Welding electrode design for expelling organic material in weld bonding

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

• providing a workpiece stack-up assembly that has a first side and an opposed second side, the workpiece stack-up assembly including an aluminum workpiece and an adjacent steel workpiece that overlaps the aluminum workpiece, and further including an intermediate organic material layer disposed between confronting faying surfaces of the overlapping aluminum and steel workpieces, wherein the aluminum workpiece is proximate the first side of the stack-up assembly and the steel workpiece is proximate the second side of the stack-up assembly;
• pressing a weld face of a radially slotted welding electrode against the first side of the workpiece stack-up assembly, the weld face of the radially slotted welding electrode including a central upstanding plateau and a convex dome portion that surrounds the central upstanding plateau and which includes a plurality of trapezoidal weld face sections that are circumferentially spaced on the base surface around the central upstanding plateau, each of the trapezoidal weld face sections including a plurality of transverse upstanding arcuate ridges that are radially spaced apart along the base surface of the convex dome portion, and, wherein, an annular channel surrounds the central upstanding plateau and a plurality of radial slots communicate with the annular channel and extend outwardly towards an outer circumference of the base surface between the trapezoidal weld face sections;
• pressing a weld face of a second welding electrode against the second side of the workpiece stack-up assembly, the weld face of the second welding electrode being facially aligned with the weld face of the radially slotted welding electrode when the weld faces of the radially slotted and second welding electrodes are pressed against their respective first and second sides of the workpiece stack-up assembly;
• passing an electrical current between the weld face of the radially slotted welding electrode and the weld face of the second welding electrode to create a molten weld pool contained within the aluminum workpiece, the molten weld pool wetting the faying surface of the steel workpiece;
• and
• terminating passage of the electrical current between the weld face of the radially slotted welding electrode and the weld face of the second welding electrode to allow the molten weld pool to solidify into a weld joint that bonds the aluminum and steel workpieces together.