Two-step spot welding process using concentric electrodes

Abstract

The present disclosure provides a concentric electrode. The concentric electrode includes a cylindrical base and a tip disposed on the cylindrical base. The cylindrical base extends in the axial direction of the rotating shaft, and the cylindrical base comprises conductive metal. The tip includes a concentric surface extending axially from the cylindrical base and defined by an outer shoulder having an outer diameter and an inner shoulder having an inner diameter. The tip is configured to provide an electrical current to the steel workpiece and remove a coating from at least a portion of the steel workpiece.

Description

Technical Field

[0001] This disclosure relates to a steel workpiece, and more specifically, to a steel workpiece including a coating and a method for spot welding multiple steel workpieces. Background Technology

[0002] The application of a galvanized coating (commonly known as galvanized steel) to steel components has been used in many industries to inhibit corrosion. The automotive industry is one of the industries that has benefited from the use of galvanized steel. Without a corrosion inhibitor (such as zinc), wear during driving and exposure of vehicles to various environments (such as water, oxygen, and chloride de-icing agents) make the steel components of a vehicle susceptible to corrosion.

[0003] However, galvanized coatings are prone to adverse properties during welding. More specifically, when heated by the current in the welding process, zinc liquefies and penetrates into the steel. Zinc promotes the liquid metal embrittlement (LME) phenomenon known in the art, in which the tensile ductility of the steel decreases sharply and / or brittle fracture occurs due to the penetrating zinc. Therefore, previously galvanized steels were prone to mechanical failure after welding.

[0004] The practical significance of LME (Low Elongation at Elongation) is particularly evident in various steels, such as Gen3 steel, which are well-suited for automotive applications to improve material strength and reduce weight. Gen3 steel is defined herein as steel with a minimum tensile strength (e.g., at most 1500 MPa or higher) and elongation (e.g., 20%). Gen3 steel has been observed to exhibit ductility loss and cracking during welding, and catastrophic LME cracking is possible, with high crack propagation rates observed. Therefore, the use of certain ideal materials (e.g., galvanized Gen3 benefiting from corrosion-resistant coatings) in automobiles is limited.

[0005] LME cracking can occur in known zinc and aluminum coatings during welding operations, such as resistance spot welding used to join metal parts to a low melting temperature coating. LME cracking can occur due to differences in metal melting temperature and the pressure applied during resistance welding, particularly at the interface between the substrate (e.g., steel) and the coating material. LME cracking weakens the substrate material, thereby reducing its fundamental strength.

[0006] While existing methods and systems attempt to provide coating removal for spot welding and can achieve their specific purposes, these offline surface treatment processes are both time-consuming and expensive. Therefore, there remains a need for a new and improved method to remove coatings prior to spot welding. Summary of the Invention

[0007] According to several aspects of this disclosure, a concentric electrode is provided. The concentric electrode includes a cylindrical base and a tip disposed on the cylindrical base. The cylindrical base extends axially along a rotation axis and comprises a conductive metal. The tip includes a concentric surface extending axially from the cylindrical base and defined by an outer shoulder having an outer diameter and an inner shoulder having an inner diameter. The tip is configured to provide current to a steel workpiece and remove coating from at least a portion of the steel workpiece.

[0008] According to another aspect of this disclosure, the concentric electrode includes a tip having a flat concentric surface.

[0009] According to another aspect of this disclosure, the concentric electrode includes an outer diameter of about 8 mm and an inner diameter of about 5 mm.

[0010] According to another aspect of this disclosure, the concentric electrode includes a flat concentric surface having a thickness of about 3 mm, and said thickness extends axially from the cylindrical base.

[0011] According to another aspect of this disclosure, the concentric electrode includes a flat concentric surface having an annular width of about 1.5 mm between the outer diameter and the inner diameter.

[0012] According to another aspect of this disclosure, the concentric electrode includes a tip, the tip comprising a circular concentric surface.

[0013] According to another aspect of this disclosure, the concentric electrode includes a steel workpiece comprising third-generation advanced high-strength steel (AHSS).

[0014] According to another aspect of this disclosure, the concentric electrode includes a coating formed of zinc.

[0015] According to another aspect of this disclosure, concentric electrodes are connected to a welding robot.

