8. 21 The EB welding technology used on the work bed will be discussed next followed by a more detailed study of the fabrication procedure. VIII,5.2: Electron Beam Welding VIII.5.2.1: Advantages of EB Welding: The advantages of EB welding in the given application are numerous. Perhaps the most important are the high speed of welding achieved by EB devices and the fact that material of two different thicknesses can be welded together easily (8.7) (8.8). The fast welding rate is due to the very high power density in the electron beam (~ 10 6 watts/m 2 (8.9)) and the large penetration ratio of 20 to l into the work. Taken together,these features account for the fact that a high power EB device can make a singlepass through-weld in steel as thick as 250 mm and in aluminum as thick as 360 mm (8.10) and a through weld at rates as high as 3m/min in plates of 30-60 mm thick. Because it makes one quick pass, EB welding leaves a smaller fusion zone and head affected zone than other welding techniques. The resulting lower total heat input reduces thermal distortion and the welding process has less effect on material allowables. Weld efficiency is, of course, dependent on the material in use. Some high strength steels can be welded after heat treating without a significant lowering of tensile properties (8.7). The most conservative estimate of weld efficiency found was 70% (for an EB weld in 2219-T87 (8.8)). Because fatigue design considerations lead to low design stress levels in the hull, this high weld efficiency is not the most important advantage of EB welding in colony construction. A final advantage of EB welding is the large distance between the gun and the work. This makes remote observation of the weld and automatic control of the head much easier. In fact, a completely automatic EB welder with feedback capable of adjusting the beam directly to the joint is expected to be developed by the end of 1976 (8 .11). VIII.5.2.2: Disadvantages of EB Welding; Alternative Fastener Systems: There are several disadvantages to EB welding. The major one is that there must be an almost total vacuum around the electron gun. This
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