Patent Publication Number: US-2017361392-A1

Title: Multistep electrode weld face geometry for weld bonding aluminum to steel

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 62/351,110 filed on Jun. 16, 2016. The entire contents of the aforementioned provisional application are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The technical field of this disclosure pertains to the formation of resistance spot weld joints between an aluminum workpiece and a steel workpiece and, more specifically, to a spot welding electrode with a multistep weld face geometry that facilitates such weld bonding, particularly when an intermediate organic material is disposed between the faying surfaces of the aluminum and steel workpieces. 
     Introduction 
     Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together metal workpieces during the manufacture of structural frame members (e.g., body sides and cross members) and vehicle closure members (e.g. vehicle doors, hoods, trunk lid, and lift-gates), among others. A number of spot welds are typically formed along a peripheral edge of the metal workpieces or some other selected bonding region to ensure the part is structurally sound. While spot welding has typically been practiced to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum-to-aluminum-the desire to incorporate lighter weight materials into the vehicle body structure has generated interest in joining a steel workpiece to an aluminum workpiece by way of resistance spot welding. 
     Resistance spot welding relies on the resistance to the flow of an electrical current through overlapping metal workpieces and across their faying interface(s) to generate heat. To carry out such a welding process, two opposed spot welding electrodes are clamped at diametrically aligned spots on opposite sides of the overlapping workpieces at a predetermined weld zone. The clamping force is typically in the range of about 600-1200 pounds force. An electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of this electrical current generates heat within the metal workpieces and at their faying interface. When the metal workpieces being spot welded include an aluminum workpiece and an adjacently positioned steel workpiece, the heat generated within the bulk of the workpieces and at their faying interface rapidly melts the aluminum workpiece and creates a molten aluminum weld pool within the aluminum workpiece. This molten weld pool wets the adjacent surface of the steel workpiece and, upon termination of the current flow, solidifies into a weld joint that weld bonds the aluminum and steel workpieces together. 
     In practice, however, spot welding an aluminum workpiece to a steel workpiece is challenging since a number of characteristics of those two metals can adversely affect the strength—most notably the peel strength and the cross-tension strength—of the weld joint. Regarding the properties of the dissimilar metals, aluminum has a relatively low melting point (˜600° C.) and relatively low electrical and thermal resistivities, while steel has a relatively high melting point (˜1500° C.) and relatively high electrical and thermal resistivities. As a result of these physical differences, most of the heat is generated within the steel workpiece during current flow such that a heat imbalance exists between the steel workpiece and the aluminum workpiece. The combination of the heat imbalance created during current flow and the high thermal conductivity of the aluminum workpiece means that, immediately after the flow of electrical current is terminated, a situation occurs where heat is not disseminated symmetrically from the weld zone. Instead, heat is conducted from the hotter steel workpiece through the aluminum workpiece towards the spot welding electrode on the other side of the aluminum workpiece, which creates a steep thermal gradient in that direction. 
     The development of a steep thermal gradient between the steel workpiece and the spot welding electrode on the other side of the aluminum workpiece is believed to weaken the resultant weld joint in several ways. First, because the steel workpiece retains heat longer than the aluminum workpiece after the flow of electrical current is terminated, the molten aluminum weld pool created during current flow solidifies directionally, starting from the region nearest the colder spot welding electrode (often water cooled) proximate the aluminum workpiece and propagating towards the faying interface of the aluminum and steel workpieces. A solidification front of this kind tends to sweep or drive defects—such as gas porosity, shrinkage voids, and micro-cracking—towards and along the faying interface. Second, the sustained elevated temperature in the steel workpiece promotes the growth of a hard and brittle Fe—Al intermetallic layer at and along the faying interface. Having a dispersion of weld defects together with excessive growth of the Fe—Al intermetallic layer along the faying interface tends to reduce the peel and/or cross-tension strength of the weld joint. 
     The challenges that tend to complicate the resistance spot welding of aluminum and steel workpieces extends beyond their materially divergent properties. Each of the aluminum and steel workpieces may, in some instances, include applied or natural surface layers that differ in composition from their underlying base substrates. The aluminum workpiece, for example, may contain a surface layer comprised of a refractory oxide material. This oxide material is typically composed of aluminum oxide compounds, although other oxide compounds may also be present such as, for example, magnesium oxide compounds when the aluminum workpiece contains a magnesium-containing aluminum alloy. When composed of the refractory oxide material, the surface layer present on the aluminum workpiece is electrically insulating and mechanically tough. As a result, a residual oxide film that includes remnants of the original surface layer tends to remain intact at and alongside the faying surface of the steel workpiece where it can hinder the ability of the molten aluminum weld pool to wet the steel workpiece, which can adversely affect the strength of the joint, especially when combined with other weld joint defects that may be swept towards the faying interface due to direction solidification of the molten aluminum weld pool. 
     The complications attributed to the surface layer of the aluminum workpiece can be magnified when an intermediate organic material layer, such as a layer of uncured, heat-curable adhesive, is present between the faying surfaces of the aluminum and steel workpieces at the weld zone. An uncured yet heat-curable adhesive layer may be disposed between the faying surfaces of the stacked workpieces to provide additional bonding between the workpieces over a broad interfacial area around and between weld zones. In clamping the workpieces together by the forceful pressure applied by the spot welding electrodes, and prior to exchanging current, some of the adhesive is squeezed laterally out of the weld zone. The remaining adhesive is then decomposed at the location of the weld joint during current flow. Upon completion of the spot welding procedure, the adhesive-containing regions of the welded workpieces are heated, for example, in an ELPO-bake oven (ELPO refers to an electrophoretic priming operation). The applied heating cures the adhesive layer to attain strong supporting adhesion between the confronting faying surfaces of the metal workpieces around the site(s) where spot welding has been practiced. 
     The intermediate organic material layer has a tendency to interact with the refractory oxide material of the surface oxide layer to form a more tenacious material at spot welding temperatures. Specifically, it is believed that residues obtained from the thermal decomposition of the intermediate organic material layer—such as carbon ash, filler particles (e.g., silica, rubber, etc.), and other derivative materials—combine with the residual oxide film to form a composite residue film that is more resistant to mechanical break down and dispersion during current flow as compared to the residual oxide film alone. The formation of a tougher composite residue film results in fragments of that film remaining grouped and compiled at and along the faying surface of the steel workpiece in a much more disruptive manner as compared to instances in which an organic material layer is not present between the steel and aluminum workpieces. In that regard, it is believed that the composite residue film blocks the diffusion of iron into the molten aluminum weld pool, which can result in excessive thickening of the hard and brittle Fe—Al intermetallic layer and, thus, weaken the joint. Additionally, any gases produced during decomposition of the organic material can become trapped in the molten aluminum weld pool and may eventually lead to porosity within the solidified weld joint. Still further, the composite residue film may provide a ready crack path along the bonding interface of the weld joint and the steel workpiece which, again, can weaken the weld joint. 
     In light of the aforementioned challenges, previous efforts to spot weld an aluminum workpiece and a steel workpiece have employed a weld schedule that specifies higher currents, longer weld times, or both (as compared to spot welding steel-to-steel), in order to try and obtain a reasonable weld bond area. Such efforts have been largely unsuccessful in a manufacturing setting and have a tendency to damage the spot welding electrodes. Given that previous spot welding efforts have not been particularly successful, mechanical fasteners including self-piercing rivets and flow-drill screws have predominantly been used instead. Mechanical fasteners, however, take longer to put in place and have high consumable costs compared to spot welding. They also add weight to the vehicle—weight that is avoided when joining is accomplished by way of spot welding—that offsets some of the weight savings attained through the use of an aluminum workpiece in the first place. Advancements in spot welding that would make it easier to join aluminum and steel workpieces would thus be a welcome addition to the art. 
     SUMMARY OF THE DISCLOSURE 
     A spot welding electrode according to one embodiment of the present disclosure may include a body and a weld face supported on an end of the body. The weld face has a multistep conical geometry that includes a series of steps centered on a weld face axis and contained within an outer perimeter of the weld face. The series of steps may comprise an innermost first step in the form of a central plateau and, additionally, one or more annular steps that surround the central plateau and cascade radially outwardly from the central plateau towards the outer perimeter of the weld face. The central plateau has a top plateau surface and each of the one or more annular steps has a top annular step surface. Moreover, the weld face has a conical cross-sectional profile in which a periphery of the top plateau surface of the central plateau and a periphery of the top annular step surface of each of the one or more annular steps are contained within a conical sectional area defined by an upper linear boundary line and a lower linear boundary line. The upper linear boundary line and the lower linear boundary line intersect at the periphery of the top plateau surface and extend downwardly and outwardly from a horizontal plane extending from the periphery of the top plateau surface to a horizontal plane extending from the outer perimeter of the weld face. The upper linear boundary line is inclined at an angle of 5° from the horizontal plane extending from the periphery of the top plateau surface and the lower linear boundary line is inclined at an angle of 15° from the horizontal plane extending from the periphery of the top plateau surface. 
