A spot weld may be formed between an aluminum workpiece and an adjacent overlapping steel workpiece with the use of opposed spot welding electrodes that have mating weld faces designed for engagement with the outer surfaces of the workpiece stack-up assembly. The electrode that engages the stack-up assembly proximate the aluminum workpiece includes a central ascending convex surface and the electrode that engages the stack-up assembly proximate the steel workpiece has an annular surface. The mating weld faces of the first and second spot welding electrodes distribute the passing electrical current along a radially outwardly expanding flow path to provide a more uniform temperature distribution over the intended spot weld interface and may also produce a deformed bonding interface within the formed weld joint. Each of these events can beneficially affect the strength of the weld joint.