EXTERNAL HEAT-ASSISTED WELDING OF UNEQUAL METAL WORKPIECES

Abstract

Method for joining an aluminium workpiece (12, 44, 202) to a steel workpiece (14, 204), the method comprising: the provision of a stack of workpieces (10) comprising a first side (24, 50) represented by an aluminium workpiece surface (26) and a second side (28) represented by a steel workpiece surface (30), wherein the stack of workpieces (10) comprises an aluminium workpiece (12, 44, 202) and an adjacent overlapping steel workpiece (14, 204), wherein a mating surface (32, 48) of the aluminium workpiece (12, 44, 202) contacts a mating surface (36) of the steel workpiece (14, 204) to form a mating interface (40, 206) between the workpieces (12, 44, 202, 14, 204); the provision of a first welding electrode (18) and a second welding electrode (20), wherein the first welding electrode (18) has a welding surface (78) opposite the first side (24, 50) of the workpiece stack (10) and the second welding electrode (20) has a welding surface (86) opposite the second side (28) of the workpiece stack (10); heating the first welding electrode (18) to raise the temperature of the first welding electrode (18) to an increased temperature that does not exceed the melting point of the aluminium workpiece (12, 44, 202); pressing the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20) against the first side (24, 50) and the second side (28) of the workpiece stack (10) while the first welding electrode (18) has the increased temperature, wherein the first welding surface (78) and the second welding surface (86) are pressed against their respective first and second sides (24, 50, 28) of the workpiece stack (10) in opposite orientation to each other at a welding point (16); the passing of electric current between the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20) to generate a weld pool (160) of molten aluminium within the aluminium workpiece (12, 44, 202), wherein the weld pool (160) of molten aluminium wets the mating surface (36) of the steel workpiece (14, 204); the interruption of the conduction of the electric current between the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20) such that the weld pool (160) of molten aluminium solidifies in a weld seam (162, 200) which welds the overlapping aluminium and steel workpieces (12, 44, 202, 14, 204) together; and controlling the temperature of the first welding electrode (18) after the electric current has stopped flowing between the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20), while the first welding electrode (18) is still in contact with the aluminium workpiece (12, 44, 202), in order to maintain the temperature of the first welding electrode (18) at a holding temperature that prevents the complete solidification of the weld pool (160) of molten aluminium at least until the steel workpiece (14, 204) at the weld point (16) cools down to below the melting point of the aluminium workpiece (12, 44, 202).

Description

TECHNICAL FIELD

[0001] The technical field of this disclosure relates generally to resistance spot welding and, more particularly, to a method for resistance spot welding an aluminum workpiece and an adjacent overlapping steel workpiece. BACKGROUND

[0002] Resistance spot welding is a process used by a number of industries to join two or more metal workpieces together. For example, the automotive industry often uses resistance spot welding to join prefabricated metal workpieces during the manufacture of vehicle closure parts (e.g., doors, hoods, decklids, liftgates) and vehicle body panels (e.g., body sides and cross members). A series of spot welds is typically formed along a peripheral edge of the metal workpieces or another joining area to ensure the structural integrity of the part.While 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 integrate lighter materials into a vehicle body has sparked interest in joining steel workpieces to aluminum workpieces using resistance spot welding. Other manufacturing industries, including the aircraft, marine, railroad, and construction industries, are also interested in developing effective and repeatable methods for joining such dissimilar metal workpieces.

[0003] Resistance spot welding relies on the resistance to the flow of an electric current through overlapping metal workpieces and across their mating interface(s) to generate heat. To perform this welding process, a set of opposing spot welding electrodes is clamped at aligned points on opposite sides of the workpiece stack, which typically consists of two or three metal workpieces arranged in an overlapping configuration, at a predetermined weld location. Electric current is then passed from one welding electrode to the other through the metal workpieces. The resistance to the flow of this electric current generates heat within the metal workpieces and at their mating interface(s).If the workpiece stack contains an aluminum workpiece and an adjacent overlapping steel workpiece, the heat generated at the mating interface and in the raw material of these dissimilar metal workpieces triggers a growing molten aluminum weld pool within the aluminum workpiece. The molten aluminum alloy weld pool wets the adjacent mating surface of the steel workpiece and, once the current flow is stopped, solidifies into a weld joining the two dissimilar workpieces.

[0004] In practice, however, spot welding a steel workpiece to an aluminum workpiece is challenging because several properties of these two metals can negatively impact the strength—and especially the peel strength and shear stress—of the weld. First, the aluminum workpiece typically contains a mechanically tough, electrically insulating, and self-healing, heat-resistant oxide layer on its surface. The oxide layer is typically comprised of aluminum oxide compounds, although other oxide compounds, such as magnesium oxide compounds, may also be present if the aluminum workpiece contains a magnesium-containing aluminum alloy.As a result of its physical properties, the refractory oxide layer tends to remain intact at the mating interface of aluminum and steel workpieces, not only hindering the ability of the molten aluminum weld pool to wet the steel workpiece but also providing a source of near-interface defects. The insulating nature of the surface oxide layer also increases the electrical contact resistance of the aluminum workpiece—specifically at the workpiece / electrode contact point—making it difficult to effectively control and concentrate heat within the aluminum alloy workpiece.

[0005] Aside from the challenges of the refractory oxide surface layer, the aluminum and steel workpieces also possess different properties that can adversely affect the strength and properties of the weld. In particular, aluminum has a relatively low melting point (~600 °C) and relatively low electrical and thermal resistances, while steel has a relatively high melting point (~1500 °C) and relatively high electrical and thermal resistances. As a result of these physical differences, most of the heat is generated in the steel workpiece during current flow, resulting in a thermal imbalance between the steel workpiece (higher temperature) and the aluminum workpiece (lower temperature).The combination of the temperature gradient created during current flow with the high thermal conductivity of the aluminum workpiece means that immediately after the electric current stops, a situation arises in which heat is not distributed symmetrically away from the weld point. Instead, heat is conducted from the hotter steel workpiece through the aluminum workpiece toward the welding electrode on the other side of the aluminum workpiece, creating a steep temperature gradient in that direction.

[0006] The development of a steep temperature gradient between the steel workpiece and the welding electrode on the other side of the aluminum workpiece is believed to weaken the integrity of the resulting weld in two primary ways. First, because the steel workpiece retains heat for a longer duration than the aluminum workpiece after the electric current is interrupted, the weld pool of molten aluminum solidifies directionally, starting from the area closest to the cooler welding electrode (often water-cooled) bonded to the aluminum workpiece and spreading toward the mating interface. A solidification front of this type is prone to flow defects—such as gas porosity, shrinkage voids, and microcracking—toward and along the mating interface within the weld, where oxide film residue defects already exist.Second, the sustained elevated temperature in the steel workpiece promotes the growth of a hard and brittle Fe-Al intermetallic layer within the weld at the adjacent mating interface of the steel workpiece. The distribution of weld defects, along with excessive growth of Fe-Al intermetallic compounds along the mating interface, tends to reduce the peel strength of the weld.

