process is controlled so that the degree of alloying among the different components is only small. This retains much of the desirable high-compressibility characteristics of the original elemental powder mixture and minimizes or eliminates the demixing tendency of such a powder. As a bonus, green strength is usually increased.

Binder-Treated Powder. In recent years powder producers have developed proprietary binders that are used in small amounts to bond fine graphite onto the coarser iron particles. Such powders reduce the dusting of a powder, and improve several important engineering properties of the sintered parts due to the more uniform concentration of carbon in the final parts.

Prealloyed Powders. Despite the widespread use of powder mixtures, the use of prealloyed powders in the structural parts industry has been increasing. In the low-alloy steel segment, nickel, molybdenum, and manganese are used as alloying elements to provide greater hardenability than is possible with admixed copper or nickel. Admixed nickel requires economically prohibitive sintering times (even at high sintering temperatures) for complete alloying. Higher alloyed powders, such as stainless steels, tool steels, and superalloys, are all prealloyed powders. The requirement of complete alloying in these alloy classes makes the elemental powder approach impractical. The problem of low

compressibility, that is, low green density and/or low green strength, is solved by high-temperature and liquid phase sintering, and through the use of special consolidation methods, such as hot isostatic pressing and extrusion.

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