Two-stage spot welding process using a concentric electrode

Abstract

Method for two-stage spot welding, comprising: Providing a stack of workpieces (10) containing a first steel workpiece (14, 14A) and a second steel workpiece (14, 14B), wherein the first steel workpiece (14, 14A) and the second steel workpiece (14, 14B) contain a coating (16); Touching the first steel workpiece (14, 14A) with a first concentric electrode (12, 12A) and touching the second steel workpiece (14, 14B) with a second concentric electrode (12, 12B), wherein the first concentric electrode (12, 12A) and the second concentric electrode (12, 12B) each have an outer diameter (OD), an inner diameter (ID) and a tip (24); Removing the coating (16) from the first steel workpiece (14, 14A) to form a first locally exposed section (28, 28A), and from the second steel workpiece (14, 14B) to form a second locally exposed section (28, 28B), by passing an electric current between the first concentric electrode (12, 12A) and the second concentric electrode (12, 12B) and through the workpiece stack (10), wherein the locally exposed sections (28, 28A, 28B) are formed in the shape of a concentric ring with outer diameter (OD) and inner diameter (ID) and are defined by an outer shoulder region (30) and an inner shoulder region (32); and Spot welding of the first steel workpiece (14, 14A) and the second steel workpiece (14, 14B) using a first welding electrode (36A) and a second welding electrode (36B), respectively, wherein the first welding electrode (36A) and the second welding electrode (36B) have a weld end face diameter that is larger than the inner diameter (ID) and smaller than the outer diameter (OD).

Description

INTRODUCTION

[0001] The present disclosure relates to a method for two-stage spot welding of several steel workpieces, in particular using concentric electrodes.

[0002] A concentric electrode for spot welding is known, for example, from DE 11 2018 007 461 T5. Further concentric electrodes for spot welding are disclosed in DE 10 2015 101 869 A1 or US 2003 / 0192 863 A1.

[0003] The application of a zinc coating to steel components (generally referred to as galvanized steel) has been used in numerous industries to prevent steel corrosion. One industry that has benefited from the use of galvanized steel is the automotive industry. Due to wear and tear from driving and the vehicles' exposure to the elements (e.g., water, oxygen, and chloride-based de-icing agents), the vehicle's steel components are susceptible to corrosion if they are not coated with a corrosion inhibitor such as zinc.

[0004] However, a zinc coating tends to exhibit an undesirable property during the welding process. Specifically, the zinc liquefies and penetrates the steel when heated by the electric current of a welding process. The zinc promotes a phenomenon known in engineering as liquid metal embrittlement (LME), in which the steel experiences a drastic loss of tensile strength and / or suffers brittle fracture due to the penetrating zinc. Earlier galvanized steels were therefore susceptible to mechanical defects after welding.

[0005] The practical significance of low-molecular-weight steels (LMS) is evident in several steels, such as Gen3 steels, which are desirable for use in motor vehicles to achieve increased material strength and reduced weight. Gen3 steels are defined here as steels with a minimum tensile strength (e.g., up to 1500 MPa or more) and elongation (e.g., 20 percent). It has been observed that Gen3 steels experience ductility losses and cracking during welding, and that LMS cracking can be catastrophic, with high crack growth rates observed. Certain desirable materials, such as zinc-coated Gen3, which benefit from a corrosion-resistant coating, have therefore been restricted from use in motor vehicles.

[0006] Low-melting-point metal arc (LME) cracking can occur in known zinc and aluminum coating materials during welding operations, such as resistance spot welding, used to join metal components to coating materials with low melting points. LME cracking can arise due to differences in the metal melting points and the pressure exerted during the resistance welding process, particularly at the interface between the base material, such as steel, and the coating material. LME cracking weakens the substrate material and reduces its inherent strength.

[0007] While existing methods and systems attempt to provide coating removal for spot welding and can fulfill their specific purpose, these offline surface treatment processes are time-consuming and expensive. There remains a need for a new and improved process for coating removal prior to spot welding.

