Probaly late here.
A weld of that thickness would probably go best in the mig process. That's a
big hole to fill.
As far as I know-there is no Invar 36 filler.
I'd have to research compatibility with available fillers.I'm sure that a
compatible filler is available.
I weld Kovar (a related alloy) on a very regular basis. Thinner stuff-1/4"
or less. Have been using a nickel 55 or 60 cast iron filler with good
success on tig. I don't used de-fluxed stick rod. This bare wire is
available, but you have to ask.
The last load I got was from Natweld/Inweld. I'm sure there are other
suppliers as well.
Invar is "invariable" over a wide temperature range.
Fiberglass wing molds have to be able to hold dimension even as the
fiberglass heats up while curing.
I got the spool of Nilo 36 and then called Inco to find out what it was.
Ni = Nickle , Lo = low thermal expansion , 36 = 36% nickle
They were very surprised to find that Boeing was dumping it at Salvage
because they were paying Inco about $50/lb. for it specifically to
repair wing molds.
I tried using it to MIG weld some cast iron once.
It kind of worked, but TIG and pure nickle rod worked better.
I still have 2 spools of the stuff.