[0016] According to several aspects of this disclosure, a two-step spot welding method is provided. The method includes providing a workpiece stack, contacting a first concentric electrode with a first steel workpiece and contacting a second concentric electrode with a second steel workpiece, removing a coating from the first steel workpiece to form a first partial opening by passing a current between the first and second concentric electrodes and through the workpiece stack, removing a coating from the second steel workpiece to form a second partial opening, and spot welding the first and second steel workpieces respectively using a first welding electrode and a second welding electrode. The first and second steel workpieces include a coating. Both the first and second concentric electrodes have an outer diameter, an inner diameter, and a tip. The partial opening is formed as a concentric circle having an outer diameter and an inner diameter and is defined by an outer shoulder region and an inner shoulder region. The first and second welding electrodes have a welding surface diameter greater than the inner diameter and smaller than the outer diameter.

[0017] According to another aspect of this disclosure, the method includes a coating formed of zinc.

[0018] According to another aspect of this disclosure, the method includes the tip of a first concentric electrode and the tip of a second concentric electrode including a flat concentric surface.

[0019] According to another aspect of this disclosure, the outer diameter is about 8 mm and the inner diameter is about 5 mm.

[0020] According to another aspect of this disclosure, the flat concentric surface has a thickness of about 3 mm, and the thickness extends axially from the cylindrical base.

[0021] According to another aspect of this disclosure, the flat concentric surface has an annular width of about 1.5 mm between the outer diameter and the inner diameter.

[0022] According to another aspect of this disclosure, the tip includes a circular concentric surface.

[0023] According to another aspect of this disclosure, the steel workpiece includes third-generation advanced high-strength steel (AHSS).

[0024] According to several aspects of this disclosure, a combined electrode is provided. The combined electrode includes a welding electrode and a retractable concentric electrode movably coupled to and radially surrounding the welding electrode. The welding electrode has a welding surface diameter greater than its inner diameter and smaller than its outer diameter. The welding electrode is configured to weld a stack of steel workpieces. The concentric electrode includes a cylindrical base and a tip disposed on the cylindrical base. The cylindrical base includes a conductive metal. The tip includes a concentric surface extending axially from the cylindrical base and defined by an outer shoulder having an outer diameter and an inner shoulder having an inner diameter. The tip is configured to supply current to the steel workpiece stack and remove coating from at least a portion of the steel workpiece stack.

[0025] According to another aspect of this disclosure, the combined electrode has an outer diameter of about 8 mm and an inner diameter of about 5 mm.

[0026] According to another aspect of this disclosure, the combined electrode has a flat, concentric surface and a thickness of approximately 3 mm. The thickness extends axially from the cylindrical base, and the annular width between the outer and inner diameters is approximately 1.5 mm.

[0027] The above-described features and advantages, as well as other features and advantages, of the currently disclosed systems and methods will become apparent when taken in conjunction with the accompanying drawings, based on the detailed description and embodiments, including the claims. Attached Figure Description

[0028] The invention will be more fully understood through detailed description and accompanying drawings, wherein:

[0029] Figure 1This is a side sectional view showing a plurality of steel workpieces according to the present disclosure and a plurality of concentric electrodes for removing coatings from the steel workpieces.

[0030] Figure 2A This illustrates, according to the present disclosure, as follows Figure 1 The diagram shows a perspective view of concentric electrodes, which include a cylindrical base and a tip.

[0031] Figure 2B This illustrates, according to the present disclosure, as follows Figure 1 The cross-sectional view of the concentric electrode shown includes a cylindrical base and a tip.

[0032] Figure 3 This illustrates the effect after a portion of the coating has been removed using a concentric electrode according to this disclosure. Figure 1 A top view of the open portion of the steel workpiece shown.

[0033] Figure 4 This is a side sectional view showing a plurality of steel workpieces and a plurality of welding electrodes according to the present disclosure, the plurality of welding electrodes being used for spot welding the steel workpieces after a portion of the coating has been removed using concentric electrodes.

[0034] Figure 5 This is a side sectional view showing a combined electrode including a concentric electrode and a welding electrode according to the present disclosure, wherein the concentric electrode is extended to remove a portion of the coating on a steel workpiece.

[0035] Figure 6 This illustrates, according to the present disclosure, as follows Figure 5 The side sectional view of the combined electrodes shown shows the concentric electrodes retracted and the welding electrode in place to provide spot welding.

[0036] Figure 7 This is a flowchart illustrating a two-step spot welding method according to the present disclosure, the method using... Figure 1-2B and Figure 4 The concentric electrodes and welding electrodes shown are used. Detailed Implementation

[0037] Reference will now be made in detail to several embodiments of the present disclosure illustrated in the accompanying drawings. Wherever possible, the same or similar reference numerals are used in the drawings and description to denote the same or similar parts or steps. The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application, or its uses.

