Abstract:
A process is provided for forming a resistance welding electrode. The process includes the step of providing a billet formed from a high conductivity metal. The billet includes a first portion having a first inner cavity formed therein. The process further includes the steps of inserting a dispersion strengthened copper insert into the first inner cavity of the billet thereby forming an insert-containing billet, and deforming the insert-containing billet so as to mechanically lock the insert in place in the billet. The deformed insert-containing billet comprises the resistance welding electrode.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to welding electrodes and a process for forming same. 
     Resistance welding has long been used as a quick and effective method of joining metal members. The workpieces to be welded are placed in abutting relationship and a large current is caused to flow through the workpieces by a pair of opposed electrodes. The current causes the abutting surfaces of the workpieces to be heated sufficiently to effect the formation of a weld nugget. Typically, the electrodes apply significant pressure to the workpieces during welding. This facilitates the welding process by urging the material together and, also, reducing electrical resistance between each electrode tip and the adjacent workpiece material. 
     Since welding is accomplished by resistance heating of the material being welded, it will be appreciated that the electrodes will also be heated substantially. It is important to have an electrode of high electrical conductivity in order to minimize the power loss in the electrode and the resulting heating of the electrode. 
     Over time, the repeated heating and pressing operations involved in resistance welding cause breakdown, softening, mushrooming and other deformation of the electrodes. As this occurs, electrical current requirements increase with the enlarged welding tip face contacting the workpiece material until ultimately, redressing or replacement of the electrode is required. Accordingly, it is also important to have an electrode which is capable of withstanding significant distorting force at the elevated temperatures which result from the welding process so as to minimize the number of times it becomes necessary to redress or replace the electrode within a given period of time. 
     It is known in the art to form resistance welding electrodes by combining a copper electrode body with an anneal resistant high hardness insert. Typically, the insert performs much better than the copper material from which the electrode body was formed. However, the insert material is much more expensive than the copper used to form the electrode body. 
     The insert may be brazed onto the shank. The brazing step is disadvantageous as it adds an additional step to the electrode manufacturing process and, hence, increases the cost of the electrode. Furthermore, the brazing operation may anneal and soften the electrode body. 
     It is also known to force the insert into an electrode body via a press-fit operation. The steel welded today is often galvanized, or coated with a zinc or other softer metal coating. As a result, the resistance welding electrodes tend to stick to the coated metal. An electrode tip joined to an electrode body via a press-fit operation may pull out of the shank during resistance welding of coated materials, thus requiring replacement of the electrode. 
     Accordingly, there is a need for an improved resistance welding electrode which can be manufactured via an efficient and more cost effective process and, yet, is capable of performing in an acceptable manner. 
     SUMMARY OF THE INVENTION 
     This need is met by the present invention, whereby an improved resistance welding electrode and process for forming same are provided. The process involves providing a billet having an inner cavity, inserting a dispersion strengthened copper insert into the billet and deforming the insert-containing billet via cold-working operations so as to mechanically lock the insert in place in the billet. The forming operations are capable of being performed in a single process such that the electrode can be manufactured in an efficient and cost effective manner. Furthermore, because the insert is mechanically locked in place within the billet, it is unlikely that the normal amount of sticking that occurs during resistance welding of coated steel will pull the insert out of the billet. It is also noted that the billet is preferably formed from a CDA C10700 silver bearing copper which is a high conductivity material. Previously, it was generally thought that silver bearing copper should not be used in forming welding electrodes as it was thought that such material would anneal at the temperatures involved in resistance welding. However, by virtue of cooling water located in an inner cavity of the silver bearing copper main body portion and because the main body portion makes only limited, if any, contact with a workpiece, annealing of the main body portion is prevented. 
     In accordance with a first aspect of the present invention, a process is provided for forming a resistance welding electrode. The process includes the step of providing a billet formed from a high conductivity metal. The billet includes a first portion having a first inner cavity formed therein. The process further includes the steps of inserting a dispersion strengthened copper insert into the first inner cavity of the billet thereby forming an insert-containing billet, and deforming the insert-containing billet so as to mechanically lock the insert in place in the billet. The deformed insert-containing billet comprises the resistance welding electrode. 
