we are currently welding aircraft moulds .the material is invar and the welding consumable is nilo 36 we are struggling to find a compatable gas at the moment we have changed from argon pureshield over to astec 75 but we are having poor results any assistance would be greatly appreciated many thanks
I have no experience, but did look around on the internet. Apparently it makes a difference on whether you are Spray Mig welding or if you are Short Arc Mig welding. So you might find out which you are using. And if you ar e having problems try switching to the other type of MIG welding.
And one recommendation was for 75% Argon 25% Helium. The Astec 75 is 75 % Helium and 25% Argon. Have no clue which is better, but the 75% Argon ou ght to be cheaper.
Dan
The problem, is not your gas, it is your heat. Invar/Nilo-36 is 36% Nickel. With that high of a nickel content you have to be very careful about overheating, and rapid cooling. Shrinkage is brutal.
Preheat your parts to 500 degF. Clean thoroughly with a stainless steel brush. Use the lowest amperage possible, slow cool. Back purge everything, and brush thoroughly between weld passes on large welds. If you see any porosity, you have to grind it out with a carbide die grinder bit. No abrasive wheels.
I consulted for a company making wing molds for Boeing with the same problem.
The Helium will be MUCH hotter
As I said previously, I have no experience. But I am really confused. You say shrinkage is brutal, but I thought that Invar had almost no shrinkage.
Dan
Invar is only "invariable" between a narrow range of temperatures. Notably 20 to 100 degC. (68 to 212 degF) It is used extensively for scientific gauges and measuring instruments, but it is not intended for high temperature use.
I weld invar 36 daily on aero space tooling. Best results come from 75/25 argon/helium. Good luck with the mig if you're machining it cause its junk. Tig works real good but base metal must be perfectly clean in weld areas.
I'm am welding Invar 36 right now and the best gas we have found is 92%argon8%carbon dioxide and running on pulse. It welds ok on cv( spray), but I like it better on pulse.
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