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. 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 (sintering) 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 (hot isostatic 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. China has become a major production hub for metal AM powders, supported by government-led R&D programmes and a rapidly expanding base of gas atomization capacity for alloys including titanium, nickel-based superalloys, and cobalt-chromium.
Overview The powder metallurgy "press and sinter" process generally consists of three basic steps: powder blending (or pulverisation), 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 (<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:
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