Another way of making joints in tubes is to place a tube in a "T" shaped die, plug the ends, and pump fluid into the pipe under high pressure until a bubble forms in the T-opening.
This is a standard manufacturing method in plumbing tube industry. The results are much stronger than a welded joint, but tooling costs are high.
Schwinn may have used this method in their heyday.
Amir Avitzur
Ramat-Gan, Israel