Page:Advanced Automation for Space Missions.djvu/186



Joining techniques involving elevated temperatures and materials fusion include welding, brazing, and soldering. Welding is a process leading to the permanent joining of materials (usually metals) through a suitable combination of temperature and pressure (DeGarmo, 1979). Approximately 40 different welding techniques have been utilized in terrestrial situations (Lindberg, 1977). Brazing and soldering require the use of a molten filler to join metal workpieces. The workpieces themselves are not melted; rather, capillary action facilitates the joining process. Brazing occurs when filler material reaches a melting temperature above 723 K (840°F); soldering uses fillers with melting points below 723 K (DeGarmo, 1979; Schey, 1977).

Within the three basic classes there are numerous joining alternatives for space manufacturing operations. Analysis is greatly simplified by reducing the 61 welding, brazing, and soldering techniques identified in table 4.17 to the following six major categories: electric arc welding, oxyfuel gas welding (i.e., gas-oxygen flame welding), resistance welding, solid-state welding, electronic welding, and brazing/ soldering. While some overlap is inevitable, this approach appears effective in providing first-order discrimination between immediately useful and less-feasible joining technologies appropriate for SMF deployment.

4E.1 Metals Joining Analysis

To determine the suitability of various joining processes for space and lunar manufacturing applications, selection criteria for SMF options (table 4.18) were applied to each major terrestrial welding, brazing, and soldering technique. These criteria include usefulness in the production of other manufacturing equipment; production rates and required consumables; energy of production; preparatory steps leading to the manufacture of the process itself or products it can help build; mandatory environmental characteristics to enable processing to proceed; feasibility of automation/teleoperation and people roles required (if necessary); further R&amp;D needed to develop promising alternatives; and a qualitative mass-multiplication ratio or "Tukey Ratio" (see chapter 5), an indication of the extent to which nonterrestrial (i.e., lunar) materials can be utilized as opposed to costly up-shipment of feedstock from Earth (Heer, 1980, unpublished draft notes of the Proceedings of the Pajaro Dunes Coal-Setting Workshop, June 1980.)

4E.1.1 Electric arc welding

Electric-arc-welding techniques include shielded or unshielded metal, gas metal (pulsed, short circuit, electrogas, spray transfer), gas tungsten, flux-cored, submerged, plasma arc, carbon arc, stud, electroslag, atomic hydrogen, plasma-MIG, and impregnated tape welding. The SMF suitability assessment is as follows:


 * Make other equipment - A basic joining process is needed.
 * Production rates - Houldcroft (1977) gives a figure of 3-140 mm2/sec and estimates a metal deposition rate of 1-12 kg/hr. Schwartz (1977) cites a 27 kg/hr figure for plasma are plus hot-wire welding.
 * Required consumables - Varies widely according to technique used. Electrodes, flux, wire, and gas (especially argon and helium, often in combination with H2, CO2, or O2) are all used in electric arc welding. Some techniques require only one of these four consumables; many use two. Stud welding demands special collars or ferrules, and 1-2 kg/m of metal also is needed (Houldcroft, 1977). Productivity varies with welding speed, current amplitude, and plate thickness.
 * Production energy - Required voltage ranges from 10-70 V, current from 2-2000 A (Schey, 1977; Schwartz, 1979). Romans and Simcns (1968) give a maximum value of 10,000 A for electroslag welding. A particularly useful quick survey of various electric arc techniques may be found in Lindberg(1977).
 * Preparation steps - A variety of hoses, valves, wire, switches, a power supply and gun are needed to make a welding unit. The amount of preparation required may be extensive in some cases (e.g., securing and aligning pieces, plates to contain slag, etc.). Other techniques require relatively little preparation.
 * Production environment - A pressurized welding environment is needed to use flux or slag processes.
 * Automation/teleoperation potential - Many of these techniques are already automated in terrestrial applications.
 * People roles - Other than design, none required.