Metals fuse together in space
You may have heard that two pieces of similar metal will bond together if they touch each other in space. In theory this is correct, although in real life we wouldn't expect it to happen quite that easily.
If two pieces of similar metals touch in a vacuum, and if both pieces are perfectly flat and polished, they will indeed fuse to effectively make one new piece. Atoms in the metals share electrons and bond permanently. This is called cold welding.
The reason this doesn't happen in everyday life is that there is always some kind of barrier between the pure metals. In particular, most metals form an oxide layer on any surface that is exposed to air. This acts as a shield to prevent bonding.
In the vacuum of space, there is no air so metals wouldn't form the protective layer. In practice, however, any metals that astronauts use should still have their oxidation layer from when they were exposed to air. In addition, astronauts' tools are coated with plastic.
Although it's not a common problem in space, it has happened1 and space engineers need to be aware of the issue2.
Note that there are numerous variations of cold welding. Some types of cold welding can be done on Earth without a vacuum. A particular type of alignment or friction may be required for the bonding to take place.
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Author: Dave Owen
Footnotes & References
1. Johnson, Michael R. (1994). The Galileo High Gain Antenna Deployment Anomaly (PDF). NASA Jet Propulsion Laboratory. Retrieved 2020-10-10
2. A. Merstallinger; M. Sales; E. Semerad; B. D. Dunn (2009). Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum (PDF). European Space Agency. Retrieved 2020-10-10.