A friend asked me if mild steel could be welded to stainless in
production. That's all I know thus far: I don't know anything yet
about thicknesses, joint design, or expected volume.

While you're waiting for the answer from the expert Ernie, I'll mention
that I've done this exact thing many times. Only because I bought litterally
a truck load of stainless for a song and its cheaper to use than buying new
mild sheet steel. Of course, you lose the stainless properties, other than
that, I've used both mig and tig for welding. Haven't had the ocassion but I
bet stick would work also. For welding, it acts just like a similar
thickness of mild steel. Torch won't work on stainless though.

Karl