ERCuSi-A is commonly used for GTAW on sheet metal. It's OK for HVAC, but not for auto body work since you can't paint over it very well. I have also used it occasionally on brass fittings with success.
A friend of mine just showed me some very nice tig welds joining stainless steel to galvanized sheet metal using silicon bronze filler wire.
It can be used for a variety of base metals, but is normally used for brazing, including GTA brazing, so the base metal shouldn't be melted.
Silicon bronze works well on galvanized sheet metal, can also use on copper and bronze.
I can be used for bronze, too!
I accidentally welded up a rebar trellis with silicon bronze filler. Still holding up, six months later...
Chris
If you can get away from the hype-you'll have much better luck with aluminum bronze, A2.
The aluminum bronze is much more forgiving, more ductile, and has many more applications.
It does not centerline crack like si-bronze and can join most any alloy out there. No hot-shortness on aluminum bronze A2.
To be sure, si-bronze is somewhat cheaper, but I think you'll find better results with aluminum bronze A2.
This alloy has saved my pudgy pink behind more times than I can remember.
Also runs DC- or AC, if tig welding!!
Good Luck
brande