[0008] Therefore, one of the aims of the invention is to improve spot welding. SUMMARY

[0009] To solve this problem, a method with the features of claim 1 is provided. Advantageous embodiments can be found in the dependent claims, the description, and the accompanying drawings.

[0010] According to the invention, a method for two-stage spot welding is provided. The method comprises providing a stack of workpieces, contacting the first steel workpiece with a first concentric electrode and contacting the second steel workpiece with a second concentric electrode, removing a coating from the first steel workpiece to form a first locally exposed section, and from the second steel workpiece to form a second locally exposed section, by passing an electric current between the first concentric electrode and the second concentric electrode and through the stack of workpieces, and spot welding the first steel workpiece and the second steel workpiece using a first welding electrode and a second welding electrode, respectively. The first steel workpiece and the second steel workpiece contain a coating.The first and second concentric electrodes each have an outer diameter, an inner diameter, and a tip. The locally exposed sections are shaped like concentric circles with the outer and inner diameters and are defined by an outer shoulder and an inner shoulder. The first and second welding electrodes have a weld end face diameter that is larger than the inner diameter and smaller than the outer diameter.

[0011] According to one aspect of the disclosure, at least one of the concentric electrodes comprises a cylindrical base, with the tip arranged on the cylindrical base. The cylindrical base extends axially along an axis of rotation and contains a conductive metal. The tip comprises a concentric surface extending axially from the cylindrical base and defined by an outer shoulder with an outer diameter and an inner shoulder with an inner diameter. The tip is configured to supply electrical current to the steel workpiece and to remove the coating from at least one section of the steel workpiece.

[0012] According to another aspect, the tip of the first concentric electrode and the tip of the second concentric electrode contain a flat concentric surface.

[0013] According to another aspect of the revelation, the outer diameter is about 8 millimeters and the inner diameter is about 5 millimeters.

[0014] According to another aspect of the disclosure, the flat concentric surface has a thickness of about 3 millimeters and extends the thickness axially from a cylindrical base.

[0015] According to another aspect of the disclosure, the flat concentric surface has a ring width between the outer diameter and the inner diameter of about 1.5 millimeters.

[0016] According to another aspect of the revelation, the tip contains a rounded concentric surface.

[0017] According to another aspect of the revelation, the steel workpiece contains an advanced third-generation high-strength steel (AHSS).

[0018] According to another aspect, the coating is a zinc coating.

[0019] According to another aspect of the revelation, the or each concentric electrode is coupled to a welding robot.

[0020] According to several aspects of the present disclosure, a combined electrode is provided. The combined electrode comprises a welding electrode and a retractable concentric electrode, which is movably coupled to the welding electrode and radially surrounds it. The welding electrode has a welding face diameter that is larger than an inner diameter and smaller than an outer diameter. The welding electrode is configured to weld a stack of steel workpieces. The concentric electrode comprises a cylindrical base and a tip arranged on the cylindrical base. The cylindrical base contains a conductive metal. The tip comprises a concentric surface that extends axially from the cylindrical base and is defined by an outer shoulder with an outer diameter and an inner shoulder with an inner diameter.The tip is configured to provide an electric current to the stack of steel workpieces and to remove a coating from at least one section of the stack of steel workpieces.

[0021] According to another aspect of the disclosure, the combined electrode has an outer diameter of about 8 millimeters and an inner diameter of about 5 millimeters.

[0022] According to another aspect of the disclosure, the combined electrode has a concentric surface that is flat and approximately 3 millimeters thick. The thickness extends axially from the cylindrical base, with a ring width of approximately 1.5 millimeters between the outer and inner diameters.

