Abstract:
A programmable polarity module that permits rapid on-demand control of the polarities assigned to the welding electrodes retained on a welding gun is disclosed. The programmable polarity module is electrically connectable to the welding gun and a direct current power supply unit to provide direct current to the welding electrodes for exchange during spot welding. A first interchangeable polarity output lug and a second interchangeable polarity output lug of the programmable polarity module permit the polarities of the welding electrodes to be switched without having to electrically disconnect the module from the welding gun.

Description:
[0001]    This application claims the benefit of U.S. provisional patent application No. 61/774,227, filed on Mar. 7, 2013, the entire contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The technical field of this disclosure relates generally to a programmable polarity module that can be connected to a direct current (“DC”) power supply for a resistance spot welding gun. The programmable polarity module allows the polarity of the welding gun&#39;s electrodes to be controlled, as needed, to best accommodate the resistance spot welding process being practiced at the time. 
       BACKGROUND 
       [0003]    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 pre-fabricated sheet metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among others. A number of spot welds are typically formed along a peripheral edge of the metal workpieces or some other bonding region to ensure the part is structurally sound. The most common metal workpieces used today in the automotive industry are those made of steel and an aluminum alloy. The desire to incorporate aluminum alloys into a vehicle has made it enviable to spot weld an aluminum alloy workpiece to another aluminum alloy workpiece or, alternatively, to a steel workpiece. 
         [0004]    The resistance spot welding process is performed by an automated robotic or pedestal welding gun that includes two arms. Each of these arms holds a welding electrode typically comprised of a suitable copper alloy. The welding gun arms can be positioned on opposite sides of a workpiece stack-up and clamped to press the two electrodes against their respective metal workpieces at diametrically common spots. A momentary electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of electrical current through the metal workpieces and their faying interface (i.e., the contacting interface of the metal workpieces) generates heat at the faying interface. This heat forms a molten weld pool which, upon stoppage of the current flow, solidifies into a weld nugget. After the spot weld is formed, the welding arms release their clamping force, and the spot welding process is repeated at another weld site. 
         [0005]    The electric current that is passed between the opposed electrodes and through the metal workpieces is received from a DC power supply carried by the welding gun. The DC power supply may, for example, be a medium-frequency integrated transformer and rectifier package configured to deliver high DC amperage in accordance with a specified weld schedule. This type of DC power supply, and other similar types as well, furnishes the opposed electrodes with fixed opposite polarities when electrically connected to the welding gun; that is, after the DC power supply has been installed, one electrode is always the positive electrode and the other is always the negative electrode. 
         [0006]    The polarity assigned to the welding electrodes is not inconsequential. It has been found, for instance, that a polarity bias exists when spot welding (1) an aluminum alloy workpiece to another aluminum alloy workpiece, and (2) an aluminum alloy workpiece to a steel workpiece. A less pronounced polarity bias also exists when spot welding a steel workpiece to another steel workpiece and in certain practices of projection welding. The ability to control which electrode has the positive/negative polarity while the welding gun and the DC power supply remain electrically connected—including the ability to switch electrode polarities at any time—would permit more operationally effective spot welding practices to be developed in at least these instances, and possibly others. Such electrode polarity control cannot be achieved with conventional DC power supplies. In fact, when a conventional DC power supply is employed, the only way to change the polarity of the electrodes is to physically disconnect the power supply from the welding gun, and then re-connect the power supply in reverse polarity orientation, which is a time-consuming and laborious process. 
       SUMMARY 
       [0007]    A programmable polarity module that permits rapid on-demand control of the polarities assigned to the welding electrodes retained on a welding gun is disclosed. The programmable polarity module is electrically connectable to the fixed polarity output lugs of a DC power supply in any known fashion to provide a multi-component DC power supply unit. It is also electrically connectable to the welding gun, and thus the welding electrodes, by way of a first interchangeable polarity output lug and a second interchangeable polarity output lug. The first and second interchangeable polarity output lugs can assign either a positive polarity or a negative polarity to their associated welding electrodes. 