     The spot welding electrode of the aforementioned embodiment may include other features or be further defined. For example, the top plateau surface of the central plateau and the periphery of the top annular step surface of each of the one or more annular steps may be aligned along a linear tangent line of constant slope that is inclined to a horizontal plane extending from the periphery of the top plateau surface by an angle that ranges from 5° to 15°. The outer perimeter of the weld face may also be aligned on the linear tangent line of constant slope along with the periphery of the top plateau surface of the central plateau and the periphery of the top annular step surface of each of the one or more annular steps. As another example, the weld face may be upwardly displaced from the end of the body by a transition nose. In yet another example, the weld face axis may be collinearly aligned with an axis of the body. And, still further, the one or more annular steps may include between two and six annular steps. 
     The sizes and shapes of the various features of the weld face may vary. For instance, the top plateau surface may be circular in plan view with a diameter that ranges from 2 mm to 8 mm, and a plateau side surface of the central plateau that surrounds and extends downwardly from the top plateau surface may have a height that ranges from 30 μm to 300 μm and may flare radially outwardly from the top plateau surface at an incline angle that ranges from 5° to 60°. The top plateau surface may also be either planar or convexly domed. As for the one or more annular steps, the top annular step surface of each of the one or more annular steps may have a width that ranges from 0.3 mm to 2.0 mm, and a step side surface that surrounds and extends downwardly from the top annular step surface of each of the one or more annular steps may flare radially outwardly from the top annular step surface at an incline angle that ranges from 5° to 60°. The top annular step surface of each of the one or more annular steps may also be either planar or convexly domed. 
     In one particular implementation of the aforementioned embodiment of the spot welding electrode, the central plateau may include a plateau side surface that extends downwardly from the top plateau surface and flares radially outwardly from the top plateau surface, and the one or more annular steps that surround the central plateau may comprise at least a first annular step contiguous with the central plateau, a second annular step contiguous with the first annular step, and a third annular step contiguous with the second annular step. The first annular step may have a first top annular step surface that extends radially outwardly from the plateau side surface of the central plateau to a first step side surface that extends downwardly from the first top annular step surface and flares radially outwardly from the first top annular step surface. Likewise, the second annular step may have a second top annular step surface that extends radially outwardly from the first step side surface of the first annular step to a second step side surface that extends downwardly from the second top annular step surface and flares radially outwardly from the second top annular step surface. And, similarly, the third annular step may have a third top annular step surface that extends radially outwardly from the second step side surface of the second annular step to a third step side surface that extends downwardly from the third top annular step surface and flares radially outwardly from the third top annular step surface. 
     A spot welding electrode according to another embodiment of the present disclosure may include a body and a weld face supported on an end of the body. The weld face may have a multistep conical geometry that includes a series of steps centered on a weld face axis and contained within an outer perimeter of the weld face. The series of steps may comprise an innermost first step in the form of a central plateau and, additionally, one or more annular steps that surround the central plateau and cascade radially outwardly from the central plateau towards the outer perimeter of the weld face. The central plateau has a top plateau surface and a plateau side surface that extends downwardly from the top plateau surface and flares radially outwardly from the top plateau surface, and each of the one or more annular steps has a top annular step surface and a step side surface that extends downwardly from the top annular step surface and flares radially outwardly from the top annular step surface. Moreover, the weld face has a conical cross-sectional profile in which a periphery of the top plateau surface of the central plateau and a periphery of the top annular step surface of each of the one or more annular steps are contained within a conical sectional area defined by an upper linear boundary line and a lower linear boundary line. The upper linear boundary line and the lower linear boundary line intersect at the periphery of the top plateau surface and are inclined at an angle of 5° and 15°, respectively, from a horizontal plane extending from the periphery of the top plateau surface. 
     The spot welding electrode of the aforementioned embodiment may include other features or be further defined. For example, the top plateau surface may circular in plan view with a diameter that ranges from 2 mm to 8 mm, and the plateau side surface may have a height that ranges from 30 μm to 300 μm and may flare radially outwardly from the top plateau surface at an incline angle that ranges from 5° to 60°. As for the one or more annular steps, the top annular step surface of each of the one or more annular steps may have a width that ranges from 0.3 mm to 2.0 mm, and the step side surface of each of the one or more annular steps may have a height that ranges from 30 μm to 300 μm and may flare radially outwardly from the top annular step surface at an incline angle that ranges from 5° to 60°. As another example, the one or more annular steps on the weld face may include between two and six annular steps. 
     A method of resistance spot welding a workpiece stack-up that includes an aluminum workpiece and an adjacent overlapping steel workpiece may include several steps according to one embodiment of the present disclosure. In one step, a workpiece stack-up is provided that includes an aluminum workpiece and a steel workpiece that overlaps with the aluminum workpiece to establish a faying interface between the aluminum and steel workpieces. The workpiece stack-up has an aluminum workpiece surface that provides a first side of the stack-up and a steel workpiece surface that provides an opposed second side of the stack-up. In another step, the workpiece stack-up is positioned between a weld face of a first spot welding electrode and a weld face of a second spot welding electrode. The weld face of the first spot welding electrode may comprise a series of steps that includes an innermost first step in the form of a central plateau and, additionally, one or more annular steps that surround the central plateau and cascade radially outwardly from the central plateau. The central plateau has a top plateau surface and each of the one or more annular steps has a top annular step surface. The weld face also has a conical cross-sectional profile in which a periphery of the top plateau surface of the central plateau and a periphery of the top annular step surface of each of the one or more annular steps are aligned along a linear tangent line of constant slope. 
     In another step, and once the workpiece stack-up is in place, the weld face of the first spot welding electrode is pressed against the first side of the workpiece stack-up such that the top plateau surface of the central plateau makes first contact with the first side of the workpiece stack-up and any pressure exerted by the weld face of the first welding electrode on the first side of the workpiece stack-up is at least initially directed through the top plateau surface of the central plateau. Also, in another step, the weld face of the second spot welding electrode is pressed against the second side of the workpiece stack-up in facial alignment with the weld face of the first spot welding electrode at a weld zone. In still a further step, an electrical current is passed between the weld face of the first spot welding electrode and the weld face of the second spot welding electrode, and through the workpiece stack-up, to grow a molten aluminum weld pool within the aluminum workpiece that wets an adjacent faying surface of the steel workpiece. The weld face of the first spot welding electrode impresses further into the first side of the workpiece stack-up during growth of the molten aluminum weld pool such that the top annular step surface of at least some of the one or more annular steps are brought into contact with the first side of the workpiece stack-up. 
     The method of the aforementioned embodiment may include additional steps or be further defined. For example, the workpiece stack-up may further comprise an intermediate organic material layer applied between the aluminum and steel workpieces at the faying interface. In that regard, in another step of the method, a preliminary electrical current may be passed between the weld face of the first spot welding electrode and the weld face of the second spot welding electrode, and through the workpiece stack-up, before passing the electrical current that grows the molten aluminum weld pool. The passage of the preliminary electrical current heats the intermediate organic material layer and renders it less viscous without melting the aluminum workpiece that lies adjacent to the steel workpiece. In particular, for example, if the intermediate organic material layer is a heat-curable adhesive layer, the passage of the preliminary electrical current between the weld face of the first spot welding electrode and the weld face of the second spot welding electrode may heat the heat-curable adhesive layer to between 100° C. and 150° C. 