[0007] Given the challenges outlined above, previous efforts to spot weld an aluminum workpiece and a steel workpiece have used a weld schedule specifying higher currents, longer welding times, or both (compared to spot welding steel to steel) to attempt to obtain an adequate weld bond area. Such efforts have been largely unsuccessful in a manufacturing environment and have a tendency to damage the welding electrodes. Given that previous spot welding efforts have not been particularly successful, mechanical fasteners such as self-piercing rivets and flow-piercing screws have predominantly been used instead. However, the application of such mechanical fasteners takes longer and has a higher material cost than spot welding.They also increase the weight of the vehicle body—weight that is avoided when joining by spot welding—which offsets some of the weight savings achieved by using aluminum workpieces. Advances in spot welding that would make the process more suitable for joining steel and aluminum workpieces would therefore be a welcome addition to the technology.

[0008] Document DE 10 2014 113 538 A1 discloses a method for resistance spot welding a steel workpiece and an aluminum or aluminum alloy workpiece, in which the aluminum-side welding electrode is preheated. Document DE 10 2012 202 612 A1 discloses a method for conditioning welding electrodes, in which an electrode is heated by means of a heating device. SUMMARY OF REVELATION

[0009] It is an object of the invention to provide a method for joining an aluminum workpiece to a steel workpiece, with which an improved strength of the weld between the workpieces can be achieved.

[0010] This object is achieved according to the invention by the features of claim 1. Advantageous further developments are defined in the dependent claims.

[0011] A workpiece stackup including an aluminum workpiece and an adjacent overlapping steel workpiece plus one or more optional aluminum and / or steel workpieces can be resistance spot welded to form a weld joining the aluminum and steel workpieces. The workpiece stackup includes a first side provided by an aluminum workpiece surface and a second side provided by a steel workpiece surface. To facilitate spot welding, one welding electrode is pressed against a first side of the workpiece stackup proximate the aluminum workpiece of the pair of adjacent aluminum and steel workpieces, and similarly, the second welding electrode is pressed against a second side of the stackup proximate the pair of adjacent aluminum and steel workpieces. The first and second welding electrodes may be the same or different from each other.

[0012] The first welding electrode is supplied with heat from an external heat source to positively influence the strength of the weld joining the aluminum and steel workpieces. External heat may be supplied to the first welding electrode before and / or after electric current is passed between the first and second welding electrodes to reduce the thermal imbalance that develops between the pair of adjacent aluminum and steel workpieces when electric current is passed between the welding electrodes and through the workpiece stack to form the weld.In particular, external heat may be supplied to the first welding electrode to accomplish the following: (1) heating the first welding electrode to an elevated temperature prior to passing the electric current between the first and second welding electrodes, (2) maintaining the first welding electrode at a holding temperature that prevents full solidification of the weld nugget of molten aluminum created within the aluminum workpiece until the temperature of the steel workpiece has dropped below the melting point (i.e., solidification temperature) of the aluminum workpiece after passing electric current between the first and second welding electrodes, or (3) performing both of the foregoing actions.

[0013] The use of an external heat source to supply heat to the first welding electrode for specific periods of time is intended to improve the strength—particularly the peel strength and transverse tensile strength—of the weld in several ways. Heating the first welding electrode to the elevated temperature introduces heat into the aluminum workpiece when the welding surface of the first welding electrode is pressed against the first side of the workpiece stack and, conversely, reduces the amount of heat that must be generated by conducting the electric current between the welding surfaces of the first and second welding electrodes. This makes it possible to minimize the growth and thickness of the brittle Fe-Al intermetallic layer that forms within the weld along the steel workpiece.Furthermore, by maintaining the first welding electrode at the holding temperature to prevent complete solidification of the molten aluminum weld nugget until the steel workpiece cools after the electric current is passed through it, heat transfer from the steel workpiece to the first welding electrode is suppressed, thus promoting lateral heat transfer from the molten aluminum weld nugget in the planes of the aluminum and steel workpieces. Such heat distribution causes the molten aluminum weld nugget to solidify in a manner that drives weld defects toward the center of the weld, where they are less likely to affect the strength and mechanical properties of the joint. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a general cross-sectional view of one embodiment of a workpiece stackup including an aluminum workpiece and an adjacent overlapping steel workpiece disposed between a set of opposing spot welding electrodes for resistance spot welding; Fig. 2 is an exploded view of the workpiece stack and parts of the set of opposing welding electrodes in Fig. 1; Fig. 3 is a general cross-sectional view of another embodiment of a workpiece stackup including an aluminum workpiece and an adjacent overlapping steel workpiece disposed between a set of opposing welding electrodes, although the workpiece stackup includes an additional aluminum workpiece (i.e., two aluminum workpieces and one steel workpiece); Fig. 4 is a general cross-sectional view of another embodiment of a workpiece stackup including an aluminum workpiece and an adjacent steel workpiece in an overlapping manner disposed between a set of welding electrodes, although the workpiece stackup includes an additional steel workpiece (i.e., one aluminum workpiece and two steel workpieces); Fig. Figure 5 shows an elevated view of the workpiece stack and the set of opposing welding electrodes together with details of a cooling pocket defined within the electrode body of each of the welding electrodes; Fig. 6 shows an elevated view of the workpiece stack and the set of opposing welding electrodes along with details associated with one embodiment of an external heat source disposed in heat transfer relationship with the welding electrode located near the aluminum workpiece adjacent to the steel workpiece; Fig. 7 is a general cross-sectional view of the workpiece stack and the set of opposed welding electrodes during the passage of an electric current between the welding surfaces of the opposed welding electrodes and the stack, wherein the passage of electric current has caused the melting of the aluminum workpiece adjacent to the steel workpiece and the formation of a weld nugget of molten aluminum within the aluminum workpiece; Fig. 8 is a general cross-sectional view of the workpiece stack and the set of welding electrodes after the electric current has ceased to flow between the welding surfaces of the opposing welding electrodes and through the stack, allowing the weld pool of molten aluminum to solidify into a weld joining the pair of adjacent aluminum and steel workpieces; Fig. Figure 9 illustrates the direction of the solidification front in a weld pool of molten aluminum solidifying from the point closest to the cooler welding electrode near the aluminum workpiece toward the mating interface as is common in conventional spot welding practices; Fig. 10 illustrates the direction of the solidification front in a weld pool of molten aluminum solidifying from its outer periphery toward its center using features of the disclosed method; Fig. 11 shows an elevated view of an alternative embodiment of an external heat source disposed in heat transfer relationship with the welding electrode and located near the aluminum workpiece overlapping and adjacent to the steel workpiece; and Fig. Figure 12 shows an elevated view of an alternative embodiment of an external heat source disposed in heat transfer relationship with the welding electrode and located near the aluminum workpiece overlapping and adjacent to the steel workpiece. DETAILED DESCRIPTION

[0014] The preferred and exemplary embodiment of the disclosed method for resistance spot welding an aluminum workpiece and an adjacent overlapping steel workpiece is described in Fig. 1-12. The disclosed method attempts to counteract the thermal imbalance that naturally develops between the aluminum and steel workpieces during current flow by applying heat from an external source to the welding electrode, if necessary, located near the aluminum workpiece. The introduction of external heat to the aluminum side of the welding electrode can be performed in conjunction with limiting or maintaining the flow of coolant fluid through the electrode, if desired.By applying heat to the aluminum side of the welding electrode at specific times during the resistance spot welding process, the growth of the Fe-Al intermetallic layer can be inhibited and / or the solidification behavior of the weld pool created from molten aluminum within the aluminum workpiece can be modified to minimize a broad distribution of defects at and along the interface of the weld and the steel workpiece. Each of these effects likely contributes to improving weld strength.

