Have used ERCuNi, ERNi-1, and ERNiCu-7 sucessfully for joining 90/10 CuNi materials over the years.
If your specification insists on a 90Ni/10Cu filler, I don't know what to suggest, since SFA-5.7 and SFA-5.14 do not list a filler with that exact composition.
ERCuNi is about 67%Cu/30%Ni
ERNiCu-7 is about 65%Ni/27%Ni
ERNi-1 is about 94%Ni
Here are some sample parameters with ERCuNi, 1/8" diameter bare wire and the GTAW process that may give you a starting point to begin some testing for your procedure development:
70F preheat minimum
250F interpass maximum
DC/SP
175-200 Amps
15-17 Volts
75%Helium25%Argon gas mix
Shielding flow @ 20-30 CFH
Internal purge @ 4-5 CFH
Suggest less than 2% Oxygen content on backside prior to welding, or run the purge gas flow until you have about 6 volume changes

Your customer must have made a typo on the filler metal requirements. You said you need to weld 90%Cu-10%Ni base metal (also known as copper alloy 706) and the customer requires 90%Ni-10%Cu filler metal. This makes no sense and is not likely what they really want for seawater service. I am not aware of any 90/10 CuNi matching fillers that meet any AWS standard, but there are some non-U.S. filler specs and may be some non-classified specialty wires. The non-U.S. specs are:

U.K. - BS2901; Part3:70 Type C16
Germany - DIN 1733 S-Cu Ni 10 Fe
India - IS5898

I've not welded 90/10, but have welded 70/30 a bit with GTAW and SMAW. Clean the area to be welded thoroughly prior to welding by abrasive flapper wheels or stiff stainless steel wire hand brush not previously used on other materials. Wipe with acetone or ethanol after initial cleaning. Clean the filler wire the same way with abrasive cloth followed by acetone wiping. Use stringer beads only. With GTAW the puddle is very sluggish and has an oxide film floating in the puddle. You need to keep the tip of the filler wire in the argon shield flow at all times when feeding, don't pull it away or it will get an oxide on the tip that will produce an inclusion on x-ray. Also, clean the end of the beads with a carbide burr to remove the oxide that solidifies on the surface of the puddle.

I know the sound, kind of like a hissing noise. Seems to be common on copper alloys and brasses when using GTAW. I always thought it was due to the high copper content

Hope this helps,
Marty

Dale,

For some reason the email you sent won't open. However, someone has given you the wrong information on this 90/10 project. Our WPS utilizes ERCuNi, which is an F34 filler and matches the P34 base metal. We use ERNiCu-7 (F43) for P1 to P34.

What happened is this. We were given some WPSs that another yard developed (to help us get started). For P34 to P34 they used ERNiCu-7. We originally went down the same path. However, when the tensile specimens were prepared, the bimetallic nature of the joint became very obvious. The nickel copper filler really stands out against the copper nickel base metal. I didn't remember any of the systems on the Navy ships I worked on looking like this. To make a long story short; Even though the PQR was acceptable according to ASME Section IX, I asked the Lloyds representative for his opinion and it came back as a recommendation to use a different filler metal (ERCuNI). So we ran another PQR using the recommended filler.

Having said all that, in accordance with Section IX: for welder performance qualification testing, we are allowed to administer the test using any of the following base metals: P1 - P11, P34 and P41 - P47. We are using 90/10 and may shift to (P1) carbon steel in the future. We are also allowed to use any of the following filler metals: F34 and F41 - F45.

It gets cumbersome to try and keep up with all this, but after awhile it starts to make sense.

Charles Hall