There was an arc welding system used briefly after the war (I think) that didn't use any filler material, the tubes were pushed together at high pressure and then a high current was passed through them that fused them together at the joints. It never really caught on, I'm guessing the tooling was not versatile enough for the bike industry.
I think the post war welding process Dallas refers to is known as flash butt welding. Dayton used it, but their frames had a reputation for breaking - so maybe that's why it didn't catch on.
There was an arc welding system used briefly after the war (I think) that didn't use any filler material, the tubes were pushed together at high pressure and then a high current was passed through them that fused them together at the joints. It never really caught on, I'm guessing the tooling was not versatile enough for the bike industry.