Porosity and lack of fusion in Copper welds

We have a customer that wants us to TIG weld two dissimilar alloys. The first alloy is Bronze 4640 (Nickel, Aluminum, Bronze)and the second alloy is UNS C10100 (Copper). We will also be using the UNS C10100 as filler material. We have done trials with a Miller Maxstar 200SD welder, AC voltage, 100% argon, and 2% thoriated electrodes. We have experienced a large amount of porosity and lack of fusion between the two alloys. Can you offer any suggestions to help? I have heard that DC voltage may work better. How about throwing in some Helium?
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Are you sure you used AC current? a miller maxstar is a DC machine. maybe you meant miller dynasty 200sd.

in any case, dc current will penetrate more. and yes helium would help tremendously. Either uhp pure helium, or a mix of about 80 percent helium, and 20 argon.

also a preheat of around 200-300f would help with better fusion as well as less porosity