Patent Publication Number: US-2023143471-A1

Title: Slug weld with increased surface contact area

Description:
TECHNICAL FIELD 
     The present invention relates generally to forge welding and, more particularly, to an apparatus and method of forge welding components together. 
     BACKGROUND AND SUMMARY 
     During the production of axle assemblies for vehicles, various techniques, such as welding, have been used to join an axle tube to an axle housing or to a differential carrier. For instance, resistance welding with a cylindrical plug is one process conventionally used in the production of axle assemblies. In this process, a housing having a neck that forms an opening for receiving the tube is provided. A cylindrical aperture is formed on the neck of the housing to receive the plug. The tube is press-fit into the opening and the plug is inserted into the aperture. Electrodes then apply pressure to force the plug against the tube while electrical current passes through the interface between the plug and the tube. Heat generated by the current deforms the plug as the interface reaches a plastic state. The plug cools to become welded to the tube after the current is shut off. The welded plug acts like a fastener to secure the tube to the housing. 
     The inventors have recognized several drawbacks with these welding techniques. For instance, in the cylindrical plug welding described above, the contact area of the weld (e.g., between the plug and the axle tube) is determined by the size of the aperture. While a larger contact area is desired for increasing the torsional strength and resistance of the weld, increasing the size of the aperture may compromise the strength of the housing at the neck where the aperture is located. Since defective axle assemblies are expensive to replace once incorporated into a larger system, ensuring that the tube is properly joined to the housing is imperative to, for instance, avoid premature degradation of the axle components, expensive warranty repairs, and/or product recall. Therefore, the inventors have recognized an unmet need for a technique for joining an axle tube to a housing with a welded connection having a larger contact area, without increasing the size of the aperture. 
     To resolve at least a portion of the aforementioned issues, the inventors have developed a method for joining first and second workpieces, particularly first and second axle components. In one example, the method includes forming an aperture in a first axle component that is mounted to a second axle component, where the aperture includes a chamfer at one end thereof. The method further includes inserting an object, which is sized to fit into the aperture, into the aperture. The method further includes resistance welding at the aperture while applying a variable pressure to press on the object, where the pressure is adjusted to a first, higher level during a first portion of the welding and subsequently adjusted to a lower level during a second, later portion of the welding as the object fills the chamfer. In this way, by filling the chamfer with the object during welding, the welded connection formed by the object between the first and second axle components has a contact surface area larger than the area of the aperture, thereby providing increased torsional strength and resistance at the connection. Further, by forming the aperture in the first axle component with a chamfer in this manner, this welded connection having increased contact area can be realized without increasing the size of the aperture, so that the strength of the first axle component is not compromised. 
     In another example, the method may include adjusting the pressure based on an angle of the chamfer. In such an example, a higher chamfer angle corresponds to a higher first pressure level. In this way, the variable pressure can be adjusted to effectively force the object to substantially fill the chamfer during the first portion of the welding to provide a secure welded connection, as described above, for apertures having various chamfer angles in different applications. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1    shows a perspective view of an axle assembly having an axle tube joined to an axle housing. 
         FIG.  2    shows a detailed cross-sectional view of a portion of the axle assembly depicted in  FIG.  1   , particularly illustrating a welded joint between the axle tube and the axle housing. 
         FIG.  3    is a schematic block diagram illustrating a welding apparatus for creating the welded joint shown in  FIG.  2   . 
         FIG.  4    shows a deformable rivet positioned for joining the axle tube to the axle housing, according to one example. 
         FIG.  5    is a graph illustrating the position of the hot forging electrode as a function of rivet deformation, the rivet temperature, and the pressure on the deformable rivet during the deformation sequence. 
         FIG.  6    is a graph illustrating pressure applied to the rivet during a portion of welding as a function of a chamfer angle, according to two examples. 
         FIG.  7    is a flow chart illustrating a method for forming a welded joint according to one example. 
         FIGS.  8 A- 8 B  show different views of another axle assembly having an axle tube joined to a differential carrier. 
         FIGS.  1 - 2  and  8 A- 8 B  are drawn approximately to scale. However, other relative component dimensions may be used in other embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to a method and apparatus for joining first and second axle components, including an axle tube and an axle housing. In an example, the approach described herein may be applied to joining an axle tube to a differential carrier trunnion. 
