So I'm putting a flange in the bottom of a bike tank to take submersible pump and filter etc. As I've just started tig welding I find myself putting too much heat into things and I don't want to warp the flange. Its a 5mm thick flange going onto quite thin steel so perhaps it will keep its shape regardless. Was thinking I could tack it down in quite a few places and then solder the rest of the way around using either lead solder or silver solder (or something else). I'm guessing you wouldn't need any flux if you soldered with the tig torch. Anyone see any issues with this?

If it's a good mechanical joint (wide mating surfaces) then you could probably use one of the harder grades of soft solder. Soft solder loses its structural strength somewhere above 100C - 150C, but that's not a problem at the bottom of a fuel tank. Don't bother to tack-weld as you'll cause distortion, just clamp it in place securely before soldering. Also, use the proper flux to match the solder in use. Don't use electrical cored solder as its resin flux isn't much good on steel (it may work, but maybe it won't). One type of harder solder is Comsol, available here amongst other places (but that place is expensive, so worth shopping around).

Background: I used to make live-steam models, and the harder grades of solder were used to make boilers in the past. It's strong stuff, used properly.

Silver solder won't be much better than welding for distortion, as you have to heat the work up to red-hot to make it flow.