Powder metallurgy
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.
[5] 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 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.
In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing.
[12] Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic,[12][15] and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds.
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.
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.
[12] 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.
[16] 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.
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.
[12] Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow.
Pumping energy is applied to droplet formation with very low efficiency (on the order of 1%) and control over the size distribution of the metal particles produced is rather poor.
Unfortunately, it is difficult to eject metals through orifices smaller than a few millimeters in diameter, which in practice limits the minimum size of powder grains to approximately 10 μm.
Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and remelting a significant fraction of the grain boundary.
As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls.
The chamber wall could be rotated to force new powders into remote collection vessels, and the electrode could be replaced by a solar mirror focused at the end of the rod.
[12] An alternative approach capable of producing a very narrow distribution of grain sizes but with low throughput consists of a rapidly spinning bowl heated to well above the melting point of the material to be powdered.
[citation needed] The dominant technology for the forming of products from powder materials, in terms of both tonnage quantities and numbers of parts produced, is die pressing.
[6] Sintering of powder metals is a process in which particles under pressure chemically bond to themselves in order to form a coherent shape when exposed to a high temperature.
The first zone, commonly coined the burn-off or purge stage, is designed to combust air, burn any contaminants such as lubricant or binders, and slowly raise the temperature of the compact materials.
Hydrogen, nitrogen, dissociated ammonia, and cracked hydrocarbons are common gases pumped into the furnace zones providing a reducing atmosphere, preventing oxide formation.
[19] This procedure, together with explosion-driven compressive techniques is used extensively in the production of high-temperature and high-strength parts such as turbine disks for jet engines.
[12] Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres or simple vacuum during forming and cooling stages.
[26] 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.
[27] Resistance sintering techniques are consolidation methods based on temperature, where heating of the mold and of the powders is accomplished through electric currents, usually with a characteristic processing time of 15 to 30 minutes.
Tubes, complex sections, and spiral drill shapes are manufactured in extended lengths and diameters varying in the range 0.5–300 mm (0.020–11.811 in).
Through a combination of high pressure and a complex strain path the powder particles deform, generate a large amount of frictional heat and bond together to form a bulk solid.
[28] 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.
