AWs D1.6 is designed for sheet stainless steels of more than 2mm. Which standard should we use to weld 1.5mm stainless steel sheets? in table 4.2.B of AWS D1.6 minimum qualified thickness is 3mm! so what should we do about the lesser thicknesses?
The engineer can elect to use D1.6 when the thickness is 1/16 inch or thicker per AWS D1.6-2007. The lower limit of 1/8 inch is listed in D1.1 Structural Welding Code/Steel.
Thinner material can still be welded per D1.6, but only with the engineer's (owner's representative) approves it.
Thinner materials can be welded to the sheet metal code and the procedures qualified to that code as well.
The question I would have is the nature of the imposed loads. If the sheet metal structure is load carrying as is the case for "structures" I would favor the structural welding codes. If the loads are very minor considerations as is the case for ventilation ducts I would favor the sheet metal code.
AWS D9.1 covers sheet metal. "This code cover sheet metal up to and including 6.4mmm." It covers steel, aluminum, copper, stainless, etc. Most processes as well.