Almost certainly resistance welding. It's very fast and passes current only through the two pieces to be welded together, so avoids all the issues of stray currents and the other safety precautions needed with arc welding processes like TIG that generate UV and showers of molten metal.

19:50 in this video shows one in use in making a vacuum tube:

https://www.youtube.com/watch?v=mx3IOlzMKp4