Making metal shapes by compressing powdered metal into a finished or near-finished shape. First used for tungsten lamp filaments, it is now used for such products as tungsten carbide cutting tools and self-lubricating bearings. It can be used on iron, tin, nickel, copper, aluminium, and titanium. The powders are made a specific and regular size by atomization (cooling a spray of molten metal) or controlled chemical precipitation. The material is pressed into shape and then heat-treated (sintered). In an alternative method, the powder is fed from a hopper into a gap between rollers to produce a strip. The advantages of powder metallurgy are economy of manufacture and porosity where needed.
Powder metallurgy is a forming and fabrication technique consisting of three major processing stages. First, the primary material is physically powdered - divided into many small individual particles. Next, the powder is injected into a mold or passed through a die to produce a weakly cohesive structure (via cold welding) very near the true dimensions of the object ultimately to be manufactured.
History and capabilities
The history of powder metallurgy and the art of metals and ceramics sintering are intimately related. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. In these early manufacturing operations, iron was extracted by hand from 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 using powder processes than from direct alloying of fused materials.
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 forming, 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.
Powder Metallurgy products are today used in a wide range of industies, from automotive and aerospace applications to power tools and household appliances.
Powder metallurgy in space-based manufacturing
Powder metallurgy in zero-g airless space or on the Moon offers several potential advantages over similar applications on Earth. Gravitational settling in polydiameter powder mixtures can largely be avoided, permitting the use of broader ranges of grain sizes in the initial compact and correspondingly lower porosities. it should be possible to selectively coat particles with special films which artificially inhibit contact welding until the powder mixture is properly shaped. (The film is then removed by low heat or by chemical means, forming the powder in zero-g conditions without a mold.)
Powder Production Techniques
Any fusible material can be atomized. Several techniques have been developed which permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. On Earth various chemical- and flame-associated powdering processes are adopted in part to prevent serious degradation of particle surfaces by atmospheric oxygen.
Atomization
Atomization is accomplished by forcing a molten metal stream through an orifice at moderate pressures. On Earth, air and powder streams are segregated using gravity or cyclonic separation.
Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. 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.
Centrifugal disintegration
Centrifugal disintegration of molten particles offers one way around these problems. Metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle.
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. Liquid metal, introduced onto the surface of the basin near the center at flow rates adjusted to permit a thin metal film to skim evenly up the walls and over the edge, breaks into droplets, each approximately the thickness of the film.
Other techniques
Another powder-production technique involves a thin jet of liquid metal intersected by high-speed streams of atomized water which break the jet into drops and cool the powder before it reaches the bottom of the bin. In subsequent operations the powder is dried.
Finally, mills are now available which can impart enormous rotational torques on powders, on the order of 2.0×107 rpm.
Powder production in space-based manufacturing
Powders prepared in the vacuum of space will largely avoid this problem, and the availability of zero-g may suggest alternative techniques for the production of spherical or unusually shaped grains.
Two powdering techniques which appear especially applicable to space manufacturing are atomization and centrifugal disintegration. The two major energy input stages - powder manufacturing and sintering - require 19 MJ/kg and 17 MJ/kg, respectively.
Powder pressing
Although many products such as pills and tablets for medical use are cold-pressed directly from powdered materials, normally the resulting compact is only strong enough to allow subsequent heating and sintering.
In some pressing operations (such as hot isostatic pressing) compact formation and sintering occur simultaneously. In most applications of powder metallurgy the compact is hot-pressed, heated to a temperature above which the materials cannot remain work-hardened. Also it permits better dimensional control of the product, lessened sensitivity to physical characteristics of starting materials, and allows powder to be driven to higher densities than with cold pressing, resulting in higher strength. Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres during forming and cooling stages.
One recently developed technique for high-speed sintering involves passing high electrical current through a powder to preferentially heat the asperities.
Continuous powder processing
The phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time.
In a simple compression process, powder flows from a bin onto a two-walled channel and is repeatedly compressed vertically by a horizontally stationary punch. An even easier approach is to spray powder onto a moving belt and sinter it without compression.
Powders can be rolled into sheets or more complex cross-sections, which are relatively weak and require sintering. Powder rolling is normally slow, perhaps 10 to 100 mm/s. This is due in part to the need to expel air from compressed powder during manufacture. In one type, the powder is mixed with a binder or plasticizer at room temperature; in the other, the powder is extruded at elevated temperatures without fortification. Hard metal wires 0.01 cm diam have been drawn from powder stock.
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.
Extrusion Temperatures Of Common Metals And Alloys| Metals and alloys | Temperature of extrusion, K |
|---|---|
| Aluminium and alloys | 673-773 |
| Magnesium and alloys | 573-673 |
| Copper | 1073-1153 |
| Brasses | 923-1123 |
| Nickel brasses | 1023-1173 |
| Cupro-nickel | 1173-1273 |
| Nickel | 1383-1433 |
| Monel | 1373-1403 |
| Inconel | 1443-1473 |
| Steels | 1323-1523 |
Special products
Many special products are possible with powder metallurgy technology. light bulb filaments made with powder technology;
Extremely thin films and tiny spheres exhibit high strength.
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