The immediate reply is yes, however it may be neither easy nor advisable in some cases if brittle structures are designed by the different metals mixing from the molten nugget.
In exacting austenitic steel and carbon steel will not be generally spot welded jointly because the resulting nugget arrangement risks being hard and brittle, community . can be studied and modified utilizing the Schaeffer diagram and particularly conceived warmth treating cycles.
Materials having widely dissimilar properties need that the warmth balance be exercised by compensation. The additional conductive material, electrically and thermally, must be heated extra because it provides fewer resistive warmth, plus the warmth is lost with less effort by conduction.
An ordinary method uses an electrode of smaller face diameter and superior resistivity facing any additional conductive material, or by inserting a foil of poorly conductive material one of them.
Concerning the amount of sheets weldable having a single nugget, usual practice advices not more than three layers, although four sheets are infrequently spot welded jointly. In any case exactely the thickest on the thinnest sheet shouldn’t go beyond three.