Various methods of powder manufacture, even for the same type of material, influence the compacting, the sintered size, and the physical characteristics as well. Different manufacturing methods affect the particle size distribution, the particle shape, the particle hardness, and particle purity. All of these affect the apparent density (loose powder), the flow, the compressibility, the springback (incremental increase from die size) (Fig. 1), and the green (unsintered) strength of the compact. Variations in any of these parameters can have an effect on the size or tolerance control of the compact. The method of powder manufacture can affect size change in sintering, especially when the powders are blended with carbon, copper, nickel, or other alloys and combinations. The condition of the alloying powders such as particle size and oxide content also affect the size and physical characteristics of the sintered part.

Fig. 1 Springback versus density. Source: Höganäs Metal Powder Data Sheets, Sweden

The choice of the powders to be used may be based on many factors. These include price, availability, ease of compacting, green and sintered physical characteristics, and familiarity with the powder and its use. When good control of tolerances and physical properties is essential, the ability of the powder producer to make a material that is very consistent in all characteristics is a key factor.

The sintered size of a compact varies with the amount and type of the alloy, the density, and the sintering conditions. The size change plotted against the change in alloy or the change in density or the change in sintering conditions can be almost linear or a curve (Fig. 2, 3, 4). For optimal dimensional control, it is desirable to operate on the flattest part of a curve, if possible, as slight variations will have the least effect.







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Fig. 2 Dimensional change for several alloys versus density. Source: Hoeganaes Corporation, Riverton, NJ

Fig. 3 Dimensional change versus density, copper, and graphite. Source: Hoeganaes Corporation, Riverton, NJ
Fig. 4 Dimensional change versus sinter temperature for various bronze powders. The "PMB" designations indicate variation in particle size distribution. Source: AMG Americas

To obtain optimal part size control based on the powder, there must be very tight control on all stages of powder manufacture. It has been shown that even the water used for atomizing affects the sintered size change (Ref 1). High sodium water (softened) shows less growth when parts are sintered in dissociated ammonia (DA) compared with low sodium water and shows the opposite effect when sintered in a nitrogen atmosphere.

Most powder suppliers now use some form of statistical process control and may operate under an appropriate ISO standard to supply the user with the most consistent powder possible. Even when powder is shipped with extremely narrow control limits on all of the parameters that can affect the tolerance control of the part, there are variables beyond the manufacturing steps that can affect the part as pressed and sintered. Powder may be altered by shipping and transport conditions. Vibration and impact such as trucking long distances over poor roads can have an effect on the flow and apparent density. Powder in containers that have not been properly sealed or that have been left open for long periods may be changed by the atmosphere resulting in oxidation and/or moisture pickup.

If the part manufacturer blends or reblends the powder, variations can occur. Changes in blending time, the amount of load placed in the blender (percent of blender capacity), the type of blender, and the various lubricants can affect the resulting apparent density, flow rate, and green strength (Ref 2). Blend uniformity can be affected by the method of loading the materials into the blender, the accuracy of the weight of the blended material, and the blender time and action itself. Any of these variables can affect the compacted and/or the sintered part. The advantage of a single large blend by the powder producer is that all of such a large blend, 10,000 lb or more for example, should be quite consistent, whereas twenty 500 lb blends may show variation from blend to blend.

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