[0015] With reference to the Fig. 1-4, a workpiece stackup 10 is illustrated in preparation for resistance spot welding according to the disclosed method. The workpiece stackup 10 includes at least one aluminum workpiece 12 and one steel workpiece 14 overlapped and adjacent to each other at a weld location 16. Indeed, as will be described in more detail below, the disclosed spot welding method is broadly applicable to workpiece stackup embodiments including the pair of adjacent aluminum and steel workpieces 12, 14. For example, the workpiece stackup 10 may include only the two aluminum and steel workpieces 12, 14, or it may include an additional aluminum workpiece (aluminum-aluminum-steel) or another steel workpiece (aluminum-steel-steel), as long as the two workpieces of the same base metal composition are arranged proximate to each other in the tool stackup 10.The workpiece stack 10 may even contain more than three workpieces such as an aluminum-aluminum-steel-steel stack, an aluminum-aluminum-aluminum-steel stack or an aluminum-steel-steel-steel stack.

[0016] The workpiece stack 10 is in Fig. 1 together with first and second welding electrodes 18, 20 mechanically and electrically configured on a welding gun arm 22 (partially shown). The workpiece stack 10 has a first side 24 provided by an aluminum workpiece surface 26 and a second side 28 provided by a steel workpiece surface 30. The two sides 24, 28 of the workpiece stack 10 are accessible for adjusting the first and second welding electrodes 18, 20 respectively at the welding location 16; i.e., the first welding electrode 18 is arranged to contact and be pressed against the first side 24 of the workpiece stack 10, while the second welding electrode 20 is arranged to contact and be pressed against the second side 28.And while only one weld location 16 is shown in the figures, those skilled in the art will appreciate that spot welding can be performed at various different weld locations 16 on the same workpiece stack 10. . Fig. Figure 2 shows the workpiece stack 10 and opposing spot welding electrodes 18, 20 in a known manner to assist in describing the disclosed method.

[0017] The aluminum workpiece 12 includes an aluminum substrate that is either coated or uncoated. The aluminum substrate may be made of unalloyed aluminum or an aluminum alloy containing at least 85 wt.% aluminum. Some well-known 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. If coated, the aluminum substrate may include a surface layer of a refractory oxide material comprising aluminum oxide compounds and optionally other oxide compounds, as well as magnesium oxide compounds if the aluminum substrate is an aluminum-magnesium alloy.The aluminum substrate may also be coated with a layer of zinc, tin, or a metal oxide passivation coating made of oxides of titanium, zirconium, chromium, or silicon, as described in US 2014 / 0 360 986 A1. The surface layer may have a thickness of 1 nm to 10 µm and may be present on each side of the aluminum substrate. Taking into account the thickness of the aluminum substrate and any optional coating present, the aluminum workpiece 12 has a thickness 120 that ranges from 0.3 mm to approximately 6.0 mm, or more narrowly from 0.5 mm to 3.0 mm, at least at the weld location 16.

[0018] The aluminum substrate of the aluminum workpiece 12 can be provided in wrought or cast form. For example, the aluminum substrate can be made from a 4xxx, 5xxx, 6xxx, or 7xxx series wrought aluminum alloy as a foil layer, extrusion, forging, or other formed part. Alternatively, the aluminum substrate can be made from a 4xx.x, 5xx.x, 6xx.x, or 7xx.x series aluminum alloy casting. Some more specific types of aluminum that can form the aluminum substrate include, but are not limited to, AA5754 and AA5182 aluminum-magnesium alloys, AA6111 and AA6022 aluminum-magnesium-silicon alloys, AA7003 and AA7055 aluminum-zinc alloys, and Al-10Si-Mg aluminum die-cast alloys. The aluminum substrate can be further processed to a variety of material conditions including tempered (O), work hardened (H) and solution treated (T), if desired.

[0019] The steel workpiece 14 includes a steel substrate of any strength and grade, which is either coated or uncoated. The steel substrate can be hot-rolled or cold-rolled and can be made of structural steel, IF (Interstitial-Free) steel, bake-hardenable steel, high-strength low-alloy (HSLA) steel, two-phase (DP) steel, complex-phase (CP) steel, martensitic (MART) steel, TRIP (Transformation-Induced Plasticity) steel, and boron steel, as if the steel workpiece 14 included press-hardened (PHS) steel. If coated, the steel substrate preferably includes a surface layer of zinc (e.g., hot-dip galvanized or electrogalvanized), a zinc-iron alloy (e.g.,galvanized), a zinc-nickel alloy, nickel, aluminum, an aluminum-magnesium alloy, an aluminum-zinc alloy, or an aluminum-silicon alloy, any of which may have a thickness of up to 50 µm and which may be present on either side of the steel substrate. Taking into account the thickness of the steel substrate and any optional coating present, the steel workpiece 14 has a thickness 940 that is in a range of 0.3 mm to 6.0 mm, or more narrowly from 0.6 mm to 2.5 mm, at least at the weld 16.

[0020] If the two workpieces 12, 14 are to be spot welded in a "2T" stack embodiment as shown in Fig. 1, the aluminum workpiece 12 and the steel workpiece 14 respectively constitute the first and second sides 24, 28 of the workpiece stack 10. In particular, the aluminum workpiece 12 includes a mating interface 32 and a back surface 34, and the steel workpiece 14 also includes a mating interface 36 and a back surface 38. The mating surfaces 32, 36 of the two workpieces 12, 14 overlap and contact each other to form a mating interface 40 that extends through the weld 16. On the other hand, the outer peripheral surfaces 34, 38 of the aluminum and steel workpieces 12, 14 generally face away from each other at the weld 16 in opposite directions and respectively constitute the aluminum workpiece surface 26 and the steel workpiece surface 30 of the first and second sides 24, 28 of the workpiece stack 10.

[0021] The term "mating interface 40" is used broadly in the present disclosure and is intended to encompass any overlap and opposing relationship between the mating surfaces 32, 36 of the workpieces 12, 14 at which resistance spot welding can be performed. The surfaces 32, 36 may be in direct contact with each other when they are physically adjacent to each other and not separated by a discrete intervening layer of material. In another example, the mating surfaces 32, 36 may be 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 extensive physical interfacial engagement experienced with direct contact—but are still close enough that resistance spot welding can still be performed.This type of indirect contact between the mating surfaces 32, 36 of the steel and aluminum workpieces 12, 14 typically results when an intermediate optional organic material layer 42 (shown in . Fig. 2, but not Fig. 1) is applied between the surfaces 32, 36 before spot welding the workpiece stack 10.

[0022] The organic material layer 42, which may be present between the surfaces 32, 36 of the aluminum and steel workpieces 12, 14, is typically an adhesive layer comprising a structural thermosetting adhesive matrix. The structural thermosetting adhesive matrix may consist of any structurally curable adhesive, including, for example, a thermosetting epoxy resin or thermosetting polyurethane. Some specific examples of thermosetting structural adhesives may include DOW Betamate 1486, Henkel 5089, and Uniseal 2343 as the adhesive matrix, all of which are commercially available. In addition, the adhesive layer may further comprise optional filler particles, such as silica particles, dispersed in the thermosetting adhesive matrix to alter the viscosity or other mechanical properties of the adhesive matrix during the curing process.In other embodiments, the organic material layer 42 may be an insulating material or a sound-damping material. The organic material layer 42 preferably has a thickness through the weld 96 of 0.1 mm to 2.0 mm, or more narrowly, of 0.2 mm to 1.0 mm.

