Mechanical performance of al-steel weld joints

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

• pre-welding an aluminum shim to a steel workpiece to form a supporting weld joint within the aluminum shim that establishes a bonding interface with a faying surface of the steel workpiece;
• positioning an aluminum workpiece over the aluminum shim and the steel workpiece to assemble a workpiece stack-up in which the aluminum shim is disposed between a faying surface of the aluminum workpiece and the faying surface of the steel workpiece, the workpiece stack-up having a first side and an opposite second side, the first side being provided by an aluminum workpiece surface and the second side being provided by a steel workpiece surface;
• pressing a weld face of a first welding electrode against the first side of the workpiece stack-up;
• pressing a weld face of a second welding electrode against the second side of the workpiece stack-up in facial alignment with the weld face of the first welding electrode;
• passing an electric current between the weld face of the first welding electrode and the weld face of the second welding electrode to create a molten aluminum weld pool that penetrates through the aluminum shim within the supporting weld joint and into the aluminum workpiece, the molten aluminum weld pool consuming a portion of the supporting weld joint, including a portion of the bonding interface of the supporting weld joint, to wet an exposed portion of the faying surface of the steel workpiece;
• and
• terminating passage of the electric current so that the molten aluminum weld pool solidifies into a weld joint that bonds the aluminum workpiece to the steel workpiece through the aluminum shim, the weld joint being surrounded by an unconsumed portion of the supporting weld joint and establishing a bonding interface with the exposed portion of the faying surface of the steel workpiece, and wherein a notch root is created between the aluminum workpiece and the aluminum shim and is displaced away from the faying surface of the steel workpiece.

A workpiece stack-up comprising:

• a steel workpiece having a faying surface;
• an aluminum workpiece that overlaps the steel workpiece, the aluminum workpiece having a faying surface that confronts the faying surface of the steel workpiece;
• an aluminum patch disposed between the steel workpiece and the aluminum workpiece;
• and
• a weld joint that bonds the aluminum and steel workpieces together, the weld joint establishing a bonding interface with the faying surface of the steel workpiece and extending through the aluminum patch and into the aluminum workpiece, and wherein the aluminum patch is adhered to the faying surface of the steel workpiece outside of and around the weld joint.