[0038] Liquid metal embrittlement (LME) cracking during spot welding is one of the reasons limiting the application of coated Gen3 steel in vehicle body structures. Previous zinc coating removal methods (such as laser ablation) employed offline surface pretreatment, which can be both time-consuming and expensive. The spot welding process disclosed herein is an online process that reduces both time and cost while significantly reducing LME cracking. This online zinc coating removal method enables the application of Gen3 steel in vehicle body structures.

[0039] refer to Figure 1 A cross-sectional view of a steel workpiece stack 10 and a concentric electrode 12 according to the present disclosure is shown. The steel stack 10 includes at least one workpiece 14 and a coating 16 disposed on the at least one workpiece 14. Although Figure 1 A coating 16 is shown on one surface of workpiece 14, but coating 16 may be applied to both surfaces of workpiece 14. Workpiece 14 may comprise at least one of steel or steel alloy (e.g., third-generation advanced high-strength steel (AHSS)). It should be understood that each workpiece 14 may comprise different grades of steel or steel alloy with different thicknesses. Additionally, each coating 16 may have a different thickness. In one embodiment, the steel workpiece 14 is also defined as a sheet metal component for a vehicle, such as a body panel. However, the steel workpiece 14 may be used with any component in a vehicle interior or any suitable non-vehicle application.

[0040] In one embodiment, workpiece 14 comprises iron, about 1.4 wt% to about 2.0 wt% aluminum, and about 0 wt% to about 1.0 wt% silicon. In another embodiment, workpiece 14 comprises about 1.5 wt% to about 1.9 wt% aluminum and about 0.2 wt% to about 0.8 wt% silicon. In yet another embodiment, workpiece 14 comprises about 1.6 wt% to about 1.8 wt% aluminum and about 0.4 wt% to about 0.6 wt% silicon. Throughout this document, the term "about" is known to those skilled in the art. Alternatively, the term "about" can be understood to mean adding or subtracting 0.15 wt%.

[0041] The workpiece 14 may also include about 0.17 to about 0.35 wt% carbon, about 2.0 to about 4.0 wt% manganese, about 0 to about 0.01 wt% sulfur, about 0 to about 0.2 wt% copper, about 0 to about 0.008 wt% nitrogen, about 0 to about 0.005 wt% boron, and about 0 to about 0.04 wt% phosphorus. Iron constitutes the remaining components of the workpiece 14. The workpiece 14 may also include other elements in a weight percentage of less than 0.02. Other elements are those not listed above but present in the alloy matrix as impurities. In this document, the term "about" is known to those skilled in the art. Alternatively, the term "about" may be understood to mean adding or subtracting 0.15 wt%.

[0042] The steel workpiece stack 10 also includes a coating 16, such as zinc. The coating 16 provides corrosion protection to the workpiece 14, where one or more elements (e.g., iron) are readily oxidized. In one embodiment, the coating 16 is applied to the workpiece 14 by galvanizing (e.g., by immersion in molten zinc, electroplating, etc.). However, the coating 16 can be applied in any suitable manner. Multiple steel workpieces 14 are assembled adjacent to each other to form the workpiece stack 10, such as… Figure 1 As shown. It should be understood that, although Figure 1 Only two workpieces 14 are shown, but other workpieces can be added to the stack 10 (e.g., three workpieces, four workpieces, etc.). The steel workpieces 14 can then be spot-welded together. When the galvanized steel workpieces 14 are spot-welded, the zinc coating 16 liquefies and penetrates into the steel grain boundaries. The zinc coating 16 promotes the liquid metal embrittlement phenomenon known in the art, in which the tensile ductility of the stack 10 decreases sharply and / or brittle fracture occurs due to the penetrating zinc, which is undesirable.

[0043] Still referencing Figure 1 The concentric electrode 12 can be positioned adjacent to the steel workpiece stack 10 to remove a portion of the coating 16 from the steel workpiece. Figure 1 In the embodiment shown, the first concentric electrode 12A is disposed on the first side 18 of the steel workpiece stack 10, and the second concentric electrode 12B is disposed on the second side 20 of the steel workpiece stack 10.

[0044] Figure 2A and Figure 2B Perspective and cross-sectional views of the concentric electrode 12 are shown. In this embodiment, the concentric electrode 12 includes a cylindrical base 22. The cylindrical base 22 extends axially along the rotation axis A. The cylindrical base 22 comprises a conductive metal, such as copper.

