Any copper alloy can be a problem when preheat or interpass approached 1000 degrees F.
Copper at this high heat likes to absorb CO and other gases/impurities at that temp.
As a rule of thumb, keep your preheat to a minimum, your weldment will warm up in time.
If you are using a wire process, consider using a hi helium mix, which will help out.
Additionally, a 400 amp power source would be considered a minimum, again if a wire process is chosen.
Personally, I have had good success with the pulse process in this app.
Contact me for details!
Good Luck
Brande