       FIG.  1    depicts an example electric drive system  100  for providing power to an axle assembly  102  of a vehicle. The vehicle may take a variety of different forms in different examples, such as a light, medium, or heavy duty vehicle. Additionally, the electric drive system  100  may be adapted for use in front and/or rear axles, as well as steerable and non-steerable axles. To generate power, the electric drivetrain  102  may include an electric machine  104 . In some examples, the electric machine  104  may be an electric motor-generator and may thus include conventional components such as a rotor, a stator and the like within an electric machine housing  105  for generating mechanical power as well as electric power during a regenerative mode, in some cases. Further, in other examples, the vehicle may include an additional motive power source, such as an internal combustion engine (ICE) (e.g., a spark and/or compression ignition engine), for providing power to another axle. As such, the electric drive system  100  may be utilized in an electric vehicle (EV), such as a hybrid electric vehicle (HEV) or a battery electric vehicle (BEV). 
     In some examples, the electric machine housing  105  may be coupled via fasteners, such as bolts, for instance, to a housing  107  of a gearbox  106 . Further, the electric machine  104  may provide mechanical power to a differential  110 , housed in an axle housing  112 , via the gearbox  106  to provide rotational power to axle shafts  114 ,  116  of the axle assembly  102 . As such, the differential  110  may distribute torque, received from the electric machine  104  via the gearbox  106 , to drive wheels attached to the axle shafts  114 ,  116 , during certain operating conditions. In some examples, the differential  110  may be a locking differential, an electronically controlled limited slip differential, or a torque vectoring differential. 
     The gearbox  106  may be a single-speed gearbox, where the gearbox operates in one gear ratio. However, other gearbox arrangements have been envisioned, such as a multi-speed gearbox that is designed to operate in multiple distinct gear ratios. Further, in one example, the electric machine  104 , the gearbox  106 , and the differential  110  may be incorporated into the axle assembly  102 , forming an electric axle (e-axle) in the vehicle. The e-axle, among other functions, provides motive power to drive wheels of the vehicle during operation. Specifically, in the e-axle embodiment, the electric machine and gearbox assembly may be coupled to and/or otherwise supported by an axle housing. In one particular example, the e-axle may be an electric beam axle where a solid piece of material (e.g., a beam, a shaft, and/or a housing) extends between the drive wheels). The e-axle may provide a compact arrangement for delivering power directly to the axle. In other examples, however, the electric machine  104  and the gearbox  106  may be included in an electric transmission in which the gearbox and/or electric motor are spaced away from the axle. For instance, in the electric transmission example, mechanical components such as a driveshaft, joints (e.g., universal joints), and the like may provide a rotational connection between the electric transmission and the drive axle. 
       FIGS.  1 - 2    illustrate an example where an axle tube is joined with axle housing, however, the approach described herein may be applied to joining various axle components, and specifically may apply to joining an axle tube to a differential carrier trunnion. In one example, the axle shafts  114 ,  116  may be disposed in axle tubes  118 ,  120 , respectively, which may be coupled to the axle housing  112 . More specifically, the axle tubes  118 ,  120  may be joined, respectively, to neck portions  122 ,  124  of the axle housing  112 . The neck portions  122 ,  124  may extend outward in opposing axial directions (e.g., along the x-axis) from a central portion  126  of the axle housing  112  where the differential  110  resides. The axle tubes  118 ,  120  may be received within an opening defined in each of the respective neck portions  122 ,  124 , and joined thereto, such that the axle shafts  114 ,  116  may extend through the respective axle tubes and neck portions of the axle housing to receive mechanical power via the differential as desired. As such, the axle shafts  114 ,  116  may be at least partially enclosed within the axle housing  112 . 
     In some examples, the axle tubes  118 ,  120  may be joined to the neck portions  122 ,  124  of the axle housing  112  via a welding process (e.g., a resistance welding process). For instance, each of the neck portions  122 ,  124  may be provided with a deformable rivet  130 ,  132 , respectively, in one example. In such an example, the deformable rivets may be subjected to heat and pressure in the welding process to join each axle tube to a respective neck portion. The deformable rivets may be cylindrical plugs, balls, or the like made of a metal or other material suitable for welding to the respective axle tube. For simplicity, the axle tube  124  (and deformable rivet  132 ) will not be discussed further, but it will be understood that the both of the axle tubes, neck portions, and deformable rivets may be similarly configured and/or constructed. Details of the welded connection between the axle tube  118  and the axle housing  112 , and formation thereof, will be elaborated on with reference to  FIGS.  2 - 7   . 