     The step of providing a billet may comprise the steps of providing a generally cylindrical cut-off portion of high conductivity copper, and upsetting and forward extruding the cut-off portion so as to form a billet having a first inner cavity therein. 
     The upsetting and forward extruding step preferably further comprises the step of locating the cylindrical cut-off portion adjacent to an inner cavity of an upsetting and forward extruding die. The inner cavity of the upsetting and forward extruding die is open at one end and has an inner diameter substantially equal to an outer diameter of the billet. The die includes a forming pin located axially within the inner cavity and extends into the inner cavity from an end opposite to the open end of the inner cavity. The pin has an outer diameter substantially equal to an inner diameter of the billet first inner cavity. The upsetting and forward extruding step additionally comprises the steps of inserting the cut-off portion into the inner cavity via an insertion pin, and applying pressure to the cut-off portion via a punch to cause forward extrusion of the cut-off portion over the pin, whereby a billet is formed having an outer diameter which is greater than an outer diameter of the cut-off portion and including a first inner cavity. 
     The deforming step preferably comprises the steps of: placing the insert-containing billet into a first inner cavity of a first insert-containing billet forming die, the first inner cavity having a first generally rounded lower portion; applying pressure with a first forming punch to a second portion of the insert-containing billet such that the insert-containing billet is initially deformed so as to have a first shape; placing the insert-containing billet having the first shape into a second inner cavity of a second insert-containing billet forming die, the second inner cavity having a second generally rounded lower portion; applying pressure with a second forming punch to the second portion of the initially deformed insert-containing billet such that the initially deformed insert-containing billet is further deformed so as to have a second shape; placing the insert-containing billet having the second shape into a third inner cavity of a third insert-containing billet forming die; and applying pressure with a third forming punch to the second portion of the further deformed insert-containing billet such that the further deformed insert-containing billet is additionally deformed so as to have a third shape. The insert of the additionally deformed insert-containing billet has a substantially hourglass shape such that the insert is mechanically locked in place within the billet. The additionally deformed insert-containing billet comprises the resistance welding electrode. 
     The steps of applying pressure with the first, second and third forming punches effects the formation of a second inner cavity in the second billet portion via back extrusion. 
     Preferably, the process further comprises the step of staking the first portion of the billet after the inserting step and before the deforming step. 
     In accordance with a second aspect of the present invention, a resistance welding electrode is provided. The electrode comprises a main body formed from a high conductivity metal and including a first portion having a first inner cavity, and a dispersion strengthened copper insert provided in the first inner cavity. The main body first portion and the insert are shaped such that the insert is mechanically locked in place in the main body. 
     Preferably, the insert is shaped substantially like a hyperboloid and the inner cavity has a substantially similar shape. 
     The main body further includes a second inner cavity provided in its second portion. The second inner cavity is adapted to be supplied with a cooling fluid during a resistance welding process. 
     Preferably, the insert is formed from an internally oxidized copper-aluminum alloy. Preferably, the main body is formed from a high conductivity copper, such as a silver bearing copper. 
     In accordance with a third aspect of the present invention, a resistance welding electrode is provided and is formed from a process comprising the steps of: providing a billet formed from a high conductivity metal, the billet including a first portion having a first inner cavity formed therein; inserting a dispersion strengthened copper insert into the first inner cavity of the billet thereby forming an insert-containing billet; and deforming the insert-containing billet so as to mechanically lock the insert in place in the billet. The deformed insert-containing billet comprises the resistance welding electrode. 
