Advanced Automation for Space Missions/Appendix 4E

From Wikisource
Jump to: navigation, search

Appendix 4E Review Of Welding, Brazing, And Soldering Techniques[edit]

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[edit]

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&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[edit]

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.
  • R&D required - Not a promising future line of inquiry.
  • Qualitative Tukey Ratio - Moderately poor. Some of the consumables, especially gases, are comparatively difficult to obtain in quantity from lunar soil.

4E.1.2 Oxyfuel welding[edit]

Included among the oxyfuel gas-welding techniques are oxyacetylene, methylacetylene propadiene (MAPP), airacetylene, oxyhydrogen, and pressure gas. The SMF suitability assessment produced the following results:

  • Make other equipment - Need a basic joining process.
  • Production rates - Estimates include 0.6 to 10 m/hr (2 to 30 ft/hr) (Romans and Simons, 1968), 1.6 to 5.4 mm2/sec (Houldcroft, 1977), and 0.3 to 0.6 kg/hr (Schey, 1977).
  • Required consumables - Main consumables are gases, especially acetylene and oxygen. Filler rods and flux may or may not be required (Houldcroft, 1977).
  • Production energy - Romans and Simons (1968) claim that vertical welding at a rate of 2-5 m/hr requires 70-500 liters/hr of acetylene gas at STP.
  • Preparation steps - Need gases in pressure tanks, a simple valve/regulator structure, gauges, hoses, torch and torch tip assemblies (Griffen et al., 1978). Surface preparation of workpieces requires a basic cleaning process. Jigs typically are used to hold the workpiece in the proper positions.
  • Production environment - Gases necessitate a pressurized environment.
  • Automation/teleoperation potential - Already automated in many terrestrial manufacturing applications (Phillips, 1963; Yankee, 1979).
  • People roles - Could conceivably be used by astronauts to perform quick, portable repair welding operations in a pressurized environment.
  • R&D required - Not a promising future line of inquiry.
  • Qualitative Tukey Ratio - Very poor in the near-term due to heavy dependence on gases comprised of chemical elements having low lunar abundances (eg., acetylene, MAPP, hydrogen).

4E.1.3 Resistance welding[edit]

Resistance techniques include spot, projection, seam, flash butt, upset, and percussion welding. (High-frequency resistance welding is discussed as an electronic welding technology.) The assessment follows:

  • Make other equipment - Basic joining process is needed.
  • Production rates - Ranges from 16 to 107 mm2/sec are given by Houldcroft (1977) for resistance welding generally, and Romans and Simons (1968) estimate 1 to 4 m/min (36 to 144 in./min) for seam welding.
  • Required consumables - Air (gas) or water are necessary to provide high pressures, and water is needed for cooling. Substitutes can probably be found among available nonterrestrial materials.
  • Production energy - Considerable electrical energy is required, typically 1000 to 100,000 A at 2 to 20 V (Moore and Kibbey, 1965; Romans and Simons, 1968). Romans and Simons give a range of 1 to 140 W/hr per spot weld.
  • Preparation steps - Modest resistance welding machines require very large power supplies. Pressure-producing cylinders for larger equipment are somewhat complex, and sophisticated timing devices are necessary. However, little preparation of materials is needed, perhaps the key reason why resistance welding is so popular on Earth (Moore and Kibbey, 1965).
  • Production environment - Moore and Kibbey (1965) indicate that air must be supplied for operation of the electrodes, so a pressurized environment may be necessary.
  • Automation/teleoperation potential - These techniques have already been largely automated on Earth.
  • People roles - None required other than design.
  • R&D required - Not a promising future line of inquiry.
  • Qualitative Tukey Ratio - Moore and Kibbey (1965) note that resistance welding electrodes are subjected to 10,800 A/cm2 at 410 MN/m2 (70,000 A/in.2 at 60,000 psi). It seems unlikely that lunar-abundant aluminum could even come close to replacing copper-bronze and copper-tungsten alloys used to make electrodes on Earth. Also, it is questionable whether aluminum could be incorporated in the massive high-current power transformers required. The Tukey Ratio appears quite poor in this case.

