I think most flywheel materials are a high grade cast iron. I think they are generally either grey cast iron or CGI (compacted graphite iron). CGI is stronger than your normal grey iron, partly because the graphite particles are smaller and slightly rounded (hence the name compacted). It is kinda like a cross between grey cast iron and SGI (spheroidal cast iron). CGI is difficult to weld properly.
Have a look at the 4th, 5th and 6th pic on this page -
http://www.offroadvw.net/bajawes/V6_baja/december.html
I made my flywheel by putting a VW ring gear on the outside of the mazda flywheel. I did this by having a vw ring gear machines off an 1800 flywheel (as on those the ring gear is machined into the actual flywheel, and is therefore the same cast iron material as the flywheel). This meant the two should weld and work together quite well as they should be similar cast irons. My engine sees up to 8,000rpm every now and again and hasn't explode yet.
So I don't think your welding idea is good because you wouldn't want to weld steel parts to cast iron parts.
If you cut the center out of one flywheel and weld it into the center of another cast iron flywheel, then that's a whole other story and would work fine (and has been done for ages by off-road racers.
Wes -
www.offroadvw.net - 200HP Quad Cam V6 in a VW Baja - with climate control...
🙂
www.taylorcycles.com.au - My DH MTB racing brothers shop.