     Accordingly, it is an object of the present invention to provide an improved low cost resistance welding electrode and process for forming same. It is further an object of the present invention to provide a resistance welding electrode having a dispersion strengthened copper insert which is mechanically locked in position within a main body formed from a high conductivity metal. These and other objects and advantages of the present invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are views, partially in cross section, of a press adapted to form a resistance welding electrode in accordance with the present invention; 
         FIG. 2  is a view, partially in cross section, of a second forming station showing a billet after is has received an insert in the second forming station; 
         FIG. 3  is a view, partially in cross section, of a portion of a die assembly forming part of the second forming station; 
         FIG. 4  is a view, partially in cross section, of a stripper mechanism stripping a workpiece from a punch at the fourth forming station; 
         FIG. 5  is a view, partially in cross section, of the fourth forming station after an insert-containing billet is further deformed to a second shape; 
         FIG. 6  is a side view of a cut-off portion of copper wire; 
         FIG. 7  is a side view, in cross section, of a billet; 
         FIG. 8 , is a side view, in cross section, of an insert-containing billet; 
         FIG. 9  is a view of the underside of the insert-containing billet illustrated in FIG.  8  and showing indentations in the billet portion of the insert-containing billet; 
         FIG. 10  is a cross-sectional view of the insert-containing billet after it has been initially deformed in the third forming station in the press illustrated in  FIGS. 1A and 1B ; 
         FIG. 11  is a cross-sectional view of the insert-containing billet after it has been further deformed in the fourth forming station in the press illustrated in  FIGS. 1A and 1B ; 
         FIG. 12  is a cross-sectional view of the insert-containing billet after it has been additionally deformed in the fifth forming station in the press illustrated in  FIGS. 1A and 1B ; 
         FIG. 13  is an enlarged cross-sectional view of a portion of the insert-containing billet illustrated in  FIG. 10 ; 
         FIG. 14  is an enlarged cross-sectional view of a portion of the insert-containing billet illustrated in  FIG. 11 ; and 
         FIG. 15  is an enlarged cross-sectional view of a portion of the insert-containing billet illustrated in FIG.  12 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1A and 1B , a press  10  is provided having a stationary bed portion  20  and a ram portion  30  which is caused to move back and fourth relative to the bed portion  20  by a conventional drive apparatus (not shown). The bed and ram portions  20  and  30  include respectively first and second electrode forming tooling  22  and  32  which are provided at first, second, third, fourth and fifth forming stations  40 ,  50 ,  60 ,  70  and  80 . Positioned adjacent to the first forming station  40  is a conventional cutting station  90 . A high conductivity copper wire  92  is fed to the cutting station  90  where it is cut into discrete, generally cylindrical cut-off portions  94 , see  FIG. 6 , for use in forming resistance welding electrodes  400 , one of which is shown in FIG.  12 . Conventional work transfer fingers  110  (shown only schematically in  FIGS. 1A ,  1 B,  2  and  5 ) are provided for moving each of the discrete cut-off portions  94  from the cutting station  90  to the first forming station  40  and from the first forming station  40  through the remaining forming stations  50 ,  60 ,  70  and  80 . 
     The first forming station  40  comprises an upsetting and forward extruding station where the cut-off portion  94  is formed into a billet  100 , see  FIG. 7 , having a first inner cavity  102 . The forming station  40  includes an upsetting and forward extruding die  42  which is fixedly coupled to the bed portion  20  and, hence, is stationary. The die  42  includes an inner cavity  44 , which is open at one end  44 a and has an inner diameter substantially equal to an outer diameter of the billet  100 . The die  42  also includes a forming pin  42 a located axially within the inner cavity  44  and extends into the inner cavity  44  from an end opposite to the open end  44 a of the inner cavity  44 . The pin  42 a has an outer portion with an outer diameter substantially equal to an inner diameter of the billet first inner cavity  102 . 
     The first forming station  40  further includes an insertion pin  46  and a first station punch  48 . The punch  48  is fixedly coupled to the ram portion  30  so as to move with the ram portion  30 . The pin  46  extends through a bore  48 a in the punch  48  and is biased in a direction toward the die  42  via a spring  46 a. As the ram portion  30  moves toward the bed portion  20 , the insertion pin  46  engages the cut-off portion  94  held adjacent to the die  42  via a pair of the work transfer fingers  110  and inserts the cut-off portion  94  into the inner cavity  44  of the die  42 . The punch  48  then engages the cut-off portion  94  and applies sufficient pressure to the cut-off portion  94  to effect forward extrusion of the cut-off portion  94  over the pin  42 a such that a billet  100  is formed, see FIG.  1 B. The billet  100  has an outer diameter which is greater than an outer diameter of the cut-off portion  94 . 