         [0008]    Each of the first and second interchangeable polarity output lugs is associated with a pair of high-amperage silicon controlled rectifiers (SCR&#39;s). Within each pair of SCRs, one SCR is associated with a positive polarity and the other SCR is associated with a negative polarity. The pairs of SCR&#39;s can thus be controlled to assign each interchangeable output polarity lug—and the welding electrode associated with each lug—with a positive polarity or a negative polarity. This type of control permits the polarity designations of the two welding electrodes to be dictated in any conceivable way so that the particulars of a variety of spot welding processes can be accommodated. The polarity of each welding electrode can even be rapidly switched without having to disconnect the DC power supply from the welding gun. 
         [0009]    The term “high-amperage silicon controlled rectifier” and its abbreviation, “SCR,” as used herein, are meant to broadly encompass a single thyristor or an arrangement of one or more thyristors that act in tandem. Thyristors are electrical switching devices that include alternating p-type and n-type semiconductor layers that can be controlled to permit or block current flow based on the magnitude (or lack thereof) of a voltage applied to a control terminal (also known as a gate). The number of thyristors employed in each SCR depends on the magnitude of the current that needs to be managed through the first and second interchangeable polarity output lugs. For example, each SCR in the pairs of SCR&#39;s associated with the first and second interchangeable polarity output lugs may be a single thyristor or, if the current capacity of a single thyristor is not sufficient for whatever reason, an arrangement of several thyristors connected in parallel that, together, can accommodate the magnitude of the current that needs to be controlled. 
         [0010]    The programmable polarity module can be used to cure the effects of an electrode polarity bias that exists within a resistance spot welding process. For example, when spot welding an aluminum alloy workpiece to another aluminum alloy workpiece with a pair of copper alloy electrodes, the current exchanged between the welding electrodes may create a heat differential at the electrode/workpiece interfaces due to the flow of electrons across the aluminum alloy-copper alloy junctions. Specifically, more heat may be generated at the positive welding electrode than at the negative welding electrode, which causes the positive welding electrode to wear at a faster rate. The programmable polarity module could be used here to switch the polarities of the two electrodes every so often, preferably after every spot weld, to keep one electrode from wearing faster than the other. 
         [0011]    As another example, an electrode polarity bias may exist when spot welding dissimilar metal workpieces with a pair of copper alloy electrodes. The dissimilar metal workpieces may be a pair of aluminum alloy sheet metal layers of considerably different thicknesses, or an aluminum alloy sheet metal layer and an aluminum alloy casting, or an aluminum alloy workpiece and a steel workpiece, to name but a few. The spot welding of such dissimilar metal workpieces, like before, may create a heat imbalance at the electrode/workpiece interfaces in which more heat is generated at the positive welding electrode and less heat is generated at the negative welding electrode. Better quality spot welds can generally be achieved by using this heat differential to offset differences in the electrical conductivities and/or the melting points of the dissimilar metal workpieces. The programmable polarity module could be used here to ensure that the welding electrodes are assigned the polarity that results in the best weld quality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a generalized illustration of a welding gun for use in resistance spot welding applications; 
           [0013]      FIG. 1A  is a magnified view of the encircled portion of  FIG. 1  identified as  1 A; 
           [0014]      FIG. 2  is a generalized illustration of a welding electrode that can be used to perform resistance spot welding; 
           [0015]      FIG. 3  is a generalized illustration of a DC power supply unit, which includes a DC power supply and a programmable polarity module, that can be carried by the welding gun shown in  FIG. 1 ; 
           [0016]      FIG. 4  is a schematic illustration of the programmable polarity module illustrated in  FIG. 3 ; 
           [0017]      FIG. 5  is a picture of a welding electrode that has been provided with a negative polarity during repeated spot welding of an aluminum alloy sheet metal layer to another aluminum alloy sheet metal layer; 
           [0018]      FIG. 6  is a picture of a welding electrode that has been provided with a positive polarity during repeated spot welding of an aluminum alloy sheet metal layer to another aluminum alloy sheet metal layer; 
           [0019]      FIG. 7  is a cross-sectional photomicrograph of a resistance spot weld formed between an aluminum alloy sheet metal layer and a steel sheet metal layer in which the welding electrode that engaged the aluminum alloy sheet metal layer had the negative polarity; 
           [0020]      FIG. 8  is a cross-sectional photomicrograph of a resistance spot weld formed between an aluminum alloy sheet metal layer and a steel sheet metal layer in which the welding electrode that engaged the aluminum alloy sheet metal layer had the positive polarity; 
           [0021]      FIG. 9  is a cross-sectional photomicrograph of a resistance spot weld formed between an aluminum alloy sheet metal layer and an aluminum alloy casting in which the welding electrode that engaged the aluminum alloy casting had the negative polarity; and 