[0023] The organic material layer 42, if present, can be easily spot welded at the temperatures and electrode clamping pressures at the weld 16 during current flow. Under spot welding conditions, the organic material of layer 42 is initially displaced laterally under the high compressive force exerted by the welding electrodes 18, 20, and any remaining portion of the layer 42 is burned away within the weld 16 during current flow, leaving only residue (e.g., carbon ash, filler particles, etc.) near the mating interface 36 of the steel workpiece 14. However, outside the weld 16, the organic material layer 42 generally remains undisturbed and can provide its intended function.For example, if the organic material layer 42 is an adhesive layer, additional bonding can be achieved between the surfaces 32, 36 of the aluminum and steel workpieces 12, 14 outside and around the weld 16. To achieve such additional bonding, the workpiece stack 10 can be heated in an ELPO oven or other heating device after spot welding to cure the structural thermosetting adhesive matrix of the adhesive layer still intact around the weld 16 between the aluminum and steel workpieces 12, 14.

[0024] Of course, as in the Fig. 3-4, the workpiece stack 10 is not limited to the inclusion of only the pair of adjacent aluminum and steel workpieces 12, 14. The workpiece stack 10 may also include at least one additional aluminum workpiece or one additional steel workpiece—in addition to the adjacent aluminum and steel workpieces 12, 14—as long as the additional workpiece is adjacent to the workpiece 12, 14 of the same base metal composition; i.e., each additional aluminum workpiece is adjacent to the aluminum workpiece 12 and each additional steel workpiece is adjacent to the steel workpiece 14. With respect to the characteristics of additional workpieces, the descriptions of the aluminum workpiece 12 and the steel workpiece 14 provided above are applicable to any additional aluminum or steel workpiece that could be included in the workpiece stack 10.It should be noted that, while the same general descriptions apply, there is no requirement that the additional aluminum workpiece(s) and / or the additional steel workpiece(s) each be identical in composition, thickness, or shape (e.g., forged or cast) to the aluminum workpiece 12 and the steel workpiece 14 that are adjacent to each other in the workpiece stack 10.

[0025] For example, in Fig. 3, the workpiece stack 10 may include the above-described adjacent aluminum and steel workpieces 12, 14 with a further aluminum workpiece 44. Here, as shown, the additional aluminum workpiece 44 overlaps the adjacent aluminum and steel workpieces 12, 14 and is disposed adjacent to the aluminum workpiece 12. With the additional aluminum workpiece 44 so disposed, the outer peripheral surface 38 of the steel workpiece 14 forms, as before, the steel workpiece surface 30, which provides the second side 28 of the workpiece stack 10, while the aluminum workpiece 12, adjacent to the steel workpiece 14, now includes a pair of opposing mating surfaces 32, 46. The mating surface 32 of the aluminum workpiece 12, which faces the adjacent mating surface 36 of the steel workpiece 14, further represents the mating interface 40 between the two workpieces 12, 14, as previously described.The other mating surface 46 of the aluminum workpiece 12 overlaps and faces a mating surface 48 of the additional aluminum workpiece 44. In this sense, in this particular arrangement of overlapping workpieces 12, 14, 44, an outer circumferential surface 50 of the additional aluminum workpiece 44 now forms the aluminum workpiece surface 26, which provides the first side 24 of the workpiece stack 10.

[0026] In another example, as in Fig. 4, the workpiece stack 10 includes the above-described adjacent aluminum and steel workpieces 12, 14 along with a further steel workpiece 52. Here, as shown, the additional steel workpiece 52 overlaps with the adjacent aluminum and steel workpieces 12, 14 and is arranged adjacent to the steel workpiece 14. With the additional steel workpiece 52 so arranged, the outer circumferential surface 34 of the aluminum workpiece 12 forms, as before, the aluminum workpiece surface 26, which provides the first side 24 of the workpiece stack 10, while the steel workpiece 14, which is adjacent to the aluminum workpiece 12, now includes a pair of opposing mating surfaces 36, 54. The mating surface 36 of the steel workpiece 14, which faces the adjacent mating surface 32 of the aluminum workpiece 12, further establishes the mating interface 40 between the two workpieces 12, 14 as previously described.The other mating surface 54 of the steel workpiece 14 faces and is in overlapping contact with a mating surface 56 of the additional steel workpiece 52. In this sense, in this particular arrangement of overlapping workpieces 12, 14, 52, an outer peripheral surface 58 of the additional steel workpiece 52 now forms the steel workpiece surface 30, which provides the second side 28 of the workpiece stack 10.

[0027] Back now to Fig. 1, where the first welding electrode 18 and the second welding electrode 20 are used to conduct the electric current through the workpiece stack 10 and across the mating interface 40 of the adjacent aluminum and steel workpieces 12, 14 at the weld location 16, regardless of whether additional workpiece(s) are present. Each of the welding electrodes 18, 20 is carried by the welding gun arm 22, which may be of any suitable type, including a C-type gun or an X-type gun. The spot welding operation may require the welding gun 22 to be mounted on a robot capable of moving the welding gun 22 around the workpiece stack 10 as needed, or it may be necessary to configure the welding gun 22 as a stationary pedestal-like type in which the workpiece stack 10 is handled and moved relative to the welding gun 22.Additionally, as shown schematically here, the welding gun arm 22 may be associated with a power supply 60 that supplies electrical current between the welding electrodes 18, 20 according to a programmed welding schedule managed by a welding controller 62. The welding gun 22 may also be equipped with coolant hoses and associated control module for providing a cooling fluid such as water to each welding electrode 18, 20.

[0028] The welding gun 22 includes a first gun arm 64 and a second gun arm 66. The first gun arm 64 is fitted into a first shaft 68 that holds and secures the first welding electrode 18, and the second gun arm 66 is fitted into a second shaft 70 that holds and secures a second welding electrode 20. The secure retention of the welding electrodes 18, 20 on their respective shafts 68, 70 can be achieved via shaft adapters 72, 74 arranged at the axial free ends of the shafts 68, 70. With respect to their positioning relative to the workpiece stack 10, the first welding electrode 18 is positioned for contact with the first side 24 of the stack 10, and consequently, the second welding electrode 20 is positioned for contact with the second side 28 of the stack 10.The first and second welding gun arms 64, 66 are operable to bring or push the welding electrodes 18, 20 toward each other and to apply a clamping force to the workpiece stack 10 at the welding location 16 once the electrodes 18, 20 have been brought into contact with their respective workpiece stack sides 24, 28.

[0029] The first and second welding electrodes 18, 20 may be constructed similarly or differently from each other and may have a variety of electrode designs. As shown in Fig. 2 and Fig. 5, the first welding electrode 18 includes an electrode body 76 carrying a first welding surface 78 and optionally including a transition tab 80 that displaces the first welding surface 78 upwardly away from the electrode body 76. The electrode body 76 of the first welding electrode 18 defines an internal recess 82 that is open at one end of the body 76 opposite the first welding surface 78. This internal recess 82 is accessible to and receives the shank adapter 72 associated with the shank 68 of the first gun arm 64. The second welding electrode 20 is constructed similarly to the first welding electrode 18 and thus includes an electrode body 84 carrying a second welding surface 86 and optionally including a transition tab 88 that displaces the second welding surface 86 upwardly away from the electrode body 84.The electrode body 84 of the second welding electrode 20 also defines an internal recess 90 that is open at one end of the body 84 opposite the second welding surface 86. This internal recess 90 is accessible to and receives the shank adapter 74 associated with the shank 70 of the second gun arm 66.

