Advanced Automation for Space Missions/Appendix 4C
Appendix 4C Review Of 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 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. Powder metallurgy technologies may be utilized by minimum initial support facilities to prepare a widening inventory of additional manufacturing techniques, and offer the possibility of creating "seed factories" able to grow into more complex production facilities which can generate many special products in space. The following sections review the basics of powder metallurgy (Jones, 1960).
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. (Jones, 1960). 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 aluminum/iron powders poses major problems (Sheasby, 1979). Other substances that are especially reactive with atmospheric oxygen, such as tin (Makhlouf et at, 1979), are sinterable in special atmospheres or with temporary coatings. Such materials may be manipulated far more extensively in controlled environments in space.
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 (Kahn, 1980), 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.
4C.1 Cold Welding
Cold or contact welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. It is now known that the force of adhesion following first contact can be augmented by pressing the metals tightly together, increasing the duration of contact, raising the temperature of the workpieces, or any combination of the above. Research has shown that even for very smooth metals, only the high points of each surface, called "asperites," touch the opposing piece. Perhaps as little as a few thousandths of a percent of the total surface is involved. However, these small areas of taction develop powerful molecular connections - electron microscope investigations of contact points reveal that an actual welding of the two surfaces takes place after which it is impossible to discern the former asperitic interface. If the original surfaces are sufficiently smooth the metallic forces between them eventually draw the two pieces completely together and eliminate even the macroscopic interface.
Exposure to oxygen or certain other reactive compounds produces surface layers which reduce or completely eliminate the cold welding effect. This is especially true if, say, a metal oxide has mechanical properties similar to those of the parent element (or softer), in which case surface deformations do not crack the oxide film. Fortunately, the extremely low concentrations of contaminating gases in free space (less than 10-14 torr is achievable) should produce minimal coating, so cold welding effects can persist on fresh metal surfaces for very long periods. Contact welding promises a convenient and powerful capability for producing complex objects from metallic powders in space with a minimum of support equipment.
Powders use cold welding to best advantage because they present large surface areas over which vacuum contact can occur. For instance, a 1 cm cube of metal comminuted into 240-100 mesh-sieved particles (60-149 μm) yields approximately 1.25×106 grains having a total surface area of 320 cm2. This powder, reassembled as a cube, would be about twice as big as before since half the volume consists of voids.
If a strong final product is desired, it is important to obtain minimum porosity (that is, high starting density) in the initial powder-formed mass. Minimum porosity results in less dimensional change upon compression of the workpiece as well as lower pressures, decreased temperatures, and less time to prepare a given part. Careful vibratory settling reduces porosity in monodiameter powders to less than 40%. A decrease in average grain size does not decrease porosity, although large increases in net grain area will enhance the contact welding effect and markedly improve the "green strength" of relatively uncompressed powder. In space applications cold welding in the forming stage may be adequate to produce usable hard parts, and molds may not even be required to hold the components for subsequent operations such as sintering.
Hard monodiameter spheres packed like cannonballs into body-centered arrays give a porosity of about 25%, significantly lower than the ultimate minimum of 35% for vibrated collections of monodiameter spheres. (The use of irregularly shaped particles produces even more porous powders.) Porosity further may be reduced by using a selected range of grain sizes, typically 3-6 carefully chosen gauges in most terrestrial applications. Theoretically. this should permit less than 4% porosity in the starting powder, but with binary or tertiary mixtures 15-20% is more the rule. Powders comprised of particles having a wide range of sizes, in theory can approach 0% porosity as the finest grains are introduced. But powder mixtures do not naturally pack to the closest configuration even if free movement is induced by vibration or shaking. Gravitational differential settling of the mixture tends to segregate grains in the compress, and some degree of cold welding occurs immediately upon formation of the powder compress which generates internal frictions that strongly impede further compaction. Considerable theoretical and practical analyses already exist to assist in understanding the packing of powders (Dexter and Tanner. 1973; Criswell, 1975; Powell. 1980a. 1980b; Shahinpoor, 1980; Spencer and Lewis, 1980, Visscher and Bolsterzi, 1973).
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.)
Moderate forces applied to a powder mass immediately cause grain rearrangements and superior packing. Specifically, pressures of 105 Pa (N/m2) decrease porosity by 1-4%; increasing the force to 107 Pa gains only an additional 1-2%. However, at still higher pressures or if heat is applied the distinct physical effects of particle deformation and mass flow become significant. Considerably greater force is required mechanically to close all remaining voids by plastic flow of the compressed metal.
