Multi-stage resistance spot welding method for workpiece stack-up having adjacent steel and aluminum workpieces

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

• providing a workpiece stack-up that has a first side and a second side, the workpiece stack-up comprising a steel workpiece disposed adjacent to and overlapping an aluminum workpiece, the steel workpiece having a faying surface that contacts a faying surface of the aluminum workpiece to establish a faying interface between the workpieces;
• bringing a weld face of a first welding electrode and a weld face of a second welding electrode into pressed contact with the first side and the second side of the workpiece stack-up, respectively, the weld face of the first welding electrode and the weld face of the second welding electrode being facially aligned with one another at a weld site when brought into pressed contact with their respective sides of the workpiece stack-up;
• resistance spot welding the steel workpiece and the aluminum workpiece together at their faying interface by heating and cooling the weld site in multiple successive stages, each of the multiple successive stages of heating and cooling the weld site comprising passing an electrical current between the first and second welding electrodes and through the weld site at a current level within a working current level range of 10 kA to 40 kA for a duration of 10 ms to 1000 ms to generate heat within the weld site, followed by reducing the current level of the electrical current to within a reduced current level range of 0 kA to 5 kA for a duration of 10 ms to 1000 ms to allow the weld site to cool, and wherein the multiple successive stages of heating and cooling the weld site includes at least:
• a weld joint origination stage in which a weld joint is initially formed that joins the steel and aluminum workpieces together at the faying interface, the weld joint extending into the aluminum workpiece and having an interfacial weld bond area adjacent to and joined with a faying surface of the steel workpiece at the faying interface;
• a first weld joint refining stage in which at least a portion of the weld joint formed in the weld joint origination stage is remelted and resolidified into a first refined weld joint, wherein remelting of at least a portion of the weld joint produces a remelted portion of the weld joint that encompasses at least 50% of the interfacial weld bond area of the weld joint;
• a second weld joint refining stage in which at least a portion of the first refined weld joint formed in the first weld joint refining stage is remelted and resolidified into a second refined weld joint, wherein remelting of at least a portion of the first refined weld joint produces a remelted portion of the first refined weld joint that encompasses at least 50% of an interfacial weld bond area of the first refined weld joint;
• and
• removing the first welding electrode and the second welding electrode from pressed contact with the first side and the second side of the workpiece stack-up, respectively, after resistance spot welding of the steel and aluminum workpieces.

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

• providing a workpiece stack-up that comprises a steel workpiece and an aluminum workpiece disposed adjacent to and overlapping the steel workpiece, the steel workpiece having a faying surface that contacts a faying surface of the aluminum workpiece to establish a faying interface between the adjacent workpieces;
• forming initially a weld joint between the steel workpiece and the aluminum workpiece, the weld joint extending from the faying surface of the steel workpiece into the aluminum workpiece and having an interfacial weld bond area adjacent to and joined with to the faying surface of the steel workpiece;
• remelting and resolidifying at least a portion of the weld joint at the interfacial weld bond area without causing interfacial expulsion along the faying interface established between the steel workpiece and the aluminum workpiece, the remelting and resolidifying forming a first refined weld joint that includes remelted and resolidified material from the weld joint and any portions of the weld joint that did not undergo remelting and resolidification;
• and
• remelting and resolidifying at least a portion of the first refined weld joint at an interfacial weld bond area of the first refined weld joint without causing interfacial expulsion along the faying interface established between the steel workpiece and the aluminum workpiece, the remelting and resolidifying forming a second refined weld joint that includes remelted and resolidified material from the first refined weld joint and any portions of the first refined weld joint that did not undergo remelting and resolidification.