On June 3, 1965, after America’s first spacewalk was complete, the two Gemini IV astronauts, spacewalker Edward H. White II and command pilot James A. McDivitt, had trouble getting the hatch closed. They managed to, and got back to Earth safely, but what had caused the hatch to stick? Was it due to something called cold welding?
Cold welding or contact welding is a solid-state welding process… Unlike in the fusion-welding processes, no liquid or molten phase is present in the joint.
Cold welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. Newly discovered micro- and nano-scale cold welding has already shown great potential in the latest nanofabrication processes.
“The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.” — Richard Feynman, The Feynman Lectures, 12–2 Friction
In the end, the Gemini IV mission didn’t have a cold welding challenge as much as a stuck door challenge, but cold welding is still an issue in space. Veritasium‘s Derek Muller explains in this video on Welding in Space.
From ESA: More info on cold welding (pdf). Riveting related reading: 50 Years Ago, The First Spacewalk Nearly Ended In Tragedy (but didn’t), a story about cosmonaut Alexey Leonov.