[0030] A wide range of electrode welding surface designs can be implemented for the welding electrodes 18, 20. For example, each of the first and second weld surfaces 78, 86 can have a diameter at its widest dimension ranging from 3 mm to 20 mm, wherein the first weld surface 78 preferably has a diameter in the range of 6 mm to 20 mm, or more narrowly 8 mm to 15 mm, and the second weld surface 86 preferably has a diameter in the range of 3 mm to 16 mm, or more narrowly 4 mm to 8 mm. Additionally, the first weld surface 78 includes a first base weld surface 92, and the second weld surface 86 includes a second base weld surface 94, and each of the first and second base weld surfaces 92, 94 can be planar or spherically curved.When spherically curved, the base weld surfaces 92, 94 may have a radius of curvature of 15 mm to 400 mm, wherein the first base weld surface 92 preferably has a radius of curvature of 15 mm to 300 mm, or more narrowly 20 mm to 50 mm, and the second base weld surface 94 preferably has a radius of curvature of 25 mm to 400 mm, or more narrowly 25 mm to 100 mm. Additionally, each of the first and second base weld surfaces 92, 94 may be smooth or roughened, or may comprise a series of upstanding concentric rings of circular ridges, such as the ridges disclosed in US 8,222,560 B2; US 8,436,269 B2; US 8,927,894 B2; or in US 2013 / 0 200 048 A1.

[0031] The first and second welding electrodes 18, 20 are equipped to receive a flow of cooling fluid during resistance spot welding operations, as best described in Fig. 5. In fact, and referring for the moment to the first welding electrode 18, the outwardly facing portion of the inner recess 82 of the electrode body 76, located near the first welding surface 78, forms a cooling pocket 96 when the first welding electrode 18 is held in the shank adapter 72 connected to the shank 68 of the first gun arm 64. In particular, the shank adapter 72 includes an outer housing 98 and a cooling fluid supply line 100 extending through an internal bore 102 of the housing 98. The cooling fluid supply line 100 is operable to introduce a flow 104 of cooling fluid, preferably water, into the cooling pocket 96. The flow 104 of cooling fluid passes through the cooling pocket 96 and ultimately out of the cooling pocket 96 through an annular return channel 106 of the inner bore 102, which is fluidly connected to the cooling pocket 96 and surrounds the cooling fluid supply line 100.The flow 104 of the cooling fluid through the first welding electrode 18 can be set anywhere between 0 gal / min, which occurs when the flow 104 is stopped, to a maximum flow rate typically ranging from 0.5 gal / min to 5 gal / min, and the temperature of the flow 104 of the cooling fluid is typically 32°C or less.

[0032] The second welding electrode 20 is configured to receive a flow of cooling fluid in substantially the same manner as the first welding electrode 18. That is, the outwardly facing portion of the inner recess 90 of the electrode body 84, located proximate the second welding surface 86, forms a cooling pocket 108 when the second welding electrode 20 is held in the shank adapter 74 connected to the shank 70 of the second gun arm 66. Specifically, as before, the shank adapter 74 includes an outer housing 110 and a cooling fluid supply line 112 extending through an internal bore 114 of the housing 110. The cooling fluid supply line 112 is operable to introduce a flow 116 of cooling fluid, preferably water, into the cooling pocket 108.The flow 116 of cooling fluid passes through the cooling pocket 108 and ultimately out of the cooling pocket 108 through an annular return channel 118 of the inner bore 114, which fluidly communicates with the cooling pocket 108 and surrounds the cooling fluid supply line 112. The flow 116 of cooling fluid through the second welding electrode 20 can be adjusted to anywhere between 0 gal / min, which occurs when the flow 116 is stopped, up to a maximum flow rate, typically ranging from 0.5 gal / min to 5.0 gal / min, and the temperature of the flow 116 of cooling fluid is typically 32°C or less.

[0033] The first and second spot welding electrodes 18, 20 are each preferably formed from an electrically conductive material, such as a copper alloy. A specific example is a C15000 zirconium-copper alloy (CuZr), which contains 0.10 wt.% to 0.20 wt.% zirconium and copper as the balance material. Other copper alloys can of course be used, including a copper-chromium alloy (CuCr) or a copper-chromium-zirconium alloy (CuCrZr). A specific example of each of the copper alloys mentioned is a C18200 copper-chromium alloy containing 0.6 wt% to 1.2 wt% chromium and a balance of copper, and a C18150 copper-chromium-zirconium alloy containing 0.5 wt% to 1.5 wt% chromium, 0.02 wt% to 0.2 wt% zirconium, and a balance of copper. Other alloy compositions possessing suitable mechanical and electrical conductivity properties may also be used.For example, at least the welding surface 78 of the first welding electrode 18 may be made of a refractory metal (e.g., molybdenum or tungsten) or a refractory metal composite material (e.g., tungsten-copper) as disclosed in US 2016 / 0 279 732 A1 (filed on March 18, 2016).

[0034] In order to provide a mechanism for introducing heat into the first welding electrode 18, which allows the temperature of the first welding electrode 18 to be controlled both before and after the flow of electric current between the welding electrodes 18, 20 during spot welding, an external heat source 130 is arranged in heat transfer relationship with the first welding electrode 18. This is specifically in Fig. 6. In other words, the external heat source 130 is configured to supply heat to the first welding electrode 18 when activated. In a preferred embodiment, as shown here in Fig. 6, the external heat source 130 includes a resistive heating element 132 that contacts and encircles the electrode body 76 and is electrically coupled to a heating circuit power supply 134. The resistive heating element 132 may be a resistive wire, such as a nickel-chromium wire, that wraps around the electrode body 76 multiple times, and the heating circuit power supply 134 may use any power source capable of conducting electrical current through the resistive heating element 132 to generate heat. Alternative embodiments of the external heat source 130 are, of course, possible, and several such alternative embodiments are described in the Fig. 11-12 actually described.

[0035] The external heat source 130 is controlled by a heating process controller 150, which generally includes a processing unit, programmable I / O peripherals, and internal or external memory that stores computer-readable operating instructions for operating the external heat source 130, as well as other functions such as A / D converters and timers. The heating process controller receives temperature data inputs from a temperature sensor 152 and a flow meter 154 associated with the first welding electrode 18, as well as a flow meter 156 associated with the second welding electrode 20. The temperature sensor 152 measures the temperature of the first welding electrode 18 at any given time and transmits this data to the controller 150.The temperature sensor 152 may be a thermocouple mounted on or within the first welding electrode 18, or it may be an optical infrared thermometer in the form of an IR spot thermometer or a thermal imager located near the first welding electrode 18 as needed for non-contact temperature measurement. The flow meters 154, 156 measure the rate at which the flow 104 of cooling fluid moves through the first welding electrode 18 and the rate at which the flow 116 of cooling fluid moves through the second welding electrode 20, respectively, at any given time and transmit the data to the controller 150 as well.

[0036] The heating process controller 150 controls the external heat source 130 by directing how much electrical current is passed through the resistive heating element 132 based on information received and derived from the temperature sensor 152 and the flow meters 154, 156. In particular, the operating instructions loaded onto the controller 150 may include a lookup table, programmed equations, or algorithms based on simulated modeling or experimental data, or some other data processing mechanism that can determine the appropriate operating conditions of the external heat source 130 at any given time based on certain desired results or measurement results of the disclosed method and the output signals required to achieve that condition.As applicable to the disclosed method, the heating process controller 150 manages the amount of heat externally supplied to the first welding electrode 18 before and / or after the electric current flows between the first and second welding electrodes 18, 20 to compensate for the heat imbalance that develops between the pair of adjacent aluminum and steel workpieces 12, 14 as electric current flows through the workpiece stack 10.