[0023] The above features and advantages, as well as other features and advantages of the presently disclosed system and method, are readily apparent from the detailed description, including the claims, and the examples, when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present revelation becomes more fully understandable through the detailed description and the accompanying drawings; they show: Fig. 1 a lateral cross-sectional view illustrating several steel workpieces and several concentric electrodes for removing a coating from the steel workpieces according to the present disclosure; Fig. 2A a perspective view showing the in Fig. Figure 1 illustrates the concentric electrode shown, wherein the concentric electrode comprises a cylindrical base and a tip, according to the present disclosure; Fig. 2B a cross-sectional view showing the Fig. Figure 1 illustrates the concentric electrode shown, wherein the concentric electrode comprises a cylindrical base and a tip, according to the present disclosure; Fig. 3 a top view showing an exposed section of the Fig. 1 illustrated by the steel workpiece shown, after a section of the coating has been removed using the concentric electrode, according to the present disclosure; Fig. 4 a lateral cross-sectional view illustrating several steel workpieces and several welding electrodes for spot welding the steel workpieces after a section of the coating has been removed using the concentric electrodes, according to the present disclosure; Fig. 5 a lateral cross-sectional view illustrating a combined electrode comprising a concentric electrode and a welding electrode, wherein the concentric electrode is extended to remove a section of the coating on the steel workpiece, according to the present disclosure; Fig. 6 a side cross-sectional view showing the in Fig. Figure 5 illustrates the combined electrode shown, wherein the concentric electrode is retracted and the welding electrode is positioned to provide a weld spot, according to the present disclosure; and Fig. 7 a process plan describing a procedure for two-stage spot welding using the concentric electrode and the welding electrode, which are in the Fig. Figures 1-2B and 4 are shown, as illustrated in the present disclosure. DETAILED DESCRIPTION

[0025] Detailed reference will now be made to several examples of the disclosure, which are illustrated in the accompanying drawings. Whenever possible, the same or similar reference symbols are used in the drawings and in the description to refer to the same or similar parts or steps. The following description is merely exemplary and is not intended to limit the present disclosure, application, or uses.

[0026] Liquid metal embrittlement (LME) cracking during spot welding is a reason for limiting the use of coatings on Gen3 steels for automotive body structure applications. Previous zinc coating removal methods, such as laser ablation, require offline surface pretreatment, which can be time-consuming and expensive. The spot welding process disclosed here is an inline process and reduces time and costs while significantly minimizing LME cracking. The zinc coating removal method disclosed here enables Gen3 steel applications for automotive body structures.

[0027] In Fig. Figure 1 illustrates a cross-sectional view of a stack of steel workpieces 10 and a concentric electrode 12 according to the present disclosure. The steel stack 10 contains at least one workpiece 14 and a coating 16 arranged on the at least one workpiece 14. Even if in Fig. As illustrated in Figure 1, a coating 16 is applied to one surface of the workpiece 14. The coating 16 can be applied to both surfaces of the workpiece 14. The workpiece 14 can contain at least steel and / or a steel alloy (e.g., an advanced third-generation high-strength steel (AHSS)). It is recognized that each workpiece 14 can contain different grades of steel or steel alloys with varying thicknesses. Additionally, each coating 16 can have a different thickness. By way of example, the steel workpiece 14 is further defined as a sheet metal component for a vehicle, such as a body panel. However, the steel workpiece 14 can be used with any component within a vehicle or in any suitable non-vehicle application.

[0028] According to one example, workpiece 14 contains iron, approximately 1.4 to approximately 2.0 wt. percent aluminum, and approximately 0 to approximately 1.0 wt. percent silicon. According to another example, workpiece 14 comprises approximately 1.5 to approximately 1.9 wt. percent aluminum and approximately 0.2 to approximately 0.8 wt. percent silicon. According to yet another example, workpiece 14 comprises approximately 1.6 to approximately 1.8 wt. percent aluminum and approximately 0.4 to approximately 0.6 wt. percent silicon. In this context, the term "approximately" is familiar to those skilled in the field. Alternatively, the term "approximately" can be understood to mean plus or minus 0.15 wt. percent.

