Here's a broken bolt I removed from an aluminum jack earlier this summer. This was my fault for trying to remove the bolt without removing the steel roll pin first. I didn't see the roll pin driven into the side of the aluminum, and I snapped off the head while trying to remove the bolt. After welding the washer and nut on I forced it out but gummed up the threads when I forced it past the roll pin. It was lony after the broken bolt was removed that I finally spotted the small hole in the aluminum with the roll pin driven in.
I hold the wire feed in the center of the washer and nut and let the liquid steel flow out to fill the hole. This way it puts the most heat on the broken bolt.