[0037] With reference to the Fig. 1 and 7-8, the disclosed method will now be described below with respect to the "2T" workpiece stackup, which includes only the pair of adjacent aluminum and steel workpieces 12, 14. The presence of additional workpieces in the workpiece stackup 10, including, for example, the aluminum or steel workpieces 44, 52 described above, does not affect the manner in which the spot welding process is performed or has any significant impact on the joining mechanism that takes place at the mating interface 40 of the adjacent aluminum and steel workpieces 12, 14. The more detailed explanation given below thus applies to cases in which the workpiece stackup 10 is a "3T" stackup, which includes the additional aluminum workpiece 44 ( Fig. 3) or the additional steel workpiece 52 ( Fig. 4) and “4T” workpiece stacks, despite the fact that these additional workpieces in Fig. 1 and 7-8 are not explained.

[0038] At the beginning of the resistance spot welding process, which was Fig. 1, the workpiece stack 10 is disposed between the first welding electrode 18 and the opposite second welding electrode 20. The welding surface 78 of the first welding electrode 18 is positioned to contact the aluminum workpiece surface 26 of the first side 24 of the stack 10, and the welding surface 86 of the second welding electrode 20 is positioned to contact the steel workpiece surface 30 of the second side 28. The welding gun 22 will then guide the first and second welding electrodes 18, 20 toward each other such that their respective welding surfaces 78, 86 are pressed against the opposite first and second sides 24, 28 of the stack 10 at the weld location 16. The welding surfaces 78, 86 are typically aligned with each other at the weld location 16 under a clamping force applied to the workpiece stack 10.The applied clamping force is preferably between 400 lb (about 181 kg) and 2000 lb (about 907 kg) or more narrowly between 600 lb (about 272 kg) and 1300 lb (about 590 kg).

[0039] The first welding electrode 18 is heated to raise its temperature to an elevated temperature above ambient conditions before electrical current is passed between the opposing first and second welding electrodes 18, 20. This may involve heating the first welding electrode 18, and in particular the first welding surface 78, to an elevated temperature not exceeding the melting temperature of the aluminum in the aluminum workpiece 12. Raising the temperature of the first welding electrode 18 to between 200°C and 500°C is particularly preferred. The process of raising the temperature of the first welding electrode 18 may also be performed at any time before current flows between the spot welding electrodes 18, 20, as long as the first welding electrode 18 is still at the elevated temperature when it contacts the aluminum workpiece surface 26 of the workpiece stack 10.For example, the temperature of the first welding electrode 18 may be increased at any of the following times: (1) Initiated before contact between the welding surface 78 and the aluminum workpiece surface 26 of the stack 10 when the first welding electrode 18 is mounted at a distance from the first side 24 of the workpiece stack 10, (2) while the first welding surface 78 contacts the aluminum workpiece surface 26 of the workpiece stack 10, or (3) during both of the aforementioned specific time periods (i.e., periods (1) and (2)).

[0040] Back to the Fig. 5 and Fig. 7 For the time being, heating the first welding electrode 18 to the elevated temperature can be achieved by cooperatively limiting or completely stopping the flow 104 of cooling fluid through the first welding electrode 18 and supplying heat to the first welding electrode 18 from the external heat source 130. The precise degree of flow 104 of cooling fluid is limited or stopped, and the exact amount of heat supplied from the external heat source 130 to the first welding electrode 18 is controlled by the heating process controller 150.The importance of heating the first welding electrode 18 to an elevated temperature prior to current flow between the welding electrodes 18, 20 and contacting the aluminum workpiece surface 26 of the first side 24 of the workpiece stack 10 with the first welding surface 78 while the first welding electrode 18 is at the elevated temperature will be described in more detail below, including the effect such heating has on the strength of the weld ultimately formed.

[0041] After the welding surfaces 78, 86 of the first and second welding electrodes 18, 20 are pressed against the first and second sides 24, 28 of the workpiece stack 10, respectively, the electrical current is passed between the aligned welding surfaces 78, 86. The exchanged electrical current is preferably a direct current (DC) supplied by the power supply 60, which, as shown, electrically communicates with the first and second welding electrodes 18, 20. The power supply 60 is preferably a medium-frequency direct current (MFDC) inverter power supply that includes an inverter and an MFDC transformer. An MFDC inverter power supply can be purchased from a variety of suppliers, including RoMan Manufacturing (Grand Rapids, MI, USA), ARO Technologies (Chesterfield Township, MI, USA), and Bosch Rexroth (Charlotte, NC, USA).The characteristics of the supplied electrical current are controlled by the weld controller 62. In particular, the weld controller 62 allows a user to program a welding schedule that directs how the electrical current is exchanged between the welding electrodes 18, 20. The welding schedule allows, among other things, individual control of the current intensity at any given time and the duration of current flow at any given current intensity, and it further allows such electrical current attributes to respond to changes in very small time increments, down to fractions of a millisecond.

[0042] The electrical current passed between the welding electrodes 18, 20 flows through the workpiece stack 10 and across the mating interface 40 between the pair of adjacent aluminum and steel workpieces 12, 14. The exchanged electrical current may be constant or pulsed over time, or a combination of the two, and typically has a current intensity of 5 kA to 50 kA, except for possible "off" periods between pulses and lasting approximately 40 ms to 2500 ms. The resistance to the flow of an electrical current generates heat and eventually melts the aluminum workpiece 12 to create a weld pool 160 of molten aluminum within the aluminum workpiece 12, as shown in Fig. 7. The weld pool 160 of molten aluminum wets the adjacent mating surface 36 of the steel workpiece 14 and extends into the aluminum workpiece 12 toward the back surface 34 (or the opposite mating surface 46). The weld pool 160 of molten aluminum can penetrate into the aluminum workpiece 12 a distance ranging from 20% to 100% of the thickness 120 of the aluminum workpiece 12 at the weld location 16. And, in terms of its composition, the aluminum weld pool 160 consists predominantly of molten aluminum material from the aluminum workpiece 12.

[0043] The aluminum weld pool 160 of molten aluminum solidifies into a weld 162 that joins the aluminum and steel workpieces 12, 14 after the electric current has flowed between the welding surfaces 78, 86 of the first and second welding electrodes 18, 20 and is terminated, as shown in Fig. 8. The weld seam 162 includes an aluminum weld nugget 164 and an Fe-Al intermetallic layer 166. The aluminum weld nugget 164 is comprised of solidified aluminum and extends into the aluminum workpiece 12 a distance often in the range of 20% to 100% of the thickness 120 of the aluminum workpiece 12 at the weld location 16, as does the pre-existing weld pool 160 of molten aluminum. The Fe-Al intermetallic layer 166 is located between the aluminum weld nugget 164 and the mating surface 36 of the steel workpiece 14. The Fe-Al intermetallic layer 166 is formed by a reaction between the molten aluminum weld pool 160 and the steel workpiece 14 at spot welding temperatures and typically includes FeAl3 compounds, Fe2Al5 compounds, and optionally other intermetallic compounds, and typically has a combined total thickness of 1 µm to 5 µm.