[0045] Still referencing Figure 2A and Figure 2BThe concentric electrode 12 includes a tip 24. The tip 24 is disposed on a cylindrical base 22 and configured to supply current to the steel workpiece 14 to remove coating 16 from a portion of the steel workpiece 14. The tip 24 is defined by an outer diameter OD and an inner diameter ID and has concentric surfaces 26 or faces. Figure 2A and 2B As depicted in the embodiments, the concentric surface 26 may have a flat surface (e.g., a plane). In other embodiments, the concentric surface 26 may be dome-shaped and / or circular. It should be understood that the concentric surface 26 may be configured and / or shaped in a variety of other ways.

[0046] Figure 3 A top view of the opening 28 of the steel workpiece 14 after a portion of the coating 16 has been removed using the concentric electrode 12 is shown. In this embodiment, the opening 28 is circular or concentric. The opening 28 is defined by an outer diameter OD and an inner diameter ID. The outer diameter OD also defines an outer shoulder 30 of the coating 16, and the inner diameter ID also defines an inner shoulder 32 of the coating 16. The inner shoulder 32 defines a central portion 34 of the coating 16, which remains after the coating 16 has been removed by the concentric electrode 12. Figure 3 In the illustrated embodiment, the opening 28 has an inner diameter ID of approximately 5.00 mm and an outer diameter OD of approximately 8.00 mm. It should be understood that the opening 28 may include a variety of outer diameters (e.g., 7.00 mm, 7.50 mm, 8.50 mm, etc.) and may include a variety of inner diameters (e.g., 4.00 mm, 4.50 mm, 5.50 mm, etc.). The term "approximately" is known to those skilled in the art herein. Alternatively, the term "approximately" may be understood to mean adding or subtracting 0.15 mm. Furthermore, the depth or thickness of the removed coating 16 is greater than 0 micrometers (μm) and less than 200 μm. In one embodiment, the coating 16 is approximately 50 μm. It should be understood that the opening 28 may include various depths or thicknesses (e.g., 25 μm, 75 μm, 100 μm, 185 μm, etc.). The term "approximately" is known to those skilled in the art herein. Alternatively, the term "approximately" may be understood to mean adding or subtracting 5 μm.

[0047] refer to Figure 4 Then, a welding electrode 36, for example, connected to a welding robot, is placed at the location where it will be welded to the steel workpiece 14. The welding electrode 36 may include, for example, a copper spot welding electrode. The welding electrode 36 has a welding surface 38, the diameter of which is greater than the inner diameter ID and less than the outer diameter OD. Figure 4In the illustrated embodiment, a first workpiece 14A and a second workpiece 14B are positioned together, and a first welding electrode 36A is positioned close to and in contact with the first workpiece 14A, while a second welding electrode 36B is positioned close to and in contact with the second workpiece 14B. Welding current flows from the first welding electrode 36A and the second welding electrode 36B through and across the first workpiece 14A and the second workpiece 14B, thereby generating resistance heat at the weld point 40 to produce molten metal. The molten metal then cools and forms a weld nugget 42, which connects the first workpiece 14A and the second workpiece 14B.

[0048] Figure 5 and Figure 6 A cross-sectional view of a combined electrode 44 disposed on one side of the workpiece stack 10 is shown. However, in practice, the combined electrode 44 is disposed on both sides of the stack 10 (e.g., a first combined electrode 44 is disposed on a first side of the stack 10, and a second combined electrode 44 is disposed on a second side of the stack 10 opposite to the first side). The combined electrode 44 includes a concentric electrode 12 and a welding electrode 36 of combined construction. The welding electrode 36 may be cylindrical and configured as part of the contact coating 16. The concentric electrode 12 may be concentric and retractably surround the welding electrode 36.

[0049] like Figure 5 As shown, the combined electrode 44 is placed near the workpiece 14, and the concentric electrode 12 extends and contacts the workpiece 14. Then, the concentric electrode 12 is used to remove a portion of the coating 16 to create the opening 28 as described above.

[0050] After the open section 28 is formed, as Figure 6 As shown, the concentric electrode 12 is retracted, and the welding electrode 36 is positioned to contact the workpiece 14 for spot welding to the workpiece stack 10. Although Figure 5 and Figure 6 Only one combined electrode 44 is depicted, but multiple combined electrodes 44 can be used to produce spot welds. For example, actuators and / or motors can be used to extend and / or retract the concentric electrodes 12.

[0051] refer to Figure 7 This illustrates a method 100 for two-step spot welding of a workpiece stack 10 according to the present disclosure. The method begins at frame 102.

