Beryllium Powder Metallurgy

Almost all beryllium in use is a PM product.12 PM is required for a number of reasons, the primary one being that beryllium castings contain too much porosity and other casting defects to allow their use in critical applications. In addition to the casting defects, the grain size of cast beryllium is too coarse (>100^m). Since the strength and ductility of beryllium depends on a fine grain size, grain sizes of less than 15 ^m are required to obtain satisfactory mechanical properties. The effects of grain size on beryllium properties are shown in Fig. 3.9. PM techniques can produce grain sizes as fine as 1-10 ^m when required. Beryllium powders are consolidated into near net shapes by either VHP or HIP to obtain parts within 99.5% of theoretical density.

Fig. 3.9. Effect of Grain Size on Beryllium Mechanical Properties9

Fig. 3.9. Effect of Grain Size on Beryllium Mechanical Properties9

To produce beryllium powder, castings are first poured and then turned into chips using a lathe with multi-head cutting tools. The chips are ground into powder using impact milling to produce powders of differing sizes and shapes. Impact milling is the most prevalent method because it produces the best powder product. In impact milling, beryllium particles are driven against a beryllium target using a high velocity gas stream. A blocky powder particle is produced that has less tendency to align preferentially during powder compaction than those produced by ball or attrition milling. When the powder is compacted under heat and pressure, it is more uniform, resulting in greater ductility. Since the low ductility of beryllium has always been a concern, improvements in room temperature ductility have been achieved by control of preferred orientation, improved purity, reduction of inclusions, control of inclusion distribution, and by reducing grain size. Elongation values of 4-5% are currently achievable. The strength of beryllium is also a function of grain size, which is determined by particle size, oxide content, and consolidation temperature.

The impact milling system shown in Fig. 3.10 is used to produce beryllium powder. Coarse powder is fed from the feed hopper into a gas stream. As the gas borne powder is carried through a nozzle, it accelerates and impacts a beryllium target. The debris is then carried to the primary classifier where the particles drop out and fines go to a secondary classifier and are discarded. This cycle continues until the desired particle size is achieved. Impact milling enables consistent control of powder composition by reducing impurity contamination and oxidation of powder particles. The process also yields improved powder

Fig. 3.10. Impact Milling System9

Platen Pressure

Platen Pressure

Platen Pressure

Vaccum Hot Pressing

Fig. 3.11. Vacuum Hot Pressing and Hot Isostatic Pressing13

Platen Pressure

Isostatic Gas Pressure Hot Isostatic Pressing

Vaccum Hot Pressing

Fig. 3.11. Vacuum Hot Pressing and Hot Isostatic Pressing13

configuration and morphology, resulting in improved isotropy and a cleaner microstructure of the final consolidated product.9

Powder compaction is conducted by either VHP or HIP, as shown schematically in Fig. 3.11. In VHP, the powder is loaded into a graphite die and vibrated, the die is placed into a vacuum hot press, a vacuum is established, and the powder is consolidated under 500-2000 psi at temperatures between 1830 and 2020° F. Densities in excess of 99% of theoretical are obtained in diameters of 8-72 in. The HIP process is similar except that the powder must be enclosed in a mild steel can prior to consolidation. After canning, the can is degassed under vacuum at 1100-1300° F to remove all air and gases absorbed on the particle surfaces. The can is then sealed and put through the HIP consolidation cycle at 15 ksi and 1400-2010° F. Although HIP is generally more expensive than VHP, it allows the best control of grain size because it allows greater latitude in temperature selection. HIP is capable of attaining 100% of theoretical density. A comparison of the two methods on the properties of grade S-200 is shown in Fig. 3.12. The improvement in anisotropy for the HIP-processed material is evident and is primarily a result of being able to use lower temperatures to keep the grain size fine and apply uniform and high pressures in all directions during consolidation.

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