COOLING TO CONTROL THERMAL STRESS AND SOLIDIFICATION FOR WELDING OF DISSIMILAR MATERIALS

• US 20170008118A1
• Filed: 07/07/2015
• Published: 01/12/2017
• Est. Priority Date: 07/07/2015
• Status: Active Application

• Current Assignee: GM Global Technology Operations LLC (General Motors Company)
• Original Assignee: GM Global Technology Operations LLC (General Motors Company)
• Inventors: Yang, David, Carlson, Blair E., Sigler, David R.
• Granted Patent: US 10,252,369 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: B23K 11/115 by means of two electrodes placed opposite one another on both sides of the welded parts
• CPC Class Codes: B23K 11/20 of different metals
• CPC Class Codes: B23K 2103/20 Ferrous alloys and aluminium or alloys thereof
• CPC Class Codes: B23K 37/003 Cooling means

Abstract

A workpiece stack-up that includes at least a steel workpiece and an aluminum-based workpiece can be resistance spot welded by a spot welding method in which the welding current is controlled to perform one or more stages of weld joint development. When it is desired to terminate weld current flow and to solidify a liquid weld pool into a weld nugget (of mostly aluminum-based composition), additional cooling is applied to the outer surface of the aluminum-based workpiece around the contact area of the spot welding electrode engaging the surface of the aluminum-based workpiece surface. The additional cooling is applied and controlled so as to increase the rate of solidification of the liquid aluminum-based material and to control the direction of solidification of the weld nugget to better confine impurities, and the like, originally in the melt, at the surface of the steel workpiece.

Claims

1. A method of resistance spot welding a workpiece stack-up, which includes an aluminum-based workpiece and a steel workpiece so as to form a resistance spot weld between the aluminum-based workpiece and the steel workpiece, 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 at least one stage of weld joint development that includes growing a molten weld pool in the aluminum-based workpiece that extends from the faying interface into the aluminum-based workpiece; and

cooling the molten weld pool to solidify it into a weld nugget that includes a weld bond area joined to the faying surface of the steel workpiece, the cooling of the molten weld pool comprising cooling the opposed side surface of the aluminum-based workpiece in the area of the opposed side surface surrounding the area contacted by the spot welding electrode, the cooling being controlled to increase the peel strength of the spot weld between the stacked workpieces and to reduce residual stress in the workpieces at the spot weld site.

2. The method as set forth in claim 1 wherein a round molten weld pool is formed in the aluminum-based workpiece, the molten weld pool having a radius dimension and a perimeter dimension at the faying interface and decreasing radii dimensions and perimeter dimensions as the molten pool extends into the aluminum-based workpiece, and the cooling at the opposed side surface of the aluminum-based workpiece is controlled to cause solidification of the molten weld pool to proceed radially inwardly from its perimeter dimensions so as to displace solid material other than the aluminum-based composition toward the center portion of the solidified weld nugget at the faying interface.

3. The method set forth in claim 1, wherein a stream of a cooling gas is directed at the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode.

4. The method set forth in claim 1, wherein a stream of a cooling gas is directed at the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode, the cooling gas being at a temperature in the range of 15°

C. to 0°

C. as it flows against the surface of the workpiece.

5. The method as set forth in claim 3, wherein the flow rate of the cooling gas is greater than 10 cm³/s and up to a gas flow rate that does not result in a further increase in the peel strength of the spot weld.

6. The method as set forth in claim 3, wherein the flow rate of the cooling gas is greater than 50 cm³/s and up to a gas flow rate that does not result in a further increase in the peel strength of the spot weld.

7. The method as set forth in claim 1, wherein a surface of a heat exchanger is placed in contact with the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode.

8. The method as set forth in claim 1, wherein a surface of a heat exchanger is placed in contact with the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode, the contacting surface of the heat exchanger being at a temperature in the range of 15°

C. to 0°

C.

9. The method set forth in claim 2, wherein a stream of a cooling gas is directed at the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode.

10. The method set forth in claim 2, wherein a stream of a cooling gas is directed at the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode, the cooled gas being at a temperature in the range of 15°

C. to 0°

C. as it flows against the surface of the workpiece.

11. The method as set forth in claim 10, wherein the flow rate of the cooling gas is greater than 10 cm³/s and up to a gas flow rate that does not result in a further increase in the peel strength of the spot weld.

12. The method as set forth in claim 10, wherein the flow rate of the cooling gas is greater than 50 cm³/s and up to a gas flow rate that does not result in a further increase in the peel strength of the spot weld.

13. The method as set forth in claim 10, wherein a surface of a heat exchanger is placed in contact with the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode.

14. The method as set forth in claim 10, wherein a surface of a heat exchanger is placed in contact with the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode, the contacting surface of the heat exchanger being at a temperature in the range of 15°

C. to 0°

C.

15. A method of resistance spot welding a workpiece stack-up, which includes an aluminum-based workpiece and a steel workpiece so as to form a resistance spot weld between the aluminum-based workpiece and the steel workpiece, 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 two stages of weld joint development, each stage including growing a molten weld pool in the aluminum-based workpiece that extends from the faying interface into the aluminum-based workpiece and re-solidifying the re-formed molten weld pool; and

, following the second stage of growth of a molten weld pool,

cooling the second molten weld pool to solidify it into a weld nugget that includes a weld bond area joined to the faying surface of the steel workpiece, the cooling of the molten weld pool comprising cooling the opposed side surface of the aluminum-based workpiece in the area of the opposed side surface surrounding the area contacted by the spot welding electrode, the cooling being controlled to increase the peel strength of the spot weld between the stacked workpieces and to reduce residual stress in the workpieces at the spot weld site.

16. The method as set forth in claim 15, wherein a round molten weld pool is formed in the aluminum-based workpiece, the molten weld pool having a radius dimension and a perimeter dimension at the faying interface and decreasing radii dimensions and perimeter dimensions as the molten pool extends into the aluminum-based workpiece, and the cooling at the opposed side surface of the aluminum-based workpiece is controlled to cause solidification of the molten weld pool to proceed radially inwardly from its perimeter dimensions so as to displace solid material other than the aluminum-based composition toward the center portion of the solidified weld nugget at the faying interface.

17. The method set forth in claim 16, wherein a stream of a cooling gas is directed at the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode.

18. The method set forth in claim 16, wherein a stream of a cooling gas is directed at the opposed side surface of the aluminum-based workpiece surrounding the area contacted by the spot welding electrode, the cooled gas being at a temperature in the range of 15°

C. to 0°

C. as it flows against the surface of the workpiece.

19. The method set forth in claim 16, wherein the flow rate of the cooling gas is greater than 10 cm³/s and up to a gas flow rate that does not result in a further increase in the peel strength of the spot weld.

20. The method set forth in claim 16, wherein the flow rate of the cooling gas is greater than 50 cm³/s and up to a gas flow rate that does not result in a further increase in the peel strength of the spot weld.