           [0022]      FIG. 10  is a cross-sectional photomicrograph of a resistance spot weld formed between an aluminum alloy sheet metal layer and an aluminum alloy casting in which the welding electrode that engaged the aluminum alloy casting had the positive polarity. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIGS. 1-1A  generally depict a welding gun  10  that can be used to resistance spot weld a metal workpiece stack-up  12  at one or more predetermined spot weld sites  14 . The workpiece stack-up  12  includes, for example, a first metal workpiece  16  and a second metal workpiece  18 . These metal workpieces  16 ,  18  overlap one another to provide a faying interface  20  at the weld site  14  where the spot welding process forms a weld nugget  22  that metallurgically joins the metal workpieces  16 ,  18  together. The term faying interface  20 , as used herein, encompasses instances of direct overlapping contact between the workpieces  16 ,  18  as well as instances where the workpieces  16 ,  18  may not be touching, but are nonetheless overlapping in close proximity to one another, such as when a thin layer of adhesive, sealer, or some other intermediate material is present. Each of the first and second metal workpieces  14 ,  16  may have a thickness  160 ,  180  that ranges from about 0.3 mm to about 6.0 mm, and preferably ranges from about 0.6 mm to about 3.0 mm, at the weld site  14 . 
         [0024]    At least one of the first or second metal workpieces  16 ,  18  is composed of an aluminum alloy. The aluminum alloy may be an aluminum-magnesium alloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy, or an aluminum-zinc alloy. Some specific aluminum alloys of this kind are 5754 aluminum-magnesium alloy, 6022 aluminum-magnesium-silicon alloy, and 7003 aluminum-zinc alloy. The other of the first or second metal workpieces  16 ,  18  may be composed of an aluminum alloy, like the ones just mentioned, or it may be composed of steel. The steel may be a low carbon steel, a galvanized low carbon steel, or a galvanized advanced high strength steel (AHSS). Some specific steels of this kind include interstitial-free (IF) steel, dual-phase (DP) steel, transformation-induced plasticity (TRIP) steel, and press-hardened steel (PHS). The term “metal workpiece” and its aluminum alloy and steel variations are used broadly in the present disclosure to include a sheet metal layer, a casting, an extrusion, and other aluminum alloy or steel pieces that are resistance spot weldable. 
         [0025]    It should be noted that the weld nugget  22  shown in  FIG. 1A  is a generic illustration that is meant to be representative of the wide variety of weld nugget compositions—and weld nugget locations—that can be formed at the faying interface  20  of first and second metal workpieces  16 ,  18 . For example, if the first and second metal workpieces  16 ,  18  are aluminum alloy sheet metal layers, the weld nugget formed at the faying interface of the two layers will penetrate into each layer to some extent. A typical penetration depth of the weld nugget into each aluminum alloy sheet metal layer is about 10% to about 80% of the thickness of the layer. As another example, if the first metal workpiece  16  is a steel sheet metal layer and the second metal workpiece  18  is an aluminum alloy sheet metal layer, the weld nugget formed at the faying interface of the two layers will penetrate mainly into the aluminum alloy sheet metal layer, primarily because aluminum alloys melt at a significantly lower temperature than steel. The weld nugget  22  depicted in  FIG. 1A  is thus intended to represent a more inclusive variety of weld nugget and weld nugget locations than what is generically shown, including the specific examples described here as well as any others that may be formed between the different combinations of workpiece materials that can be employed. 
         [0026]    The welding gun  10  includes a first gun arm  24  and a second gun arm  26 . The first gun arm  24  includes a shank  28  that retains a first welding electrode  30 . Likewise, the second gun arm  26  includes a shank  32  that retains a second welding electrode  34 . The first and second gun arms  24 ,  26  may be stationary (pedestal welder) or robotically moveable, as is customary in the art, and are operated during spot welding to press the first and second welding electrodes  30 ,  34  against oppositely-facing surfaces  36 ,  38  of the first and second metal workpieces  16 ,  18  in diametric alignment with one another at the weld site  14 . The clamping force assessed by the gun arms  24 ,  26  establishes good mechanical and electrical contact between the welding electrodes  30 ,  34  and their respective engaged metal workpiece surfaces  36 ,  38 . 
         [0027]    The first and second welding electrodes  30 ,  34  are preferably water-cooled copper alloy welding electrodes that include a body  40  and a weld face  42  at the end of the body  40 , as illustrated in  FIG. 2 . The weld face  42  is the part of the electrode that contacts the surface  36 ,  38  of the metal workpiece  16 ,  18  being engaged by the electrode  30 ,  34 . And it may incorporate any of a wide variety of designs that are suitable for spot welding an aluminum alloy workpiece or a steel workpiece. If the welding electrode  30 ,  34  is intended to engage an aluminum alloy workpiece, for example, the weld face  42  is preferably domed, as shown in  FIG. 2 , and may further be smooth, textured, or include surface features such as protruding ringed ridges. Some examples of these types of copper alloy welding electrodes are described in U.S. Pat. Nos. 6,861,609, 8,222,560, 8,274,010, and 8,436,269, and U.S. Patent Application Publication No. 2009/0255908. If the welding electrode  30 ,  34  is intended to engage a steel workpiece, the weld face  42  is preferably flat or domed as has long been known in the art. An electrode weld face design that may be used to weld both aluminum alloy and steel workpieces is described in U.S. Pat. No. 8,525,066. 
