Larry Fishel wrote:

On the welding group, several people have reported success welding a
washer to the end of the screw and then welding a nut to the washer.
Probably easier with larger bolts and may require more welding skill
than I have. I'll have to try it some day though.

I have a little mig welder at home and we have a couple at work. I always
use this method and it has never failed. I just build up the broken bolt
a little at a time. Bzzt, bzzt, bzzt... Either grab the nub with vice grips
or weld a nut over it. It might take a few tries. And not too hot-- don't
want too much penetration as to weld the bolt completely over! I used
to do this all the time in an automotive machine shop.