     When performing the method of the aforementioned embodiment, the pressing of the weld face of the first spot welding electrode against the first side of the workpiece stack-up may drive lateral displacement of the intermediate organic material layer along the faying interface of the aluminum and steel workpieces and outside of at least a central area of the weld zone. This may occur as a result of at least initially directing any pressure exerted by the weld face of the first welding electrode on the first side of the workpiece stack-up through the top plateau surface of the central plateau at a middle of the weld zone prior to passing the electrical current between the weld face of the first welding electrode and the weld face of the second welding electrode. The method may be performed on a variety of workpiece stack-up configurations. For instance, in one implementation, the aluminum workpiece includes a faying surface and a back surface, and the steel workpiece includes a faying surface and a back surface. The faying surface of the aluminum workpiece and the faying surface of the steel workpiece may confront one another to establish the faying interface between the aluminum and steel workpieces. On the other hand, he back surface of the aluminum workpiece and the back surface of the steel workpiece may constitute the aluminum workpiece surface that provides the first side of the workpiece stack-up and the steel workpiece surface that provides the second side of the workpiece stack-up, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a spot welding electrode that includes a multistep weld face geometry according to one embodiment of the disclosure 
         FIG. 2  is a partial cross-sectional view of the spot welding electrode depicted in  FIG. 1  according to one embodiment of the disclosure; 
         FIG. 3  is a magnified partial cross-sectional view of the side wall of one of the steps of the weld face depicted in  FIG. 2  according to one embodiment of the disclosure; 
         FIG. 4  is general cross-sectional view of one embodiment of a workpiece stack-up situated between a set of opposed spot welding electrodes in preparation for resistance spot welding, wherein the workpiece stack-up includes an aluminum workpiece and an adjacent overlapping steel workpiece along with an optional intermediate organic material layer disposed between the two workpieces, and wherein each of the opposed spot welding electrodes includes a multistep weld face geometry according to one embodiment of the disclosure; 
         FIG. 5  is an exploded view of the workpiece stack-up and portions of the set of opposed spot welding electrodes shown in  FIG. 1 ; 
         FIG. 6  is a general cross-sectional view of another embodiment of a workpiece stack-up situated between a set of opposed spot welding electrodes in preparation for resistance spot welding, wherein each of the opposed spot welding electrodes includes a multistep weld face geometry according to one embodiment of the disclosure and the workpiece stack-up includes an aluminum workpiece and an adjacent overlapping steel workpiece along with an intermediate organic material layer disposed between the two workpieces, although here the workpiece stack-up includes an additional aluminum workpiece (i.e., two aluminum workpieces and one steel workpiece); 
         FIG. 7  is a general cross-sectional view of another embodiment of a workpiece stack-up situated between a set of opposed spot welding electrodes in preparation for resistance spot welding, wherein each of the opposed spot welding electrodes includes a multistep weld face geometry according to one embodiment of the disclosure and the workpiece stack-up includes an aluminum workpiece and an adjacent overlapping steel workpiece along with an intermediate organic material layer disposed between the two workpieces, although here the workpiece stack-up includes an additional steel workpiece (i.e., two steel workpieces and one steel workpiece); 
         FIG. 8  is a general view of the workpiece stack-up (in cross-section) and the set of opposed spot welding electrodes during initial clamping of the workpiece stack-up, which may include passing a preliminary electrical current through the workpiece stack-up and between the opposed spot welding electrode at the weld zone while the welding electrodes are clamped against their respectively opposed sides of the workpiece stack-up; 
         FIG. 9  is a general view of the workpiece stack-up (in cross-section) and the set of opposed spot welding electrodes during passage of electrical current between the weld faces of the electrodes and through the stack-up, which occurs after the stack-up is initially clamped at the weld zone, wherein the passage of electrical current causes melting of the aluminum workpiece that lies adjacent to the steel workpiece and the creation of a molten aluminum weld pool within the aluminum workpiece; 
         FIG. 10  is a general view of the workpiece stack-up (in cross-section) and the set of opposed spot welding electrodes after passage of electrical current between the weld faces of the electrodes and through the stack-up has terminated, thus allowing the molten aluminum weld pool to solidify into a weld joint that weld bonds the pair of adjacent aluminum and steel workpieces together; 
         FIG. 11  is a general perspective view of a second spot welding electrode that may be used in conjunction with the first spot welding electrode (e.g., the spot welding electrode depicted in  FIGS. 1-3 ) to resistance spot weld the workpiece stack-up; and 
         FIG. 12  is a partial cross-sectional view of the spot welding electrode depicted in  FIG. 1  showing the conical sectional area that defines the conical cross-sectional weld face profile of the multistep weld face geometry of the spot welding electrode of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Resistance spot welding an aluminum workpiece and a steel workpiece presents some notable challenges due to the materially different properties of the dissimilar workpieces. Specifically, the refractory surface oxide layer of the aluminum workpiece is difficult to breakdown and disintegrate, which hinders the ability of the molten aluminum weld pool to wet the steel workpiece and may also contribute to near-interface defects. Moreover, the steel workpiece is more thermally and electrically resistive than the aluminum workpiece, meaning that the steel workpiece acts as a heat source and the aluminum workpiece acts as a heat conductor. The resultant heat imbalance established between the workpieces during and just after electrical current flow has a tendency to drive the weld defects, such as porosity and micro-cracks, towards and along a bonding interface of the weld joint and the steel workpiece, and also contributes to the formation and growth of a brittle Fe—Al intermetallic layer contiguous with the steel workpiece. The challenges attendant in forming a weld joint between the aluminum and steel workpieces are further complicated when an intermediate organic material layer is disposed between the faying surfaces of the overlapping workpieces. 
     A spot welding electrode  10  that is useful in resistance spot welding applications is shown generally in  FIGS. 1-3 . In particular, the spot welding electrode  10  has a weld face defined by a multistep conical geometry. The spot welding electrode  10  may be used in conjunction with another spot welding electrode having a similar or dissimilar weld face geometry to spot weld a workpiece stack-up that includes at least an aluminum workpiece and an overlapping and adjacent steel workpiece, as will be described in more detail below with reference to  FIGS. 4-10 . For example, the spot welding electrode  10  is operable to spot weld a “2T” workpiece stack-up ( FIGS. 4-5 ) that includes only the adjacent and overlapping pair of aluminum and steel workpieces. As another example, the spot welding electrode  10  is operable to spot weld a “3T” workpiece stack-up ( FIGS. 6-7 ) that includes the adjacent and overlapping pair of aluminum and steel workpieces plus an additional aluminum workpiece or an additional steel workpiece so long as the two workpieces of the same base metal composition are disposed next to each other (e.g., aluminum-aluminum-steel or aluminum-steel-steel). The spot welding electrode  10  may even be used to spot weld “4T” workpiece stack-ups (e.g., aluminum-aluminum-steel-steel, aluminum-aluminum-aluminum-steel, or aluminum-steel-steel-steel). 
     Referring now to  FIGS. 1-3 , the spot welding electrode  10  includes an electrode body  12  and a weld face  14 . The electrode body  12 , which is preferably cylindrical in shape, has a front end  16  that presents and supports the weld face  14  and a back end  18  that facilitates mounting of the electrode  10  to a weld gun. The front end  16  of the electrode body  12  has a diameter  161  that lies within the range of 12 mm to 22 mm or, more narrowly, within the range of 16 mm to 20 mm, and the back end  18  of the electrode body  12  has a diameter  181  that is typically the same as the diameter  161  of the front end  16 , particularly if the electrode body  12  is shaped as a cylindrical. Moreover, as shown generally in  FIG. 1 , the back end  18  of the electrode body  12  defines an opening  20  to an internal recess  22  for insertion of, and attachment with, an electrode mounting device, such as a shank adapter (not shown), that can secure the spot welding electrode  10  to a gun arm of the weld gun and also enable a flow of cooling fluid (e.g., water) through the internal recess  22  in order to manage the temperature of the electrode  10  during spot welding operations. 
     The weld face  14  is the portion of the spot welding electrode  10  that, during spot welding, is designed to contact a side of the workpiece stack-up under pressure and to pass electrical current through the stack-up in conjunction with the weld face of an opposed and facially aligned spot welding electrode on the opposite side of the stack-up. The weld face  14  may be upwardly displaced from the front end  16  of the electrode body  12  by a transition nose  24  or it may transition directly from the front end  16  (termed a “full face electrode”). When the transition nose  24  is present, the weld face  14  may be upwardly displaced from the front end  16  by a distance  26  that preferably lies between 2 mm to 10 mm. The transition nose  24  may be frustoconical or truncated spherical in shape, although other shapes are certainly possible. If frustoconical, the angle of truncation  241  of the nose  24  is preferably between 30° and 60° from a horizontal plane (also plane  208  as described below) at the intersection of the nose  24  and the weld face  14 . If truncated spherical, the radius of curvature of the nose  24  is preferably between 6 mm and 12 mm. 
     The weld face  14  has a multistep conical geometry that includes a series of steps  28  centered on a weld face axis  30  and contained within an outer perimeter  32  of the weld face  14 . The weld face outer perimeter  32  has a diameter  34  that preferably ranges from 6 mm to 20 mm, or more narrowly from 8 mm to 15 mm, and it may be oriented relative to the front end  16  of the electrode body  12  in different ways. For example, as shown here in  FIGS. 1-2 , the outer perimeter  32  of the weld face  14  may be parallel to the front end  16  of the electrode body  12 , in which case the weld face axis  30  may be parallel and collinearly aligned with an axis of the electrode body  12  or those two axes may be offset such as in the case of a double-bent welding electrode. In other embodiments, however, the outer perimeter  32  of the weld face  14  may be tilted relative to the front end  16  of the electrode body  12 , in which case the weld face axis  30  and an axis of the electrode body  12  are angled with respect to one another. The latter configuration of the spot welding electrode  10  may be employed to help gain access to a weld zone of the workpiece stack-up that would otherwise be difficult to reach. 
     The series of steps  28  on the weld face  14  includes an innermost first step  36  in the form of a central plateau  38  and, additionally, one or more annular steps  40  that surround the central plateau  38  and cascade radially outwardly from the plateau  38  towards the outer weld face perimeter  32 . The central plateau  38  includes a top plateau surface  42  and a surrounding plateau side surface  44 , as shown best in  FIGS. 2-3 . Similarly, each of the one or more annular steps  40  includes a top annular step surface  46  and a surrounding step side surface  48 . The transition between the top plateau surface  42  and the surrounding plateau side surface  44 , as well as the top annular step surface  46  and the surrounding step side surface  48  of each annular step  40 , is preferably a defined edge or a rounded shoulder having a radius of curvature that ranges from 30 μm to 300 μm or, more narrowly, from 50 μm to 200 μm. Anywhere from one to ten annular steps  40  may be included on the weld face  14  around the central plateau  38  with two to six annular steps  40  being preferred in many instances. 