     An axis system  140  is provided in  FIG.  1   , as well as  FIGS.  2 - 4   , for reference. The z-axis may be a vertical axis (e.g., parallel to a gravitational axis), the x-axis may be a lateral axis (e.g., horizontal axis), and/or the y-axis may be a longitudinal axis, in one example. However, the axes may have other orientations, in other examples. 
     Turning now to  FIG.  2   , a cross-sectional view of the axle assembly  102  is shown, as defined by a lateral cut taken along dashed line  2 - 2  shown in  FIG.  1   . As illustrated in  FIG.  2   , the neck portion  122  of the axle housing  112  may have an aperture  200  formed therein (e.g., extending between an outer surface/diameter  202  and an inner surface/diameter  204  of the neck) sized to receive the deformable rivet  130 . The aperture  200  may therefore be cylindrical and may be formed by drilling, in one example. Further, a chamfer  210  may be formed at one end of the aperture. More specifically, the chamfer  210  may be formed at an inner end of the aperture  200  closer to the axle tube  116 . As illustrated, for example, the chamfer may be formed between an interior surface  201  of the aperture  200  and the inner surface  204  of the neck portion  122  of the axle housing  112 . The chamfer  210  may be formed, for instance, by cutting away material between the aperture and the inner surface  204 . Further, the chamfer  210  may have a chamfer angle  212 , as measured between the interior surface  201  of the aperture  200  and the chamfer. 
     Further, prior to deformation of the deformable rivet  130 , the chamfer  210  may form a void  214  defined between the neck portion  122 , the axle tube  216 , and the deformable rivet  130 . Even further, during deformation of the rivet  130 , a pressure (e.g., a downward pressure) applied by a joining apparatus may be controlled (e.g., adjusted), based at least in part on the size of the chamfer angle  212 , to adequately force material of the deformable rivet into the void  214 , where the size of the void is dependent on the chamfer angle. In other words, the pressure and heat applied during the welding process may be controlled so that the deformable rivet  130  fills the void  214  created by the chamfer. In this way, the rivet  130  deforms to create weld having a contact surface area with the axle tube  116  greater than the area of the aperture  200 , as will be elaborated on herein with reference to  FIGS.  3 - 7   , thereby providing additional torsional strength and resistance at the welded joint. 
     To elaborate, an end portion  216  of the axle tube  116  may be press-fit or otherwise inserted into an opening  218  in the axle housing  112 . The deformable rivet  130  extends through the aperture  200  in the neck portion  122  to meet the axle tube  126 . Further, when the deformable rivet  130  is initially inserted into the aperture  200 , the rivet may sit proud of the neck portion  122  of the axle housing  112 , so as to have an initial height  201  as measured above the neck portion  122 . In some examples, the initial height  201  may be at least 5 millimeters (mm). This initial offset between the deformable rivet and the axle housing may allow for better positional control during installation. 
     The deformable rivet  130  is then welded to the axle tube  116  through the axle housing  112  (e.g., at the aperture  200 ) causing the axle tube  160  to be joined to the axle housing  112 . Although only one deformable rivet is shown extending through the aperture  200 , more apertures may be provided to fit respective deformable rivets, in other examples. Further, as noted above while the deformable rivet  130  is described herein for joining an axle tube with an axle housing, it will be understood that the rivet may be used to join two different components (e.g., axle components), where one of the components includes an aperture sized to receive the rivet and having a chamfer at one end thereof proximate the other of the components, in other examples. 
     For instance, the deformable rivet may be inserted into an aperture in a trunnion of a differential carrier to join the carrier to an axle tube that is inserted into an opening of the trunnion, in one example, where the aperture again includes a chamfer at an end proximate the axle tube. In such an example, where element  112  denotes a portion of a differential carrier trunnion, it can be joined to axle tube  116 . One specific example is illustrated in  FIGS.  8 A and  8 B , showing an e-axle assembly  800 , which may be implemented in an electric drivetrain in a light vehicle application. The e-axle assembly  800  may include a differential  802  having a differential carrier  804 . Further, the differential carrier  804  may include trunnions  806 ,  808  integrated therewith and extending in opposing axial directions therefrom. The carrier trunnions  806 ,  808  further include apertures  810 ,  812 , respectively, formed therein, which apertures may include a chamfer formed at an inner end, similar to the aperture  200  formed in neck portion  122  of the axle housing  112 , shown in  FIGS.  2 - 4   . 