[0052] Box 102 depicts providing a workpiece stack 10. The workpiece stack 10 includes a first steel workpiece 14 and a second steel workpiece 14. Both the first steel workpiece 14 and the second steel workpiece 14 include a coating 16, which may be zinc as described above. The method then moves to box 104.

[0053] Box 104 depicts contacting the first concentric electrode 12A with the first steel workpiece 14A and contacting the second concentric electrode 12B with the second steel workpiece 14B. In an embodiment, the first concentric electrode 12A is coupled to and controlled by a welding robot, and the second concentric electrode 12B is coupled to and controlled by a second welding robot. The method then proceeds to box 106.

[0054] Box 106 depicts removing the coating from a first steel workpiece 14A to form a first partial opening 28A and removing the coating from a second steel workpiece 14B to form a second partial opening 28B. The coating removal 16 may include allowing current to pass between the first concentric electrode 12A and the second concentric electrode 12B and through the workpiece stack 10. The first partial opening 28A and the second partial opening 28B are shaped as concentric circles or rings defined by an outer diameter OD and an inner diameter ID, and are defined by an outer shoulder 30 and an inner shoulder 32. The method then moves to box 108.

[0055] Box 108 depicts spot welding of a first steel workpiece 14A and a second steel workpiece 14B using a first welding electrode 36A and a second welding electrode 36B. Spot welding includes the use of the first welding electrode 36A and the second welding electrode 36B. Both the first welding electrode 36A and the second welding electrode 36B have a welding surface diameter greater than the inner diameter ID and less than the outer diameter OD.

[0056] The steel workpiece and spot welding method disclosed herein are advantageous and beneficial compared to the prior art. The concentric electrode 12 and the method for removing the zinc-containing coating 16 are in-line processes, reducing time and cost while significantly reducing LME cracking. The in-line zinc coating removal method disclosed herein enables the application of Gen3 steel in vehicle body structural components.

[0057] This description is merely illustrative in nature and is in no way intended to limit this disclosure, its application, or its use. The broad teachings of this disclosure can be implemented in many forms. Therefore, while this disclosure includes specific embodiments, its true scope should not be so limited, as other modifications will become apparent upon examination of the drawings, description, and appended claims.

Claims

1. A concentric electrode, comprising: A cylindrical base extending axially along a rotation axis, the cylindrical base comprising a conductive metal; as well as A tip, disposed on a cylindrical base, the tip comprising concentric surfaces extending axially from the cylindrical base and defined by an outer shoulder having an outer diameter and an inner shoulder having an inner diameter. The tip is configured to supply current to the steel workpiece and remove coating from at least a portion of the steel workpiece.

2. The concentric electrode according to claim 1, wherein, The tip includes a flat, concentric surface.

3. The concentric electrode according to claim 2, wherein, The outer diameter is approximately 8 mm, and the inner diameter is approximately 5 mm.

4. The concentric electrode according to claim 2, wherein, The flat concentric surface has a thickness of approximately 3 mm, wherein the thickness extends axially from the cylindrical base.

5. The concentric electrode according to claim 2, wherein, The flat concentric surface has an annular width of approximately 1.5 mm between the outer diameter and the inner diameter.

6. The concentric electrode according to claim 1, wherein, The tip includes a circular concentric surface.

7. The concentric electrode according to claim 1, wherein, The steel workpiece includes third-generation advanced high-strength steel (AHSS).

8. The concentric electrode according to claim 1, wherein, The coating is zinc.

9. The concentric electrode according to claim 1, wherein, The concentric electrodes are connected to the welding robot.

10. A two-step spot welding method, comprising: A workpiece stack including a first steel workpiece and a second steel workpiece is provided, wherein the first steel workpiece and the second steel workpiece include a coating; The first concentric electrode is brought into contact with the first steel workpiece, and the second concentric electrode is brought into contact with the second steel workpiece, wherein both the first concentric electrode and the second concentric electrode have an outer diameter, an inner diameter, and a tip; By passing an electric current between the first and second concentric electrodes and through the workpiece stack, a coating is removed from the first steel workpiece to form a first partial opening, and a coating is removed from the second steel workpiece to form a second partial opening, wherein the partial opening is shaped as a concentric circle having the outer diameter and the inner diameter and is defined by an outer shoulder region and an inner shoulder region; and The first steel workpiece and the second steel workpiece are spot welded using a first welding electrode and a second welding electrode, respectively, wherein the first welding electrode and the second welding electrode have a welding surface diameter that is larger than the inner diameter and smaller than the outer diameter.

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