     An ejection sleeve  49  is positioned about the pin  42 a and is movable relative to the pin  42 a. The ejection sleeve  49  ejects the billet  100  from the die  42  after the billet  100  has been formed. In the illustrated embodiment, a base portion  49 a is coupled to three pins  49 b (only two of which are illustrated in  FIG. 1B ) which, in turn, are fixedly coupled to the sleeve  49 . The base portion  49 a and the three pins  49 b effect movement of the sleeve  49  to eject the billet  100  in response to movement of a timing cam (not shown) which engages the base portion  49 a. 
     The second forming station  50  comprises a punch  52  which is fixedly coupled to the ram portion  30  so as to move with the ram portion  30 . The forming station  50  also includes a die assembly  54  coupled to the bed portion  20 , see  FIGS. 1B ,  2  and  3 . The die assembly  54  comprises a die  55  having an inner cavity  55 a, a die support block  56  which is movable relative to the bed portion  20 , and an insert pin  57  which is movable relative to the die block  56 . In the illustrated embodiment, the die block  56  is biased in a direction toward the punch  52  via three springs  56 a (only one of which is shown in FIGS.  1 B and  2 ). The die  55  is fixedly coupled to the die block  56  so as to move with the die block  56 . 
     The die assembly  54  further includes an insert supply mechanism  58  for supplying dispersion strengthened copper inserts  120  one at a time into the path of movement of the insert pin  57  such that the insert pin  57  inserts a copper insert  120  into a first inner cavity  102  of a billet  100  to form an insert-containing billet  100 a, one of which is shown in FIG.  8 . The insert supply mechanism  58  comprises a supply conduit  59  having a plurality of inserts  120  therein. The inserts  120  are fed to the conduit  59  via a feed drive (not shown). The supply conduit  59  extends through a bore  20 a provided in a support block  20 b which is fixedly coupled to the bed portion  120 . The supply conduit  59  is capable of moving within the bore  20 a. The conduit  59  also extends through a first bore  56 b in the die block  56  and is fixedly connected to the die block  56  so as to move with the die block  56 . A distal end  59 a of the conduit  59  terminates at an insert receiving channel  56 c in the die block  56  such that the conduit  59  supplies inserts  120  to the channel  56 c. 
     The supply mechanism  58  further includes a reciprocating pin  130  which extends into the channel  56 c. A spring  132  biases the pin  130  toward an outer stationary member  134  which forms part of the lower die assembly  54 . The pin  130  has an outer surface  130 a which is adapted to engage an outer camming surface  134 a of the stationary member  134 , see FIG.  3 . Upon downward movement of the die block  56 , the outer surface  130 a of the pin  130  moves along the outer camming surface  134 a of the stationary member  134  such that the pin  130  is moved inward against the force of the spring  132 . As the pin  130  moves inward, it pushes two inserts  120  located in the channel  56 c in a direction toward the path of movement of the insert pin  57  such that one of the inserts  120  is moved into the path of movement of the pin  57  and the other insert  120  is moved from a first position to a second position within the channel  56 c. In  FIG. 2 , the pin  130  is shown in its home position with the single insert  120  in the channel  56 c positioned in the second insert position.  FIG. 3  illustrates the channel  56 c after an insert  120  has been supplied to the channel  56 c via the supply conduit  59  such that two inserts  120  are located within the channel  56 c in the first and second positions. 