4E.1.4 Solid-state welding[edit]

Included within this category are ultrasonic, explosive, diffusion, friction, inertia, forge, vacuum (cold), and roll welding. The SMF assessment is as follows:

  • Make other equipment - Need a basic joining process.
  • Production rates - On thin materials, roller-seam ultrasonic welds can be produced at rates up to 10 m/min.
  • Required consumables - Air, water, or other pressure-producing agents are needed for cold, friction, and roll welding. Explosion welding uses explosive sheets with TNT, ammonium nitrate, amatol, and others. Ultrasonic welding requires a transmission medium for sound waves.
  • Production energy - Vacuum (cold) welding requires only a very light pressure. Ultrasonic welders are rated at up to 25 kW (Schwartz, 1979).
  • Preparation steps - Materials to be cold welded under vacuum need only be appropriately positioned for application of modest pressure, though the exact preparation steps for a vacuum welding machine are unknown. Explosion welding involves placing an explosive sheet on the workpieces. Friction and inertia welding require a driving system, hydraulic cylinder, bearing, bearing enclosure, etc. Ultrasonic welding utilizes a rigid anvil, a welding tip consisting of a piezoelectric crystal and a transducer with horn, and a force-application mechanism. Parts alignment is a crucial step in all joining processes.
  • Automation/teleoperation potential - Most, if not all, of these techniques should readily be automatable.
  • People roles - None, other than original design.
  • R&D required - Cold welding has the highest appeal as a simple joining process. A system of applying small pressures without accidentally contact welding the machine to the workpiece must be devised. One simple method is a vise made of insulated metal parts (teflon- or oxide-coated). More must be learned about cold-welding properties of various materials.
  • Qualitative Tukey Ratio - Seems likely to be extremely good for cold welding. Closely related forms such as friction, inertia and roll welding should also exhibit satisfactory Tukey Ratios, since only small pressures need be applied. Forge and diffusion welding require heat as well (and hence, seem superfluous), but can probably exhibit favorable ratios with some modification. The ratio for ultrasonic welding appears relatively poor.

4E.1.5 Electronic welding[edit]

Electronic welding methods encompass forms of electron-beam, laser, induction, and high-frequency resistance welding. The following is the SMF suitability assessment:

  • Make other equipment - A basic joining process is needed. (Note: A number of these techniques, particularly the laser, can be used for many other options.)
  • Production rates - Lindberg (1977) cites a figure of 16 m/hr (50 ft/hr) for a high-power continuous-wave solid-state laser. The estimate by Schwartz (1979) is much higher: 50 to 80 m/hr (150 to 250 ft/hr). Electron-beam welders can produce up to 1800 small parts per hour in a partial vacuum (Schwartz, 1979) or up to 200 mm2/sec of welding (Houldcroft, 1977). Induction welding production rates are given as 6.5 m/min (20 ft/min) of 20 cm (8 in.) pipe (Phillips, 1963) and 3.1 m/min (122 in./min) of tube welding for typical machines (Lindberg, 1977). High-frequency resistance methods can weld seams at 50 m/min (150 ft/min) with 60% efficiency (Schwartz, 1979).
  • Required consumables - Flashlamps for solid-state lasers have a lifetime of 104 to 105 shots. Gas lasers may use a variety of gases including CO2/H2/N2, argon, krypton, neon, xenon, and others. Electron-beam filaments last 2 to 1000 hr depending on filament type. High-frequency resistance welding contacts are good for roughly 6,000 to 130,000 m (50,000 to 400,000 ft) of welding before they must be replaced (Schwartz, 1979).
  • Production energy - Lasers require up to 15 to 20 kW (Lindberg, 1977; Schwartz, 1979). Schwartz notes that gas lasers are inefficient (less than 0.1%) relative to solid-state lasers (up to 10% efficiency). Electron-beam welders draw 6 to 75 kW, with voltages in the 15 to 200 kV range. The American Welding Society (Phillips, 1963) estimates 1 to 600 kW output power for induction welding - as much as 1 MW may be needed in some cases. Energy requirements for high-frequency resistance welding are much lower than other resistance techniques due to increased resistivity at higher (400 kHz) frequencies (Lindberg, 1977; Schwartz, 1979). Schwartz claims that the most powerful high-frequency resistance welding machines in terrestrial use draw 150 kW, though many require only 1 to 50 kW.
  • Preparation steps - A solid-state laser is comprised of a rod, laser cavity, precision-ground mirrors, flashlamp, cooling system, focusing optics, and power supply. In recent years ruby rods have been increasingly replaced by Nd:YAG rods (Schwartz, 1979). Flashlamps usually are xenon- or krypton-filled (Lindberg, 1977). Gas lasers do not need rods and flashlamps of such exotic composition, but instead require gas and a heat exchanger. Electron-beam welders need a sophisticated variant of the cathode-ray tube, a very high voltage power supply, and preferably a vacuum environment. Induction welding units are characterized by a large coil at low frequencies, a high-power oscillator circuit at high frequencies, 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&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[edit]

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&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[edit]

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.

