Mating electrodes for resistance spot welding of aluminum workpieces to steel workpieces

A method comprising:

• pressing a central ascending convex surface of a weld face of a first spot welding electrode against a first outer surface of a workpiece stack-up assembly that includes a steel workpiece and an adjacent aluminum workpiece that overlaps with the steel workpiece, the first outer surface of the workpiece stack-up assembly being proximate the aluminum workpiece;
• pressing an annular surface of a weld face of a second spot welding electrode against a second outer surface of the workpiece stack-up assembly that is opposed to the first outer surface, the annular surface of the weld face of the second spot welding electrode surrounding a central descending concave surface that extends below the annular surface of the weld face of the second spot welding electrode and is complimentary in shape to the central ascending convex surface of the weld face of the first spot welding electrode, the second outer surface of the workpiece stack-up assembly being proximate the steel workpiece, and wherein the weld face of the first spot welding electrode and the weld face of the second spot welding electrode are aligned with and centered on a common axis;
• forming a resistance spot weld that bonds together the steel and aluminum workpieces, wherein forming the resistance spot weld comprises passing an electrical current between the central ascending convex surface of the weld face of the first spot welding electrode and the annular surface of the weld face of the second spot welding electrode so that the electrical current passed between the weld face of the first spot welding electrode and the weld face of the second spot welding electrode is distributed along a radially outwardly expanding flow path that extends from the first spot welding electrode to the second spot welding electrode such that a density of the electrical current is higher in the aluminum workpiece at an interface with the central ascending convex surface of the weld face of the first spot welding electrode than in the steel workpiece at an interface with the annular surface of the weld face of the second spot welding electrode.

A method comprising:

• providing a workpiece stack-up assembly that comprises a steel workpiece an adjacent aluminum workpiece that overlaps with the steel workpiece to establish a faying interface therebetween, the workpiece stack-up assembly having a first outer surface proximate the aluminum workpiece and an opposed second outer surface proximate the steel workpiece;
• pressing a first spot welding electrode against the first outer surface of the workpiece stack-up assembly, the first spot welding electrode comprising a weld face that includes a central ascending convex surface that rises above an annular surface that surrounds the central ascending convex surface, wherein, at least initially, the central ascending convex surface makes contact with the first outer surface of the workpiece stack-up assembly and the surrounding annular surface of the weld face of the first welding electrode does not;
• pressing a second spot welding electrode against the second outer surface of the workpiece stack-up assembly, the second spot welding electrode comprising a weld face that includes a central descending concave surface that drops below an annular surface that surrounds the descending concave surface, the central descending concave surface of the weld face of the second spot welding electrode being complimentary in shape to the central ascending convex surface of the weld face of the first spot welding electrode, and wherein, at least initially, the annular surface of the weld face of the second welding electrode makes contact with the second outer surface of the workpiece stack-up assembly and the descending concave surface does not;
• passing an electrical current between the weld faces of the first and second spot welding electrodes across the faying interface of the steel and aluminum workpieces to form a molten weld pool within the aluminum workpiece that wets an adjacent surface of the steel workpiece, the electrical current initially being distributed along a radially outwardly expanding flow path that extends from the first spot welding electrode to the second spot welding electrode such that a density of the electrical current is higher in the aluminum workpiece at an interface with the central ascending convex surface of the weld face of the first spot welding electrode than in the steel workpiece at an interface with the annular surface of the weld face of the second spot welding electrode, the annular surface of the weld face of the first spot welding electrode eventually coming into contact with the first outer surface of the workpiece stack-up assembly during passage of the electrical current between the weld faces of the first and second spot welding electrodes to thereby cause the radially outwardly expanding flow path of the electrical current to cease to exist;
• and
• terminating passage of the electrical current between the weld faces of the first and second spot welding electrodes so that the molten weld pool solidifies into a weld joint that bonds the steel and aluminum workpieces together, and wherein the weld joint includes a bonding interface with the adjacent surface of the steel workpiece that is deformed toward the central descending concave surface of the weld face of the second spot welding electrode.