     The central plateau  38  and the one or more annular steps  40  are contiguous with each other starting from the plateau side surface  44 . In that regard, the top annular step surface  46  of each annular step  40  extends radially outwardly from the step side surface  48  of its radially inward neighboring annular step  40  (or the plateau side surface  44  in the case of the annular step  40  that immediately surrounds the central plateau  38 ). For example, in the embodiment shown here in  FIGS. 1-2 , the weld face  14  includes three annular steps  40  that surround the central plateau  38 . Specifically, a first annular step  40 ′ is contiguous with the central plateau  38  of the innermost first step  36 , and includes a first top annular step surface  46 ′ that extends radially outwardly from the plateau side surface  44  to a first step side surface  48 ′. Continuing on, a second annular step  40 ″ is contiguous with the first annular step  40 ′, and includes a second top annular step surface  46 ″ that extends radially outwardly from the first step side surface  48 ′ of the first annular step  40 ′ to a second step side surface  48 ″. In the same way, a third annular step  40 ′″ is contiguous with the second annular step  40 ″, and includes a third top annular step surface  46 ′″ that extends radially outwardly from the second step side surface  48 ″ of the second annular step  40 ″ to a third step side surface  48 ′″. Any additional annular steps  40  that may be present on the weld face  14  outside of the third annular step  40 ′″ are contiguous wither their radially inward neighboring annular step  40  in the same way. 
     The innermost first step  36  and the one or more surrounding annular steps  40  are sized and aligned relative to one another on the weld face  14  to help support the overall spot welding process and to obtain strong and reliable weld joints between an aluminum workpiece and an adjacent steel workpiece within the workpiece stack-up undergoing spot welding. The top plateau surface  42 , for instance, may be circular in plan view and have a diameter  421  that ranges from 2 mm to 8 mm or, more narrowly, from 3 mm to 6 mm, although other profiles may be employed if desired. Moreover, in terms of its curvature, the top plateau surface  42  may be planar or it may be convexly domed. If convexly domes, the top plateau surface  42  may, for example, be spherically domed with a radius of curvature that preferably ranges from 15 mm to 300 mm or, more narrowly, from 20 mm to 200 mm. When afforded with these size and curvature dimensions, the top plateau surface  42  is able to initially concentrate and direct the pressure exerted through the spot welding electrode  10  onto a more limited area of the workpiece stack-up in order to laterally displace and substantially clear organic material such as, for example, uncured structural adhesive, if such organic material is present, from at least a central area of the weld zone, as will be described in more detail below. 
     The plateau side surface  44  that surrounds and extends downwardly from the top plateau surface  42  has a height  441  that preferably ranges from 30 μm to 300 μm or, more narrowly, from 50 μm to 250 μm, as shown in  FIG. 3 . This height dimension  441 —also referred to as a step size of the central plateau  38 —is measured as the distance between the closest points of the top plateau surface  42  and the top annular step surface  46  of the immediately surrounding annular step  40  (e.g., the top annular step surface  46 ′ of the second annular step  40 ′) parallel to the weld face axis  30 . And, to promote retractability of the weld face  14  from engaged workpiece stack-up surfaces, the plateau side surface  44  may flare radially outwardly as it extends from the top plateau surface  42  to the top annular step surface  46  of the immediately surrounding annular step  40 , as shown best in  FIG. 3 . The extent of the incline of the plateau side surface  44  can be measured by an incline angle  50 , which is the angle at which the plateau side surface  44  deviates from a line  52  that runs parallel to the weld face axis  30  and intersects the top plateau surface  42  at the outer perimeter  32  of the weld face  14 . In a preferred embodiment, the incline angle  50  of the plateau side surface  44  ranges from 5° to 60° or, more narrowly, from 20° to 50°. 
     Referring now specifically to  FIGS. 1-2 , the top annular step surface  46  of each annular step  40  is displaced axially below (along the weld face axis  30 ) the top annular step surface  46  of its radially inward neighboring annular step  40  or, in the case of the annular step  40  that immediately surrounds the central plateau  38 , the top plateau surface  42 . The top annular step surface  46  of each of the annular steps  40  has a width  461  that extends from the step side surface  48  of its radially inward neighboring annular step  40  (or the plateau side surface  44  in the case of the annular step  40  that immediately surrounds the central plateau  38 ) to its own step side surface  48  that extends downwardly from the top annular step surface  46 . The width  461  of each of the top annular step surfaces  46  preferably ranges from 0.3 mm to 2.0 mm or, more narrowly, from 0.5 mm to 1.5 mm. And, in terms of curvature, the top annular step surface  46  of each annular step  40  may be planar or it may have a spherical radius of curvature that preferably ranges from 50 mm to 300 mm or, more narrowly, from 75 mm to 200 mm. 
     The step side surface  48  of each of the annular steps  40  is fashioned similarly to the plateau side surface  44  of the central plateau  38 . Each of the step side surfaces  48 , for instance, has a height  481  measured in the same way as the plateau side surface  44  (i.e., the distance between the closest points of the relevant top annular step surfaces  46  parallel to the weld face axis  30 ) that preferably ranges from 30 μm to 300 μm or, more narrowly, from 50 μm to 250 μm. Additionally, each of the step side surfaces  48  may flare radially outwardly as it extends from the top annular step surface  46  of its respective annular step  40  to the top annular surface  46  of the next immediately surrounding and axially downward displaced annular step  40 . The extent of the incline of the step side surface(s)  48  can be measured by the same incline angle  50  that is shown in  FIG. 3  and described above in the context of the plateau side surface  44 . The previous discussion of the incline angle  40  thus applies equally to each of the step side surfaces  48  of the annular steps  40  and the fact that  FIG. 3  demonstrates the incline angle  50  in the context of the plateau side surface  44  does not make a difference. In a preferred embodiment, the incline angle  50  of each of the step side surfaces  48  ranges from 5° to 60° or, more narrowly, from 20° to 50°. 
     A notable geometric characteristic of the spot welding electrode  10  is the cross-sectional profile of the weld face  14 , as depicted best in  FIG. 12 . Indeed, the central plateau  38  and the one or more surrounding annular steps  40  are arranged to provide the weld face  14  with a conical cross-sectional weld face profile to help support the initial pressure concentration through the central plateau, followed by the application of radial outward pressure forces as the one or more annular steps  40  are brought into contact one-by-one with the workpiece stack-up, and to also contain the growing molten aluminum weld pool. The conical cross-sectional weld face profile is established when a periphery  54  of the top plateau surface  42  and a periphery  56  of the top annular step surface  46  of each of the one or more annular steps  40  are contained within a conical sectional area  200  defined by an upper linear boundary line  202  and a lower linear boundary line  204 . The upper linear boundary line  202  and the lower linear boundary line intersect at the periphery  54  of the top plateau surface  42  and extend downwardly and outwardly from a horizontal plane  206  extending through and from the periphery  54  of the top plateau surface  42  to a horizontal plane  208  extending through and from the outer perimeter  32  of the weld face  14 . The upper linear boundary line  202  is inclined at an angle α from the horizontal plane  206  extending from the periphery  54  of the top plateau surface  42  and the lower linear boundary line  204  is inclined at an angle θ from the same horizontal plane  60 . The angle of inclination of the upper linear boundary line  202  (angle α) is 5° and the angle of inclination of the lower linear boundary line  204  (angle β) is 15°. Alternatively, if a tighter conical sectional area  200  is desired, these angles α, β are 7° and 12°, respectively. 
     The periphery  54  of the top plateau surface  42  and the periphery  56  of the top annular step surface  46  of each of the one or more annular steps  40  may be aligned within the conical section area  200  or they not. For example, in one particular embodiment, and as shown in  FIG. 2 , the periphery  54  of the top plateau surface  42  and the periphery  56  of the top annular step surface  46  of each of the one or more annular steps  40  is aligned along a linear tangent line  58  of constant slope; that is, the outermost radial portions of the top plateau surface  42  and the top annular step surface(s)  46  are intersected by the linear tangent line  58  within, of course, acceptable manufacturing tolerances of ±0.1 mm. The tangent line  58  that establishes the conical cross-sectional weld face profile may be inclined to the horizontal plane  206  extending from the periphery  54  of the top plateau surface  42  by an angle  62  that preferably ranges from 5° to 15° or, more narrowly, from 7° to 12°. Accordingly, the linear tangent line  58  may be collinear with the upper linear boundary line  202 , collinear with the lower linear boundary line  204 , or lie somewhere between the upper linear boundary line  202  and the lower linear boundary line  204 . Moreover, in some instances, and as shown here in  FIG. 2 , the tangent line  58  may also intersect the outer perimeter  32  of the weld face  14 . 
