I'm not an expert on welding, but I have always heard that a properly done weld is as strong or stronger than the surrounding metal. I saw a piece if failed hand forged barrel, and the failure was along a longitudinal inclusion and not at the weld.
When I first saw your post, the idea of welding the tang came to mind, particularly if the weld would be hidden by the guard or other transition between blade and hilt.
Merry Christmas,
Larry Luck