     As the ram portion  30  moves toward the bed portion  20 , the punch  52  engages the billet  100  held adjacent to the die  55  via a pair of the work transfer fingers  110  and inserts the billet  100  into the die  55 . A punch coupling block  53  is fixedly coupled to the ram portion  30  so as to move with the ram portion  30 . The punch coupling block  53  moves toward the bed portion  20  with the punch  52  and engages the die block  56  to move the die block  56  inwardly against the force of the springs  56 a until the die block  56  engages the support block  20 b. As the billet  100  is moved to the bottom of the die inner cavity  55 a via the punch  52 , the insert pin  57  is caused to move in a direction toward the punch  52  via movement of a timing cam (not shown) which engages a base portion  57 a of the pin  57 . As the pin  57  moves toward the punch  52 , it moves an insert  120 , which has been positioned in its path of movement by the pin  130 , into the first inner cavity  102  of the billet  100  positioned in the die  55  so as form an insert-containing billet  100 a, see FIG.  8 . Preferably, the insert  120  is fully inserted into the billet  100  before the billet  100  reaches the bottom of the die inner cavity  55 a and engages the die block  56 . 
     A surface of the die block  56  which is encircled by the die  55  is provided with three staking pins (not shown). The punch  52  applies sufficient pressure to the billet  100  such that the staking pins form three indentations  100 c in a lower surface  100 b of the billet  100 , see FIG.  9 . The staking operation effects displacement of a sufficient amount of metal in the billet  100  so as to temporarily hold the insert  120  in the billet  100  as the billet  100  is moved out of the second forming station  50  and into the third forming station  60 . 
     As the punch  52  is removed from the die  55 , the pin  57  is extended further in a direction toward the punch  52  so as to eject the insert-containing billet  100 a from the die  55 , see FIG.  2 . The pin  57  is returned to its home position via a spring  57 b which is positioned about the pin  57  and engages the die block  56  and the base portion  57 a of the pin  57 . Further, the die block  56  is moved to its home position, shown in  FIG. 2 , via the springs  56 a. 
     From the second forming station  50 , the insert-containing billet  100 a is moved to the third forming station  60  where it is initially deformed so as to have a first shape. The third forming station  60  includes a first insert-containing billet forming die  62  having an inner cavity  64 . The inner cavity  64  has a generally cylindrical upper portion  64 a and a generally rounded lower portion  64 b. The third forming station  60  further includes a punch  68  which is coupled to the ram portion  30  so as to move with the ram portion  30 . 
     The third forming station  60  further includes a movable guide and ejecting pin  66  which is capable of moving into the inner cavity  64  via movement of a timing cam (not shown) which engages a base portion  66 a of the pin  66 . First and second polymeric holding members  69  frictionally engage the pin base portion  66 a and maintain the pin  66  in its extended position so that it may act as a guide for the insert-containing billet  100 a as it is moved into the die inner cavity  64  for forming. The holding members  69  are biased toward the pin base portion  66 a via springs  69  which, in turn, are adjustably supported by set screws  69 b. 
     As the ram portion  30  moves toward the bed portion  20 , the punch  66  engages the insert-containing billet  100 a held adjacent to the die  62  via a pair of the work transfer fingers  110  and inserts the insert-containing billet  100 a into the die  62 . The pin  66 , which is in its extended position before the insert-containing billet  100 a is inserted into the die  62 , moves downwardly with the insert-containing billet  100 a and acts as a guide for the insert-containing billet  100 a as it is moved into the die inner cavity  64  for forming. 
     The punch  68  applies sufficient pressure to the insert-containing billet  100 a such that it is initially deformed to a first shape. An insert-containing billet  200  having a first shape is illustrated in  FIGS. 1A ,  10  and  13 . The insert  120  of the insert-containing billet  100 a is ductile enough so as to flow with the copper billet  100  during the cold-working operation occurring in station  60 . As a result, the initially deformed insert-containing billet  200  has an insert  220  with an initially deformed central portion  220 a, see FIG.  13 . Further, the punch  68  deforms a second portion  100 d of the billet  100  such that the initially deformed insert-containing billet  200  has a slight recess  200 c in its second portion  200 b. 
     After the forming operation is completed, the pin  66  ejects the insert-containing billet  200  from the die  62 . 