     At least the weld face  14  of the spot welding electrode  10 , and preferably the entire spot welding electrode  10  including the electrode body  12 , the weld face  14 , and the transition nose  24  if present, is constructed from a material having an electrical conductivity of at least 45% IACS and a thermal conductivity of at least 180 W/mK. Some material classes that fit these criteria include a copper alloy, a dispersion-strengthened copper material, and a refractory-based material that includes at least 35 wt %, and preferably at least 50 wt %, of a refractory metal. Specific examples of suitable copper alloys include a C15000 copper-zirconium (CuZr) alloy, a C18200 copper-chromium (CuCr) alloy, and a C18150 copper-chromium-zirconium (CuCrZr) alloy. A specific example of a dispersion-strengthened copper material includes copper with a dispersal of aluminum oxide. And a specific example of a refractory-base material includes a tungsten-copper metal composite that contains between 50 wt % and 90 wt % of a tungsten particulate phase dispersed in copper matrix that constitutes the balance (between 50 wt % and 10 wt %) of the composite. Other materials not expressly listed here that meet the applicable electrical and thermal conductivity standards may, of course, also be used as well. 
     Referring now to  FIGS. 4-10 , the spot welding electrode  10  may be used to resistance spot weld a workpiece stack-up  70  that comprises at least an aluminum workpiece  72  and a steel workpiece  74  that overlap and lie adjacent to one another at a weld zone  76 . Indeed, as will be described in greater detail below, the disclosed spot welding method is broadly applicable to a wide variety of workpiece stack-up configurations that include the adjacent pair of aluminum and steel workpieces  72 ,  74 . The workpiece stack-up  70  may, for example, include only the aluminum workpiece  72  and the steel workpiece  74  as far as the number of workpieces are concerned, or it may include an additional aluminum workpiece (aluminum-aluminum-steel) or an additional steel workpiece (aluminum-steel-steel) so long as the two workpieces of the same base metal composition are disposed next to each other in the stack-up  70 . The workpiece stack-up  70  may even include more than three workpieces such as an aluminum-aluminum-steel-steel stack-up, an aluminum-aluminum-aluminum-steel stack-up, or an aluminum-steel-steel-steel stack-up. The aluminum and steel workpieces  72 ,  74  may be worked or deformed before or after being assembled into the workpiece stack-up  70  depending on the part being manufactured and the specifics of the overall manufacturing process. 
     The workpiece stack-up  70  is illustrated in  FIG. 4  along with the spot welding electrode  10  described above (hereafter referred to as the “first spot welding electrode” for purposes of identification) and a second spot welding electrode  78  that are mechanically and electrically configured on a weld gun  80  (partially shown). The workpiece stack-up  70  has a first side  82  provided by an aluminum workpiece surface  82 ′ and a second side  84  provided by a steel workpiece surface  84 ′. The two sides  82 ,  84  of the workpiece stack-up  70  are accessible to the set of first and second spot welding electrodes  10 ,  78 , respectively, at the weld zone  76 ; that is, the first spot welding electrode  10  is arranged to make contact with and be pressed against the first side  82  of the workpiece stack-up  70  while the second spot welding electrode  78  is arranged to make contact with and be pressed against the second side  84 . And while only one weld zone  76  is depicted in the figures, skilled artisans will appreciate that spot welding may be practiced according to the disclosed method at multiple different weld zones  76  within the same stack-up  70 . 
     The aluminum workpiece  72  includes an aluminum substrate that is either coated or uncoated. The aluminum substrate may be composed of unalloyed aluminum or an aluminum alloy that includes at least 85 wt % aluminum. Some notable aluminum alloys that may constitute the coated or uncoated aluminum substrate are an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, and an aluminum-zinc alloy. If coated, the aluminum substrate may include a surface layer comprised of a refractory oxide material such as a native oxide coating that forms naturally when the aluminum substrate is exposed to air and/or an oxide layer created during exposure of the aluminum substrate to elevated temperatures during manufacture, e.g., a mill scale. The refractory oxide material is typically comprised of aluminum oxide compounds and possibly other oxide compounds as well, such as magnesium oxide compounds if, for example, 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 conversion coating comprised of oxides of titanium, zirconium, chromium, or silicon, as described in US Pat. Pub. No. 2014/0360986. The surface layer may have a thickness ranging from 1 nm to 10 μm depending on its composition and may be present on each side of the aluminum substrate. Taking into account the thickness of the aluminum substrate and any surface layer that may be present, the aluminum workpiece  72  has a thickness  721  that ranges from 0.3 mm to about 6.0 mm, or more narrowly from 0.5 mm to 3.0 mm, at least at the weld site  76 . 
     The aluminum substrate of the aluminum workpiece  72  may be provided in wrought or cast form. For example, the aluminum substrate may be composed of a 4xxx, 5xxx, 6xxx, or 7xxx series wrought aluminum alloy sheet layer, extrusion, forging, or other worked article. Alternatively, the aluminum substrate may be composed of a 4xx.x, 5xx.x, 6xx.x, or 7xx.x series aluminum alloy casting. Some more specific kinds of aluminum alloys that may constitute the aluminum substrate include, but are not limited to, AA5754 and AA5182 aluminum-magnesium alloy, AA6111 and AA6022 aluminum-magnesium-silicon alloy, AA7003 and AA7055 aluminum-zinc alloy, and A1-10Si-Mg aluminum die casting alloy. The aluminum substrate may further be employed in a variety of tempers including annealed (O), strain hardened (H), and solution heat treated (T), if desired. The term “aluminum workpiece” as used herein thus encompasses unalloyed aluminum and a wide variety of aluminum alloys, whether coated or uncoated, in different spot-weldable forms including wrought sheet layers, extrusions, forgings, etc., as well as castings. 
     The steel workpiece  74  includes a steel substrate from any of a wide variety of strengths and grades that is either coated or uncoated. The steel substrate may be hot-rolled or cold-rolled and may be composed of steel such as mild steel, interstitial-free steel, bake-hardenable steel, high-strength low-alloy (HSLA) steel, dual-phase (DP) steel, complex-phase (CP) steel, martensitic (MART) steel, transformation induced plasticity (TRIP) steel, twining induced plasticity (TWIP) steel, and boron steel such as when the steel workpiece  74  includes press-hardened steel (PHS). Preferred compositions of the steel substrate, however, include mild steel, dual phase steel, and boron steel used in the manufacture of press-hardened steel. Those three types of steel have ultimate tensile strengths that, respectively, range from 150 MPa to 500 MPa, from 500 MPa to 1100 MPa, and from 1200 MPa to 1800 MPa. 
     The steel workpiece  74  may include a surface layer on one side or both sides of the steel substrate. If coated, the steel substrate preferably includes a surface layer of zinc (e.g., hot-dip galvanized), a zinc-iron alloy (e.g., galvanneal or electrodeposited), a zinc-nickel alloy (e.g., electrodeposited), 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 on each side of the steel substrate. Taking into account the thickness of the steel substrate and any surface layer that may be present, the steel workpiece  74  has a thickness  741  that ranges from 0.3 mm and 6.0 mm, or more narrowly from 0.6 mm to 2.5 mm, at least at the weld site  76 . The term “steel workpiece” as used herein thus encompasses a wide variety of steel substrates, whether coated or uncoated, of different grades and strengths. 
     When the two workpieces  72 ,  74  are stacked-up for spot welding in the context of a “2T” stack-up embodiment, which is illustrated in  FIGS. 4-5 , the aluminum workpiece  72  and the steel workpiece  74  present the first and second sides  82 ,  84  of the workpiece stack-up  70 , respectively. In particular, the aluminum workpiece  72  includes a faying surface  86  and a back surface  88  and, likewise, the steel workpiece  74  includes a faying surface  90  and a back surface  92 . The faying surfaces  86 ,  90  of the two workpieces  72 ,  74  overlap and confront one another to establish a faying interface  94  that extends through the weld zone  76  and which may optionally encompass an intermediate organic material layer  96  applied between the faying surfaces  86 ,  90 . The back surfaces  88 ,  92  of the aluminum and steel workpieces  72 ,  74 , on the other hand, face away from one another in opposite directions at the weld zone  76  and constitute, respectively, the aluminum workpiece surface  82 ′ and the steel workpiece surface  84 ′ of the first and second sides  82 ,  84  of the workpiece stack-up  70 . 
     The intermediate organic material layer  96  that may be present between the faying surfaces  86 ,  90  of the aluminum and steel workpieces  72 ,  74  may be an adhesive layer that includes a structural thermosetting adhesive matrix. The structural thermosetting adhesive matrix may be any curable structural adhesive including, for example, as a heat-curable epoxy or a heat-curable polyurethane. Some specific examples of heat-curable structural adhesives that may be used as the thermosetting adhesive matrix include DOW Betamate 1486, Henkel Terokal 5089, and Uniseal 2343, all of which are commercially available. Additionally, the adhesive layer may further include optional filler particles, such as silica particles, dispersed throughout the thermosetting adhesive matrix to modify the viscosity or other mechanical properties of the adhesive layer for manufacturing operations. In addition to an adhesive layer, the intervening organic material layer  96  may include other organic material layers such as a sound-proofing layer or an organic sealer, to name but a few other possibilities. 
