(c) Special Forming Operations
The following forming operations are considered as a group with respect to robotics applications and lunar factory criteria: conventional stretching, conventional drawing (involving nine suboperations) and deep drawing, swaging, spinning, and bending.
Stretching is a cold-forming process in which sheet metal is wrapped around an upward-moving form block. Conventional drawing involves pressing a flat metal blank into a male die while stretching the blank to force it to conform to the shape of a male die or punch. Shallow drawing is defined as a deformation cup no deeper than half its diameter with little thinning of the metal, whereas, deep drawing produces a cup whose depth may exceed its diameter with more pronounced wall thinning. Swaging is a cold-forging process in which an impact or compressive force causes metal to flow in a predetermined direction. Spinning is a forming technique for plastically deforming a rapidly rotating flat disk against a rotating male contour. Cold spinning is used for thin sheets of metal. Hot-spinning of heavier sheets up to 150 mm thick can produce axisymmetric (shell) shapes. Finally, bending is the plastic deformation of metals about a linear axis with little or no change in the surface area.
Robotics applications and space manufacturing options for these types of deformation processes are minimal, especially under vacuum conditions. If there is no oxidized film on the metal, the workpiece and die may contact weld, causing the machine to seize.
(d) Extrusion
In the extrusion process, either at high or low temperatures, metal is compressively forced through a suitably shaped die to form a product with reduced cross-section - like squeezing toothpaste from a tube. Lead, copper, aluminum, magnesium, and their many alloys are commonly employed, and hydrostatic extrusion using high-pressure fluids into the die makes possible similar processing of relatively brittle materials such as molybdenum, beryllium, and tungsten. Steel is relatively difficult to extrude because of its high-yield strength and its tendency to weld to die walls. Extrusion by pressurizing solid metals shares with other deformation processes problems of cold welding. However, the degree of such welding decreases if markedly dissimilar metals are in contact. The vacuum environment may enhance ductility for some extruded metals.
In one variant of the basic extrusion process, melts are drawn through dies to produce threads. The use of basalt in preparing spun products is well known (Kopecky and Voldan, 1965; Subramanian et al., 1975, 1979) and has numerous lunar applications (see table 4.16). A variation of the technique is the use of centrifugal force to spin the extruded threads (Mackenzie and Claridge, 1979).
In commercial spun basalt processes, molten basalt is drawn through a platinum-rhodium bushing and the final fiber blasted by a tangential gas or steam jet in the air cone as shown in figure 4.7. Fibers also may be produced without the air cone by direct pulling of a winding reel. For example, work done by Subramanian et al. (1975) showed that molten basalt flowing from a 3-mm hole in a graphite crucible, yielded fibers by simple mechanical pulling (table 4.26). The crude fibers created using this procedure were nonuniform, measured about 150 um diam, and contained many nodules - a poor product compared with air cone output. Assuming the air/steam cone can be eliminated from basalt spinning operations, a step-by-step Unimate-automatable sequence is suggested in table 4.27.
As yet no research has been performed either on vacuum or lunar basalt fiber drawing. Molten basalt on the Moon has very low viscosity which may possibly be controlled, if necessary, by additives. At present it remains unknown whether mechanical spinning of raw lunar basalts is possible or if the vacuum environment will yield a thinner, more uniform product. Still, extrusion of viscous rock melts to produce spun products appears promising and as indicated in table 4.27 is likely amenable to automation in space-manufacturing applications.
| Temperature at bottom of bushing, K | Fiber size, um | Tensile strength, | |
|---|---|---|---|
| MPa | psi | ||
| 1450 | 9-11 | 66 | 96,000 |
| 1510 | 9-10 | 134 | 196,000 |
| 13-15 | 130 | 190,000 | |
| 1525 | 7-9 | 143 | 209,000 |
| 9-11 | 163 | 238,000 | |
| 13-16 | 145 | 212,000 | |
| 1560 | 8-10 | 136 | 190,000 |
| 11-13 | 128 | 187,000 | |
| 15-18 | 132 | 193,000 | |
| 1600a | 7.5 | 149 | 218,000 |
aAverage of only five specimens.
(e) Shearing
Shearing is the mechanical cutting of sheet or plate materials using two straight cutting blades, without chip
