A method of resistance spot welding a workpiece stack-up that includes a steel workpieces and an aluminum alloy workpiece that overlie and contact one another to establish a faying interface at a weld site is disclosed. The method comprises passing a DC electrical current through the workpiece stack-up at the weld site and causing the current to assume a conical flow pattern. The conical flow pattern has a path of current flow that expands along a direction leading from a first welding electrode in electrical communication with the steel workpiece towards a second welding electrode in electrical communication with the aluminum alloy workpiece.