     The intermediate organic material layer  96 , if present, can be spot welded through at the temperatures and electrode clamping pressures attained at the weld zone  76  during current flow between the spot welding electrodes  10 ,  78 . Under spot welding conditions, the intermediate organic material layer  96  is laterally displaced with the help of the multistep conical geometry of the first spot welding electrode  10  such that very little, if any, organic material is thermally decomposed within the weld zone  76  during current flow so that only minimal, if any, residual materials (e.g., carbon ash, filler particles, etc.) are produced near the faying surface  90  of the steel workpiece  74 . Outside of the weld zone  76 , however, the intermediate organic material layer  96  remains generally undisturbed. Thus, in the case of an adhesive layer, the undisturbed adhesive outside of the weld zone  76  is able to provide additional bonding between the faying surfaces  86 ,  90  of the aluminum and steel workpieces  72 ,  74 . To achieve such additional bonding, the workpiece stack-up  70  may be heated in an ELPO-bake oven or other heating apparatus following spot welding to cure the structural thermosetting adhesive matrix of the adhesive layer that is still intact outside of and around the weld zone(s)  76 . 
     The term “faying interface  94 ” is thus used broadly in the present disclosure and is intended to encompass any overlapping and confronting relationship between the faying surfaces  86 ,  90  of the workpieces  72 ,  74  in which resistance spot welding can be practiced. The faying surfaces  86 ,  90  may, for example, be in direct contact with each other such that they physically abut and are not separated by a discrete intervening material layer (i.e., the intervening organic material layer  96  is not present). As another example, the faying surfaces  86 ,  90  may be in indirect contact with each other such as when they are separated by the intervening organic material layer  96 —and thus do not experience the type of interfacial physical abutment found in direct contact—yet are in close enough proximity to each other that resistance spot welding can still be practiced. This type of indirect contact between the faying surfaces  86 ,  90  of the aluminum and steel workpieces  72 ,  74  typically results when the intermediate organic material layer  96  is applied between the faying surfaces  86 ,  90  to a thickness at least within the weld zone  76  that ranges from 0.1 mm to 2.0 mm or, more narrowly, from 0.2 mm to 1.0 mm. 
     Of course, as shown in  FIGS. 6-7 , the workpiece stack-up  70  is not limited to the inclusion of only the aluminum workpiece  72  and the adjacent steel workpiece  74  as far as the number of workpieces are concerned. The workpiece stack-up  70  may also include at least an additional aluminum workpiece or an additional steel workpiece—in addition to the adjacent aluminum and steel workpieces  72 ,  74 —so long as the additional workpiece is disposed adjacent to the workpiece  72 ,  74  of the same base metal composition; that is, any additional aluminum workpiece is disposed adjacent to the aluminum workpiece  72  opposite the faying interface  94  and any additional steel workpiece is disposed adjacent to the steel workpiece  74  opposite the faying interface  94 . As for the characteristics of the additional workpiece(s), the descriptions of the aluminum workpiece  72  and the steel workpiece  74  provided above are applicable to any additional aluminum or any additional steel workpiece that may be included in the workpiece stack-up  70 . It should be noted, though, that while the same general descriptions apply, there is no requirement that the additional aluminum workpiece(s) and/or the additional steel workpiece(s) be identical in terms of composition, thickness, or form (e.g., wrought or cast) to the aluminum workpiece  72  and the steel workpiece  74 , respectively, that lie next to each other within the workpiece stack-up  70 . 
     As shown in  FIG. 6 , for example, the workpiece stack-up  70  may include the adjacent aluminum and steel workpieces  72 ,  74  described above along with an additional aluminum workpiece  98 . Here, as shown, the additional aluminum workpiece  98  overlaps the adjacent aluminum and steel workpieces  72 ,  74  and lies next to the aluminum workpiece  72 . When the additional aluminum workpiece  98  is so positioned, the back surface  92  of the steel workpiece  74  constitutes the steel workpiece surface  84 ′ that provides the second side  84  of the workpiece stack-up  70 , as before, while the aluminum workpiece  72  that lies adjacent to the steel workpiece  74  now includes a pair of opposed faying surfaces  86 ,  100 . The faying surface  86  of the aluminum workpiece  72  that faces the faying surface  90  of the steel workpiece  74  continues to establish the faying interface  94  between the two workpieces  72 ,  74  as previously described. The other faying surface  100  of the aluminum workpiece  72  overlaps and confronts a faying surface  102  of the additional aluminum workpiece  98 . As such, in this particular arrangement of lapped workpieces  98 ,  72 ,  74 , a back surface  104  of the additional aluminum workpiece  98  now constitutes the aluminum workpiece surface  82 ′ that provides the first side  82  of the workpiece stack-up  70 . 
     In another example, as shown in  FIG. 7 , the workpiece stack-up  70  may include the adjacent aluminum and steel workpieces  72 ,  74  described above along with an additional steel workpiece  106 . Here, as shown, the additional steel workpiece  106  overlaps the adjacent aluminum and steel workpieces  72 ,  74  and lies next to the steel workpiece  74 . When the additional steel workpiece  106  is so positioned, the back surface  88  of the aluminum workpiece  72  constitutes the aluminum workpiece surface  82 ′ that provides the first side  82  of the workpiece stack-up  70 , as before, while the steel workpiece  74  that lies adjacent to the aluminum workpiece  72  now includes a pair of opposed faying surfaces  90 ,  108 . The faying surface  90  of the steel workpiece  74  that faces the faying surface  86  of the aluminum workpiece  72  continues to establish the faying interface  94  between the two workpieces  72 ,  74  as previously described. The other faying surface  108  of the steel workpiece  74  overlaps and confronts a faying surface  110  of the additional steel workpiece  106 . As such, in this particular arrangement of lapped workpieces  72 ,  74 ,  106 , a back surface  112  of the additional steel workpiece  106  now constitutes the steel workpiece surface  84 ′ that provides the second side  84  of the workpiece stack-up  70 . 
     Returning now to  FIG. 4 , the first spot welding electrode  10  and the second spot welding electrode  78  are used to pass electrical current through the workpiece stack-up  70  and across the faying interface  94  of the adjacent aluminum and steel workpieces  72 ,  74  at the weld zone  76  regardless of whether an additional aluminum and/or steel workpiece is present. Each of the spot welding electrodes  10 ,  78  is carried by the weld gun  80 , which may be of any suitable type including a C-type or an X-type weld gun. The spot welding operation may call for the weld gun  80  to be mounted to a robot capable of moving the weld gun  80  around the workpiece stack-up  70  as needed, or it may call for the weld gun  80  to be configured as a stationary pedestal-type in which the workpiece stack-up  70  is manipulated and moved relative to the weld gun  80 . Additionally, as illustrated schematically here, the weld gun  80  may be associated with a power supply  114  that delivers electrical current between the spot welding electrodes  10 ,  78  according to a programmed weld schedule administered by a weld controller  116 . The weld gun  80  may also be fitted with coolant lines and associated control equipment in order to deliver a coolant fluid, such as water, to each of the spot welding electrodes  10 ,  78 . 
     The weld gun  80  includes a first gun arm  118  and a second gun arm  120 . The first gun arm  118  is fitted with a shank  122  that secures and retains the first spot welding electrode  10  and the second gun arm  120  is fitted with a shank  124  that secures and retains the second spot welding electrode  78 . The secured retention of the spot welding electrodes  10 ,  78  on their respective shanks  122 ,  124  can be accomplished by way of shank adapters that are located at axial free ends of the shanks  122 ,  124 . In terms of their positioning relative to the workpiece stack-up  70 , the first spot welding electrode  10  is positioned for contact with the first side  82  of the stack-up  70 , and, consequently, the second spot welding electrode  78  is positioned for contact with the second side  84  of the stack-up  70 . The first and second weld gun arms  118 ,  120  are operable to converge or pinch the spot welding electrodes  10 ,  78  towards each other and to impose a clamping force on the workpiece stack-up  70  at the weld zone  76  once the electrodes  10 ,  78  are brought into contact with their respective workpiece stack-up sides  82 ,  84 . 
     The second spot welding electrode  78  employed opposite the first spot welding electrode  10  can be any of a wide variety of electrode designs. In general, and referring now to  FIGS. 4-5 , the second spot welding electrode  78  includes an electrode body  126 , a weld face  128 , and optionally a transition nose  130  that serves to upwardly displace the weld face  128  from a front end  132  of the electrode body  126 . The weld face  128  is the portion of the second spot welding electrode  78  that makes contact with the second side  84  of the workpiece stack-up  70  opposite the weld face  14  of the first spot welding electrode  10  during spot welding. At least the weld face  128  of the second spot welding electrode  78 , and preferably the entire spot welding electrode  78  including the electrode body  126 , the weld face  128 , and the transition nose  130  if present, is constructed from a material having an electrical conductivity of at least 70% IACS, or more preferably at least 90% IACS, and a thermal conductivity of at least 300 W/mK. Some materials that meet these criteria include a C15000 copper-zirconium (CuZr) alloy, a C18200 copper-chromium (CuCr) alloy, and a C18150 copper-chromium-zirconium (CuCrZr) alloy, and a dispersion-strengthened copper material such as copper with an aluminum oxide dispersion. Other materials not expressly listed here that meet the applicable electrical and thermal conductivity standards may, of course, also be used as well. 
