With standard MIG or TIG welding, the steel welding wire won't stick to or even damage the aluminum of the cylinder head, allowing you relative safety while attempting to weld on a nut.

Welding something to the top of a broken bolt has always been enough for me to get it to extract. ... So with an M8 I TIG weld on an m6 washer and weld a nut onto that. makes my life easier, ...