     From the third forming station  60 , the insert-containing billet  200  is moved to the fourth forming station  70  where it is further deformed so as to have a second shape, see  FIGS. 11 and 14 . The fourth forming station  70  includes a second insert-containing billet forming die  72  having an inner cavity  74 . The inner cavity  74  has a generally cylindrical upper portion  74 a and a generally rounded lower portion  74 b. The fourth forming station  70  further includes a punch  78  which is coupled to the ram portion  30  so as to move with the ram portion  30 . 
     The fourth forming station  70  further includes a movable guide and eject pin  76  which is capable of moving into the inner cavity  74  via movement of a timing cam (not shown) which engages a base portion  76 a of the pin  76 . First and second polymeric holding members  79  frictionally engage the pin base portion  76 a to maintain the pin  76  in its extended position so that it may act as a guide for the insert-containing billet  200  as it is moved into the die inner cavity  74  for forming. The holding members  79  are biased toward the pin base portion  76 a via springs  79  which, in turn, are adjustably supported by set screws  79 b. 
     As the ram portion  30  moves toward the bed portion  20 , the punch  78  engages the insert-containing billet  200  held adjacent to the die  72  via a pair of the work transfer fingers  110  and inserts the insert-containing billet  200  into the die  72 . The pin  76 , which is in its extended position before the insert-containing billet  200  is inserted into the die  72 , moves downwardly with the insert-containing billet  200  and acts to guide the insert-containing billet  200  into the die cavity  74 . 
     The punch  78  applies sufficient pressure to the insert-containing billet  200  which it is in the die  72  such that the insert-containing billet  200  is further deformed to a second shape. An insert-containing billet  300  having a second shape is illustrated in  FIGS. 11 and 14 . The insert  220  of the insert-containing billet  200  flows with the copper billet  200 a during the cold-working operation occurring in station  70 . As a result, the further deformed insert-containing billet  300  has an insert  320  with a further deformed central portion  320 a. As noted above, while in the third forming station  60 , the central portion of the insert is deformed inwardly an initial amount. Further occurring in station  70  is the back extrusion of the second portion  200 b of the billet  200 a about the punch  78 . As a result, the insert-containing billet  300  includes a second inner cavity  300 c in its second portion  300 b. 
     A stripper mechanism  170  is provided for stripping the insert-containing billet  300  from the punch  78  as the punch  78  is moved away from the die  72 , see  FIGS. 4 and 5 . The stripper mechanism  170  includes a workpiece engaging member  172  which is positioned about the punch  78  and is movable relative to the punch  78 . The engaging member  172  is fixedly connected to a reciprocating stripper element  174  which in turn is fixedly coupled to first and second rod members  176 . Springs  177  are positioned about the rod members  176  and engage enlarged portions  176 a of the rod members  176  and a recess portion  177 a of a block member  177 b which is fixedly coupled to the ram portion  30  so as to move with the ram portion  30 . The rod members  176  are fixedly coupled to a pin engaging member  178  and extend through the block member  177 b. 
     After the cold working operation is completed at station  70 , a pin  179 , which is held in the position illustrated in  FIG. 4  via a timing cam (not shown), engages the pin engaging member  178  such that the engaging member  172  engages the insert-containing billet  300  and strips it from the punch  78  as the punch  78  moves away from the die  72 , see FIG.  4 . Once the punch  78  has moved a sufficient distance from the die  72  such that the insert-containing billet  300  is removed from the punch  78 , the timing cam allows the pin  179  to move to the position shown in  FIG. 5 , such that the springs  177  return the engaging member  172  to its home position. 
     After the forming operation is completed, the pin  76  ejects the insert-containing billet  300  from the die  72 , see FIG.  5 . 
     From the fourth forming station  70 , the insert-containing billet  300  is moved to the fifth forming station  80  where it is additionally deformed to a third shape, which is the final shape of the resistance welding electrode  400 , see  FIGS. 12 and 15 . The fifth forming station  80  includes a third insert-containing billet forming die  82  having an inner cavity  84 . The inner cavity  84  has a generally cylindrical upper portion  84 a and a generally rounded lower portion  84 b. The fifth forming station  80  further includes a punch  88  which is coupled to the ram portion  30  so as to move with the ram portion  30 . 