     The e-axle assembly  800  may further include axle tubes  816 ,  818  may be inserted (e.g., in a press-fit manner) into respective carrier trunnions The e-axle  800  may further include axle tubes  816 ,  818  inserted (e.g., in a press-fit manner) into the trunnions  806 ,  808 , respectively, and coupled thereto by a deformable rivet inserted and welded into the apertures  810 ,  812 . Thus, it will be understood that the apertures formed in the differential carrier  804  are configured to receive deformable rivets such as the deformable rivet  130  discussed with regard to  FIGS.  1 - 4   , and a similar joining process and apparatus may be employed to couple the axle tubes  816 ,  818  to the differential carrier  804  using these deformable rivets. Further, as particularly illustrated in the view of the differential carrier  804  shown in  FIG.  8 B , the carrier trunnions  806 ,  808  may include apertures  811 ,  813  in addition to the apertures  810 ,  812 , which may provide additional points for joining the axle tubes to the carrier (e.g., by welding of deformable rivets inserted into the apertures) to strengthen the connections therebetween. 
     Returning to the cross-sectional view of the axle assembly  102  of  FIG.  2   , the deformable rivet  130  is shown to act like a fastener in the aperture  200  of the neck  122  to secure the axle tube  116  to the axle housing  112 . As will be described in detail below, the amount of deformation of the deformable rivet  130  is equal to the change in its position between an initial position and a final position. The deformable rivet  130  is in the initial position when it meets the axle tube  116 , and in the final position when it is welded to the axle tube  116 . The change in position is caused by heat and pressure acting in conjunction on the deformable rivet  130 . Further, the loading strength of the deformable rivet  130  depends upon the amount of rivet deformation. As will be expanded upon herein, the pressure applied to the deformable rivet may be controlled (e.g., adjusted) as a function of the size of the chamfer  210  in the aperture  200 , such that the rivet  130  deforms to fill the void formed between the chamfer and the axle tube  116 . In this way, the resulting weld may have a contact surface area larger than the area of the aperture, providing increased strength and resistance characteristics. 
     Referring now to  FIG.  3   , a schematic block diagram illustrating a joining apparatus  300  for joining the axle tube  116  to the axle housing  112  is shown. In one example, the joining apparatus  300  includes an assembly fixture  302  for supporting the axle tube  116  and the axle housing  112  after the portion of the axle tube  116  is inserted within the opening  218 . The assembly fixture  302  is mechanically secured to hold the axle assembly  102 . Further, the assembly fixture  302 , axle tube  116 , and axle housing  112  form a conductive path for electricity to travel. 
     The joining apparatus  300  further includes a resistance heating power supply  304 . In one example, the power supply  304  may be a readily obtainable component from various resistance welding component manufacturers such as Weltronic, Medar, or Square D. The power supply  304  has a pair of power output terminals. One of the pair of power output terminals is connected to the assembly fixture  302  in electrical communication with the axle tube  116 . The other one of the pair of power output terminals is connected to a hot forging electrode  306 . 
     Further, an actuator  308  may include an output member  307  affixed to the hot forging electrode  306 . In one example, the actuator  308  may be in the form of a fluid powered cylinder, though other linear actuators, such as mechanical, electromechanical, piezoelectric actuators and the like, have been contemplated, in other examples. The actuator  308  shifts the hot forging electrode  306  into and out of engagement with the deformable rivet  130 , as indicated by arrow  309 , to force it against the axle tube  116 . When the actuator  308  shifts the hot forging electrode  306  into engagement with the deformable rivet  130 , a closed electrical circuit forms between the power supply  304 , hot forging electrode  306 , deformable rivet  130 , axle housing  112 , axle tube  116 , and assembly fixture  302 . As such, the power output from the power supply  304  may then be applied to heat the faying surface, i.e., the interface between the deformable rivet  130  and the axle tube  116 , due to resistance in the electric current. The heat causes the deformable rivet  130  to deform and flow into the void  214  formed by the chamfer  210 , as will be explained in greater detail below. 
