Powder metallurgy

thumb|[[Iron powder is commonly used for sintering]]

Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes are sometimes used to reduce or eliminate the need for subtractive processes in manufacturing, lowering material losses and reducing the cost of the final product. This occurs especially often with small metal parts, like gears for small machines. Tungsten carbide is the largest and most important use of tungsten, consuming about 50% of the world supply. Other products include sintered filters, porous oil-impregnated bearings, electrical contacts and diamond tools.

Powder metallurgy techniques usually consist of the compression of a powder, and heating (sintering) it at a temperature below the melting point of the metal, to bind the particles together.

One of the older such methods is the process of blending fine (<180 microns) metal powders with additives, pressing them into a die of the desired shape, and then sintering the compressed material together, under a controlled atmosphere. The metal powder is usually iron, and additives include a lubricant wax, carbon, copper, and/or nickel. This produces precise parts, normally very close to the die dimensions, but with 5–15% porosity, and thus sub-wrought steel properties. This method is still used to make around 1 Mt/y of structural components of iron-based alloys.

There are several other PM processes that have been developed over the last fifty years. These include:

Powder forging: A "preform" made by the conventional "press and sinter" method is heated and then hot forged to full density, resulting in practically as-wrought properties.
Hot isostatic pressing (HIP): Here the powder, normally gas atomized and spherical, is filled into a mould, usually a metallic "can". The can is vibrated, then evacuated and sealed. To sinter the powder, it is placed in a hot "isostatic press" for several hours, where it is heated to around 0.7 times the melting point, and subjected to an external gas pressure of ~100 MPa. This results in a shaped part of full density with as-wrought or better properties. HIP was invented in the 1950-60s and entered tonnage production in the 1970-80s. In 2015, it was used to produce ~25,000 t/y of stainless and tool steels, as well as important parts of superalloys for jet engines.
Metal injection moulding (MIM): Here the powder, normally very fine (<25 microns) and spherical, is mixed with plastic or wax binder to near the maximum solid loading, typically around 65% volume, and injection moulded into a mould to form a "green" (with binder) part of complex geometry. This part is then heated or otherwise treated to remove the binder to give a "brown" (without binder) part. This part is then sintered and shrinks by ~18% to give a complex and 95–99% dense finished part (surface roughness ~3 microns). Invented in the 1970s, production has increased since 2000 with an estimated global volume in 2014 of 12,000 t worth €1265 million.
Electric current assisted sintering (ECAS) technologies use electric currents to sinter powders. This reduces production time dramatically (it can take from 15 minutes to a few microseconds), does not require a long furnace heat, and allows near-theoretical densities, but it also has the drawback of simple shapes. Powders used in ECAS do not require binders because they can be directly sintered, without needing to be pre-pressed and compacted with binders. Moulds are designed for the final part shape since the powders sinter while filling the cavity under applied pressure. This avoids the problem of shape variations caused by non-isotropic sintering, as well as distortions caused by gravity at high temperatures. The most common of these technologies is hot pressing, which has been used to make diamond tools for the construction industry. As of 2018, only hot pressing and, in a more limited way, spark plasma sintering had achieved direct industrial application.
Additive manufacturing (AM) is a relatively novel family of techniques that use metal powders (among other materials, such as plastics) to make parts by laser sintering or melting. The process was undergoing rapid growth , and as of 2018 has been used predominantly for research, prototyping or advanced applications in the aerospace industry, though also in the biomedical, defence and automotive industries. Other substances that are especially reactive with atmospheric oxygen, such as tin, are sinterable in special atmospheres or with temporary coatings.

In powder metallurgy or ceramics it is possible to fabricate components which otherwise would decompose or disintegrate. All considerations of solid-liquid phase changes can be ignored, so powder processes are more flexible than casting, extrusion, or forging techniques. and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds.

Extremely thin films and tiny spheres exhibit high strength. One application of this observation is to coat brittle materials in whisker form with a submicrometre film of much softer metal (e.g. cobalt-coated tungsten). The surface strain of the thin layer places the harder metal under compression, so that when the entire composite is sintered the rupture strength increases markedly. With this method, strengths on the order of 2.8 GPa versus 550 MPa have been observed for, respectively, coated (25% cobalt) and uncoated tungsten carbides.

Powders of the elements titanium, vanadium, thorium, niobium, tantalum, calcium, and uranium have been produced by high-temperature reduction of the corresponding nitrides and carbides. Iron, nickel, uranium, and beryllium submicrometre powders are obtained by reducing metallic oxalates and formates. Exceedingly fine particles also have been prepared by directing a stream of molten metal through a high-temperature plasma jet or flame, atomizing the material. Various chemical and flame-associated powdering processes are adopted in part to prevent serious degradation of particle surfaces by atmospheric oxygen. centrifugal atomization, The smaller the particles, the more homogeneous the microstructure will be. Particles produced this way will also have a more irregular shape Most powder compaction is done with mechanical presses and rigid tools, but hydraulic and pneumatic techniques can also be used, as well as methods that combine compaction with sintering, like hot isostatic compaction. This technique is useful for very large products, including those over 3000 tons and larger than 100 square inches.

Cold isostatic compaction

Isostatic powder compacting is an alternate method of powder compaction. This operation is generally only applicable on small production quantities, and although the cost of a mold is much lower than that of pressing dies, it is generally not reusable and the production time is much longer. Production rates are usually very low, but parts weighing up to 100 pounds can be effectively compacted. If the temperature is above the melting point of a component in the powder metal part, the liquid of the melted particles fills the pores. This type of sintering is known as liquid-state sintering. A combination of mechanical pressure and electrical current, passed through either the powder or the container, significantly reduces the sintering time compared to conventional solutions. electric discharge sintering techniques include capacitor discharge sintering.

There appears to be no limitation to the variety of metals and alloys that can be extruded, provided the temperatures and pressures involved are within the capabilities of die materials. and diameters from 0.2 to 1 m. Modern presses are largely automatic and operate at high speeds (on the order of m/s).

Risks of aluminium metal powder

Aluminium powders are known to cause lung fibrosis among people who work with it in manufacturing plants. According to the CDC, workers who are exposed to large amounts of aluminium can have other lung problems and abnormal chest X-rays. Some workers have also had decreased performance in neurological tests. Exposure to aluminum can irritate the skin and eyes.

Aluminium powder is also highly flammable.

Besides the risks of metal powder itself, there are many hazards during the production process as well.

Notes

References

Cited sources

External links

Rapid manufacturing technique developed at the KU Leuven, Belgium
Slow motion video images of metal atomization at the Ames Laboratory
APMI International "The Global Professional Society for Powder Metallurgy"[https://www.apmiinternational.org/], a non-profit organization