Multi-step direct welding of an aluminum-based workpiece to a steel workpiece

1. A method of resistance spot welding the method comprising:

contacting a workpiece stack-up with a pair of spot welding electrodes such that the spot welding electrodes make contact with opposed sides of the workpiece stack-up, the workpiece stack-up comprising an aluminum-based workpiece and a steel workpiece, the aluminum-based workpiece having a faying surface and the steel workpiece having a faying surface, and wherein the faying surfaces of the aluminum-based workpiece and the steel workpiece overlap and contact one another to provide a faying interface between the workpieces; and

controlling the passage of electrical current between the spot welding electrodes and through the aluminum-based workpiece and the steel workpiece to perform multiple stages of weld joint development that include:

growing a molten weld pool in the aluminum-based workpiece that extends from the faying interface into the aluminum-based workpiece;

allowing the molten weld pool to cool and solidify into a weld nugget that includes a weld bond area joined to the faying surface of the steel workpiece, the weld bond area of the weld nugget being at least 4(π)(t), where t is a thickness of the aluminum-based workpiece at the weld site;

re-melting at least a portion of the weld nugget including at least part of the weld bond area of the weld nugget;

allowing the re-melted portion of the weld nugget to cool and solidify.

2. The method set forth in claim 1 , wherein electrical current in the form of current pulses is passed between the spot welding electrodes to re-melt at least a portion of the weld nugget.

3. The method set forth in claim 1 , wherein the weld nugget extends into the aluminum-based workpiece from the faying surface to a penetration depth, and wherein the re-melted portion of the weld nugget does not extend to the penetration depth of the weld nugget.

4. The method set forth in claim 1 , wherein the re-melted portion of the weld nugget is entirely confined within the weld bond area.

5. The method set forth in claim 1 , wherein the re-melted portion of the weld nugget includes up to 100% of the weld bond area of the weld nugget.

6. The method set forth in claim 1 , wherein the re-melted portion of the weld nugget includes the entire weld bond area of the weld nugget and further combines with molten material of the aluminum-based workpiece outside of the weld bond area of the weld nugget to establish an enlarged weld bond area.

7. The method set forth in claim 6 , wherein the enlarged weld bond area is up to 50% greater in area than the weld bond area of the weld nugget before re-melting.

8. The method set forth in claim 1 , wherein the steel workpiece is galvanized steel or bare steel.

9. The method set forth in claim 1 , wherein the aluminum-based workpiece comprises an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, or an aluminum-zinc alloy.

10. The method set forth in claim 1 , further comprising:

expelling at least part of the re-melted portion of the weld nugget at the faying interface.

11. A method of resistance spot welding the method comprising:

providing a workpiece stack-up that includes an aluminum-based workpiece and a steel workpiece, the aluminum-based workpiece having a faying surface and the steel workpiece having a faying surface, and wherein the faying surfaces of the workpieces overlap and contact one another to provide a faying interface between the workpieces;

passing electrical current through the aluminum-based workpiece and the steel workpiece and across the faying interface for a first period of time during which a molten weld pool that extends from the faying interface into the aluminum-based workpiece is initiated and grown;

allowing the molten weld pool to cool and solidify into a weld nugget by either, for a second period of time after passage of electrical current for the first period of time, passing electrical current through the workpieces at a reduced level that allows for solidification of the molten weld pool or ceasing passage of electrical current through the workpieces; and

passing electrical current through the aluminum-based workpiece and the steel workpiece for a third period of time, after the weld nugget has solidified during the second period of time, during which at least a portion of the weld nugget is re-melted.

12. The method set forth in claim 11 , wherein the weld nugget extends into the aluminum-based workpiece to a penetration depth, and wherein re-melted portion of the weld nugget does not extend to the penetration depth of the weld nugget.

13. The method set forth in claim 11 , wherein the weld nugget includes a weld bond area joined to a faying surface of the steel workpiece at the faying interface, and wherein the re-melted portion of the weld nugget includes the entire weld bond area of the weld nugget and further combines with molten material of the aluminum-based alloy workpiece outside of the weld bond area of the weld nugget, and wherein, upon cooling and solidification, the re-melted portion of the weld nugget and the molten material of the aluminum-based workpiece outside of the weld bond area of the weld nugget establish an enlarged weld bond area.

14. The method set forth in claim 11 , further comprising:

expelling at least part of the re-melted portion of the weld nugget at the faying interface.

15. The method set forth in claim 14 , wherein expelling at least part of the re-melted portion of the weld nugget is caused by passing electrical current through the aluminum-based workpiece and the steel workpiece for a fourth period of time, after the third period of time, during which electrical current is raised to a higher level than in the third period of time.

16. The method set forth in claim 11 , wherein the steel workpiece is galvanized steel or bare steel, and wherein the aluminum-based workpiece comprises an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, or an aluminum-zinc alloy.

17. A multi-stage spot welding method comprising:

pressing a pair of spot welding electrodes against a workpiece stack-up such that the spot welding electrodes are pressed against opposed sides of the workpiece stack-up, the workpiece stack-up comprising an aluminum-based workpiece and a steel workpiece;

growing a molten weld pool in the aluminum-based workpiece that extends from a faying interface of the steel workpiece and the aluminum-based workpiece into the aluminum-based workpiece;

allowing the molten weld pool to solidify into a weld nugget that includes a weld bond area joined to a faying surface of the steel workpiece at the faying interface;

re-melting at least a portion of the weld nugget that includes at least part of the weld bond area;

expelling at least part of the re-melted portion of the weld nugget at the faying interface before allowing the re-melted portion of the weld nugget to solidify; and

allowing the re-melted portion of the weld nugget to solidify into a re-solidified portion of the weld nugget.

18. The method set forth in claim 17 , wherein, prior to re-melting at least a portion of the weld nugget, the weld bond area of the weld nugget is at least 4(π)(t), where t is a thickness of the aluminum-based workpiece at the weld site.