According to published welding literature from Russia, the quality of friction welds in certain alloys (for example, stainless steels) and refractory metals such as niobium and tantalum can be improved significantly if welding is performed in a vacuum.
Creating a vacuum around the friction welding equipment is likely to prove problematical and therefore costly: there would have to be a very strong justification for building and operating such a system.
It is interesting to consider the possibility of friction welding in Space. In time, parts of this vast, naturally occurring vacuum may become readily available for this kind of activity.
The vacuum friction welding of high-melting and high-temperature alloys. Bol'shakov M V, Ovsyankin V P, Chepukhin S I. Automatic Welding, vol.25, no.6. June 1972. pp.60-62. English translation of Avtomaticheskaya Svarka.
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