Powder metallurgy

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. Finally, the end part is formed by applying pressure, high temperature, long setting times (during which self-welding occurs), or any combination thereof.

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. There is evidence that iron powders were fused into hard objects as early as 1200 B.C. 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 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 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. Competitive characteristics of manufacturing processing (e.g., tool wear, complexity, or vendor options) also may be closely regulated.

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. For example, cold-welding effects will be far more pronounced and dependable due to the absence of undesirable surface coatings. 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. Finally. 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. Powders may be prepared by comminution, grinding, chemical reactions, or electrolytic deposition. Several of the melting and mechanical procedures are clearly adaptable to operations in space or on the Moon.

Powders of the elements Ti, V, Th, Cb, Ta, Ca, and U have been produced by high-temperature reduction of the corresponding nitrides and carbides. Fe, Ni, U, and Be submicron 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, simultaneously atomizing and comminuting the material. 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. A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. On Earth, air and powder streams are segregated using gravity or cyclone devices.

Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance index used is the Reynolds number R = fvd/n, where f = fluid density, v = velocity of the exit stream, d = diameter of the opening, and n = absolute viscosity. At low R the liquid jet oscillates, but at higher velocities the stream becomes turbulent and breaks into droplets. 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. Other techniques such as nozzle vibration, nozzle asymmetry, multiple impinging streams, or molten-metal injection into ambient gas are all available to increase atomization efficiency, produce finer grains, and to narrow the particle size distribution. 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.

Centrifugal disintegration

Centrifugal disintegration of molten particles offers one way around these problems. Extensive experience is available with iron, steel, and aluminium. Metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle. Opposite the spindle tip is an electrode from which an are is established which heats the metal rod. As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls. A circulating gas sweeps particles from the chamber. Similar techniques could be employed in space or on the Moon. The chamber wall could be rotated to force new powders into remote collection vessels (DeCarmo, 1979), and the electrode could be replaced by a solar mirror focused at the end of the rod.

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. Such forces cause grains to disintegrate into yet finer particles.

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. Direct Solar energy can be used to melt the working materials, so the most energy-intensive portion of the operation requires a minimum of capital equipment mass per unit of output rate since low-mass solar collectors can be employed either on the Moon or in space. The two major energy input stages - powder manufacturing and sintering - require 19 MJ/kg and 17 MJ/kg, respectively. At a mean energy cost of $0.007/MJ, this corresponds to about $0.13/kg. Major savings might be possible in space using solar energy.

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. Release of the compact from its mold is usually accompanied by small volume increase called "spring-back."

In some pressing operations (such as hot isostatic pressing) compact formation and sintering occur simultaneously. This procedure, together with explosion-driven compressive techniques, is used extensively in the production of high-temperature and high-strength parts such as turbine blades for jet engines. In most applications of powder metallurgy the compact is hot-pressed, heated to a temperature above which the materials cannot remain work-hardened. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. 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. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders.

Continuous powder processing

The phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however, the term refers to processes whose products are much longer in one physical dimension than in the other two. Compression, rolling, and extrusion are the most common examples.

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. After stripping the compress from the conveyor the compact is introduced into a sintering furnace. An even easier approach is to spray powder onto a moving belt and sinter it without compression. Good methods for stripping cold-pressed materials from moving belts are hard to find. One alternative that avoids the belt-stripping difficulty altogether is the manufacture of metal sheets using opposed hydraulic rams, although weakness lines across the sheet may arise during successive press operations.

Powders can be rolled into sheets or more complex cross-sections, which are relatively weak and require sintering. It is possible that rolling and sintering processes can be combined, which necessitates relatively low roller speeds. 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. Considerable work also has been done on rolling multiple layers of different materials simultaneously into sheets.

Extrusion processes are of two general types. 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. Extrusions with binders are used extensively in the preparation of tungsten-carbide composites. Tubes, complex sections, and spiral drill shapes are manufactured in extended lengths and diameters varying from 0.05-30 cm. Hard metal wires 0.01 cm diam have been drawn from powder stock. At the opposite extreme, large extrusions on a tonnage basis may be feasible.

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 lengths may range from 3 to 30 m and diameters from 0.2 to 1 m. Modern presses are largely automatic and operate at high speeds (on the order of m/s).

Special products

Many special products are possible with powder metallurgy technology. A nonexhaustive list includes Al₂O₃ whiskers coated with very thin oxide layers for improved refractories; 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 submicron 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% Co) and uncoated tungsten carbides. It is interesting to consider whether similarly strong materials could be manufactured from aluminium films stretched thin over glass fibers (materials relatively abundant in space).