It is always best to do a test weld on a sample of the same material and thickness to determine heat setting.
As for the thicker material, it is easier to wash into the thinner material going from thick to the thin. Make sure you square and tack in at least four spots .
A picture might help too.

 

Set the machine to the thinner material so you don't burn through. And favor the thicker side and let it wash into the tube.

That's what I would do

 

That is what I would do as well.

 

Well good thing I asked! I was wondering about burning through with the heat set to thicker material.

BD1 - I will get some pictures tonight if I remember.

 

Preheat the assembly well with gas torch and weld with welder set for thin stuff...Welding is a heat thang, gents.

If you decide to weld it cold, set welder not for either thickness, but in the middle and diret the puddle onto the thick and allow edges to wash into the thin.

 

Good info guys. If preheating, what temp should I target?

I finally remembered to get some pictures too.

 

I just welded 4 of those V band flanges on the 3" stainless exhaust on my son's '69 Camaro to make it easy to remove the dual stainless mufflers. I did it with mig to save time (since we hand-made the whole system using straights and pre-bends cut to size and hand fitting everything). I used tri-mix and a Lincoln 180. I set wire speed to favor the heavier flange and then did mostly spot welds for a second or two, welding the joint mostly at the edge of the flange to allow the wash onto the thinner pipe. When all the spots joined it looked very clean and didn't need grinding. No burn though that way. When we did the actual pipe sections, I turned the power down and ran beads.

 

Put that vband on some aluminum. They can warp easily