         [0028]    A DC power supply unit  44 , as shown best in  FIG. 3 , is carried by the welding gun  10 . The DC power supply unit  44  supplies a direct current for passage between the welding electrodes  30 ,  34  when they are pressed against the oppositely-facing surfaces  36 ,  38  of their respective metal workpieces  16 ,  18 . This supplied DC is sufficient to initiate a weld pool at the faying interface  20  according to a defined weld schedule. Additionally, the DC power supply unit  44  allows the polarity designations of the first and second welding electrodes  30 ,  34  to be controlled. And it can perform these functions while remaining installed on the welding gun  10 . The DC power supply unit  44  includes, as shown, a DC power supply  46  and a programmable polarity module  48 . 
         [0029]    The DC power supply  46  is configured to receive, for example, an input single-phase medium frequency (˜1000 Hz) alternating current (AC) from a weld control (not shown), and to convert that input AC into a higher-amperage welding DC, typically between about 5 kA and about 65 kA, that is supplied to the welding gun  10 . The DC power supply  44  may be any known type that is suitable for conducting resistance spot welding. For example, as shown in  FIG. 3 , the DC power supply  44  may be a water-cooled, medium-frequency DC power supply that includes, as an integrated package, a transformer  50  and a rectifier  52 . This type of DC power supply is commercially available from a number of suppliers including ARO Welding Technologies (US headquarters in Chesterfield Township, MI) and Bosch Rexroth (US headquarters in Charlotte, N.C.). Other types of DC power supplies may of course be used, including those configured to receive a single-phase 60 Hz AC. 
         [0030]    The transformer  50  receives the input AC at an input port  54 . The input AC is fed through a primary winding and is “stepped down” to create a lower voltage, higher amperage secondary AC in a secondary winding. This secondary AC is then fed to the rectifier  52  where a collection of semiconductor diodes converts it into the welding DC. The rectifier  52  includes a fixed positive polarity output lug  56  and a fixed negative polarity output lug  58  that are composed of copper, a copper alloy, or some other highly electrically conductive material. These output lugs  56 ,  58  deliver the welding DC from the rectifier  52 . Skilled artisans will know and understand the function and operation of the transformer  50  and the rectifier  52  and, as such, a more comprehensive description of these two components and their integration into a single package need not be provided here. 
         [0031]    The programmable polarity module  48  is electrically connectable to the rectifier  52  of the DC power supply  46 . Here, as shown in  FIGS. 1 and 3 , the programmable polarity module  48  includes a pair of fixed polarity input lugs  60 . Each of these input lugs  60  is composed of copper, a copper alloy, or some other highly electrically conductive material. One of the input lugs  60  is electrically connectable to the positive polarity output lug  56  of the rectifier  52  and the other is electrically connectable to the negative polarity output lug  58  of the rectifier  52 . When electrically connected, as is the case in  FIG. 1 , the input lugs  60  assume the polarity of whichever output lug  56 ,  58  they are associated with—i.e., the input lug  60  connected to the fixed positive polarity output lug  56  is afforded a positive polarity (designated positive lug  602 ) and the input lug  60  connected to the fixed negative polarity output lug  58  is afforded a negative polarity (designated negative lug  604 ). Bolting or any other suitable type of connection features may be used to physically fasten the programmable polarity module  48  and the DC power supply  46  together. 
         [0032]    The programmable polarity module  48  is also electrically connectable to the welding gun  10  so that the welding DC can be delivered to the welding electrodes  30 ,  34 . The programmable polarity module  48  may include a first interchangeable polarity output lug  62  and a second interchangeable polarity output lug  64  to facilitate such a connection. The first interchangeable polarity output lug  62  is electrically connectable to a first bus bar  66  and the second interchangeable polarity output lug  64  is electrically connectable to a second bus bar  68 . The first and second bus bars  66 ,  68  are composed of copper, a copper alloy, or some other highly electrically conductive material, and are configured to electrically communicate with the first and second gun arms  24 ,  26  and ultimately the first and second welding electrodes  30 ,  34 , respectively. Like before, bolting or any other suitable type of connection features may be used to physically fasten the programmable polarity module  48  and the welding gun  10  together. 