Sintering is the increased adhesion between particles as they are heated. In most cases the density of a collection of grains increases as material flows into voids causing a decrease in overall size. Mass movements which occur during sintering consist of the reduction of total porosity by repacking, followed by material transport due to evaporation and condensation with diffusion. In the final stages metal atoms move along crystal boundaries to the walls of internal pores, redistributing mass from the internal bulk of the object and smoothening pore walls.
Most, if not all, metals may be sintered. This is especially true of pure metals produced in space which suffer no surface contamination. Many nonmetallic substances also sinter, such as glass, alumina, silica, magnesia, lime, beryllia, ferric oxide, and various organic polymers. The sintering properties of lunar materials have been examined in detail (Simonds, 1973). A great range of material properties can be obtained by sintering with subsequent reworking. Physical characteristics of various products can be altered by changing density, alloying. or heat treatments. For instance, the tensile strength En of sintered iron powders is insensitive to sintering time, alloying, or particle size in the original powder, but is dependent upon the density (D) of the final product according to En/E = (D/d)3.4, where E is Young's Modulus and d is the maximum density of iron.
Particular advantages of this powder technology include: (1) the possibility of very high purity for the starting materials and their great uniformity; (2) preservation of purity due to the restricted nature of subsequent fabrication steps; (3) stabilization of the details of repetitive operations by control of grain size in the input stages: (4) absence of stringering of segregated particles and inclusions as often occurs in melt processes: and (5) no deformation is required to produce directional elongation of grains (Clark, 1963). There exists a very large literature on sintering dissimilar materials for solid/solid phase compounds or solid/melt mixtures in the processing stage. As previously noted (and see below), any substance which can be melted may also be atomized using a variety of powder production techniques. Finally, when working with pure elements, scrap remaining at the end of parts manufacturing may be recycled through the powdering process for reuse.
4C.3 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. 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. Kaufman (1979) has presented estimates of the total energy input of the complete powdering process in the production of iron parts. The two major energy input stages - powder manufacturing and sintering - require 5300 kW-hr/t and 4800 kW-hr/t, respectively. At a mean energy cost of $0.025/kW-hr, this corresponds to $250/t or about $0.11/kg. Major savings might be possible in space using solar energy.
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. Cyclone separators could be used in space, although an additional step would be required - introduction of the powder into a pumping chamber so that the working gas may be removed and reused. Evacuated metal would then be transferred to the zero-pressure portion of the manufacturing facility. Figures 4.24 and 4.25 present schematics of major functional units of terrestrial facilities for metal atomization (DeCarmo, 1979; Jones, 1960).
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 um. Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and remelting a significant fraction of the grain.
Centrifugal disintegration of molten particles offers one way around these problems, as shown in figure 4.25(a). Extensive experience is available with iron, steel, and aluminum (Champagne and Angers, 1980). 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 arc 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 (Jones, 1960).
Figure 4.25(b) illustrates another powder-production technique. A thin jet of liquid metal is 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. In space applications it would be necessary to recycle the water or other atomizing fluid.
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. Operations in free space should permit a variety of related approaches.
4C.4 Powder Pressing
An extensive literature on the various aspects of powder pressing is available and growing rapidly. 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 space, compact strength should far exceed that on Earth due to powerful cold-welding effects on pristine grain surfaces.
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-amperage 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 (DeCarmo, 1979).
4C.5 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 (Jones, 1960).
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 0.01-0.1 m/sec. This is due in part to the need to expel air from compressed powder during terrestrial manufacture, a problem which should be far less severe in space applications. 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, Jones (1960) considers that large extrusions on a tonnage basis may be feasible. He anticipates that problems associated with binder removal, shrinkage from residual porosity during sintering, and maintenance of overall dimensional accuracies are all controllable. Low die and pressure cylinder wear are expected. Also, it seems quite reasonable to extrude into a vacuum.
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. Table 4.25 lists extrusion temperatures of various common metals and alloys. Extrusion lengths may range from 3-30 m and diameters from 0.2-1.0 m. Modern presses are largely automatic and operate at high speeds (on the order of m/sec). Figure 4.26 illustrates seven different processes for generating multilayer powder products by sheathed extrusion.
|Metals and alloys||Temperature of extrusion, K|
|Aluminum and alloys||673-773|
|Magnesium and alloys||573-673|
4C.6 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. The product list can be considerably expanded using terrestrial materials. A profitable line of research would be to determine which elements if brought to LEO could offer especially large multiplier effects in terms of the ratio of lunar-materials mass to Earth-supplied mass.
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 aluminum films stretched thin over glass fibers (materials relatively abundant in space).
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