Powder Processing

Powder Manufacture. Although crushed/irregular, mechanically alloyed, and hydride/dehydride powders can be used for HIP processing, prealloyed, spherical powder produced via gas atomization is most commonly used throughout the HIP industry. The primary advantage of spherical powders is their high pre-HIP packing density (e.g., 60 to 80% or greater of theoretical density) and relatively low variation in packing, which provides repeatable packing density in production. As the initial packing density increases, the probability of predicting final fully dense shape is higher, thus the more reproducible the HIP P/M process becomes.

For HIP P/M parts, the most commonly used powder-making process is inert gas atomization, which entails creation of powder by impinging high-pressure inert gas on a molten metal stream to disintegrate the metal into tiny liquid droplets. Due to surface tension considerations, the liquid droplets form spheres that rapidly solidify into powder particles at a rate of 104 to 108 °C/s (Ref 17). This rapid solidification provides a powder particle with superior chemical homogeneity not possible with conventional ingot metallurgy. In general, several alloy systems can be gas atomized successfully, namely those based on iron, nickel, cobalt, niobium, titanium, aluminum, copper, silver, and so forth with the molten metal containment (i.e., refractory or skull melt) being the limiting factor.

In addition to gas atomization, spherical powder can be produced by other methods that utilize centrifugal force as the energy mechanism to break up liquid metal. The rotating electrode process (REP) and the plasma rotating electrode process (PREP) are two such methods. During REP, the face of a rotating bar is melted by a direct-current electric arc maintained between the electrically negative tungsten tip and the positive alloy bar (Ref 18). The liquid metal is centrifugally ejected from the outer edge of the rotating bar and solidifies in flight as spherical particles, which are collected in a sealed chamber. While REP is normally used to produce iron-base powders, applications requiring superior cleanliness (e.g., titanium, nickel alloys, etc.) can be produced by PREP. During PREP, the plasma torch uses a dc arc to melt the rotating bar with a water-cooled tungsten cathode that is protected by helium gas to minimize erosion (Ref 18). Another mechanical atomizing technique is the rotating disk process where a molten metal stream is impinged on a rapidly spinning disk. As with the first two processes, the liquid metal is spun off the disk and centrifugally atomized into spherical droplets, which solidify in flight. Particle cooling can be enhanced by blasting the emerging particles with a stream of helium (Ref 19). Centrifugally produced powder typically has a narrower particle size distribution and lower packing density than gas atomized powder.

Another process that is commonly used to make titanium powder is the hydride/dehydride process. Titanium sponge is charged with hydrogen and thus embrittled. The particles are mechanically crushed and dehydrided to yield fine powder particles. This powder is not as pure as that made by the PREP or inert gas atomization, nor is it spherical. However, it can be used to make HIP components. Powder of other refractory metals such as molybdenum and niobium can also be made in this way. It is also possible to make alloys of these elements using this process.

Powders can also be made by crushing and grinding ingot material. This process is usually used for highly alloyed materials and is not ideal for HIP P/M because of the inherently low packing density associated with this manufacturing method. These powders are often used with the HIP cladding process because they can be highly alloyed to achieve a specific property such as wear and/or corrosion resistance. These powders have a tendency to be extremely nonhomogeneous.

Powder Particle Size Classification. Classification may involve separating powder by particle size, typically via vibratory screening. Air classification through specially designed cyclones has also been employed. Bulk screening processes involve feeding raw powder onto the vibratory screener. Two exit ports from the screener are connected to separate collection bins to segregate the oversize and undersize fractions. The prime powder (usually the undersize) is used to manufacture HIP P/M parts, and the other fraction (usually the oversize) is reprocessed through melting.

Powder blending is performed to homogenize the particle size distribution. Hot vacuum blending is used to remove adsorbed gases from the surface of powder particles. Uniform particle size distribution is essential to HIP P/M near-net shape parts to minimize distortion during the consolidation step of the process. If a near-net shape is not being produced, then blending of the powder is not an essential step in the process and is often omitted. For some applications, blending can be used to achieve the desired chemistry of the end product. This can be as drastic as blending of elemental powders to make the entire composition or blending prealloyed powders that are close in chemistry as a refinement to meet a requirement for a specification. Blending for chemistry reasons is not considered a high-quality process and would not be utilized for technically demanding applications. After blending, powder is ready for loading into containers for HIP.

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