Manual GTAW Welding Aged Inconel 718 with 625 Filler

[croy]

We are attempting to weld small tubular sections (7/8" OD x 1/2" ID) of aged Inconel 718 with 625 filler. We're using 625 filler for two main reasons; 1. Availability vs. 718 or 725 filler 2. Lower tendency for weld cracking (we are told) compared to 718/725. We need the strength of 718 in one end of the tubular assembly, but not at the weld, which allows use of 625 filler. The structure cannot be PWHT anyway because there are elastomer seals in one part of it that are captured after welding and could not be removed and replaced after heat treatment.

The only information I have come upon is that low heat input is recommended to reduce the tendency of HAZ cracking. It seems that this would be similar to an in-situ repair of an aged 718 structure that cannot be PWHT (aircraft engines?). We plan on conducting extensive microstructure examination (deleterious phases, micro-fissuring) as part of the weld procedure qualification. Any additional input on developing a sound WPS for this material/filler combination would be most appreciated. Thanks.

Chris

 

[metengr]

croy:
I did a little research on the above item in my spare time considering I deal with many welding related issues. From the literature reviews that I conducted (Haynes, Allvac and Welding Metallurgy by Kou, just to name a few), I just don't see where Inconel 718 either solution treated or aged will have the severity of problems you reported in comparison to other Ni-base precipitation hardening alloys.

Various publications on Inconel 718 aged or solution treated recommend Inconel 718 filler metal. I really don't see why you would use 625 for this, and to imply it would result in less susceptibility to cracking. Most of the literature indicates a favorable weldability of Inconel 718 because of the slower aging characteristics that would work in your favor during fabrication welding.

 

[AeroWeldEng]

Croy: I have to agree with metengr, I believe you will be fine with Inco 718 filler wire instead of 625. Are you planning to weld manually, or try to automate the process? There are orbital GTAW systems on the market with wire feed capability that could be used for your application, if the volume supports it. In terms of a good WPS, do you have a welding engineer? If not, you could investigate aerospaceweldingconsultant.com, there appear to be several happy clients there. You should also do the welding in the solution HT condition, if possible.