If you go to heavy springs, in the 8-9kg range, this is what WILL eventually happen.
I was warned by 2 different people, then it happened to mine, and since it's happened to 2 others that I know of.
Just a heads up.
It's the chain side of the swing arm near the pivot. Spring was an 8.3kg.
This was the second time it cracked. You can see the fresh TIG weld. Possibly it could be braced, but this was the reason I went to the full RMZ rear end swap out.