[0029] The workpiece 14 may further comprise approximately 0.17 to approximately 0.35 wt% carbon, approximately 2.0 to approximately 4.0 wt% manganese, approximately 0 to approximately 0.01 wt% sulfur, approximately 0 to approximately 0.2 wt% copper, approximately 0 to approximately 0.008 wt% nitrogen, approximately 0 to approximately 0.005 wt% boron, and approximately 0 to approximately 0.04 wt% phosphorus. Iron comprises the remainder of the composition of workpiece 14. Workpiece 14 may also comprise other elements comprising less than 0.02 wt%. These other elements are those not listed above but present as impurities in the alloy substrate. In this context, the term "approximately" is familiar to those skilled in the art. Alternatively, the term "approximately" may be understood to mean plus or minus 0.15 wt%.

[0030] The steel workpiece stack 10 further comprises the coating 16, e.g., zinc. The coating 16 provides corrosion protection for the workpiece 14, where one or more of the elements (e.g., iron) are susceptible to oxidation. According to one example, the coating 16 is applied to the workpiece 14 by electroplating (e.g., by immersion in molten zinc, zinc plating, etc.). However, the coating 16 can be applied in any suitable manner. Several steel workpieces 14 are assembled adjacent to one another to form a workpiece stack 10, as shown in Fig. 1 is shown. It is recognized that, although in Fig. Figure 1 illustrates only two workpieces 14; additional workpieces (e.g., three workpieces, four workpieces, etc.) can be added to the stack 10. 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 the steel grain boundary. The zinc coating 16 promotes a phenomenon known in engineering as liquid metal embrittlement, in which the stack 10 experiences a drastic loss of tensile ductility and / or suffers brittle fracture due to the penetrating zinc, which is undesirable.

[0031] In Fig. 1. The concentric electrode 12 can be positioned adjacent to the stack of steel workpieces 10 to remove a section of the coating 16 from the steel workpiece. According to the Fig. In the example shown, a first concentric electrode 12A is arranged on a first side 18 of the steel workpiece stack 10, while a second concentric electrode 12B is arranged on a second side 20 of the steel workpiece stack 10.

[0032] The Fig. 2A and Fig. Figure 2B illustrates a perspective view and a cross-sectional view of the concentric electrode 12. According to this example, the concentric electrode 12 contains a cylindrical base 22. The cylindrical base 22 extends axially along a rotation axis A. The cylindrical base 22 contains a conductive metal, e.g., copper.

[0033] In the Fig. 2A and Fig. 2B contains the concentric electrode 12 with a tip 24. The tip 24 is arranged on the cylindrical base 22 and is configured to supply an electric current to the one or more steel workpieces 14 in order to remove the coating 16 from a section of the one or more steel workpieces 14. The tip 24 is defined by an outer diameter OD and an inner diameter ID and has a concentric surface 26 or end face. As per the Fig. 2A and Fig. As illustrated in the example shown in Figure 2B, the concentric surface 26 can have a flat (e.g., planar) surface. According to other examples, the concentric surface 26 can be curved and / or rounded. It should be recognized that the concentric surface 26 can be configured and / or shaped in various other ways.

[0034] Fig. Figure 3 illustrates a top view of an exposed section 28 of the steel workpiece 14 after a section of the coating 16 has been removed using the concentric electrode 12. According to this example, the exposed section 28 is circular or concentric. The exposed section 28 is defined by an outer diameter OD and an inner diameter ID. The outer diameter OD further defines an outer shoulder 30 of the coating 16, while the inner diameter ID further defines an inner shoulder 32 of the coating 16. The inner shoulder 32 defines a central section 34 of the coating 16 that remains after the coating 16 has been removed by the concentric electrode 12. According to the Fig. In the example shown in Figure 3, the exposed section 28 has an inner diameter ID of approximately 5.00 millimeters (mm) and an outer diameter OD of approximately 8.00 mm. It should be recognized that the exposed section 28 may contain various outer diameters (e.g., 7.00 mm, 7.50 mm, 8.50 mm, etc.) and various inner diameters (e.g., 4.00 mm, 4.50 mm, 5.50 mm, etc.). In this context, the term "approximately" is familiar to those skilled in the art. Alternatively, the term "approximately" can be understood to mean plus or minus 0.15 millimeters. Additionally, the depth or thickness of the removed coating 16 is greater than 0 micrometers (µm) and less than 200 µm.