         [0033]    The polarities of the first and second interchangeable polarity output lugs  62 ,  64  are not fixed; rather, they can be switched between positive or negative at any time in accordance with any conceivable welding plan. The ability to switch the polarity of the first and second interchangeable polarity output lugs  62 ,  64  ultimately permits the polarities of the first and second welding electrodes  30 ,  34  to be switched in a corresponding way. This is because the designated polarity of the first and second interchangeable polarity output lugs  62 ,  64  establishes a matching polarity of the first and second welding electrodes  30 ,  34 . For instance, if the first interchangeable polarity output lug  62  is designated positive and, consequently, the second interchangeable polarity output lug  64  is designated negative, then the first and second welding electrodes  30 ,  34  will be designated positive and negative, respectively, until the polarities of the lugs  62 ,  64  are switched. And when the polarities of the output lugs  62 ,  64  are switched, the polarities of the welding electrodes  30 ,  34  will be switched as well in the same way. 
         [0034]    A circuit design that may be incorporated into the programmable polarity module  48  to switch the polarities of the first and second interchangeable polarity output lugs  62 ,  64  is shown schematically in  FIG. 4 . As shown, a first pair  70  of SCR&#39;s (silicon controlled rectifiers) is associated with the first interchangeable polarity output lug  62  and a second pair  72  of SCR&#39;s is associated with the second interchangeable polarity output lug  64 . The first pair  70  of SCR&#39;s includes a forward positive polarity SCR  74  and a reverse negative polarity SCR  76 . Similarly, the second pair  72  of SCR&#39;s includes a forward negative polarity SCR  78  and a reverse positive polarity SCR  80 . The forward positive polarity SCR  74  and the reverse positive polarity SCR  80  are associated with one of the fixed polarity input lugs  60 , and the reverse negative polarity SCR  76  and the forward negative polarity SCR  78  are associated with the other input lug  60 . It should be reiterated that, even though  FIG. 4  shows the several SCR&#39;s  74 ,  76 ,  78 ,  80  as a single thyristor, each of the forward positive polarity SCR  74 , the reverse negative polarity SCR  76 , the forward negative polarity SCR  78 , and the reverse positive polarity SCR  80  may also be an arrangement of one or more thyristors connected in parallel such that they act in tandem to achieve the same cumulative function as a single thyristor would, but with the added possibility of greater current capacity. 
         [0035]    Each of the SCR&#39;s  74 ,  76 ,  78 ,  80  includes a gate  740 ,  760 ,  780 ,  800 . These gates  740 ,  760 ,  780 ,  800  can be controlled to turn their respective SCR&#39;s  74 ,  76 ,  78 ,  80  “on” (gated)—which means current can flow through the SCR—or “off” (ungated)—which means current cannot flow through the SCR. Whether the SCR&#39;s  74 ,  76 ,  78 ,  80  are turned “on” or “off” depends on whether a voltage is applied to their gates  740 ,  760 ,  780 ,  800  that meets or exceeds a gate voltage, which is typically anywhere between about 1V-10V. To turn any of the SCR&#39;s  74 ,  76 ,  78 ,  80  “on,” and to thus permit current flow, a voltage is applied to the relevant gate  740 ,  760 ,  780 ,  800  that is equal to or greater than the required gate voltage. To turn any of the SCR&#39;s  74 ,  76 ,  78 ,  80  “off,” and to thus block current flow, no voltage (i.e., 0V) or a voltage that is less than the gate voltage is applied to the relevant gate  740 ,  760 ,  780 ,  800 . A controller  82  may be incorporated into the circuit design to control which SCR&#39;s are turned “on” or “off” at any given time. The controller  82  may be a microcontroller of any known kind, and it may interface with the gates  740 ,  760 ,  780 ,  800  through conventional circuitry known to skilled artisans. 
         [0036]    Two modes for turning the SCR&#39;s  74 ,  76 ,  78 ,  80  “on” and “off” are applicable here: a forward polarity mode and a reverse polarity mode. In the forward polarity mode, the forward positive polarity SCR  74  and the forward negative polarity SCR  78  are turned “on” while the reverse negative polarity SCR  76  and the reverse positive polarity SCR  80  are turned “off.” This mode coordinates the positive input lug  602  with the first interchangeable polarity output lug  62  and the negative input lug  604  with the second interchangeable polarity output lug  64 . Such coordination assigns a positive polarity to the first interchangeable polarity output lug  62  and a negative polarity to the second interchangeable polarity lug  64  within the context of the electrical circuit shown in  FIG. 4 . 
         [0037]    The reverse polarity mode achieves the opposite effect at the interchangeable polarity output lugs  62 ,  64 . Specifically, in the reverse polarity mode, the reverse negative polarity SCR  76  and the reverse positive polarity SCR  80  are turned “on” while the forward positive polarity SCR  74  and the forward negative polarity SCR  78  are turned “off.” This mode coordinates the positive input lug  602  with the second interchangeable polarity output lug  64  and the negative input lug  604  with the first interchangeable polarity output lug  62 . Such coordination assigns a positive polarity to the second interchangeable polarity output lug  64  and a negative polarity to the first interchangeable polarity output lug  62  within the context of the electrical circuit shown in  FIG. 4 . Table 1 below summarizes the forward polarity mode and the reverse polarity mode as just described. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Polarity of 
               
