Cold welding or contact welding is a solid-state welding process in which joining takes place without fusion or heating at the interface of the two parts to be welded. Unlike the fusion-welding processes, no liquid or molten phase is present in space.
On the contrary, it is also believed that nothing of this sort actually happens. It requires vacuum for cold welding to work, and that is quite easy to set up in real life and it demands only clean surfaces. An experiment was conducted in which large structures were used that were cleaned carefully with solvents and baked to remove dirt so that they might not contaminate the vacuum system. Now if you screw a clean stainless steel screw into a clean tapped hole in a stainless steel part, you’ll be lucky to get it out again. This proves that metals do not stick together in vacuum.
But an evidence of cold welding came forward in 1991, when the Galileo spacecraft’s antenna got locked down and it never opened. It was later revealed that the umbrella shaped antenna, which was locked before the spacecraft’s take off got cold welded after the spacecraft left Earth.
The metal tools used outside the space station in the vacuum of space are coated or covered with plastic so as to prevent them from sticking together.