[0035] For example, coating 16 is approximately 50 µm thick. It should be noted that the exposed section 28 can have different depths or thicknesses (e.g., 25 µm, 75 µm, 100 µm, 185 µm, etc.). In this context, the term "approximately" is familiar to those skilled in the field. Alternatively, the term "approximately" can be understood to mean plus or minus 5 µm.

[0036] In Fig. 4. A welding electrode 36, which is coupled to a welding robot, for example, is then positioned to be welded to one or more steel workpieces 14. The welding electrode 36 can, for example, be a copper spot welding electrode. The welding electrode 36 has a welding end face 38 with a diameter larger than the inner diameter ID and smaller than the outer diameter OD. According to the Fig. In the illustrated example 4, a first workpiece 14A and a second workpiece 14B are positioned together, with a first welding electrode 36A located directly next to and in contact with the first workpiece 14A, while a second welding electrode 36B is located directly next to and in contact with the second workpiece 14B. A welding current flows from the first welding electrode 36A and the second welding electrode 36B through the first workpiece 14A and the second workpiece 14B, generating resistance heat at the weld point 40 to produce molten metal. The molten metal then cools and forms a weld core 42 that joins the first workpiece 14A and the second workpiece 14B.

[0037] The Fig. 5 and Fig. Figure 6 illustrates a cross-sectional view of a combined electrode 44 arranged on one side of the workpiece stack 10. In practice, however, a combined electrode 44 is arranged on both sides of the stack 10 (e.g., a first combined electrode 44 is arranged on a first side of the stack 10, while a second combined electrode is arranged on a second side of the stack 10 opposite the first side). The combined electrode 44 incorporates the concentric electrode 12 and the welding electrode 36 in a combined configuration. The welding electrode 36 can be cylindrical and configured to contact a section of the coating 16. The concentric electrode 12 can be concentric and retractably surrounds the welding electrode 36.

[0038] As in Fig. As shown in Figure 5, the combined electrode 44 is positioned directly next to the workpiece 14, with the concentric electrode 12 extended and in contact with the workpiece 14. The concentric electrode 12 is then used to remove a section of the coating 16 to create the exposed section 28, as described above.

[0039] After the exposed section 28 has been formed, and as in Fig. As shown in Figure 6, the concentric electrode 12 is retracted, positioning the welding electrode 36 to contact the workpiece 14 in order to provide a weld point on the workpiece stack 10. Although in the Fig. 5 and Fig. Figure 6 shows only one combined electrode 44; however, several combined electrodes 44 are used to create a weld spot. The concentric electrode 12 can be extended and / or retracted, for example, using an actuator and / or motor.

[0040] In Fig. Figure 7 describes a method 100 for two-stage spot welding of the workpiece stack 10 according to the present disclosure. The method begins in block 102.

[0041] Block 102 represents the provision of a workpiece stack 10. The workpiece stack 10 contains a first steel workpiece 14 and a second steel workpiece 14. The first steel workpiece 14 and the second steel workpiece 14 each contain a coating 16, which can consist of zinc, as described above. The process then proceeds to block 104.

[0042] Block 104 depicts the contact of the first steel workpiece 14A with a first concentric electrode 12A and the contact of the second steel workpiece 14B with a second concentric electrode 12B. In one example, the first concentric electrode 12A is coupled to and controlled by a welding robot, while the second concentric electrode 12B is coupled to and controlled by a second welding robot. The process then continues to Block 106.

[0043] Block 106 describes the removal of a coating from the first steel workpiece 14A to form a first locally exposed section 28A, and from the second steel workpiece 14B to form a second locally exposed section 28B. The removal of the coating 16 may involve passing an electric current between the first concentric electrode 12A and the second concentric electrode 12B and through the workpiece stack 10. The first locally exposed section 28A and the second locally exposed section 28B are formed in the shape of a concentric circle or ring defined by the outer diameter OD and the inner diameter ID, and are defined by an outer shoulder 30 and an inner shoulder 32. The procedure then proceeds to Block 108.