     In a preferred embodiment, the second spot welding electrode  78  is constructed similarly to the first spot welding electrode  10  and, accordingly, the description above regarding the first spot welding electrode  10  and the contents of  FIGS. 1-3  are equally applicable here. In other words, the structure of the electrode body  126 , the weld face  128 , and the optional transition nose  130  of the second spot welding electrode  78  has the same structural features and is consistent with the discussion above regarding the structure of the electrode body  12 , the weld face  14 , and the optional transition nose  24  of the first spot welding electrode  10 . And while the second spot welding electrode  78  can have a similar structure to the first spot welding electrode  10 , the first and second spot welding electrodes  10 ,  78  do not necessarily have to be identical and indistinguishable in every facet. To be sure, the first and second spot welding electrodes  10 ,  78  can share a similar structure, especially if they both employ a multistep conical weld face geometry, while still exhibiting some structural distinctions that fall within the permitted numerical variances detailed herein. 
     In an alternative embodiment, and referring now to  FIG. 11 , the second spot welding electrode  78  may be constructed differently from the first spot welding electrode  10 , most notably in the geometry of its weld face  128 . In particular, the electrode body  126  of the second spot welding electrode  78 , which is preferably cylindrical in shape, has the front end  132  that presents and supports the weld face  128  and a back end  134  that facilitates mounting of the electrode  78  to the weld gun  80 . The front end  132  of the electrode body  126  has a diameter  1321  that lies within the range of 12 mm to 22 mm or, more narrowly, within the range of 16 mm to 20 mm, and the back end  134  of the electrode body  126  has a diameter  1341  that is typically the same as the diameter  1321  of the front end  132 , particularly if the electrode body  126  is shaped as a cylindrical. Moreover, the back end  134  of the electrode body  126  defines an opening  136  to an internal recess  138  for insertion of, and attachment with, an electrode mounting device, such as a shank adapter (not shown), that can secure the spot welding electrode  78  to the second gun arm  120  of the weld gun  80  and also enable a flow of cooling fluid (e.g., water) through the internal recess  138  in order to manage the temperature of the electrode  78  during spot welding operations. 
     The weld face  128  may be upwardly displaced from the front end  132  of the electrode body  126  by the transition nose  130  or it may transition directly from the front end  132  (a “full face electrode”). When the transition nose  130  is present, the weld face  128  may be upwardly displaced from the front end  132  by a distance  140  that preferably lies between 2 mm to 10 mm. The transition nose  130  may be frustoconical or truncated spherical in shape, although other shapes are certainly possible. If frustoconical, an angle of truncation  142  of the nose  130  is preferably between 15° and 40° from a horizontal plane at the intersection of the nose  130  and the weld face  128 . If truncated spherical, the radius of curvature of the nose  130  is preferably between 6 mm and 12 mm. 
     A broad range of electrode weld face designs may be implemented for the second spot welding electrode  78 . The weld face  128 , for example, may have a diameter  144  that ranges from 3 mm to 16 mm or, more narrowly, from 4 mm to 8 mm, and may include a base weld face surface  146  that is either planar or convexly domed. If convexly domed, the base weld face surface  146  ascends upwardly and inwardly from its outer perimeter. In one embodiment, for example, the base weld face surface  146  may be spherically domed and have a radius of curvature that ranges from 15 mm to 400 mm or, more narrowly, from 25 mm to 100 mm. Moreover, the base weld face surface  146  may be smooth, roughened, or may include a series of upstanding concentric rings of circular ridges such as the ridges disclosed in U.S. Pat. Nos. 8,222,560; 8,436,269; 8,927,894; or in U.S. Pat. Pub. No. 2013/0200048. Several specific examples of additional weld face designs that may be employed on the second spot welding electrode  78  are a weld face having a smooth, 25-mm radius spherically domed base weld face surface  146  or a 25-mm radius spherically domed base weld face surface  146  with anywhere from three to eight concentric circular rings of ridges that project outwardly from the base weld face surface  146 . The ridges may have heights in the range of 20 μm to 400 μm and have blunted cross-sectional profiles while being radially spaced apart (midpoint to midpoint of adjacent ridges) on the base weld face surface  146  by a distance that ranges from 50 μm to 1800 μm. 
     The power supply  114  that delivers electrical current for passage between the first and second spot welding electrodes  10 ,  78  during spot welding of the workpiece stack-up  70  is preferably a medium-frequency direct current (MFDC) inverter power supply that electrically communicates with the spot welding electrodes  10 ,  78 . A MFDC power supply generally includes an inverter and a MFDC transformer. Such a transformer is commercially available from a number of suppliers including ARO Welding Technologies (US headquarters in Chesterfield Township, Mich.), RoMan Manufacturing Incorporated (US headquarters in Grand Rapids, Mich.) and Bosch Rexroth (US headquarters in Charlotte, N.C.). The MFDC inverter power supply is configured to pass direct current (DC) between the spot welding electrodes  10 ,  78  at current levels up to 50 kW. Other types of power supplies may certainly be used to conduct the disclosed method despite not being expressly identified here. 
     The power supply  114  is controlled by the weld controller  116  in accordance with programmed weld schedule tailored to carry out spot welding of the workpiece stack-up  10 . The weld controller  116  interfaces with the power supply  114  and allows a user or operator to set the waveform of the electrical current being passed between the spot welding electrodes  10 ,  78  in order to initiate and grow a molten aluminum weld pool that ultimately solidifies into a weld joint that weld bonds the aluminum and steel workpieces  72 ,  74  together at the weld zone  76 . Indeed, the weld controller  116  allows for customized control of the current level at any given time and the duration of current flow at any given current level, among others, and further allows for such attributes of the electrical current to be responsive to changes in very small time increments down to fractions of a millisecond. 
     The resistance spot welding method will now be described with reference to  FIGS. 4 and 8-10 , which depict only the aluminum and steel workpieces  72 ,  74  that overlap and lie adjacent to one another so as to establish the faying interface  94 . The presence of additional workpieces in the workpiece stack-up  70  including, for example, the additional aluminum or steel workpieces  98 ,  106  described above, does not affect how the spot welding method is carried out or have any substantial effect on the joining mechanism that takes place at the faying interface  94  of the adjacent aluminum and steel workpieces  72 ,  74 . The more-detailed discussion provided below thus applies equally to instances in which the workpiece stack-up  70  is a “3T” stack-up that includes the additional aluminum workpiece  98  ( FIG. 6 ) or the additional steel workpiece  106  ( FIG. 7 ), as well as “4T” stack-ups, despite the fact that those additional workpieces are not illustrated in  FIGS. 4 and 8-10 . 
     The disclosed method involves first assembling, if needed, the workpiece stack-up  70  including the pair of adjacent aluminum and steel workpieces  72 ,  74  together with the optional intermediate organic material layer  96  that extends through the weld zone  76  over a broader joining region. Once assembled, the workpiece stack-up  70  is positioned between the first spot welding electrode  10  and the opposed second spot welding electrode  78 . The weld face  14  of the first spot welding electrode  10  is positioned to contact the aluminum workpiece surface  82 ′ of the first side  82  of the workpiece stack-up  70  and the weld face  128  of the second spot welding electrode  78  is positioned to contact the steel workpiece surface  84 ′ of the second side  84  of the stack-up  70 . The weld gun  80  is then operated to converge the first and second spot welding electrodes  10 ,  78  relative to one another so that their respective weld faces  14 ,  128  are pressed against the opposite first and second sides  82 ,  84  of the stack-up  70  at the weld zone  76 . The weld faces  14 ,  128  are typically facially aligned with each other at the weld zone  76  under a clamping force imposed on the workpiece stack-up  70  that ranges from 400 lb (pounds force) to 2000 lb or, more narrowly, from 600 lb to 1300 lb. 
     As a function of the multistep conical geometry of the weld face  14  of the first spot welding electrode  10 , the pressure exerted by the first spot welding electrode  10  is initially concentrated and directed through the top plateau surface  42  of the central plateau  38  onto a corresponding limited area of the first side  82  of the workpiece stack-up  70 , as illustrated in  FIG. 8 . The focused direction of the clamping pressure through a limited area stresses and distorts the faying surfaces  86 ,  90  of the aluminum and steel workpieces  72 ,  74  together at the middle of the weld zone  76  and, furthermore, drives lateral displacement of the intermediate organic material layer  96 , if present, along the faying interface  94  and outside of at least a central area  148  of the weld zone  76 . Such lateral displacement of the intermediate organic material layer  96  (if present) substantially clears the organic material from at least the central area  148 , which may be between 2 mm and 4 mm in diameter, leaving behind only minimal organic material of less than 0.1 mm in thickness, if any. 
