Helmets should fit the wearer well, and this requires some skill from practice.
Cold-formed, a helmet is often welded nowadays, but your welding skills must be excellent; such will be worn by fighters in live, non-choreographed fights. Welding helms is not considered to be an authentic crafting method, but in our modern age where time is critical, it makes sense economically.
Authentic construction however, is done with heat as from a forge. It’s called “raising a helmet”. Raising takes a lot more time than welding but yields a piece considered far superior in both strength and authenticity. Done well, raised helms command a much higher price when sold too.
Here is a well-known image depicting a history of medieval helmet evolution:
