DE102017102032B4 - Milling tool for welding electrodes and method for using it - Google Patents

Abstract

Milling tool (10) capable of reworking asymmetric welding side geometries of first and second welding electrodes (200, 220), the milling tool (10) comprising:
a body (12) extending longitudinally along a central axis (18) between a first end (22) and a second end (26), and
a milling element (14) within the body (12) forming a first milling receptacle (32) accessible through a first opening (32) at the first end (22) of the body (12) and further forming a second milling receptacle (34) accessible through a second opening (24) at the second end (26) of the body (12), wherein the milling element (14) includes a milling groove (58) containing a milling blade having axially spaced and opposed first and second shearing surfaces (68, 70) at least partially defining the first and second milling receptacles (32, 34), respectively, wherein the first shearing surface (68) includes a lower end portion (72) profiled to mill a first weld face geometry comprising a planar or curved weld face base surface (210), and wherein the second shearing surface (70) has a lower end portion (86) profiled to mill a second weld side geometry comprising a curved weld side base surface (230),
characterized in that
the second weld face geometry further comprises a series of upstanding circular ridges (232) projecting outwardly from the curved weld face base surface of the second weld face geometry; and that
the body (12) and the milling element (14) are formed in one piece.

Description

TECHNICAL FIELD

The present invention generally relates to a milling tool for reworking welding electrodes or welding caps used for resistance spot welding of workpiece stacks containing dissimilar workpieces, such as an aluminum workpiece and an adjacent steel workpiece. In particular, the invention relates to a milling tool according to the preamble of claim 1, as it is essentially US 7 458 139 B2 is known.

Furthermore, the US 2013 / 0 015 164 A1 a milling tool for simultaneously reworking an upper electrode and a lower electrode by means of upper and lower cutting edges, one of the electrodes having a series of upright circular ridges.

Further state of the art also results from the DE 103 45 714 A1 .

INTRODUCTION

Resistance spot welding is a process used in a number of industries to join two or more metal workpieces together. The automotive industry, for example, often uses resistance spot welding to join metal workpieces during the manufacture of structural frame members (e.g., body side and cross members) and vehicle closure elements (e.g., vehicle doors, hoods, deck lids, and liftgates), among others. A number of spot welds are often formed at various points along an edge of the metal workpieces or at another joining region to ensure the part is structurally sound. Although spot welding has typically been practiced to join certain similarly composed metal workpieces—such as steel to steel and aluminum to aluminum—the desire to incorporate lighter materials into a vehicle body structure has generated interest in joining steel workpieces to aluminum workpieces using resistance spot welding. The aforementioned desire to resistance spot weld dissimilar metal workpieces is not unique to the automotive industry; in fact, it extends to other industries that can use spot welding, including the aerospace, marine, railway, and construction industries.

Resistance spot welding generally relies on the flow of an electric current through overlapping metal workpieces to generate heat. To perform this welding process, a set of opposing welding electrodes is pressed in facing alignment against opposite sides of the workpiece stack, which typically contains two or three metal workpieces arranged in an overlapping configuration. An electric current is then passed from one welding electrode through the metal workpieces to the other. Resistance to the flow of this electric current generates heat in the metal workpieces and at their mating surfaces. If the workpiece stack contains an aluminum workpiece and an adjacent overlapping steel workpiece, the heat generated at the mating surface and within the mass of these dissimilar metal workpieces causes a molten aluminum pool to form, penetrate from the mating surface into the aluminum workpiece, and cause it to grow. This molten pool of molten aluminum wets the adjacent butt surface of the steel workpiece and, after the current flow has stopped, solidifies into a weld joint that joins the two workpieces together by welding.

Each welding electrode used to perform resistance spot welding includes a welding face located at one end of an electrode body. The welding face is the portion of the welding electrode that contacts and electrically communicates with the workpiece stack. Over the course of repeated resistance spot welding operations, the welding faces of the welding electrodes are susceptible to deterioration due to the large amount of heat generated at the welding faces during current flow and the high compressive force used to press the welding faces against the workpiece stack. This deterioration can include plastic deformation of the welding face and/or the development of contamination resulting from a reaction between the electrode and the respective contacted workpiece at elevated temperatures. To extend the service life of the welding electrodes, especially in a production environment, the original geometry of the welding faces of the welding electrodes can be periodically restored. This restoration process should be fast, convenient, and accurate so that it does not interrupt production operations by making the welding electrodes unavailable for extended periods.

Resistance spot welding of an aluminum workpiece to a steel workpiece is fraught with challenges. Aside from the need for periodic rework of weld faces, which undergo different deterioration mechanisms, the vastly different properties of the two workpieces and the presence of a mechanically strong, electrically insulating, and self-healing heat-resistant oxide layer (or multiple layers) on the aluminum workpiece make it difficult to consistently achieve welds with adequate peel and transverse stress strength. Since previous spot welding efforts have not been particularly successful, mechanical fasteners, including self-piercing rivets and flow-drill screws, have been primarily used to join aluminum and steel workpieces. However, mechanical fasteners require more time to install and have high consumable costs compared to spot welding. They also add weight to the vehicle body structure—weight that is avoided when joining using spot welding—which counteracts some of the weight savings achieved primarily by using aluminum workpieces. Furthermore, mechanical fasteners can introduce sites for galvanic corrosion on the aluminum workpiece, since the fasteners are typically made of steel.

SUMMARY

According to the invention, a milling tool for reworking asymmetric welding side geometries of first and second welding electrodes is presented, which is characterized by the features of claim 1.

The design of the milling tool is subject to a certain degree of variability without losing its ability to rework. For example, each of the one or more milling grooves may include an elongated foot piece that supports the milling blade against the inner surface of the body.

As another example of a specific design of the milling tool, the milling element may include a first milling groove having a first milling blade, a second milling groove having a second milling blade, a third milling groove having a third milling blade, and a fourth milling groove having a fourth milling blade. The first, second, third, and fourth milling blades are circumferentially spaced from one another such that each of the first, second, third, and fourth milling blades is oriented transversely to each of its two circumferentially adjacent milling blades. Additionally, each of the first, second, third, and fourth milling blades includes axially spaced and opposing first and second shear surfaces. The first shear surfaces of the first, second, third, and fourth milling blades define the first milling receptacle, and the second shear surfaces of the first, second, third, and fourth milling blades define the second milling receptacle.

When the milling element further includes the first, second, third, and fourth milling blades, both the first shearing surface of the first milling blade and the first shearing surface of the third milling blade, which are aligned with each other, have a lower end portion having an upwardly profiled leading edge and an upwardly profiled trailing edge offset by a positive clearance angle below the leading edge. Similarly, both the second shearing surface of the second milling blade and the second shearing surface of the fourth milling blade, which are aligned with each other but oriented transversely to the first shearing surface of the first milling blade and the first shearing surface of the third milling blade, have a lower end portion having an upwardly profiled leading edge and an upwardly profiled trailing edge offset by a positive clearance angle below the leading edge, and further comprising a plurality of penetration grooves extending from the leading edge at least part of the way to the trailing edge. The penetration grooves extending across the lower end portion of the second shearing surface of the second milling blade and across the second shearing surface of the fourth milling blade may be curved or straight.

The first shearing surface of the first and third milling blades and the second shearing surface of the second and fourth milling blades may include additional structures in addition to their respective lower end portions. For example, the first shearing surface of each of the first and third milling blades may also include an upper end portion extending from its respective lower end portion and having a convex shape. The upper end portion of the first shearing surface of each of the first and third milling blades has a leading edge and a trailing edge. Similarly, the second shearing surface of each of the second and fourth milling blades may include an upper end portion extending from its respective lower end portion and having a convex shape. The upper end portion of the second shearing surface of each of the second and fourth milling blades has a leading edge and a trailing edge.

Furthermore, specific examples of the first and second weld side geometries that can be reworked by the milling tool can be further defined. The first weld side geometry can, for example, comprise a spherically curved weld side base surface having a diameter between 3 mm and 16 mm and a radius of curvature between 8 mm and 400 mm. With regard to the second weld side geometry, it can comprise a spherically curved weld side base surface having a diameter between 8 mm and 20 mm and a radius of curvature between 15 mm and 300 mm, and it can further include between two and ten upright circular ridges surrounding a weld side axis and whose diameter increases from an innermost upright circular ridge immediately surrounding the weld side axis to an outermost upright circular ridge furthest from the weld side axis. The upstanding circular burrs may be spaced on the curved weld side base surface by a distance of 50 µm to 1800 µm and each of the upstanding circular burrs may have a burr height ranging from 20 µm to 500 µm.

Another embodiment of a milling tool capable of finishing asymmetric weld side geometries of first and second welding electrodes includes a body and a milling element within the body. The body extends longitudinally along a central axis between a first end and a second end. The milling element defines a first milling receptacle accessible through the first opening at the first end of the body and further defines a second milling receptacle accessible through a second opening at the second end of the body. The milling element includes a milling groove including a milling blade having axially spaced and opposing first and second shearing surfaces that at least partially define the first and second milling receptacles, respectively. The first shear surface includes a lower end portion profiled for milling a first weld face geometry comprising a flat or curved weld face base surface, and the second shear surface includes a lower end portion profiled for milling a second weld face geometry comprising a curved weld face base surface and a series of upstanding circular ridges projecting outwardly from the curved weld face base surface.

Specific examples of the first and second weld face geometries that can be reworked by the milling tool can be further defined. For example, the first weld face geometry can comprise a spherically curved weld face base surface having a diameter between 3 mm and 16 mm and a radius of curvature between 8 mm and 400 mm. With regard to the second weld face geometry, it can comprise a spherically curved weld face base surface having a diameter between 8 mm and 20 mm and a radius of curvature between 15 mm and 300 mm, and it can further include between two and ten upstanding circular ridges surrounding a weld face axis and increasing in diameter from an innermost upstanding circular ridge immediately surrounding the weld face axis to an outermost upstanding circular ridge furthest from the weld face axis. The upstanding circular burrs may be spaced apart on the curved weld side base surface by a distance of 50 µm to 1800 µm, and each of the upstanding circular burrs may have a burr height ranging from 20 µm to 500 µm.

The design of the milling tool is subject to a certain degree of variability without losing its ability to be reworked. For example, the milling element can comprise a first milling groove with a first milling blade, a second milling groove with a second milling blade, a third milling groove with a third milling blade, and a fourth milling groove with a fourth milling blade. The first, second, third, and fourth milling blades are circumferentially spaced from one another such that each of the first, second, third, and fourth milling blades is oriented transversely to each of its two circumferentially adjacent milling blades. In addition, each of the first, second, third, and fourth milling blades includes axially spaced and opposing first and second shear surfaces. The first shear surfaces of the first, second, third, and fourth milling blades define the first milling socket, and the second shear surfaces of the first, second, third, and fourth milling blades define the second milling socket.

When the milling element includes the first, second, third, and fourth milling blades, furthermore, both the first shearing surface of the first milling blade and the first shearing surface of the third milling blade, which are aligned with each other, have a lower end portion having an upwardly profiled leading edge and an upwardly profiled trailing edge offset by a positive clearance angle below the leading edge. Similarly, both the second shearing surface of the second milling blade and the second shearing surface of the fourth milling blade, which are aligned with each other but oriented transversely to the first shearing surface of the first milling blade and the first shearing surface of the third milling blade, a lower end portion having an upwardly profiled leading edge and an upwardly profiled trailing edge offset by a positive clearance angle below the leading edge, and further comprising a plurality of penetration grooves extending from the leading edge over at least part of the distance to the trailing edge.

As another example of a specific design of the milling tool, the milling element can be supported within the body in several different ways. In one specific embodiment, for example, the milling groove can include an elongated foot piece that supports the milling blade against the inner surface of the body.

A method for reworking welding electrodes having asymmetrical weld face geometries comprises, in accordance with one embodiment of the invention, several steps. Specifically, a milling tool having the features of claim 1 is provided. A first weld face of a first welding electrode is received in the first milling receptacle, and a second weld face of a second welding electrode is received in the second milling receptacle. Once the first and second weld faces are received in the first and second milling receptacles, respectively, the milling tool is rotated to mill and rework a first weld face geometry in the first weld face and a second weld face geometry in the second weld face. The first weld face geometry includes a flat or curved weld face base surface, and the second weld face geometry includes a curved weld face base surface and a series of upstanding circular ridges projecting outwardly from the curved weld face base surface.

The method for reworking welding electrodes having asymmetrical weld face geometries can be implemented with certain preferences. For example, the milling tool can be rotated between one and ten complete revolutions around axes of the first and second weld faces, such that a material depth in the range of 10 µm to 500 µm is removed from both the first weld face and the second weld face upon restoration of the first weld face geometry and the second weld face geometry. Furthermore, a set of ten to one hundred weld joints can be formed between overlapping and adjacent steel and aluminum workpieces before the first weld face is mounted in the first milling shank of the milling tool and the second weld face is mounted in the second milling shank of the milling tool. Of course, many other variations of the resistance spot welding method can be implemented.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1 is a perspective view of a milling tool in accordance with an embodiment of the present invention and specifically the first milling socket of the milling tool;
• 2 is a perspective view of the 1 shown milling tool, and especially the second milling holder of the milling tool;
• 3 is an exploded view of the 1 - 2 illustrated milling tool showing a body and a milling element separated from each other in accordance with an embodiment of the invention;
• 4 is a cross-sectional view of the 1 - 2 illustrated milling tool in accordance with an embodiment of the invention;
• 5 is a top view of the first milling fixture of the 1 - 2 illustrated milling tool in accordance with an embodiment of the invention;
• 6 is a top view of the second milling fixture of the 1 - 2 illustrated milling tool in accordance with an embodiment of the invention;
• 7 is a cross-sectional view of a blade portion of one of the milling grooves taken along section lines 7-7 in 5 ;
• 8 is a cross-sectional view of a blade portion of one of the milling grooves taken along section lines 8-8 in 6 ;
• 9 is a partially enlarged view of the second shear surfaces of the 2 shown milling element, in which two of these surfaces define a plurality of penetration grooves;
• 10 is a general perspective view of a welding electrode or welding cap incorporating a first welding side geometry in accordance with an embodiment of the invention;
• 11 is a general perspective view of a welding electrode or welding cap including a second welding side geometry that differs from the first welding side ten geometry, in accordance with an embodiment of the invention;
• 12 is an enlarged cross-sectional view of the welding side of the 11 welding electrode shown;
• 13 is a general cross-sectional view of a workpiece stackup including a steel workpiece and an adjacent aluminum workpiece arranged in an overlapping manner and sandwiched between a first welding electrode and a second welding electrode, the first and second welding electrodes having different weld side geometries;
• 14 is a general cross-sectional view of a workpiece stackup including a steel workpiece and an adjacent aluminum workpiece arranged in an overlapping manner and sandwiched between a first welding electrode and a second welding electrode, the first and second welding electrodes having different weld side geometries, although here the workpiece stackup includes an additional steel workpiece (i.e., two steel workpieces and one aluminum workpiece), in accordance with one embodiment of the invention;
• 15 is a general cross-sectional view of a workpiece stackup including a steel workpiece and an adjacent aluminum workpiece arranged in an overlapping manner and sandwiched between a first welding electrode and a second welding electrode, the first and second welding electrodes having different weld side geometries, although the workpiece stackup here includes an additional aluminum workpiece (i.e., two aluminum workpieces and one steel workpiece), in accordance with one embodiment of the invention;
• 16 is a general cross-sectional view of the workpiece stack and welding electrodes used in 13 are shown, during the passage of an electric current between the welding electrodes and through the stack, and wherein the passage of the electric current has caused melting of the aluminum workpiece adjacent to the steel workpiece and the creation of a molten aluminum pool within the aluminum workpiece;
• 17 is a general cross-sectional view of the workpiece stack and welding electrodes used in 13 are shown after the passage of the electric current between the welding electrodes and through the stack has ceased and wherein the molten pool of molten aluminium has solidified into a weld joint which joins the adjacent workpieces of aluminium and steel together by a weld joint; and
• 18 represents a reworking of at least the welding sides of the first and second welding electrodes, wherein the first welding electrode is received in the first milling holder of the milling tool and the second welding electrode is received in the second milling holder.

EXACT DESCRIPTION

A milling tool is disclosed that can simultaneously mill and restore asymmetric weld face geometries of two welding electrodes subject to different deterioration mechanisms. The milling tool can be used as part of a method for resistance spot welding a workpiece stackup containing adjacent and overlapping steel and aluminum workpieces. Specifically, a first welding electrode having a first weld face and a second welding electrode having a second weld face can be used to pass an electric current through the workpiece stackup at a welding station. The geometry of the first weld face and the geometry of the second weld face are asymmetric due to the need to compensate for the different physical properties of the adjacent steel and aluminum workpieces. Over time, the first and second weld faces deteriorate to such an extent that spot welding operations are adversely affected. To address this problem, the milling tool can be used to periodically rework both the first and second weld faces of the first and second welding electrodes, respectively. Remachining the weld sides involves receiving the first weld side in a first milling fixture and receiving the second weld side in a second milling fixture, and then rotating the milling tool around the axes of the first and second weld sides to mill the weld sides and restore their geometries.

A milling tool and a method for using the milling tool in the context of resistance spot welding a workpiece stack containing adjacent and overlapping workpieces made of steel and aluminum are described with reference to 1 - 18 described. The milling machine The tool is designed to simultaneously rework the weld faces of both welding electrodes, although the weld face geometries are designed asymmetrically due to the significant differences in physical properties between steel and aluminum workpieces (e.g., melting point, thermal conductivity, electrical conductivity, strength at elevated temperatures, etc.). In this way, the weld faces of both welding electrodes can be periodically reworked with the same milling tool in a single operation to restore their original unique geometries, rather than being reworked separately with their own dedicated milling tools. Reworking the weld faces with the same milling tool is more efficient and can be performed whenever it is desired to counteract weld face deterioration mechanisms, which, if allowed to progress without intervention, would rapidly compromise the quality of the welding electrodes and the weld spot, ultimately rendering them unsuitable for continued use in spot welding operations. The milling tool can be used to rework the weld sides as much as possible until the weld sides can no longer support rework due to the accumulated material loss resulting from the rework operations.

A preferred embodiment of the milling tool is shown in 1 - 9 and designated by reference numeral 10. The milling tool 10 comprises a body 12 and a milling element 14. The body 12 defines a through hole 16 extending longitudinally along a central axis 18 between a first opening 20 at a first end 22 of the body 12 and a second opening 24 at a second end 26 of the body 12. Each of the openings 20, 24 is perpendicular to the central axis 18 of the through hole 16 such that a plane 28 of the first opening 20 and a plane 30 of the second opening 22 are parallel to each other and are intersected by the central axis 18 at 90° angles, as shown in 4 is shown. The milling element 14 is rigidly held by the body 12 within the through-hole 16. The milling element 14 forms a first milling receptacle 32 and a second milling receptacle 34. The first milling receptacle 32 is accessible through the first opening 20 of the body 12, and the second milling receptacle 34 is accessible through the second opening 24 of the body 12.

The body 12 and the milling element 14 are constructed of a hard material capable of withstanding rework operations on welding electrodes. For example, both the body 12 and the milling element 14 may be formed of a tool steel, such as an S7 or M2 tool steel. Furthermore, the milling element 14 may be rigidly retained by the body 12 in a variety of ways that prevent these two portions of the tool 10 from moving relative to each other when the tool 10 is in operation. Contrary to the invention, the body 12 and the milling element 14 may be discrete individual pieces that are assembled and secured together to form the milling tool 10. This may be accomplished in a number of ways, including mechanical interlocking, fusion welding, brazing, soldering, adhesive bonding, or a combination of any of these techniques. However, according to the invention, the body 12 and the milling element 14 are integrally formed, e.g., machined from a single solid piece of tool steel to form a single integral piece in the sense that the body 12 and the milling element 14 did not previously exist as discrete objects.

The body 12 includes an annular wall 36 extending between the axially spaced first and second ends 22, 26 of the body 12. The annular rim 36 has an inner surface 38 and an outer surface 40. The inner surface 38 of the annular wall 36 defines the through-hole 16, which extends through the body 12 including the first and second openings 20, 24. The inner surface 38 has a base surface 42 and one or more depressed surfaces 44 depressed into the annular wall 36 to define one or more retention channels 46. The one or more retention channels 46 serve to retain the milling element 14 within the through-hole 16 in the event that the body 12 and the milling element 14 are not integrally formed, contrary to the invention. And the retaining channels 46 can, as shown here in 3 shown, include a plurality of axial retention channels 46a extending axially from the first end 22 of the body 12 to the second end 26 and spaced circumferentially around the inner surface 38, and may further include a circumferential retention channel 46b extending circumferentially around the inner surface 38 and intersecting each of the axial retention channels 46a.

The outer surface 40 of the annular wall 36 includes an integrated retaining nut 48 and an integrated radial flange 50. The integrated retaining nut 48 protrudes from a central portion of the annular wall 36 between the first and second ends 22, 26 of the body 12 and has a plurality of planar surfaces 52 located at circumferentially spaced apart axial edges 54 cut ( 2 ). In a preferred embodiment, the integrated retaining nut 48 includes six planar surfaces 52 of equal size arranged hexagonally around the outer surface 40 of the annular wall 36. The integrated radial flange 50 abuts and supports an axial end of the integrated retaining nut 48 near either the first or second end 22, 26 of the body 12. Here, the integrated radial flange 50 is in 1 - 9 disposed near the first end 22, although, if desired, it could just as easily be disposed near the second end 26. The integrated radial flange 50 extends radially outward beyond the planar surfaces 52 of the integrated retaining nut 48 to provide a semicircular bearing surface 56 projecting transversely to each of the planar surfaces 52, as shown in 2 and 6 is best shown. The combination of the integrated retaining nut 48 and the integrated radial flange 50 enables the milling tool 10 to be received and coupled in a rotatable milling tool holder, such as a collet chuck.

The milling element 14 includes one or more milling grooves 58 that form the first and second milling receptacles 32, 34. The one or more milling grooves 58 are designed to rework weld faces received in the first and second milling receptacles 32, 34 and to restore asymmetric geometries on those weld faces through a shearing action that results when the milling tool 10 is rotated about the central axis 18 of the through hole 16. Each of the milling grooves 58 includes a blade 60 that is supported on the inner surface 38 of the annular wall 36 by an elongated foot piece 62 that spans the entire axial dimension of the annular wall 36. One to four milling grooves 58 may be present as part of the milling element 14. In a preferred embodiment, as shown herein 1 - 6 As shown, the one or more milling grooves 58 include a first milling groove 58a, a second milling groove 58b, a third milling groove 58c, and a fourth milling groove 58d. The blades 60 and the elongated foot pieces 62 of the four milling grooves 58a, 58b, 58c, 58d are in 1 - 6 consequently designated by reference numerals 60a, 60b, 60c, 60d, and 62a, 62b, 62c, 62d, respectively.

In the embodiment shown, each of the elongated feet 62a, 62b, 62c, 62d is axially inserted into one of the axial retaining channels 46a of the inner surface 38 of the annular wall 36 and is held firmly in place by friction due to the precise complementary shape of the retaining channels 46a and the elongated feet 62, as in 3 is best illustrated. In order to further secure the milling grooves 58a, 58b, 58c, 58d in place, and specifically to prevent unwanted axial movement of the milling grooves 58a, 58b, 58c, 58d within the through-hole 16, a radial spring washer 64, biased radially outward, may be received in the circumferential retaining channel 46b and extend through a transverse groove 66 defined in a rearward end of each of the elongated foot pieces 62a, 62b, 62c, 62d. In addition to or instead of the axially insertable elongated feet 62a, 62b, 62c, 62d and the radial spring washer 64, other mechanisms may be used to rigidly hold the milling grooves 58a, 58b, 58c, 58d on the inner surface 38 of the annular wall 36 within the through-hole 16. According to the invention, the elongated feet 62a, 62b, 62c, 62d of the milling grooves 58a, 58b, 58c, 58d are formed integrally with the inner surface 38 of the annular wall 36, so that the body 12 and the milling element 14 form a single piece.

The blades 60a, 60b, 60c, 60d of the milling grooves 58a, 58b, 58c, 58d project inwardly from the inner surface 38 of the annular wall 36 and are centrally connected within the through-hole 16. The blades 60a, 60b, 60c, 60d are circumferentially spaced from one another at regular intervals around the central axis 18 such that each blade 60 is oriented transversely to each of its two circumferentially adjacent blades 60. Each of the blades 60a, 60b, 60c, 60d includes axially spaced and opposed first and second shearing surfaces 68, 70. Specifically, in this embodiment, the blade 60a of the first milling furrow 58a includes a first shearing surface 68a proximate the first end 22 of the body 12 and a second shearing surface 70a proximate the second end 26 of the body 12. The blades 60b, 60c, 60d of the other milling furrows 58b, 58c, 58d include similarly arranged first and second shearing surfaces 68b, 70b, 68c, 70c, 68d, 70d relative to the first and second ends 22, 26 of the body 12. Consequently, in this embodiment, the first and second milling receptacles 32, 34 formed by the milling furrows 58 are collectively defined by the first shearing surfaces 68a, 68b, 68c, 68d and the second shearing surfaces 70a, 70b, 70c, 70d are defined.

The first shear surfaces 68a, 68b, 68c, 68d are profiled to mill and restore an electrode welding side of a first geometry, and the second shear surfaces 70a, 70b, 70c, 70d are profiled to mill and restore an electrode welding side of a second geometry that differs from the first geometry. The different profiles of the first shear surfaces 68a, 68b, 68c, 68d and the second shear surfaces 70a, 70b, 70c, 70d enable that the milling tool 10 restores the first weld side geometry on a welding electrode received in the first milling holder 32, and simultaneously restores the second weld side geometry for another welding electrode received in the second milling holder 34, while the tool 10 is rotated about the central axis 18 of the through hole 16. In this way, the milling tool 10 is capable of reworking two welding electrodes with asymmetric weld side geometries, which is a useful reworking practice when resistance spot welding is performed with significantly different welding electrodes, for example, when the workpiece stack being welded includes an aluminum workpiece and an adjacent steel workpiece.

The first and second weld face geometries, milled from the first shear surfaces 68 and the second shear surfaces 70, respectively, are designed for resistance spot welding a workpiece stackup containing adjacent and overlapping steel and aluminum workpieces. The design of the weld face geometries is based in large part on the materially different physical properties of the steel workpiece and the aluminum workpiece being spot welded together. Specifically, the first weld face geometry, employed on the steel-side welding electrode, is designed to concentrate a current within the steel workpiece (relative to the aluminum workpiece) and also to cause a certain deformation of the steel workpiece during electrical current flow. This advantageously utilizes the low conductivity—both thermal and electrical—of the steel workpiece as well as its elevated melting point relative to the aluminum workpiece. In a slightly different way, the second weld face geometry used on the aluminum-side electrode is designed to break up the refractory oxide layers on the aluminum workpiece and to contain the molten aluminum pool growing within the aluminum workpiece. Both the size and shape of the second weld face geometry affect the content of the molten aluminum pool as it grows.

With reference now to 1 , 5 and 7 At least one of the first shear surfaces 68a, 68b, 68c, 68d includes a lower end portion 72 and an upper end portion 74. The lower end portion 72 extends at least to the central axis 18 of the through-hole 16 and has a leading edge 76 and a trailing edge 78. The leading edge 76 is profiled upwardly from a distal tip 80 and is contoured to mill at least the first weld face geometry and any surrounding transition shoulder into a welding electrode. Upon recovery, the weld face having the first geometry has a specified diameter and, in addition, a specified planar or domed shape. The trailing edge 78 of the lower end section 72 is profiled upwards like the leading edge 76, but offset below the leading edge 76, so that the shearing surface 68 within the lower end section 72 is inclined from the leading edge 76 to the trailing edge 78 at a positive clearance angle, which is in a range of 3° to 8°. The positive clearance angle is in 7 generally presented.

The upper end portion 74 of the first shearing surface 68 has a convex shape and extends from the lower end portion 72 to the elongated root portion 62 of the milling groove 58. The upper end portion 74 has a leading edge 82 and a trailing edge 84. These two edges 82, 84 may be offset by a positive clearance angle like the lower end portion 72, but do not necessarily have to be, since the upper end portion 74 is not necessarily involved in milling the first weld side geometry. Instead, the upper end portion 74 serves to center and guide the welding electrode down to the lower end portion 72 during rotation of the milling tool 10 about the axis 18 of the through hole 16. In fact, when a welding electrode is received in the first milling receptacle 32 and the milling tool 10 is rotated to restore the first weld face geometry, the upper end portion 74 of the shear surface 68 typically does not contact and therefore does not mill the adjacent regions of the welding electrode that are outside the weld face and the transition shoulder.

Included here in the embodiment of 1 , 5 and 7 Two of the aligned first shearing surfaces 68a, 68c include the just-described lower end portion 72, while the other two aligned first shearing surfaces 68b, 68d include a variation of the lower end portion 72, in which the only significant difference is that the distal tip 80 does not extend all the way to the axis 18 of the through-hole 16. Each of the four shearing surfaces 68a, 68b, 68c, 68d also includes the above-described upper end portion 74 for guiding and centering the welding electrode. All four first shearing surfaces 68a, 68b, 68c, 68d are therefore profiled to participate in milling a welding side to restore the first geometry, while also contributing to the welding side of the welding electrode and guide it into the correct position within the first milling receptacle 32. In this embodiment, the four shearing surfaces 68a, 68b, 68c, 68d are used together to make restoring the first geometry easier and less time-consuming.

A welding electrode 200 (also referred to as the “first welding electrode 200”), which includes the first welding side geometry and can be reworked within the first milling receptacle 32 by the first shearing surfaces 68 of the one or more milling grooves 58, is in 10 shown. The first welding electrode 200 includes an electrode body 202 and a welding side 204. The electrode body 202 preferably has a cylindrical shape and includes a front end 206 with a circumference 2060. A diameter 2062 of the body 202 at its front end circumference 2060 is preferably in the range of 12 mm to 22 mm, or more narrowly in the range of 16 mm to 20 mm. The welding side 204 is disposed at the front end 206 of the body 202 and has a circumference 2040 that is coincident with the circumference 2060 of the front end 206 of the body 202 (a "full-face electrode") or is displaced upwardly from the circumference 2060 of the front end 206 by a distance between 2 mm and 10 mm by a transition shoulder 208 having a frustoconical or truncated spherical shape. If the transition shoulder 208 is frustoconical, the truncation angle is preferably between 15° and 40° from a horizontal plane of the weld side perimeter 2040. If the transition shoulder 208 is spherical, the radius of curvature of the transition shoulder 208 is preferably between 6 mm and 20 mm, or more narrowly between 8 mm and 12 mm.

The welding face 204 preferably has a diameter 2042, measured at its periphery 2040, that is in the range of 3 mm to 16 mm, or more narrowly, in the range of 4 mm to 8 mm. With regard to its shape, the welding face 204 includes a welding face base surface 210, which may be flat or curved. When curved, the welding face base surface 210 rises upwardly and inwardly from the periphery 2040 of the welding face 204 to achieve an upwardly curved convex shape. For example, in a particular embodiment, the welding face base surface 210 may be spherically curved by having a spherical profile with a radius of curvature that is preferably in the range of 8 mm to 400 mm, or more narrowly, in the range of 25 mm to 100 mm. The geometry of the weld face 204—regardless of whether it has a flat or curved shape with its prescribed diameter 2042—can be milled and restored by receiving the deteriorated weld face 204 into the first milling socket 32 of the milling tool 10 and then rotating the tool 10 about an axis 212 of the weld face 204. In this way, the first shear surfaces 68 of the one or more milling grooves 58 shear away weld face material to expose new weld face material and restore the first weld face geometry.

The first welding electrode 200 may be constructed of any electrically and thermally conductive material suitable for spot welding applications and which may undergo degradation during welding. For example, the first welding electrode 200 may be constructed of a copper alloy having an electrical conductivity of at least 80% IACS, or more preferably at least 90% IACS, and a thermal conductivity of at least 300 W/mK, or more preferably at least 350 W/mK. A specific example of a copper alloy that may be used for the first welding electrode 200 is a copper-zirconium (CuZr) alloy containing about 0.10 to about 0.20 mass percent zirconium and the remainder copper. Copper alloys that meet this component composition, referred to as C15000, are preferred. Other copper alloy compositions as well as other metal compositions not explicitly mentioned here, which have suitable mechanical properties as well as electrical and thermal conductivity properties, can also be used, which include, for example, a copper-chromium alloy (CuCr alloy) C18200, a copper-chromium-zirconium alloy (CuCrZr alloy) C18150 or a heat-resistant metal composite, such as a tungsten-copper metal composite.

With reference now to 2 , 6 , 8 and 9 At least one of the second shear surfaces 70 includes a lower end portion 86 and an upper end portion 88 similar to the first shear surfaces 68. The lower end portion 86 extends at least to the central axis of the through-hole 16 and has a leading edge 90 and a trailing edge 92. The leading edge 90 is profiled upwardly from a distal tip 94 and is contoured to mill at least the second weld face geometry and any surrounding transition shoulder into a welding electrode. Upon recovery, the weld face with the second geometry has a specified diameter and, in addition, a specified domed shape that defines a plurality of upright standing annular ridges. The trailing edge 92 of the lower end section 86 is profiled upwards like the leading edge 90, but is offset below the leading edge 90 such that the shearing surface 70 within the lower end section 86 is inclined from the leading edge 90 to the trailing edge 92 at a positive clearance angle ranging from 3° to 8°. The positive clearance angle is in 8 shown.

As in 9 As best shown, the second shear surface 70 defines within the lower end portion 86 a plurality of penetration flutes 96 extending from the leading edge 90 at least a portion of the way to the trailing edge 92 such that, during rotation of the milling tool 10, upstanding ridges are milled into the weld face as part of the second weld face geometry. The penetration flutes 96, which may be straight or curved across the second shear surface 70, preferably include between two and ten flutes extending the entire distance across the shear surface 70 from the leading edge 90 to the trailing edge 92. Each of the penetration grooves 96 has a height (measured as the maximum penetration distance from the shear surface 70 at the leading edge 90) ranging from 20 µm to 500 µm, or more narrowly from 50 µm to 300 µm. Furthermore, the penetration grooves 96 are spaced apart along the shear surface 70 (measured as the distance between the centers of adjacent grooves 96 along the shear surface 70 at the leading edge 90), with the spacing ranging from 50 µm to 1800 µm, or more narrowly from 80 µm to 1500 µm. In a preferred embodiment, the underside of each of the penetration grooves 96 has a constant radius of curvature to provide a blunt or rounded cross-sectional shape, although other alternative cross-sectional shapes, including truncated, semi-circular, and triangular, are of course possible.

The penetration grooves 96 may extend from the leading edge 90 across the shearing surface 70 at a positive clearance angle that is equal to or different from the clearance angle of the second shearing surface 70. In particular, the positive clearance angle of the penetration grooves 96 from the leading edge 90 to the trailing edge 92 may be in a range of 1.5° to 20°, or more narrowly, 5° to 15°. When the penetration grooves 96 extend just across the second shearing surface 70, as in 2 and 6 As shown, the clearance angle of the grooves 96 is preferably greater than 8° to allow sufficient clearance between the groove inner walls and the upstanding ridges that are milled and restored during rotation of the milling tool 10. However, if the penetrating grooves 96 are curved across the second shearing surface 70 to match the curvature of the ridges that are milled and restored, the positive clearance angle may be equal to or even smaller (e.g., down to 1.5°) than the clearance angle of the second shearing surface 70, since the curvature of the grooves 96 naturally limits interference between the groove inner walls and the ridges that are milled and restored.

The upper end portion 88 of the second shearing surface 70 has a convex shape and extends from the lower end portion 86 to the elongated root portion 62 of the milling groove 58. The upper end portion 88 has a leading edge 98 and a trailing edge 100. These two edges 98, 100 may be offset by a positive clearance angle as in the lower end portion 86, but this is not necessarily required, since the upper end portion 88 is not necessarily involved in milling the second weld side geometry. Instead, the upper end portion 88 serves, as before, to center and guide the welding electrode downward toward the lower end portion 86 during rotation of the milling tool 10 about the central axis 18 of the through hole 16. In fact, when a welding electrode is received in the second milling receptacle 34 and the milling tool 10 is rotated to restore the second weld face geometry, the upper end portion 88 of the shear surface 70 typically does not contact and therefore does not mill the adjacent regions of the welding electrode that lie outside the weld face and the transition shoulder.

In the embodiment of 2 , 6 , 8 and 9 Two of the aligned second shear surfaces 70b, 70d comprise the lower end portion 86 just described, while the other two aligned second shear surfaces 70a, 70c include a variation of the lower end portion 86 in which penetration grooves 96 are not present and the distal tip 94 does not extend all the way to the central axis 18 of the through-hole 16. Each of the second shear surfaces 70a, 70b, 70c, 70d also includes the upper end portion 88 as described above for guiding and centering the electrode. All four second shear surfaces 70a, 70b, 70c, 70d are therefore profiled to assist in aligning and guiding the welding side of the welding electrode into the correct position within the second milling receptacle 34 and to further mill and restore the transition shoulder region of the electrode. But only the two second shearing surfaces 70b, 70d, which contain the penetration grooves 96, actually participate in milling a welding side to restore annular burrs that are part of the second Weld side geometry. Furthermore, as shown, the second shear surfaces 70b, 70d, which have distal tips 94 extending to the central axis 18 of the through-hole 16, are not present on the same blades 60 as the first shear surfaces 68a, 68c, which similarly have distal tips 80 extending to the central axis 18 of the through-hole 16. The two sets of first and second shear surfaces 68a, 68c, 70b, 70d are instead oriented transversely to each other on the milling element 14.

A welding electrode 220 (also referred to as the “second welding electrode 220”), which contains the second welding side geometry and can be reworked within the second milling receptacle 34 by the second milling surfaces 70 of the one or more milling grooves 58, is in 11 shown. The second electrode 220 includes an electrode body 222 and a welding side 224. The electrode body 222 preferably has a cylindrical shape and includes a front end 226 with a circumference 2260. A diameter 2262 of the body 222, measured at its circumference 2260 at the front end, is preferably in the range of 12 mm to 22 mm, or more narrowly in the range of 16 mm to 20 mm. The welding face 224 is disposed at the front end 226 of the body 222 and has a perimeter 2240 that coincides with the perimeter 2260 of the front end 226 of the body 222 (a "full-face electrode") or is offset upwardly from the perimeter 2260 of the front end 226 by a transition boss 228 having a frustoconical or truncated spherical shape by a distance between 2 mm and 10 mm. When the transition boss 228 is frustoconical, the truncation angle is preferably between 30° and 60° from a horizontal plane of the welding face perimeter 2240. When the transition boss 228 is spherical, the radius of curvature of the transition boss 228 is preferably between 6 mm and 12 mm.

The welding face 224 preferably has a diameter 2242, measured at its perimeter 2240, which is in the range of 8 mm to 20 mm, or more narrowly in the range of 10 mm to 15 mm. With regard to its shape, the welding face 224 includes a welding face base surface 230 that is curved. Accordingly, the welding face base surface 230 rises upwardly and inwardly from the perimeter 2240 of the welding face 224 to achieve an upwardly curved convex shape. For example, in a particular embodiment, the welding face base surface 230 may be spherically curved in that it has a spherical profile with a radius of curvature that is preferably in the range of 15 mm to 300 mm, or more narrowly in the range of 20 mm to 50 mm. In addition, the welding side 224 includes a series of upstanding circular ridges 232 that project outwardly from the welding side base surface 230. These circular ridges 232 enable the second welding electrode 220 to make good mechanical and electrical contact with a surface of an aluminum workpiece by stressing and breaking through the mechanically strong and electrically insulating refractory oxide layers that typically coat the surface of an aluminum workpiece.

The rows of upstanding circular ridges 232 are preferably centered around and surround an axis 234 of the weld side 224. The weld side base surface 230 from which the ridges 232 protrude may comprise 50% or more, and preferably between 50% and 80%, of the surface area of the weld side 224. The remaining surface area is allocated to the rows of upstanding circular ridges 232, which preferably comprise between two and ten ridges 232, or more narrowly, between three and five ridges 232. The plurality of upstanding circular ridges 232 are radially spaced from one another on the weld side base surface 230 such that the upstanding ridges 232 have a larger diameter as one moves away from the innermost upstanding ridge 232a ( 12 ), which immediately surrounds the axis 234 of the welding side 224, to the outermost upright ridge 232b ( 12 ) which is closest to the circumference 2240 of the welding side 224 and consequently furthest from the axis 234 of the welding side 224.

The size and shape of the upstanding circular ridges 232 are subject to a certain variability without losing their ability to be reworked. In an embodiment best described in 11 As shown, each of the upstanding circular ridges 232 has a closed perimeter, meaning that the perimeter of the ridge 232 is continuously curved and therefore not interrupted by significant separation areas, and is additionally defined by a cross-sectional profile that has no sharp corners while having a curved (as shown) or a flat top surface. In addition, as shown in 12 As shown, each of the circular ridges 232 also has a ridge height 232h - detected at the center of the ridge 232 - that extends upwardly from the weld side base surface 230 when viewed in cross section. The ridge height 232h of each ridge 232 is preferably in a range of 20 µm to 500 µm, or more narrowly, from 50 µm to 300 µm. And the gap between the burrs 232 on the weld-side base surface 230, measured by the distance between the centers of two adjacent burrs 232, is preferably in a range of 50 µm to 1800 µm, or more narrowly, from 80 µm to 1500 µm. Each of the circular burrs 232 is preferably semicircular, truncated semicircular, or triangular in cross-section.

The geometry of the weld face 224—specifically, the curved weld face base surface 230 with the upstanding circular ridges 232 and the prescribed diameter 2242 of the weld face 224—may be milled and restored by receiving the deteriorated weld face 224 in the second milling receiver 34 of the milling tool 10 and then rotating the tool 10 about the axis 234 of the weld face 224. In this manner, the upstanding circular ridges 232 of the weld face 224 are engaged with the penetration grooves 96 extending at least a portion of the distance of the second shear surfaces 70 of the one or more milling flutes 58, and the rotation of the milling tool 10 shears away weld face material to expose new weld face material and restore the second weld face geometry. The first and second welding side geometries of the first and second welding electrodes 200, 220 can be restored simultaneously by rotating the milling tool 10 while both the first welding side 204 and the second welding side 224 are received in the first and second milling holders 32, 34, respectively.

Like the first spot welding electrode 200, the second welding electrode 220 may be constructed of any electrically and thermally conductive material suitable for spot welding applications that may undergo degradation during welding. For example, the second welding electrode 220 may be constructed of a copper alloy having an electrical conductivity of at least 80% IACS, or more preferably at least 90% IACS, and a thermal conductivity of at least 300 W/mK, or more preferably at least 350 W/mK. As before, a specific example of a copper alloy that may be used for the second welding electrode 220 is a copper-zirconium (CuZr) alloy containing about 0.10 to about 0.20% by mass of zirconium and the remainder copper. Copper alloys meeting this constituent composition, referred to as C15000, are preferred. Other compositions of copper alloys and other metal compositions not explicitly listed here may also be used which have suitable mechanical properties as well as electrical and thermal conductivity properties and which include, for example, a copper-chromium alloy (CuCr alloy) C18200, a copper-chromium-zirconium alloy (CuCrZr alloy) C18150 or a heat-resistant metal composite, such as a tungsten-copper metal composite.

It should be appreciated that other milling groove designs designed to accommodate the asymmetric first and second weld side geometries are, of course, possible and may be used as an alternative to the milling grooves 58a, 58b, 58c, 58d—with their opposing first and second shear surfaces 68a, 68b, 68c, 68d, 70a, 70b, 70c, 70d—shown in the figures and described above. For example, the milling element 14 may include only one milling groove 58 with a first shear surface 68 and a second shear surface 70. The axially spaced first and second shear surfaces 68, 70 may include the lower end portions 72, 86 described above. In another example, the milling element 14 may include two opposing milling grooves 58, each having a first shearing surface 68 and a second shearing surface 70. The first shearing surfaces 68 and the second shearing surfaces 70 of the opposing milling grooves 58 may be constructed in the same manner as the previously described surfaces 68a, 68c and surfaces 70b, 70d, respectively.

The milling tool 10 can be used to rework, as needed, a pair of welding electrodes involved in resistance spot welding a workpiece stack 300 containing dissimilar workpieces, as shown in 13 - 18 shown. The workpiece stack 300 has a first side 302 and a second side 304 and includes at least one steel workpiece 306 and one aluminum workpiece 308 overlapping and adjacent to each other to establish a butt interface 310 passing through a weld location 312. The first side 302 of the workpiece stack 300 is provided by a surface 314 of a steel workpiece, and the second side 304 is provided by a surface 316 of an aluminum workpiece. The workpiece stack 300 may therefore be a "2T" stack containing only the adjacent pair of steel and aluminum workpieces 306, 308, or it may be a "3T" stack containing the adjacent steel and aluminum workpieces 306, 308 plus an additional steel workpiece 318 (steel-steel-aluminum, as in 14 shown) or an additional workpiece 320 made of aluminum (steel-aluminum-aluminum, as in 15 shown), as long as the two workpieces with the same base metal composition are arranged side by side. In other embodiments, the workpiece stack 300 may even be a "4T" stack, such as steel-steel-steel-aluminum, steel-steel-aluminum-aluminum, or steel-aluminum-aluminum-aluminum.

The 306 steel workpiece includes a steel substrate having any of a wide variety of strengths and grades, which is either coated or uncoated (i.e., bare). The coated or uncoated steel substrate may be hot-rolled or cold-rolled and may be made of a steel such as a mild steel, a free-interstitial steel, a sinter-hardened steel, a low-alloy high-strength (HSLA) steel, a two-phase (DP) steel, a complex-phase (CP) steel, a martensite (MART) steel, a transformation-induced plasticity (TRIP) steel, a twinning-induced plasticity (TWIP) steel, and a boron steel, such as when the 306 steel workpiece includes a pressure-hardened (PHS) steel. If the steel substrate is coated, it preferably includes a surface layer of zinc (e.g., hot-dip galvanized or electrogalvanized), zinc-iron (galvanized), a zinc-nickel alloy, nickel, aluminum, or an aluminum-silicon alloy. Thus, as used herein, the term "steel workpiece" encompasses a wide variety of steel substrates, coated or uncoated, of various grades and strengths, and further includes those that have undergone pre-welding treatments such as annealing, quenching, and/or tempering, as in the production of pressure-hardened steel. Taking into account the thickness of the steel substrate and any surface coating that may be present, the steel workpiece 306 has a 3060 thickness ranging from 0.3 mm to 6.0 mm, and more narrowly from 0.6 mm to 2.5 mm, at least at the weld location 312.

The aluminum workpiece 308 includes an aluminum substrate that is either coated or uncoated (i.e., bare). The aluminum substrate may be comprised of an unalloyed aluminum or an aluminum alloy containing at least 85% aluminum by mass. Some notable aluminum alloys that may comprise the coated or uncoated aluminum substrate are an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, or an aluminum-zinc alloy. The aluminum substrate, when coated, preferably includes a surface layer of its native heat-resistant oxide layers, or alternatively, it may include a surface layer of zinc, tin, or a metal oxide conversion layer comprised of oxides of titanium, zirconium, chromium, or silicon, as in US 2014/0360986 Taking into account the thickness of the aluminum substrate and any surface coating that may be present, the aluminum workpiece 308 has a thickness 3080 that is in the range of 0.3 mm to about 6.0 mm, and more narrowly from 0.5 mm to 3.0 mm, at least at the weld location 312.

The aluminum substrate of the aluminum workpiece 308 may be provided in machined or cast form. For example, the aluminum substrate may consist of a sheet of a machined 4xxx, 5xxx, 6xxx, or 7xxx series aluminum alloy, an extrusion, a forging, or another machined article. Alternatively, the aluminum substrate may consist of a 4xx.x, 5xx.x, or 7xx.x series aluminum alloy casting. Some more specific types of aluminum alloys that may form the aluminum substrate include, but are not limited to, aluminum-magnesium alloys AA5182 and AA5754, aluminum-magnesium-silicon alloys AA6011 and AA6022, aluminum-zinc alloys AA7003 and AA7055, and an aluminum injection molding alloy Al-10Si-Mg. The aluminum substrate may further be used in a variety of tempers, including annealed (O), hardened (H), and solution treated (T), if desired. Therefore, as used herein, the term "aluminum workpiece" encompasses unalloyed aluminum and a wide variety of aluminum alloy substrates, coated or uncoated, in various forms suitable for spot welding, including machined sheet layers, extrusions, forgings, etc., as well as castings, and further includes those that have undergone pre-welding treatments such as annealing, hardening, and solution treatment.

The surface 314 of the steel workpiece and the surface 316 of the aluminum workpiece, which provide the first and second sides 302, 304 of the workpiece stack 300, may be represented by the adjacent and overlapping steel and aluminum workpieces 306, 308. For example, if the two workpieces 306, 308 are to be spot welded in the context of the 13 shown "2T" embodiment, the steel workpiece 306 includes a butt surface 322 and an outer surface 324, and similarly, the aluminum workpiece 308 includes a butt surface 326 and an outer surface 328. The butt surfaces 322, 326 The two workpieces 306, 308 overlap and contact each other to form the butt interface 310, which extends through the weld location 312. The outer surfaces 324, 328 of the steel and aluminum workpieces 306, 308, on the other hand, face away from each other in opposite directions at the weld location 312 and form the surfaces 314, 316 of the steel and aluminum workpieces, respectively, of the workpiece stack 300.

The term "butting interface 310" is used broadly in the present invention and is intended to encompass instances of direct and indirect contact between the butting surfaces 322, 326 of adjacent steel and aluminum workpieces 306, 308. The butting surfaces 322, 326 are in direct contact with each other when they are physically adjacent and not separated by a discrete intervening layer of material. The butting surfaces 322, 326 are in indirect contact with each other when they are separated by a discrete intervening layer of material—and thus do not experience the type of physical abutment at the interface found in direct contact—but are in sufficiently close proximity to each other that resistance spot welding can still be performed. The indirect contact between the abutting surfaces 322, 326 of the steel and aluminum workpieces 306, 308 typically occurs when an optional (not shown) intermediate layer of material is applied between the abutting surfaces 322, 326 before the workpieces 306, 308 are placed on top of one another in forming the workpiece stack 300.

An intermediate material layer that may be present between the abutting surfaces 322, 326 of adjacent steel and aluminum workpieces 306, 308 is an uncured, but heat-curable structural adhesive. Such an intermediate material typically has a thickness of 0.1 mm to 2.0 mm, which allows spot welding through the intermediate layer without great difficulty. A structural adhesive may be disposed between the abutting surfaces 322, 326 of the steel and aluminum workpieces 306, 308 so that the workpiece stack 300 can be heated in an ELPO oven or other device after spot welding to cure the adhesive and provide additional adhesion between the workpieces 306, 308. A specific example of a suitable thermosetting structural adhesive is a thermosetting epoxy, which may contain filler particles, such as silica particles, to modify the viscosity or other mechanical properties of the adhesive when cured. A variety of thermosetting epoxies are commercially available, including DOW Betamate 1486, Henkel 5089, and Uniseal 2343. Of course, other types of materials may form the interlayer material instead of a thermosetting structural adhesive.

Of course, the workpiece stack 300, as in 14 - 15 is not limited to only including the steel workpiece 306 and the adjacent aluminum workpiece 308. The workpiece stack 300 may also include the additional steel workpiece 318 or the additional aluminum workpiece 320—in addition to the adjacent steel and aluminum alloy workpieces 306, 308—as long as the additional workpiece is disposed adjacent to the workpiece 306, 308 having the same base metal composition, that is, the additional steel workpiece 318 (if present) is disposed adjacent to the other steel workpiece 306, and the additional aluminum workpiece 320 (if present) is disposed adjacent to the other aluminum workpiece 308. With respect to the characteristics of the additional workpiece, the descriptions of the steel workpiece 306 and the aluminum workpiece 308 provided above are applicable to the additional steel workpiece or the additional aluminum workpiece that may be included in the workpiece stack 300. It should be noted, however, that while the same general descriptions apply, there is no requirement that the two steel workpieces or the two aluminum workpieces of a 3T stack be identical in composition, thickness, or shape (e.g., machined or cast).

For example, in 14 As shown, the workpiece stack 300 may include the adjacent steel and aluminum workpieces 306, 308 described above, along with the additional steel workpiece 318 overlapping and disposed adjacent to the steel workpiece 306. With the additional steel workpiece 318 so positioned, the outer peripheral surface 328 of the aluminum workpiece 308 forms, as before, the aluminum workpiece surface 316 providing the second side 304 of the workpiece stack 300, while the steel workpiece 306 adjacent to the aluminum workpiece 308 now includes a pair of opposing abutment surfaces 322, 330. The abutment surface 322 of the steel workpiece 306, which faces and contacts (directly or indirectly) the adjacent abutment surface 326 of the aluminum workpiece 308, forms the abutment interface 310 between the two workpieces 306, 308 as previously described. The other abutment surface 330 of the workpiece 306 of steel faces and makes overlapping contact (directly or indirectly) with an abutting surface 332 of the additional steel workpiece 318. Therefore, in this particular arrangement of overlapping workpieces 306, 308, 318, an outer surface 334 of the additional steel workpiece 318 now forms the surface 314 of the steel workpiece, providing the first side 302 of the workpiece stack 300.

In another example, as in 15 As shown, the workpiece stack 300 may include the above-described adjacent steel and aluminum workpieces 306, 308, along with the additional aluminum workpiece 320 overlapping and disposed adjacent to the aluminum workpiece 308. With the additional aluminum workpiece 320 so positioned, the outer exterior surface 324 of the steel workpiece 306 forms the steel workpiece surface 314, which, as before, provides the first side 302 of the workpiece stack 300, while the aluminum workpiece 308 adjacent to the steel workpiece 306 now includes a pair of opposed abutment surfaces 326, 336. The abutment surface 326 of the aluminum workpiece 308, which opposes and contacts (directly or indirectly) the adjacent abutment surface 322 of the steel workpiece 306, forms the abutment interface 310 between the two workpieces 306, 308, as described above. The other abutting surface 336 of the aluminum workpiece 308 faces a butting surface 338 of the additional aluminum workpiece 320, thereby forming an overlapping contact (directly or indirectly). Consequently, in this particular arrangement of overlapped workpieces 306, 308, 320, an outer peripheral surface 340 of the additional aluminum workpiece 320 now forms the surface 316 of the aluminum workpiece, providing the second side 304 of the workpiece stack 310.

With reference now to 16 - 17 The first welding electrode 200 and the second welding electrode 220 described above can be used to perform resistance spot welding of the workpiece stack 300. The welding side 204 of the first welding electrode 200 has the first geometry (which can be finished by the first milling cutter 32 of the milling tool 10), and the second welding side 224 of the second welding electrode 220 has the second geometry (which can be finished by the second milling cutter 34 of the milling tool 10). The welding electrodes 200, 220 are supported by a welding gun (not shown) of any suitable type, including a C-type or X-type welding gun, and are electrically coupled to a power supply capable of supplying electrical current—preferably a direct current in the range of 5 kA to 50 kA—between the welding electrodes 200, 220 and through the workpiece stack 300 in accordance with a programmed welding sequence. The welding gun may also be equipped with coolant lines and associated control devices for supplying a coolant fluid, such as water, to each of the welding electrodes 200, 220 during spot welding operations.

The resistance spot welding method begins by positioning the first and second welding electrodes 200, 220 relative to the workpiece stack 300 such that the first welding side 204 faces the surface 314 of the steel workpiece and the second welding side 224 faces the surface 316 of the aluminum workpiece, as shown in 16 is shown. The first weld side 204 and the second weld side 224 are then pressed against their respective steel and aluminum workpiece surfaces 314, 316 in mutually oriented alignment with an applied clamping force at the weld location 312. The applied clamping force is preferably in a range of 1780 N to 8896 N (400 lb to 2000 lb), or more narrowly, 2670 N to 5780 N (600 lb to 1300 lb). Although only the steel and aluminum workpieces 306, 308 overlapping and adjacent to each other, thereby establishing the butt interface 310, are shown in this figure, the following discussion of the resistance spot welding process applies equally to instances where the workpiece stackup 300 includes the additional steel workpiece 318 or the additional aluminum workpiece 320 ( 14 - 15 ), although these additional workpieces 318, 320 have been omitted from the figures for the sake of clarity.

As soon as the first welding side 204 and the second welding side 224 press against the surfaces 314, 316 of the steel and aluminum workpieces, respectively, of the workpiece stack 300, an electric current is conducted between the welding electrodes 200, 220 by means of their mutually oriented welding sides 204, 224. The electric current exchanged between the welding electrodes 200, 220 flows through the workpiece stack 300 and across the butt interface 310 formed between the adjacent steel and aluminum workpieces 306, 308. Resistance to the flow of the electric current, which is preferably a direct electric current with a current level in the range of 5 kA to 50 kA, melts the aluminum workpiece 308 and creates a molten aluminum pool 350 within the workpiece. 308 aluminum. The molten aluminum pool 350 wets the butt surface 322 of the steel workpiece 308 and penetrates into the aluminum workpiece 308 by a distance ranging from 20% to 100% of the thickness 3080 of the aluminum workpiece 308 at the weld location 312.

After the electrical current flow has ceased, the molten pool 350 of molten aluminum solidifies into a weld 352, which joins the workpieces 306, 308 of steel and aluminum at the welding location 312 by welding, as shown in 17 shown, without consuming the abutment surface 310 between the workpieces 306, 308. The weld joint 352 includes resolidified material of the aluminum workpiece 308 and may also include one or more reaction layers of Fe-Al intermetallic compounds adjacent to the abutment surface 322 of the steel workpiece 306. The one or more Fe-Al intermetallic layers may include FeAl ₃ compounds, Fe ₂ Al ₅ compounds, and possibly other intermetallic compounds, and typically have a combined total thickness of 1 µm to 5 µm. The weld joint 352 extends into the aluminum workpiece 308 to a distance often in the range of 20% to 100% of the thickness 3080 of the aluminum workpiece 308 at the weld location 312 (where 100% means completely through the aluminum workpiece 308), just like the pre-existing molten aluminum pool 350.

After the passage of electric current between the welding electrodes 200, 220 has ceased and the clamping force imposed by the electrodes 200, 220 is no longer required, the welding electrodes 200, 220 are withdrawn from their respective surfaces 314, 316 of the steel and aluminum workpieces. The resistance spot welding process is then repeated at other welding locations 312 on the same or a different workpiece stackup 300. Continued use of the first and second welding electrodes 200, 220 in resistance spot welding operations eventually causes the first weld side 204 and the second weld side 224 to deteriorate. This deterioration of the weld sides 204, 224 is generally unavoidable and, at some point, begins to disrupt the communication of electric current between the welding electrodes 200, 220 and through the workpiece stackup 300. If the current flow is disturbed as a result of noticeable weld side deterioration, the formation of the weld joint 352 is disturbed, making it difficult to consistently achieve good strength properties in the joint 352.

The combination of dissimilar materials in the workpiece stackup 300 and the different weld face geometries of the first and second welding electrodes 200, 220 result in different forms of deterioration occurring on the first and second weld faces 204, 224. For example, the first weld face 204 of the first welding electrode 200 may experience macrodeformation in the form of growths due to the high temperatures encountered at the surface 314 of the steel workpiece and the clamping pressure imposed on the weld face 204, especially when the steel workpiece 306 includes a high-strength steel such as DP, TRIP, or others. Furthermore, if the first weld face 204 is made of a copper alloy, it may react with zinc on the steel workpiece 306, if present, to form a copper-zinc alloy layer on the weld face 204, which accelerates the macrodeformation. On the other hand, if the second welding face 224 of the second welding electrode 220 is made of a copper alloy, it may experience a metallurgical reaction between copper and aluminum, forming a copper-aluminum reaction product. The copper-aluminum reaction product may cause the welding face 224 to chip and corrode. Furthermore, the upstanding circular ridges 232 may become deformed or flattened over time, impairing the ability of the welding face 224 to conduct electrical current into or out of the workpiece stackup 300.

The first and second welding sides 204, 224 can be periodically reworked by the milling tool 10 whenever it is desired to counteract deterioration of the welding side and thereby extend the useful life of the first and second welding electrodes 200, 220. The first and second welding electrodes 200, 220 can be specifically reworked after they have formed between 10 and 100 weld joints 352. That is, the first and second welding electrodes 200, 220 can be used to form a first set of weld joints 352, which is in the range of 10 to 100, followed by reworking by the milling tool 10. After reworking, the first and second welding electrodes 200, 220 can be used to form a second set of weld joints 352, which is again in the range of 10 to 100, followed by further reworking with the milling tool 10. For each welding electrode 200, 220, this sequence of welding and reworking can be continued until the cumulative consumption of weld side material resulting from the reworking operations puts the electrodes 200, 220 into a state in which they are not suitable for further use. Since each rework operation with the milling tool 10 removes a material depth in the range of 10 µm to 500 µm, and more preferably 50 µm to 200 µm, each of the welding electrodes 200, 220 can typically withstand between 10 and 500 rework operations before it must be replaced with a new electrode having the same welding side geometry.

Using the milling tool 10 to rework the first and second welding sides 204, 224 can be performed without removing the first and second welding electrodes 200, 220 from the welding gun. The reworking operation includes mounting the milling tool 10 in a rotatable holder. The first and second welding electrodes 200, 220 are then simultaneously moved together along the center axis 18 of the through hole 16 of the milling tool 10 such that the first welding side 204 is received in the first milling receptacle 32 and the second welding side 224 is received in the second milling receptacle 34, as shown in 18 is shown. This receiving of the weld sides 204, 224 brings them into contact with the first and second shear surfaces 68, 70, respectively, of the one or more milling grooves 58 of the milling element 14. At this time, the upstanding circular ridges 232 on the second weld side 224 are snapped onto and received in the mating penetration grooves 96 defined in the second shear surfaces 70 of the second milling receiver 34. With the first and second weld sides 204, 224 so received, the axis 212 of the first weld side 204 and the axis 234 of the second weld side 224 are coaxially aligned with the central axis 18 of the through hole 16 of the milling tool 10.

The milling tool 10 is rotated about the central axis 18 of the through hole 16 at a speed typically in the range of 100 rpm to 1000 rpm, or more narrowly, 200 rpm to 500 rpm, for a minimum of one to ten revolutions, or more narrowly, four to six complete revolutions, about the axes 212, 234 of the weld sides 204, 224. During this rotation, the leading edges 76, 90 of the shear surfaces 68, 70 of the one or more milling grooves 58 are similarly rotated about the axis 212, 234 of their respective weld sides 204, 224, while remaining in contact with the weld sides 204, 224. This rotational movement of the leading edges 76, 90 around the weld faces 204, 224 mills the first and second weld faces 204, 224 and their associated transition shoulders 208, 228 to expose new weld face material and restore the first and second weld face geometry. After the first and second weld faces 204, 224 have been appropriately reworked, the welding electrodes 200, 220 are retracted from the first and second milling receptacles 32, 34 and, still supported by the welding gun, can be quickly returned to service.

Claims (8)

A milling tool (10) capable of reworking asymmetric weld side geometries of first and second welding electrodes (200, 220), the milling tool (10) comprising: a body (12) extending longitudinally along a central axis (18) between a first end (22) and a second end (26), and a milling element (14) within the body (12) defining a first milling receptacle (32) accessible through a first opening (32) at the first end (22) of the body (12), and further defining a second milling receptacle (34) accessible through a second opening (24) at the second end (26) of the body (12), the milling element (14) comprising a milling groove (58) including a milling blade having axially spaced and opposed first and second shearing surfaces (68, 70) at least partially defining the first or second milling receptacle (32, 34), wherein the first shearing surface (68) includes a lower end portion (72) profiled to mill a first weld face geometry comprising a flat or curved weld face base surface (210), and wherein the second shearing surface (70) includes a lower end portion (86) profiled to mill a second weld face geometry comprising a curved weld face base surface (230), characterized in that the second weld face geometry further includes a series of upstanding circular ridges (232) projecting outwardly from the curved weld face base surface of the second weld face geometry; and in that the body (12) and the milling element (14) are integrally formed. Milling tool (10) according to Claim 1 , wherein the first welding side geometry comprises a spherically curved welding side base surface (210) having a diameter between 3 mm and 16 mm and a radius of curvature between 8 mm and 400 mm. Milling tool (10) according to Claim 1 , wherein the second welding side geometry (230) comprises a spherically curved welding side base surface (230) having a diameter between 8 mm and 20 mm and a radius of curvature between 15 mm and 300 mm, and further between comprises between two and ten upstanding circular ridges (232) surrounding a weld side axis (234) and increasing in diameter from an innermost upstanding ridge (232) immediately surrounding the weld side axis (234) to an outermost upstanding ridge (232) furthest from the weld side axis (234), the upstanding ridges (234) being spaced apart on the curved weld side base surface (230) by a distance of 50 µm to 1800 µm, and each of the upstanding circular ridges (232) having a ridge height ranging from 20 µm to 500 µm. Milling tool (10) according to Claim 1 , wherein the milling element (14) comprises a first milling groove (58a) having a first milling blade, a second milling groove (58b) having a second milling blade, a third milling groove (58c) having a third milling blade, and a fourth milling groove (58d) having a fourth milling blade, wherein the first, second, third, and fourth milling blades are spaced from one another in the circumferential direction such that each of the first, second, third, and fourth milling blades is oriented transversely to each of its two circumferentially adjacent milling blades, wherein each of the first, second, third, and fourth milling blades includes axially spaced and opposed first and second shearing surfaces (68, 70), wherein the first shearing surfaces (68) of the first, second, third, and fourth milling blades define the first milling receptacle (32), and the second shearing surfaces (70) of the first, second, third, and fourth milling blades define the second Define milling holder (34). Milling tool (10) according to Claim 4 , wherein both the first shearing surface (68a) of the first milling blade and the first shearing surface (68c) of the third milling blade, which are aligned with each other, have a lower end portion (72) having an upwardly profiled leading edge (76) and an upwardly profiled trailing edge (78) offset below the leading edge (76) by a positive clearance angle, and wherein both the second shearing surface (68b) of the second milling blade and the second shearing surface (70) of the fourth milling blade, which are aligned with each other but oriented transversely to the first shearing surface (68) of the first milling blade and the first shearing surface (68) of the third milling blade, have a lower end portion (86) having an upwardly profiled leading edge (90) and an upwardly profiled trailing edge (92) offset below the leading edge (90) by a positive clearance angle and further a plurality of penetration grooves (96) extending from the leading edge (90) at least part of the way to the trailing edge (92). Method for reworking welding electrodes having asymmetric welding side geometries, the method comprising: a milling tool (10) according to Claim 1 is provided; a first welding face of a first welding electrode (200) is received in the first milling receptacle (32) of the milling tool (10); a second welding face of a second welding electrode (220) is received in the second milling receptacle (34) of the milling tool (10); and the milling tool (10) is rotated to mill and restore a first welding face geometry into the first welding face and a second welding face geometry into the second welding face, wherein the first welding face geometry comprises a flat or curved weld face base surface (210) and the second welding face geometry comprises a curved weld face base surface (230) and a series of upstanding circular ridges (232) projecting outwardly from the curved weld face base surface (230). Procedure according to Claim 6 wherein rotating the milling tool (10) comprises rotating the milling tool (10) between one and ten complete revolutions about axes of the first and second weld sides such that a material depth ranging from 10 µm to 500 µm is removed from both the first weld side and the second weld side in restoring the first weld side geometry and the second weld side geometry. Procedure according to Claim 6 , further comprising: forming a set of ten to one hundred weld joints between overlapping and adjacent workpieces of steel and aluminum before receiving the first weld side in the first milling receptacle (32) of the milling tool (10) and the second weld side in the second milling receptacle (34) of the milling tool (10).

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