[0044] After the passage of electric current between the welding electrodes 18, 20 is completed and while the first welding surface 78 is still pressed against the aluminum workpiece surface 26 of the first side 24 of the stack 10, the temperature of the first welding electrode 18 and in particular the first welding surface 78 is controlled to maintain its temperature at a holding temperature that prevents full solidification of the weld pool 160 of molten aluminum into the weld seam 162 until a temperature of the steel workpiece 14 at the weld location 16 cools to below the melting point of the aluminum workpiece 12.In other words, the temperature of the first welding electrode 18 is temporarily controlled to slow the solidification of the molten aluminum weld pool 160 and to provide time for cooling the thermally more resistive steel workpiece 14, rather than the rapid cooling of the first welding electrode 18 as per conventional spot welding practices. Controlling the temperature of the first welding electrode 18 helps avoid the development of a steep thermal gradient between the steel workpiece 14 and the first welding electrode 18, which would otherwise develop due to the relatively low thermal resistance of the aluminum workpiece 12. Here, the holding temperature of the first welding electrode 18 is actually a range—and not necessarily a single temperature degree—extending from the melting point of the aluminum workpiece 12 to 100°C, or preferably from 200°C to 500°C.

[0045] The holding temperature of the first welding electrode 18 does not need to be maintained during the entire time it takes for the steel workpiece 14 to cool to ambient conditions. For example, the holding temperature may be maintained during the period in which the temperature of the steel workpiece 14 decreases from a peak temperature that occurs when the electric current between the welding electrodes 18, 20 is turned off, until the temperature of the steel workpiece 14 drops below the melting point of the aluminum workpiece 12. The melting point of the aluminum workpiece 12 can range from 570°C to 660°C due to the variety of compositions of the aluminum workpiece 12.Once the temperature of the steel workpiece 14 falls below the melting point of the aluminum workpiece 12, which is preferably assisted by the flow 116 of the cooling fluid through the second welding electrode 20 at maximum flow rate and which may take from 5 ms to 50 ms, control of the temperature of the first welding electrode 18 may be relinquished and the electrode 18 may continue to cool down normally.

[0046] The temperature of the first welding electrode 18 can be maintained at the holding temperature by cooperatively controlling the flow 104 of cooling fluid through the first welding electrode 18 and the amount of heat supplied to the first welding electrode 18 by the external heat source 130 to regulate the rate of electrode cooling. In this way, the flow 104 of cooling fluid can be restricted or stopped entirely over time to change the amount of heat removed from the first welding electrode 18, and the amount of heat supplied by the external heat source 130 over time can be changed to change the amount of heat transferred to the first welding electrode 18.The balance of extracting heat through the flow 104 of the cooling fluid and supplying heat from the external heat source 130 is precisely controlled by the process controller 150 based on input data provided by the temperature sensor 152 and the flow meters 154, 156 as well as specific data related to the cooling over time of the particular steel workpiece 14 included in the workpiece stack 10 after the current flow between the welding electrodes 18, 20 is switched off.

[0047] The first and second welding electrodes 18, 20 continue to exert the clamping force on the workpiece stack 10 until the weld pool 160 of molten aluminum has completely solidified into the weld seam 162. Once the weld seam 162 is formed, the clamping force on the workpiece stack 10 at the weld location 16 is reduced, and the first and second welding electrodes 18, 20 are retracted from their respective sides 24, 28 of the stack 10. The workpiece stack 10 can now be moved relative to the welding gun 22 so that the first and second welding electrodes 18, 20 can be arranged in facing orientation at another welding location 16, where the spot welding process is repeated. Or, instead of performing spot welding at another location 16, the workpiece stack 10 can be moved away from the welding gun 22 to make room for a similar workpiece stack 10.The spot welding process can thus be carried out many times at different welding points 16 on the same or different workpiece stacks in a manufacturing arrangement.

[0048] Weld 162 formed by the disclosed method is expected to exhibit improved strength—particularly improved peel strength—compared to welds formed by conventional spot welding practices. The improved strength may be due to the premature heating of the first welding electrode 18 prior to the initiation of electrical current flow between the welding electrodes 18, 20, which maintains the temperature of the first welding electrode 18 at the holding temperature while the steel workpiece 14 cools due to the cessation of electrical current flow between the welding electrodes 18, 20, or both.More specifically, the step of heating the first welding electrode 18 to an elevated temperature and contacting the first welding surface 78 with the first side 24 of the workpiece stackup 10 at the weld location 16 while the first welding electrode 18 is at the elevated temperature conductively introduces heat into the underlying aluminum workpiece 12 adjacent to the steel workpiece 14. This additional heat reduces the amount of heat that must be supplied from the steel workpiece 14 (via conduction of the electric current through the workpiece stackup 10) to create the weld pool 160 of molten aluminum, which is believed to reduce the thickness of the brittle Fe-Al intermetallic layer 166 located adjacent the mating interface 36 of the steel workpieces 14 within the weld seam 162.

[0049] The step of maintaining the temperature of the first welding electrode 18 at the holding temperature while the steel workpiece 14 cools below the melting point of the aluminum workpiece 12 is intended to positively contribute to increasing the strength of the weld 162 by minimizing defects within the weld 162 at and along a bonding surface 168 of the weld 162 and the steel workpiece 14. The proliferation of weld defects along a bonding surface 168 is prevented by altering the solidification behavior of the molten aluminum weld pool 160 in a manner that drives or pushes weld defects toward the center of the weld 162 and away from the outer edge of the weld 162.Directing weld defects toward the center of the weld 162 has a beneficial effect on peel strength and transverse tensile strength because the center of the weld 162 is a more benign location for the presence of weld defects than near the outer edge of the joint 162 adjacent to the notch zone surrounding the weld 162.

[0050] The influence of maintaining the temperature in the first welding electrode 18 after the current flow has ceased on the solidification behavior of the weld pool 160 of molten aluminum is generally shown in the Fig. 9-10. To provide some context, Fig. 9 shows a weld 200 formed between an aluminum workpiece 202 and a steel workpiece 204, which overlap to establish a mating interface 206. The weld 200 depicted here is representative of a weld formed by a conventional resistance spot welding process that does not utilize features of the method described above. As can be seen, weld defects 208 are distributed at and along a joining surface 210 of the weld 200 and the steel workpiece 204. These weld defects 208 may include shrinkage voids, gas porosity, surface oxide residue, and microcracks, among others. If present and distributed at and along the joining surface 210, the weld defects 208 may reduce the peel and transverse tensile strength of the weld 200.

[0051] Without being bound by theory, it is believed that the scattering of the weld defects 208 at and along the joint surface 210 is due, at least in part, to the solidification behavior of the pre-existing weld pool of molten aluminum as it transforms into the weld 200. In particular, a thermal mismatch may develop between the much hotter steel workpiece 204 and the aluminum workpiece 202 due to the different physical properties of the two materials—namely, the significantly greater thermal and electrical resistances of steel. The steel workpiece 204 therefore acts as a heat source, while the aluminum workpiece 202 acts as a heat conductor, creating a strong temperature gradient in the vertical direction that causes the weld pool of molten aluminum to expand from the area near the cooler (e.g.,water-cooled) welding electrode cools and solidifies from near the aluminum workpiece 202 toward the steel workpiece 204. The path and direction of the solidification front are shown in . Fig. 8 by the arrows 212. As the hardening front progresses along path 212, the weld defects 208 are driven toward the fitting interface 210 of the weld 200 and the steel workpiece 204 and ultimately terminate distributed along the fitting interface 210 within the weld 200.

