Resistance spot welding steel and aluminum workpieces with hot welding electrode at aluminum workpiece

A method of resistance spot welding a steel workpiece and an aluminum or aluminum alloy workpiece together, the method comprising:

• providing a workpiece stack-up that includes a steel workpiece and an aluminum or aluminum alloy workpiece;
• bringing a first welding electrode and a second welding electrode together such that a first weld face of the first welding electrode comes into direct contact with a second weld face of the second welding electrode;
• passing electrical current between the first and second welding electrodes while the weld faces of first and second welding electrodes are in direct contact and while cooling of the second electrode is restricted or halted to preheat the second welding electrode to a temperature between 200° C. and 900° C.;
• separating the first welding electrode and the second welding electrode from each other;
• pressing the first and second welding electrodes against opposite sides of the workpiece stack-up with the first welding electrode abutting the steel workpiece and the second welding electrode abutting the aluminum or aluminum alloy workpiece;
• passing electrical current between the first and second welding electrodes and the workpiece stack-up at the weld site to initiate and grow a molten weld pool within the aluminum or aluminum alloy workpiece;
• and
• ceasing the passage of electrical current between the first and second welding electrodes and allowing the molten weld pool to solidify into a weld nugget before retracting the first and second welding electrodes from the workpiece stack-up.