3.3.1.1 Welding
In vacuum equipment, components of mild and stainless steel are usually welded together for vessels and joints. In addition, it is also possible to weld aluminum components together. To ensure that the welds that are produced are vacuum-tight, it is necessary to use proper materials that are free of cracks and voids, and whose surfaces are smooth and free of grease. In addition, a special geometric design is also required that sometimes differs from the normal welded connections that are employed for non-vacuum applications. Wherever possible in terms of engineering, interior welds must be provided in order to avoid vacuum-side gaps and cracking, so-called latent or virtual leaks. If this is not possible, the weld must extend through to the vacuum side. Where necessary, a supplemental atmosphere-side weld can be employed to increase mechanical stability. In this connection, it is important that this supplemental weld not be continuous in order to allow leak detection, if necessary, and have no air inclusions.
The welding of vacuum components and chambers requires special knowledge and the welding personnel must have a professional qualification. Usually, a welding company documents this with regular tests of their welders through independent testing institutes. In addition, welding procedure tests for each welded material and the weld geometry should be carried out. Specially trained welding personnel, for example welding engineers or technicians, accompany and evaluate the welding work.
The welding heat and the relatively rapid cooling can change the properties of materials. For example, changes in the structure during welding of austenitic stainless steels can increase the magnetizability or result in pores and hot cracks occurring during welding of aluminum (this was already mentioned in Chapter 3.2.1.1 “Stainless steel” and 3.2.1.3 “Aluminum”). In addition, high residual stresses in the weld area lead to the distortion of the components, which must be kept as low as possible. If functional areas like sealing surfaces are affected, they must be reworked. If this is not possible, it can lead to a loss of the entire workpiece. However, various welding measures can be taken to prevent this, including the selection of a suitable welding method combined with a suitable weld geometry and welding sequence, welding preparation and post-weld treatment, and not least the qualifications and experience of the welder.
In vacuum technology tungsten inert gas welding (TIG) is used often. In addition, other types of gas shielded metal arc welding are used as well as special methods, such as micro-plasma welding for thin-walled components or orbital welding for pipe components. Significantly more elaborate machine procedures are laser welding and electron beam welding. Both are suitable for delicate components and for deep welds. For welding of large aluminum valve housings, friction stir welding is used, which is an elaborate machine procedure with low welding distortions.
Tungsten inert gas welding (TIG) does not require a consumable electrode, and the joint parts can be welded directly without any additional materials. If additional welds need to be made, for example, for stability reasons, then welding consumables can be used. Other advantages of this method are virtually no spatter, no slag formation and versatility: stainless steel, aluminum and also copper can be TIG welded. The TIG method is preferred if a high quality weld is desired with respect to the welding speed.
Figure 3.4: Cross section image of a laser weld
Laser beam welding, or in short laser welding, is characterized by a high welding speed and low thermal distortion. The high concentrated energy input of the laser leads to a narrow weld zone and limits the range of the heat zone. Thin films, as well as deep and narrow welds for load-bearing structures can be created by setting the focus width and the laser intensity. That way, chamber components can be can designed without additional welds, or weld-on flange rings can be deep-penetration welded to pipe ends without the need for elaborate reworking of the sealing surface geometry. Larger gap widths at the joints can be bridged to a certain degree. Additional materials used are partially used here. A disadvantage are the high investment costs.
Figure 3.5: Cross section image of WIG orbital weld
Orbital welding is a fully mechanized inert gas welding process that provides steady high seam quality, as the arc is lead mechanically and under controlled conditions around the pipes or the round component. The system expense is higher than for TIG welding. An orbital welding tong covers only a limited range of pipe diameters. Each tube outside diameter also requires a device suitable for holding the pipe.
During electron beam welding, accelerated, focused electrons provide the energy required in the weld zone. In order to prevent the scattering and absorption of the electrons, the process is carried out in a high vacuum. This also makes it possible to weld highly reactive materials. The high system price and the weld preparation with possibly a required equipment construction, it usually leads to high prices for the procedure and mainly restricts its use to series components.
After the welding of austenitic stainless steel a metallically smooth surface must be present again, so an even chromium oxide passive layer without interruptions can be formed. For example, an inert gas shielding (including for the root base) prevents scaling of the surface at temperatures above 600°C. Mechanical or chemical finishing followed by thorough rinsing removes discoloration on the surface and cleans the component.