[0052] By maintaining the temperature of the first welding electrode 18 at the holding temperature at least until the temperature of the steel workpiece 14 drops below the melting point of the aluminum workpiece 12, the Fig. 9 shows that this can be avoided. Referring now to Fig. 10 is an enlarged view of the weld 162 formed by the spot welding process described above. As can be seen, weld defects 170 in this weld 162 are clustered near the center of the joint 162, in contrast to a larger distribution along the mating interface 168, as in Fig. 9. The weld defects 170 are driven toward the center of the weld seam 162 because the new temperature distribution within the workpiece stack 10, resulting from maintaining the temperature of the first welding electrode 18 at the holding temperature, causes the weld pool 160 of molten aluminum to solidify from its outer edge toward its center. The path and direction of the solidification front are generally Fig. 10 by arrows 172. Here, the path 172 can drive weld defects 170 into the center of the weld 162, either on or away from the mating interface 168, and can consolidate the defects 170 into larger-sized defects.

[0053] The process of maintaining the temperature of the first welding electrode 18 at the holding temperature at least temporarily, as described above, starts the solidification front 172 in Fig. 10 by suppressing the formation of a steep thermal gradient between the steel workpiece 14 and the first welding electrode 18 after the current flow is removed. The more equal heat distribution on opposite sides of the aluminum workpiece 12 changes the temperature distribution within the weld 16 by creating three-dimensional radial temperature gradients around the molten aluminum weld pool 160 or magnifying pre-existing three-dimensional radial temperature gradients. These new and / or magnified radial temperature gradients slow directional heat flow into the first welding electrode 18 from the molten aluminum weld pool 160 and thus promote lateral heat flow into the planes of the aluminum and steel workpieces 12, 14, which promotes final solidification in the center of the molten aluminum weld pool 160.As a result, weld defects such as shrinkage cavities, gas cavities, microcracks, and oxide film residues are driven to and retained at the center of the weld 162, as shown, where they are less likely to affect the strength of the joint 162.

[0054] A number of variations of the above-described and in the Fig. 1-10 may be implemented including alternative embodiments of the external heat source 130. For example, as shown in Fig. 11, the external heat source 130 may include a gas outlet tube 180 directed toward the first welding electrode 18 and carried on the first welding gun 64. The gas outlet tube 180 is configured to direct a flow of heated gas 182, such as hot air, to the first welding electrode 18 to supply heat to the electrode 18. The flow of the heated gas 182 may initially affect the electrode body 76 and / or the welding surface 78 of the first welding electrode 18. In another embodiment, as shown in Fig. 12, the external heat source 130 may include a laser beam head 190 that directs a laser beam 192 onto the first welding electrode 18 such that the laser beam 192 impinges on the electrode 18 to generate and supply heat to the electrode 18 without inducing melting. The laser beam 192 may impinge on the first welding electrode 18 at any location on the electrode body 76 or the welding surface 78. Furthermore, and not shown in the figures, a tube carrying superheated steam may be wrapped around the electrode body 76 of the first welding electrode 18 as a mechanism for supplying heat to the electrode 18. The same basic control system described above may also be used to control other types of external heat sources, including those described in the Fig. 11-12 are shown.

Claims

[1] A method for joining an aluminum workpiece (12, 44, 202) to a steel workpiece (14, 204), the method comprising: providing a workpiece stack (10) comprising a first side (24, 50) represented by an aluminum workpiece surface (26) and a second side (28) represented by a steel workpiece surface (30), the workpiece stack (10) comprising an aluminum workpiece (12, 44, 202) and an adjacent overlapping steel workpiece (14, 204), a mating surface (32, 48) of the aluminum workpiece (12, 44, 202) contacting a mating surface (36) of the steel workpiece (14, 204) to form a mating interface (40, 206) between the workpieces (12, 44, 202, 14, 204); providing a first welding electrode (18) and a second welding electrode (20), wherein the first welding electrode (18) has a welding surface (78) opposite the first side (24, 50) of the workpiece stack (10) and the second welding electrode (20) has a welding surface (86) opposite the second side (28) of the workpiece stack (10); heating the first welding electrode (18) to increase a temperature of the first welding electrode (18) to an elevated temperature that does not exceed a melting point of the aluminum workpiece (12, 44, 202); pressing the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20) against the first side (24, 50) and the second side (28) of the workpiece stack (10), respectively, while the first welding electrode (18) is at the elevated temperature, wherein the first welding surface (78) and the second welding surface (86) are pressed against their respective first and second sides (24, 50, 28) of the workpiece stack (10) in opposing alignment with one another at a welding location (16); passing electrical current between the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20) to create a weld pool (160) of molten aluminum within the aluminum workpiece (12, 44, 202), the weld pool (160) of molten aluminum wetting the mating surface (36) of the steel workpiece (14, 204); suspending the passage of the electric current between the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20) such that the weld pool (160) of molten aluminum solidifies into a weld seam (162, 200) that welds the overlapping aluminum and steel workpieces (12, 44, 202, 14, 204) together; and controlling the temperature of the first welding electrode (18) after the electric current has ceased to flow between the welding surface (78) of the first welding electrode (18) and the welding surface (86) of the second welding electrode (20) while the first welding electrode (18) is still in contact with the aluminum workpiece (12, 44, 202) to maintain the temperature of the first welding electrode (18) at a holding temperature that prevents complete solidification of the weld pool (160) of molten aluminum at least until the steel workpiece (14, 204) at the weld location (16) cools to below the melting point of the aluminum workpiece (12, 44, 202). [2] The method of claim 1, wherein the step of heating the first welding electrode (18) comprises: limiting a flow of cooling fluid through the first welding electrode (18) to less than a maximum flow rate or completely stopping the flow of the cooling fluid; and supplying heat from an external heat source (130) to the first welding electrode (18). [3] The method of claim 2, wherein the step of supplying heat from the external heat source (130) to the first welding electrode (18) comprises passing an electric current through a resistive heating element (132) contacting and surrounding the first welding electrode (18). [4] The method of claim 3, wherein the resistance heating element (132) is a resistive wire surrounding an electrode body (76) of the first welding electrode (18). [5] The method of claim 2, wherein the step of supplying heat from the external heat source (130) to the first welding electrode comprises directing a stream of heated gas (182) to the first welding electrode (18). [6] The method of claim 2, wherein the step of supplying heat from the external heat source (130) to the first welding electrode (18) comprises impinging a laser beam (192) on the first welding electrode (18). [7] The method of claim 1, wherein the elevated temperature of the first welding electrode (18) is between 100 °C and 600 °C. [8] The method of claim 1, wherein the step of controlling the temperature of the first welding electrode (18) to maintain the temperature of the electrode (18) at the holding temperature comprises: Passing a flow of cooling fluid through the first welding electrode (18) at a flow rate; Supplying an amount of heat from an external heat source (130) to the first welding electrode (18); and Controlling the flow rate of the cooling fluid and the amount of heat supplied to the first welding electrode (18) to achieve the holding temperature. [9] The method of claim 8, wherein the step of supplying an amount of heat from the external heat source (130) to the first welding electrode (18) comprises at least one of the following options: (1) passing an electric current through a resistive heating element (132) contacting and surrounding the first welding electrode (18); (2) passing a stream of heated gas (182) to the first welding electrode (18); or (3) the impact of a laser beam (192) on the first welding electrode (18).

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