Method of resistance spot welding aluminum to steel

A method of resistance spot welding an aluminum workpiece and an adjacent overlapping steel workpiece, the method comprising:

• locating a source of a reactive metal in a diffusible state along a faying interface of an aluminum workpiece and an adjacent overlapping steel workpiece, the reactive metal being able to react with molten aluminum to form metal aluminide particles;
• pressing a weld face of a first welding electrode against an aluminum workpiece surface that provides a first side of the workpiece stack-up;
• pressing a weld face of a second welding electrode against a steel workpiece surface that provides a second side of the workpiece stack-up;
• passing an electrical current through a workpiece stack-up between the weld faces of the opposed first and second welding electrodes at a weld zone to melt the aluminum workpiece that lies adjacent to the steel workpiece and create a molten aluminum weld pool within the aluminum workpiece that wets a faying surface of the adjacent steel workpiece, and wherein contact between the molten aluminum weld pool and the source of a reactive metal in a diffusible state results in the formation of metal aluminide particles that migrate into the molten aluminum weld pool;
• and
• terminating passage of the electrical current between the weld faces of the opposed first and second welding electrodes to allow the molten aluminum alloy weld pool to solidify into a weld joint.