Copper is a very conductive material and therefore may require one to use modified procedures from the ordinary. It can be resistance welded to similar materials. There is so much difference in the conductivity between copper and steel that this would be considered a difficult weld to make. The steel will want to heat up before the copper. The weld charts call for Class 1 on the steel side and Class 13 or 14 on the copper side. There probably will not be a weld nugget formed. The bond would be described as a stick weld.
A way to improve this joint would be to coat the copper with zinc or nickel and let this coating act as a brazing agent during the weld. Better yet make a resistance braze by placing a piece of braze material in the joint and use resistance heat to create a braze joint. This is done regularly in the electrical industry. Reduce the force so that you don’t force the molten braze alloy out of the joint and heat long enough for a complete fillet to form. Don’t remove the force until the joint has solidified.
Low Inertia Holder
Ref: What is Resistance Brazing?
CMW Products Inc. Catalog
Tuffaloy Products Catalog
RWMA, Resistance Welding Manual, Section 6