Hello;  I work in a busy shop, which deals daily with broken studs and bolts
in various metals and various positions.  The first choice for all mechanics
on the floor is to find a welder who can weld a washer then nut onto what
remains of the stud or bolt then turn it out with the hammering motion of
the impact gun.  The heat transferred to the parts during welding helps the
dis-similar metals to detatch.  Remember to cool the parts before hammering.
If this is not possible, or can't be used then there are easy-outs available
which do not tend to spread the drilled stud/bolt.  Both Mac and Snap-on
market them, they consist of a shaped shaft which is driven into a hole
drilled into the stud/bolt, then a collar is slid over the shaft, which is
turned by a wrench. 
In all cases heat is an important ally.

Hope this helps   Larry Carter