Page:Advanced Automation for Space Missions.djvu/171

 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