Of the most promising techniques, vacuum welding and vacuum brazing seem the simplest, the least energy-consuming, and exhibit the best Tukey Ratios. Vacuum brazing requires some heat to melt filler material, but probably bonds a greater variety of materials (e.g., refractory and reactive bare metals, ceramics, graphite, and composites) than vacuum welding methods. Electronic techniques offer poorer mass multiplication ratios, especially in the case of the laser. However, both E-beams and laser beams are extremely versatile - besides welding a very wide variety of materials, lasers can drill, cut, vapor deposit, heat treat, and alloy (Schwartz, 1979). They can cast and machine as well as weld, making them excellent candidates for the initial elements of a space manufacturing bootstrap operation. High-frequency resistance and induction welding can also join a wide variety of materials, and with high efficiency. Table 4.29 compares key characteristics of laser and electron-beam processes with those of two less-promising alternatives for space and lunar applications. It is apparent that both E-beam and laser techniques are competitive in most categories whether on Earth or in space. Equipment cost of the E-beam should be much lower in a vacuum environment, since the major expense in terrestrial applications is for the maintenance of proper vacuum.

Figure 4.29 provides a useful overview of welding capabilities for various material thicknesses. While this factor has not yet been discussed it is nonetheless important, since production speed diminishes nonlinearly with penetration depth. It is interesting to note that the combination of laser and E-beam technologies spans the entire range of usual material thicknesses. No direct data were available on the vacuum-welding technique, but this range conceivably could be quite large.

Figure 4.29.- Thickness range of welding processes.

From the standpoint of automation in space, a final and most significant conclusion is that all joining processes of interest appear readily automatable. Joining should pose no insurmountable problems for space or lunar manufacturing facilities. General-purpose repair welding must probably be accomplished initially via teleoperation, as this activity requires a much higher degree of intelligence and adaptability.

Table 4.29.- Simplified Qualitative Comparisons Between Lasers, E-Beams, And Two Common Forms Of Resistance And Arc Welding
Characteristic Laser E-beam Resistance (spot) Electric arc (gas tungsten)
Heat generation Low Moderate Moderate-high Very high
Weld quality Excellent Excellent Good Excellent
Weld speed Moderate High Moderate High
Initial costs Moderate High Low Low
Operating/maintenance costs Low Moderate Low Low
Tooling costs Low High High Moderate
Controllability Very good Good Low Fair
Ease of automation Excellent Good Fair Fair
Range of dissimilar materials which can be welded Very wide Wide Narrow Narrow

4E.3 References[edit]

DeCarmo, E. P.: Materials and Processes in Manufacturing. Macmillan, New York, 1979. Fifth Edition.

Griffin, I. H.; Roden, E. M.; and Briggs, C. W.: Welding Processes, Van Nostrand Reinhold Company, New York, 1978. Second edition.

Houldcroft, P. T.: Welding Process Technology. Cambridge University Press, Cambridge, England, 1977.

Lindberg, R. A.: Processes and Materials of Manufacture. Allyn and Bacon, Boston, 1977. Second Edition. 714 pp.

Moore, H. D.; and Kibbey, D. R.: Manufacturing Materials and Processes, Irwin, Homewood, Illinois, 1965.

Phillips, A. L., ed.: Welding Handbook: Welding Processes - Gas, Are and Resistance. American Welding Society, New York, 1963. Fifth Edition.

Romans, D.; and Simons, E. N.: Welding Processes and Technology. Pitman and Sons. London, England, 1968.

Schey, J. A.: Introduction to Manufacturing Processes. McGraw-Hill, San Francisco, 1977.

Schwartz, M. M.: Metals Joining Manual. McGraw-Hill Book Co., San Francisco, 1979.

Yankee, H. W.: Manufacturing Processes. Prentice-Hall, Englewood Cliffs, New Jersey, 1979. 765 pp.