Multistep electrode weld face geometry for weld bonding aluminum to steel

A method of resistance spot welding a workpiece stack-up that includes an aluminum workpiece and an adjacent overlapping steel workpiece, the method comprising:

• providing a workpiece stack-up that includes an aluminum workpiece and a steel workpiece that overlaps with the aluminum workpiece to establish a faying interface between the aluminum and steel workpieces, the workpiece stack-up having an aluminum workpiece surface that provides a first side of the stack-up and a steel workpiece surface that provides an opposed second side of the stack-up;
• positioning the workpiece stack-up between a weld face of a first spot welding electrode and a weld face of a second spot welding electrode, the weld face of the first spot welding electrode comprising a series of steps that includes an innermost first step in the form of a central plateau and, additionally, one or more annular steps that surround the central plateau and cascade radially outwardly from the central plateau, the central plateau having a top plateau surface and each of the one or more annular steps having a top annular step surface, wherein the weld face has a conical cross-sectional profile in which a periphery of the top plateau surface of the central plateau and a periphery of the top annular step surface of each of the one or more annular steps are contained within a conical sectional area defined by an upper linear boundary line and a lower linear boundary line that intersect at the periphery of the top plateau surface and are inclined at an angle of 5° and 15°, respectively, from a horizontal plane extending from the periphery of the top plateau surface;
• pressing the weld face of the first spot welding electrode against the first side of the workpiece stack-up such that the top plateau surface of the central plateau makes first contact with the first side of the workpiece stack-up and any pressure exerted by the weld face of the first welding electrode on the first side of the workpiece stack-up is at least initially directed through the top plateau surface of the central plateau;
• pressing the weld face of the second spot welding electrode against the second side of the workpiece stack-up in facial alignment with the weld face of the first spot welding electrode at a weld zone;
• passing an electrical current between the weld face of the first spot welding electrode and the weld face of the second spot welding electrode, and through the workpiece stack-up, to grow a molten aluminum weld pool within the aluminum workpiece that wets an adjacent faying surface of the steel workpiece, wherein the weld face of the first spot welding electrode impresses further into the first side of the workpiece stack-up during growth of the molten aluminum weld pool such that the top annular step surface of at least some of the one or more annular steps are brought into contact with the first side of the workpiece stack-up.