               
                   
                 Forward 
                 Reverse 
                 Forward 
                 Reverse 
                 Polarity of First 
                 Second 
               
               
                   
                 Positive 
                 Negative 
                 Negative 
                 Positive 
                 Interchangeable 
                 Interchangeable 
               
               
                   
                 Polarity 
                 Polarity 
                 Polarity 
                 Polarity 
                 Polarity Output 
                 Output Polarity 
               
               
                 Mode 
                 SCR 
                 SCR 
                 SCR 
                 SCR 
                 Lug 
                 Lug 
               
               
                   
               
             
             
               
                 Forward 
                 ON 
                 OFF 
                 ON 
                 OFF 
                 POSITIVE 
                 NEGATIVE 
               
               
                 Polarity 
               
               
                 Mode 
               
               
                 Reverse 
                 OFF 
                 ON 
                 OFF 
                 ON 
                 NEGATIVE 
                 POSITIVE 
               
               
                 Polarity 
               
               
                 Mode 
               
               
                   
               
             
          
         
       
     
         [0038]    A resistance spot welding process that implements the programmable polarity module  48  will now be described with reference to  FIGS. 1 ,  3 , and  4 . To begin, the metal workpiece stack-up  12  is located between the first and second welding electrodes  30 ,  34  so that the weld site  14  is generally aligned with the electrodes&#39;  30 ,  34  opposed weld faces  42 . The metal workpiece stack-up  12  may be brought to such a location, as is often the case when the gun arms  24 ,  26  are part of a stationary pedestal welder, or the gun arms  24 ,  26  may be robotically moved to locate the electrodes  30 ,  34  relative to the weld site  14  of the stack-up  12 . Once the stack-up  12  is properly located, the first and second welding arms  24 ,  26  converge to press the weld faces  42  of the first and second welding electrodes  30 ,  34  against the oppositely-facing surfaces  36 ,  38  of the first and second metal workpieces at the weld site  14 . 
         [0039]    The welding DC supplied by the DC power supply unit  44  is then passed between the first and second welding electrodes  30 ,  34  and through the first and second metal workpieces  16 ,  18  and across the faying interface  20 . Resistance to the concentrated flow of the welding DC through the metal workpieces  16 ,  18  and across the faying interface  20  generates heat at the faying interface  20  at the weld site  14 . This heat initiates a molten weld pool at the faying interface  20  that penetrates into one or both of the workpiece  16 ,  18  depending on the composition and nature of the workpieces  16 ,  18 . Upon stoppage of the welding DC current, the molten weld pool solidifies into the weld nugget  22 . The first and second welding electrodes  30 ,  34  are then refracted from their engaged surfaces  36 ,  38  of the metal workpieces  16 ,  18 . Next, the workpiece stack-up  12  is re-located between the first and second welding electrodes  30 ,  34  at a different weld site  14 , or it is moved away so that another stack-up  12  can be located for spot welding. More spot welds are then formed in the same way. 
         [0040]    The programmable polarity module  48  can designate the polarities of the first and second welding electrodes  30 ,  34  as needed to best suit the particular spot welding process being performed. The programmable polarity module  48  can assign a positive polarity to the first welding electrode  30  and a negative polarity to the second welding electrode  34 , or vice versa, and can further switch the polarities of the first and second welding electrodes  30 ,  34  at any time. Such flexibility is made possible by controlling which of the SCR&#39;s  74 ,  76 ,  78 ,  80  are turned “on” and which are turned “off” Recall that in the forward polarity mode, for instance, the first interchangeable polarity output lug  62 , and thus the first welding electrode  30 , is assigned the positive polarity while the second interchangeable polarity output lug  64 , and thus the second welding electrode  34 , is assigned the negative polarity. The opposite is true in the reverse polarity mode, in which the first interchangeable polarity output lug  62 , and thus the first welding electrode  30 , is assigned the negative polarity while the second interchangeable polarity output lug  64 , and thus the second welding electrode  34 , is assigned the positive polarity. 
