Page:Advanced Automation for Space Missions.djvu/189

 and a heavy-duty power supply and cooling system. High-frequency resistance welding differs from induction joining only in that its contacts are supplied at relatively low loads. Finally, workpieces require alignment. Electron-beam and laser techniques typically are reserved for small, shallower welds demanding very precise alignment. Induction welding usually is in conjunction with a pressure-producing machine.
 * Production environment - Electron-beam welders work best in a vacuum. Gas lasers require an enclosed chamber to contain the gas. Otherwise electronic welding techniques appear fairly adaptable.
 * Automation/teleoperation potential - Lindberg (1977) notes that both E-beam and laser welding techniques are easy to automate. Induction welding also has been automated to a considerable extent in terrestrial manufacturing.
 * People roles - None required beyond the design phase.
 * R&amp;D required - Further developments in electron-beam and laser technologies are likely to be highly fruitful. Laser flashlamp lifetimes must be greatly increased.
 * Qualitative Tukey Ratio - The Ratio is somewhat poor for solid-state lasers using present-day technologies, though the components are not too massive and so could be lifted from Earth with only modest penalty. With some possible substitutions the Ratios for other electronic welding options appear favorable. Some essential materials may be difficult to obtain in sufficiently large quantities (such as the carbon for CO2 or inert gases in a gas laser).

4E.1.6 Brazing and soldering

Among the various brazing processes identified in this study are torch, induction, furnace, dip, resistance, infrared, and especially vacuum methods. Soldering includes iron, resistance, hot plate, oven. induction, dip, wave, and ultrasonic techniques. The space manufacturing suitability assessment follows:


 * Make other equipment - Since bracing and soldering make weaker bonds than welding they are somewhat less universal in common use. On the other hand, some very dissimilar materials can be brazed but not welded.
 * Production rates - No figures were given in any of the references reviewed. Wave soldering allows the processing of entire circuit boards (hundreds of components) in a few seconds.
 * Required consumables - Filler metals or alloys and fluxes usually are required, though some processes are fluxless.
 * Production energy - Highly variable. (See oxy-fuel gas welding for estimates on one common method.) The major difference between these techniques and welding with respect to production energy is that less heat is required.
 * Preparation steps - Alignment jigs are needed to position workpieces to a fairly high degree of accuracy. Flux and heat are applied first, followed by filler material. Some fluxes and fillers are combined. Vacuum brazing requires filler only.
 * Production environment - A pressurized environment is mandatory except for vacuum and fluxless brazing.
 * Automation/teleoperation potential - These processes are not extremely complex. Furnace brazing and wave soldering are contemporary examples of automated or semiautomated systems.
 * People roles - None except for design.
 * R&amp;D required - Fluxless brazing (e.g., of aluminum) and vacuum brazing appear fruitful research avenues worthy of further exploration.
 * Qualitative Tukey Ratio - The Ratio is poor in most cases. The most commonly used brazing metals (fillers) are copper and copper/silver/aluminum alloys; solders typically are tin/lead mixtures. Most flux materials are not readily available from nonterrestrial sources. However, the Tukey Ratios for vacuum and fluxless brazing of aluminum, titanium, and a few other metals seem rather promising.

4E.2 Summary of Metal-Joining Options in Space Manufacturing

Perhaps the most significant conclusion to be drawn from the preceding analysis is that NASA is on the right track in its research and development efforts on space-qualifiable joining processes. Most promising are vacuum or cold-pressure welding in the solid-state category, the various electronic welding techniques (E-beam, laser, induction, and high-frequency resistance welding), and vacuum and fluxless brazing. NASA has already done some research on electron-beam and laser welding (including successful experiments in space) and vacuum brazing. Explosion welding may be useful if an explosive can be developed from lunar materials and the shock wave made to propagate in a vacuum environment. Friction welding might usefully be combined with vacuum welding (at lower pressures than required on Earth) to quickly remove protective coatings which inhibit undesired contact welding.