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.
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.