         [0041]    The programmable polarity module  48  may be useful when spot welding an aluminum alloy workpiece to another aluminum alloy workpiece with a pair of copper alloy welding electrodes. The aluminum alloy workpieces could be, for example, a pair of aluminum alloy sheet metal layers, one of which is about 3.0 mm thick or less at the weld site  14 . They could also be, as another example, a pair of aluminum alloy castings, one of which is about 3.0 mm thick or less at the weld site  14 . It has been found that repeatedly forming spot welds between such aluminum alloy workpieces with a conventional spot welding set-up—in which one welding electrode has a fixed positive polarity and the other welding electrode has a fixed negative polarity—causes the positive welding electrode to wear at a faster rate than the negative welding electrode. The positive welding electrode may, in some instances, wear approximately twice as fast as the negative welding electrode over the course of forming 30-100 spot welds. 
         [0042]    The wear experienced at the two welding electrodes  30 ,  34  is the accumulation of a hard metal reaction product on the weld face  42  that is derived from a metallurgical reaction between the aluminum alloy of the metal workpiece and the copper alloy of the welding electrode. The accumulation of this hard metal reaction product may eventually spill and form pits in the weld face  42 . To visually demonstrate this wear mechanism,  FIGS. 5 and 6  show photomicrographs of a fixed negative polarity copper alloy welding electrode and a fixed positive polarity copper alloy welding electrode, respectively, that have been used together to form  100  spot welds in a pair of overlapping 2 mm thick aluminum alloy sheet metal layers. The fixed positive polarity welding electrode ( FIG. 6 ) has plainly experienced more aluminum alloy-copper alloy reaction product accumulation on its weld face. Because of this, the positive welding electrode needs to be periodically redressed to remove the hard metal reaction product, or replaced with a new welding electrode, more often than the negative welding electrode. 
         [0043]    The programmable polarity module  48  can mitigate the above-described polarity bias by periodically switching the polarities of the first and second welding electrodes  30 ,  34 . The polarities may be switched each time a certain number of spot welds have been performed. Preferably, the polarities of the first and second welding electrodes  30 ,  34  are switched after every 1-5 spot welds, and most preferably after every spot weld. For example, the programmable polarity module  48  may be operated in its forward polarity mode, in which the first welding electrode  30  is assigned the positive polarity and the second welding electrode  34  is assigned the negative polarity, and the welding DC may be supplied to form a first spot weld. Then, after the welding DC has stopped, the programmable polarity module  48  switches to its reverse polarity mode, in which the first welding electrode  30  is assigned the negative polarity and the second welding electrode  34  is assigned the positive polarity, and the welding DC may be supplied to form a second spot weld. The programmable polarity module  48  may then switch back to its forward polarity mode, and so on. This back-and-forth switching of the electrode polarities will even out the rates at which the two welding electrodes  30 ,  34  wear and, as a result, increase the amount of spot welds that can be formed with the two electrodes  30 ,  34  relative to the conventional fixed electrode polarity spot welding technique. 
         [0044]    The programmable polarity module  48  may also be useful when the spot welding of an aluminum alloy workpiece and a metal workpiece would create a heat imbalance in the workpieces that degrades weldability. For example, an aluminum alloy sheet metal layer and a steel sheet metal layer have different physical characteristics (e.g., melting points, thermal conductivities, hardness, etc.), and when trying to spot weld the two sheet metal layers together with a pair of copper alloy electrodes, a heat imbalance develops as current passes through them. In this case, a greater amount of localized heat is generated in the more electrically resistive steel than the less electrically resistive aluminum alloy. A heat imbalance may also develop when trying to spot weld different types of aluminum alloy workpieces—such as an aluminum alloy sheet metal layer and an aluminum alloy casting—with a pair of copper alloy electrodes. This is because an aluminum alloy casting typically has a higher electrical resistivity than an aluminum alloy sheet metal layer. 