     In those instances in which the intermediate organic material layer  96  is present between the faying surfaces  86 ,  90  of the aluminum and steel workpieces  72 ,  74 , a preliminary electrical current ranging between 3 kA rms and 15 kA rms over the preheating time period may be passed between the first and second spot welding electrodes  10 ,  78  and through the workpiece stack-up  10  while pressing the welding electrodes  10 ,  78  against the opposite sides  82 ,  84  of the stack-up  70 . Passage of the preliminary electrical current heats the faying interface  94 , and thus the intermediate organic material layer  96 , without melting the aluminum workpiece  72 . Such preheating renders the intermediate organic material layer  96  less viscous and more compliant without curing or thermally decomposing the layer  96 . While preheating of the intermediate organic material layer  96  during passage of the preliminary electrical current is subject to some variance, a preferred temperature that achieves good flowability, particularly if the layer  96  contains a structural thermosetting adhesive matrix, is between 100° C. and 150° C. or, more narrowly, between 120° C. and 140° C. The preheating of the intermediate organic material layer  96  with the preliminary electrical current, in conjunction with initially directing the pressure exerted by the first spot welding electrode  10  through the central plateau  38 , may laterally displace and substantially clear the intermediate organic material layer  96  over a larger area than using only the clamping pressure of the spot welding electrodes  10 ,  78 . 
     After the spot welding electrodes  10 ,  78  are pressed against their respective sides  82 ,  84  of the workpiece stack-up  10 , and the optional passage of the preliminary electrical current has been carried out, an electrical current is passed between the facially-aligned weld faces  14 ,  128  of the first and second spot welding electrodes  10 ,  78  to form a weld joint  150  ( FIG. 10 ). The exchanged electrical current may be constant or pulsed over time, or some combination of the two, and typically has a current level that ranges from 5 kA and 50 kA and lasts for a total duration of 40 ms to 4,000 ms. As a few specific examples, the schedule of the applied electrical current may be in the nature of the multi-step schedules disclosed in US2015/0053655 and US2017/0106466, the entire contents of each of those applications being incorporated herein by reference, or another weld schedule that is suitable for the workpiece stack-up  70 . 
     Referring now to  FIG. 9 , the electrical current flowing between the first and second spot welding electrodes  10 ,  78  heats the more electrically- and thermally-resistive steel workpiece  74  quite rapidly. This heat is transferred to the aluminum workpiece  72  and causes the aluminum workpiece  72  to begin to melt within the weld zone  76 . The melting of the aluminum workpiece  72  creates a molten aluminum weld pool  152 . The molten aluminum weld pool  152  wets the adjacent faying surface  90  of the steel workpiece  74 . And since only a minimal amount, if any, of the intermediate organic material layer  96  remains within the central area  148  of the weld zone  76  when electrical current flow is commenced, if the intermediate organic material layer  96  was originally applied in the first place, the interactions that would transpire between the residual oxide film (if present) and the thermal decomposition residues from the organic material layer  96  are not nearly as prevalent as they would otherwise be when using conventional spot welding practices. The avoidance of such interactions and the resulting formation of a tougher, more tenacious composite residue film helps maintain the wettability of the faying surface  90  of the steel workpiece  74 . 
     During the time the molten aluminum weld pool  152  is growing within the aluminum workpiece  72  to is final size, the weld face  14  of the first spot welding electrode  10  impresses further into the first side  82  of the workpiece stack-up  70 , which successively brings the one or more annular steps  40  into pressed contact with the first side  82 . The pressure exerted on the first side  82  of the workpiece stack-up  10  by each additional annular step  40  that is brought to bear against with the first side  82  may contribute to further laterally displacement the intermediate organic material layer  96  beyond that which was previously achieved prior to electrical current flow and melting of the aluminum workpiece  72 . In addition to laterally displacing the intermediate organic material layer  96 , the continued impression or indentation of the weld face  14  into the aluminum workpiece  72  causes the molten aluminum weld pool  152  to flow laterally and increase in diameter along the faying surface  90  of the steel workpiece  74 . This effect is enhanced at the center of the molten aluminum weld pool  152  by the central plateau  38 , which extends further into the weld pool  152  than any other portion of the weld face  14 . The multistep conical geometry of the weld face  14  thus has the added function of enticing lateral movement of the molten aluminum weld pool  152  and, consequently, sweeping residual oxide film and/or composite residue film that may be present, if any, away from the interface of the molten aluminum weld pool  152  and the faying surface  90  of the steel workpiece  74  and outside of the weld zone  76 . 
     The continued impression of the weld face  14  of the first spot welding electrode  10  eventually contains the molten aluminum weld pool  152  within the outer weld face diameter  32 . The molten aluminum weld pool  152  may have a diameter along the faying surface of the steel workpiece  74  that ranges from 3 mm to 15 mm, or more narrowly from 6 mm to 10 mm, and may penetrate a distance into the aluminum workpiece  72  that ranges from 20% to 100% of the thickness  721  of the aluminum workpiece  72  at the weld site  76 . And, in terms of its composition, the molten aluminum weld pool  152  is composed predominantly of aluminum material derived from the aluminum workpiece  72 . The passage of the electrical current between the weld faces  14 ,  128  of the first and second spot welding electrodes  10 ,  78  is eventually terminated, thereby allowing the molten aluminum weld pool  152  to solidify into the weld joint  150  as depicted in  FIG. 10 . The weld joint  150  is the material that weld bonds the adjacent aluminum and steel workpieces  72 ,  74  together. In particular, the weld joint  150  establishes a bonding interface  154  with the faying surface  90  of the steel workpiece  74  and includes two main components: (1) an aluminum weld nugget  156  and (2) a Fe—Al intermetallic layer  158 . In general, the bonding interface  154  of the formed weld joint  150  and the steel workpiece  74  is expected to be largely free of contaminating material derived from the thermal decomposition of the intermediate organic material layer  96  if such a layer is originally present between the aluminum and steel workpieces  72 ,  74 . And, if desired, portions of the weld joint  150  may be re-melted and re-solidified numerous times for the reasons provided in US2017/0106466. 
     The aluminum weld nugget  156  is comprised of resolidified aluminum and extends into the aluminum workpiece  72  to a distance that ranges from 20% to 100% of the thickness  721  of the aluminum workpiece  72  at the weld zone  76 . The Fe—Al intermetallic layer  158  is situated between the aluminum weld nugget  156  and the faying surface  90  of the steel workpiece  74  and is contiguous with the bonding interface  154 . The Fe—Al intermetallic layer  158  is produced due to a reaction between the molten aluminum weld pool  152  and iron that diffuses from the steel workpiece  74  at spot welding temperatures, and typically comprises FeAl 3  compounds, Fe 2 Al 5  compounds, and possibly other Fe—Al intermetallic compounds as well. The Fe—Al intermetallic layer  158  is harder and more brittle than the aluminum weld nugget  156  and often has an average thickness of 1 μm to 7 μm along the bonding interface  154  of the weld joint  150  and the steel workpiece  74 . 
     The Fe—Al intermetallic layer  158  is less liable to compromise the strength and mechanical properties of the weld joint  150  after performing the disclosed spot welding method. Indeed, the removal of the intermediate organic material layer  96 , if originally present, from within the weld zone  76  as aided by the multistep conical geometry of the weld face  14  of the first welding electrode  10  effectively minimizes or altogether eliminates the thermal decomposition residues from the layer  96  that can lead to near-interface defects within the brittle Fe—Al intermetallic layer  158 . Moreover, in the event that some quantity of thermal decomposition residues derived from the intermediate organic material layer  96  remain within the weld zone  76  and are exposed to the molten aluminum weld pool  152 , lateral flow of the molten aluminum weld pool  152  as induced by the multistep conical weld face geometry of the first spot welding electrode  12  can sweeps those residues away from the weld zone  76  and the bonding interface  154  to further improve the mechanical performance of the solidified weld joint  150 . In that regard, the wide-spread distribution of weld joint disparities that has been found to frequently occur in conventional spot welding practices when an intermediate organic material is present is, at the very least, not as prevalent when spot welding is conducted according to the presently disclosed method. 
     After the disclosed spot welding method is completed, and the weld joint  150  is formed so as to weld bond the aluminum and steel workpieces  72 ,  74  together, the clamping force imposed on the workpiece stack-up  70  at the weld zone  76  is relieved and the first and second spot welding electrodes  10 ,  78  are retracted away from their respective workpiece sides  82 ,  84 . The workpiece stack-up  70  may now be moved relative to the weld gun  80  so that the first and second spot welding electrodes  10 ,  78  are positioned in facing alignment at another weld zone  76  where the disclosed method is repeated. Once the desired number of resistance spot weld joints  150  has been formed on the workpiece stack-up  70 , which typically ranges anywhere from 1 to 50, the stack-up  70  may be subject to further processing if appropriate. For example, if an uncured yet heat-curable adhesive layer is applied between the aluminum and steel workpieces  72 ,  74  prior to spot welding, the workpiece stack-up  70  may be heated to cure the heat-curable adhesive layer that remains intact outside of the weld zone  76  of each weld joint  150 , but within the adhesive coated joining region(s) of the stack-up  70 , to attain additional adherent adhesive bonding between the faying surfaces  86 ,  90  of the aluminum and steel workpieces  72 ,  74 . The requisite heating of the workpiece stack-up  70  may be performed in an ELPO-bake oven, furnace, or other heating apparatus. 
     The above description of preferred exemplary embodiments and specific examples are merely descriptive in nature; they are not intended to limit the scope of the claims that follow. Each of the terms used in the appended claims should be given its ordinary and customary meaning unless specifically and unambiguously stated otherwise in the specification.