Powder metallurgy


Powder metallurgy 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. Some porous products, allowing liquid or gas to permeate them, are produced in this way. They are also used when melting a material is impractical, due to it having a high melting point, or an alloy of two mutually insoluble materials, such as a mixture of copper and graphite.
In this way, powder metallurgy can be used to make unique materials impossible to get from melting or forming in other ways. A very important product of this type is tungsten carbide. Tungsten carbide is used to cut and form other metals and is made from tungsten carbide particles bonded with cobalt. 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 it at a temperature below the melting point of the metal, to bind the particles together. Powder for the processes can be produced in a number of ways, including reducing metal compounds, electrolyzing metal-containing solutions, and mechanical crushing, as well as more complicated methods, including a variety of ways to fragment liquid metal into droplets, and condensation from metal vapor. Compaction is usually done with a die press, but can also be done with explosive shocks or placing a flexible container in a high-pressure gas or liquid. Sintering is usually done in a dedicated furnace, but can also be done in tandem with compression, or with the use of electric currents.
Since the advent of industrial production-scale metal powder-based additive manufacturing in the 2010s, selective laser sintering and other metal additive manufacturing processes are a new category of commercially important powder metallurgy applications.

Overview

The powder metallurgy "press and sinter" process generally consists of three basic steps: powder blending, die compaction, and sintering. Compaction of the powder in the die is generally performed at room temperature. Sintering is the process of binding a material together with heat without liquefying it. It is usually conducted at atmospheric pressure and under carefully controlled atmosphere composition. To obtain special properties or enhanced precision, secondary processing like coining or heat treatment often follows.
One of the older such methods is the process of blending fine 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 : 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 : Here the powder, normally very fine 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" part of complex geometry. This part is then heated or otherwise treated to remove the binder to give a "brown" part. This part is then sintered and shrinks by ~18% to give a complex and 95–99% dense finished part. 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 technologies use electric currents to sinter powders. This reduces production time dramatically, 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 is a relatively novel family of techniques that use metal powders to make parts by laser sintering or melting. The process was undergoing rapid growth as of 2015, 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. It has been used in the aerospace industry because traditional processes are more time-consuming, difficult, and costly. Processes include 3D printing, selective laser sintering, selective laser melting, and electron beam melting.

    History and capabilities

The history of powder metallurgy and the art of metal and ceramic sintering are intimately related to each other. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. The ancient Incas made jewelry and other artifacts from precious metal powders, though mass manufacturing of PM products did not begin until the mid or late 19th century. In these early manufacturing operations, iron was extracted by hand from a metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.
A much wider range of products can be obtained from powder processes than from direct alloying of fused materials. In melting operations, the "phase rule" applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminium/iron powders poses major problems. 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. Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic, and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds. Competitive characteristics of manufacturing processing also may be closely controlled.

Special products

Many special products are possible with powder metallurgy technology. A non-exhaustive list includes Al2O3 whiskers coated with very thin oxide layers for improved refraction; iron compacts with Al2O3 coatings for improved high-temperature creep strength; light bulb filaments made with powder technology; linings for friction brakes; metal glasses for high-strength films and ribbons; heat shields for spacecraft reentry into Earth's atmosphere; electrical contacts for handling large current flows; magnets; microwave ferrites; filters for gases; and bearings which can be infiltrated with lubricants.
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. 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 and uncoated tungsten carbides.

Powder production

Any fusible material can be atomized. Several techniques have been developed that permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. Powders may be prepared by crushing, grinding, chemical reactions, or electrolytic deposition. The most commonly used powders are copper-base and iron-base materials.
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.
Powder can be obtained through gas or water atomization, centrifugal atomization, chemically reducing particulate compounds, electrolytic deposition in appropriate conditions, simple pulverization and grinding, thermal decomposition of particulate hydrides or carbonyls, precipitation out of solution, and also condensation from vaporized metal.