What the heck kind of metal.....

Is contained in the cabinet of a rooftop Air Conditioning unit (Carrier) .... I saved the the sheet metal after taking out the fan motor, blower motor etc... when I try to "tig" the edge to a piece of mild steel tubing it seems to "run" from the puddle... I can get a quick tack on it with the tig, or push a "mig" puddle along the edge of it, but it seems to be allurgic to the tig torch.... Reminds me of welding aluminum with acetylene. Pat Landy