         [0045]    The spot welding of such workpieces, like before, creates a heat imbalance at the electrode/workpiece interfaces in which more heat is generated at the positive welding electrode and less heat is generated at the negative welding electrode. It has been found that the weld quality between the metal workpieces can be affected by controlling the electrode polarities and, by extension, the heat imbalance developed at the welding electrodes  30 ,  34 . When spot welding an aluminum alloy sheet metal layer and a steel sheet metal layer, for instance, the ability to switch the polarities of the welding electrodes  30 ,  34  allows for the electrode heat imbalance to be used to compensate for the lower electrical resistivity and the lower melting point of the aluminum alloy sheet metal layer. Either the positive welding electrode or the negative welding electrode may engage the aluminum alloy sheet metal layer to generate more or less heat, respectively, so as to obtain better weld nugget penetration, preferably approaching 50%, into the aluminum alloy sheet metal layer. When spot welding an aluminum alloy sheet metal layer to an aluminum alloy casting, the differences in electrical resistivities can usually be counteracted by engaging the more electrically resistive aluminum alloy casting with the negative welding electrode, which experiences less heat generation at the electrode/workpiece interface compared to the positive welding electrode. 
         [0046]      FIGS. 7-10  visually demonstrate the effects that electrode polarity can have on weld quality.  FIGS. 7 and 8  are cross-sectional photomicrographs of a 1 mm thick aluminum alloy sheet metal layer and a 0.55 mm thick steel sheet metal layer that have been subjected to spot welding.  FIG. 7  shows the effect of engaging the aluminum alloy sheet metal layer (bottom layer) with a copper alloy welding electrode that has been assigned the negative polarity and engaging the steel sheet metal layer (top layer) with a copper alloy welding electrode that has been assigned the positive polarity. Conversely,  FIG. 8  shows the effect of engaging the aluminum alloy sheet metal layer (top layer) with a copper alloy welding electrode that has been assigned the positive polarity and engaging the steel sheet metal layer (bottom layer) with a copper alloy welding electrode that has been assigned the negative polarity. As can be seen, in this particular example, engaging the aluminum alloy sheet metal layer with the positive polarity welding electrode ( FIG. 8 ) results in deeper weld penetration. 
         [0047]      FIGS. 9 and 10  are cross-sectional photomicrographs of a 2.5 mm thick aluminum alloy sheet metal layer and a 3 mm thick aluminum alloy casting that have been subjected to spot welding.  FIG. 9  shows the effect of engaging the aluminum alloy casting (top layer) with a copper alloy welding electrode that has been assigned the negative polarity and engaging the aluminum alloy sheet metal layer (bottom layer) with a copper alloy welding electrode that has been assigned the positive polarity. Conversely,  FIG. 10  shows the effect of engaging the aluminum alloy casting (top layer) with a copper alloy welding electrode that has been assigned the positive polarity and engaging the aluminum alloy sheet metal layer (bottom layer) with a copper alloy welding electrode that has been assigned the negative polarity. Here, it can be seen that a better-quality spot weld was produced when the negative polarity welding electrode, which generates less heat at its workpiece/electrode interface, engaged the more electrically resistive aluminum alloy casting ( FIG. 9 ), as demonstrated by the absence of the large triangular-shaped void formed below the interface of the casting (upper layer) and the welding electrode that can be seen in  FIG. 10 . 
         [0048]    The programmable polarity module  48  can accommodate the above-described polarity bias by switching the polarities of the welding electrodes  30 ,  34 , as needed, to achieve or maintain good weld quality. For instance, when spot welding a metal stack-up  12  that includes an aluminum alloy sheet metal layer and a steel sheet metal layer, the polarity assignments of the two welding electrodes  30 ,  34  will depend on the properties of stack-up  12  and the weld schedule, meaning that the aluminum alloy sheet metal layer could be engaged by either the positive welding electrode or the negative welding electrode depending on the circumstances. If the first and second welding electrodes  30 ,  34  are desired to be assigned the positive and negative polarities, respectively, then the programmable polarity module  48  would be operated in its forward polarity mode. If the opposite polarity designations are desired, however, then programmable polarity module  48  would be operated in its reverse polarity mode. Regarding the practice of spot welding a metal stack-up  12  that includes an aluminum alloy sheet metal layer and an aluminum alloy casting, the programmable polarity module  48  could be operated, in many instances, in whichever mode assigns the positive polarity to the welding electrode  30 ,  34  that engages the aluminum alloy sheet metal layer. 
         [0049]    The above description of preferred exemplary embodiments is 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.