[0044] Block 108 describes the spot welding of the first steel workpiece 14A and the second steel workpiece 14B using a first welding electrode 36A and a second welding electrode 36B. The spot welding process involves the use of the first welding electrode 36A and the second welding electrode 36B. The first welding electrode 36A and the second welding electrode 36B each have a weld end face diameter that is larger than the inner diameter ID and smaller than the outer diameter OD.

[0045] The steel workpiece and spot welding process of the present disclosure are advantageous and superior to the prior art. The concentric electrode 12 and the method for removing the zinc-containing coating 16 are an inline process and reduce time and costs, while significantly reducing LME cracking. The inline zinc coating removal process disclosed herein enables the use of Gen3 steel applications for vehicle body structure components.

[0046] This description is for illustrative purposes only and is not intended to limit the disclosure, its application or uses in any way.

Claims

[1] Method for two-stage spot welding, comprising: Providing a stack of workpieces (10) containing a first steel workpiece (14, 14A) and a second steel workpiece (14, 14B), wherein the first steel workpiece (14, 14A) and the second steel workpiece (14, 14B) contain a coating (16); Touching the first steel workpiece (14, 14A) with a first concentric electrode (12, 12A) and touching the second steel workpiece (14, 14B) with a second concentric electrode (12, 12B), wherein the first concentric electrode (12, 12A) and the second concentric electrode (12, 12B) each have an outer diameter (OD), an inner diameter (ID) and a tip (24); Removing the coating (16) from the first steel workpiece (14, 14A) to form a first locally exposed section (28, 28A), and from the second steel workpiece (14, 14B) to form a second locally exposed section (28, 28B), by passing an electric current between the first concentric electrode (12, 12A) and the second concentric electrode (12, 12B) and through the workpiece stack (10), wherein the locally exposed sections (28, 28A, 28B) are formed in the shape of a concentric ring with outer diameter (OD) and inner diameter (ID) and are defined by an outer shoulder region (30) and an inner shoulder region (32); and Spot welding of the first steel workpiece (14, 14A) and the second steel workpiece (14, 14B) using a first welding electrode (36A) and a second welding electrode (36B), respectively, wherein the first welding electrode (36A) and the second welding electrode (36B) have a weld end face diameter that is larger than the inner diameter (ID) and smaller than the outer diameter (OD). [2] Method according to claim 1, wherein at least one of the concentric electrodes (12, 12A, 12B) comprises: a cylindrical base (22) extending axially along an axis of rotation (A), wherein the cylindrical base (22) contains a conductive metal; wherein the tip (24) is arranged on the cylindrical base (22) and wherein the tip (24) includes a concentric surface (26) extending axially from the cylindrical base (22) and defined by an outer shoulder with outer diameter (OD) and an inner shoulder with inner diameter (ID), wherein the tip (24) is configured to provide electrical current to the steel workpiece (14, 14A, 14B) and to remove the coating (16) from at least one section of the steel workpiece (14, 14A, 14B). [3] Method according to claim 1 or 2, wherein the tip (24) includes a flat concentric surface (26). [4] Method according to claim 1 or 2, wherein the outer diameter (OD) is about 8 millimeters and the inner diameter (ID) is about 5 millimeters. [5] Method according to claim 3, wherein the flat concentric surface (26) has a thickness of about 3 millimeters, the thickness extending axially from the cylindrical base (22). [6] Method according to claim 3, wherein the flat concentric surface (26) has a ring width between the outer diameter (OD) and the inner diameter (ID) of about 1.5 millimeters. [7] Method according to claim 1 or 2, wherein the tip (24) includes a rounded concentric surface (26). [8] Method according to claim 1, wherein at least one of the steel workpieces (14, 14A, 14B) contains an advanced third-generation high-strength steel (AHSS). [9] Method according to claim 1, wherein the coating (16) is zinc. [10] Method according to claim 1, wherein the or each concentric electrode (12, 12A, 12B) is coupled to a welding robot.

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