     The fifth forming station  80  further includes a movable guide and eject pin  86  which is capable of moving into the inner cavity  84  via movement of a timing cam (not shown) which engages a base portion  86 a of the pin  86 . First and second polymeric holding members  89  frictionally engage the pin base portion  86 a to maintain the pin  86  in its extended position to permit it to act as a guide for the insert-containing billet  300  as it is moved into the die inner cavity  84  for forming. The holding members  89  are biased toward the pin base portion  86 a via springs  89 a which, in turn, are adjustably supported by set screws  89 b. 
     As the ram portion  30  moves toward the bed portion  20 , the punch  88  engages the insert-containing billet  300  held adjacent to the die  82  via a pair of the work transfer fingers  110  and inserts the insert-containing billet  300  into the die  82 . The pin  86 , which is in its extended position before the insert-containing billet  300  is inserted into the die  82 , moves downwardly with the insert-containing billet  300  and acts to guide the insert-containing billet  300  into the die cavity  84 . 
     The punch  88  applies sufficient pressure to the insert-containing billet  300  such that it is additionally deformed to a third shape. An insert-containing billet  400  having a third shape is illustrated in  FIGS. 12 and 15  and comprises the resistance welding electrode of the present invention. The insert  320  of the further deformed insert-containing billet  300  flows with the copper billet  300 a during the cold-working operation occurring in station  80 . As a result, the additionally deformed insert-containing billet  400  has an insert  420  with a central portion  420 a that has been deformed inwardly an additional amount, see FIG.  15 . Thus, the insert  420 , after having been deformed during the cold working operations in stations  60 ,  70  and  80 , is shaped like an hyperboloid and, as such, is mechanically locked in place within the billet  400 a. The billet  400 a has a first inner cavity  421  having a shape similar to that of the insert  420  Further occurring in station  80  is the coining of the second portion  300 b of the billet  300 a by the punch  88 . Thus, the electrode  400  has an inner cavity  400 c provided with a stepped portion  400 d. Further, the second portion  400 b includes a tapered skirt portion  400 e. 
     The billet  400 a is also referred to herein as the main body of the electrode  400 . 
     As the punch  88  is removed from the die  82 , a stripper mechanism  170 , constructed in the same manner as the stripper mechanism  170  shown in  FIGS. 4 and 5  and described above, strips the insert-containing billet  400  from the punch  88  as the punch  88  is moved away from the die  82 . The pin  86  then ejects the electrode  400  from the die  82 . 
     Preferably, the copper wire  92  comprises a CDA C10700 silver bearing copper or a CDA C10500 silver bearing copper. Alternatively, another metal which is highly electrically conductive and is substantially anneal resistance below approximately 900° F. may be used. The inserts  120  are preferably formed from a dispersion strengthened copper such as GlidCop® Al-25 or GlidCop® Al-60, which are commercially available from SCM Metal Products, Inc. Dispersion strengthened copper is resistant to heading or “mushrooming” during welding cycles and, further, is resistant to sticking to galvanized and coated steels. Of course, the inserts  120  may be formed from other materials which are resistant to sticking and mushrooming. 
     The tapered skirt portion  400 e is adapted to fit onto the arm of a conventional robotic welder. The electrode  400  is adapted to be water cooled through the second cavity  400 c. The nose portion  400 f of the electrode  400  is adapted to engage a workpiece during a resistance welding operation. 
     During the cold-working operations performed in stations  60 ,  70  and  80 , the insert is maintained in a compressive state. This is important since dispersion strengthened copper is relatively brittle and may break if it goes into tension. 
     It is further contemplated that the electrode  400  may be machined after it has been formed in the press  10 . 
     Having described the invention in detail and by reference to a preferred embodiment thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention as defined in the appended claims.