Page:Advanced Automation for Space Missions.djvu/174

 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.

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).