     The hot forging electrode  306  is movable relative to the deformable rivet  130  to track the deformation of the deformable rivet  130 . For example, when the hot forging electrode  306  engages the deformable rivet  130 , the deformation of the deformable rivet  130  may be equivalent to the change in the position of the hot forging electrode  306 . Hence, the deformation of deformable rivet  130  can be determined by knowing the position of hot forging electrode  306 . 
     The joining apparatus  300  may further include a pressure regulator  314  cooperating with a controller  110 , as indicated by line  315 , for varying the force exerted by the hot forging electrode  306  on the deformable rivet  130 . In some examples, the pressure regulator  314  can be set repeatedly to have the actuator  308  apply differing amounts of pressure. Further, the controller  310  may cooperate with the pressure regulator  314  to vary the force on the deformable rivet  130  as a function of time and of rivet deformation and, in some examples, based on the size (e.g., angle  214 ) of the chamfer  210 . An exemplary strategy for controlling (e.g., varying) pressure applied to the deformable rivet  130  via the hot forging electrode  306  will be discussed with reference to  FIG.  5   . 
     The controller  310  may also cooperate with the power supply  304 , as indicated by line  303 , to regulate the power output of said power supply  304 , as shown by line  305 . The controller  310  also cooperates with the actuator  308 , as indicated by line  311 , to further regulate and/or track its movement. The controller  310  may be a readily available component obtainable from various controller manufacturers such as Allen Bradley, Square D, Modicon, or Fanuc. 
     Further, the controller  310  may include a processor and a memory with instructions stored therein that, when executed by the processor, cause the controller to perform various methods and control techniques described herein. The processor may include a microprocessor unit and/or other types of circuits. The memory may include known data storage mediums, such as random access memory, read only memory, keep alive memory, combinations thereof, and the like. In some examples, the controller  310  may be a programmable logic controller (PLC) having associated A/D converters and a programmed instruction card or a personal computer (PC). 
     The controller may receive various signals from sensors positioned in the joining apparatus  310  and/or on the axle assembly  102 . Conversely, the controller  310  may send control signals to various actuators at different locations based on the sensor signals. For instance, the controller  310  may send command signals to the pressure regulator  314  and, in response, the pressure regulator may adjust a fluid pressure delivered to the actuator  308  to vary the pressure exerted on the deformable rivet  308  via the hot forging electrode  306  (e.g., by the output member  307  of the actuator  308 ). Other controllable components in the joining apparatus  300  may be operated in a similar manner with regard to sensor signals and actuator adjustment. 
     The joining apparatus  300  may further include a transducer  312  operative with the hot forging electrode  306 . The transducer may be a sensor such as a linear variable deformation transducer (LVDT), in one example, and may provide an output indicative of the position of the hot forging electrode. However, other types of position sensors have been contemplated, in other examples, for determining the position of the hot forging electrode. Further, the controller  310  may cooperate with the transducer  312 , as indicated by line  313 , to determine the position of the hot forging electrode  306 . 
     After the actuator  308  shifts the hot forging electrode  306  into engagement with the deformable rivet  130 , and after the power supply  304  applies power, the controller  310  monitors the position of the hot forging electrode  306  to determine the deformation of the deformable rivet  130 . The deformation of the deformable rivet  130  equals the change in position of the hot forging electrode  306  when the hot forging electrode  306  is engaged to the deformable rivet  130 . The controller  310  may also regulate the power output of the power supply  304  as a function of rivet deformation to ensure that the deformable rivet  130  properly deforms. Even further, the controller  310  may regulate the power output of power supply  304  by varying the power level and the power duration. 
     In some examples, the joining apparatus  300  may include an Infra-Red (IR) temperature sensor  316 . The temperature sensor  316  may be pointed at the deformable rivet  130  to generate a temperature signal indicative of the temperature of the deformable rivet  130 . In some cases, the temperature of the deformable rivet  130  may change to more than 2000° F. from room temperature during the welding process. The controller  310  may cooperate with the IR temperature sensor  316 , as indicated by line  317 , to determine a temperature of the deformable rivet  310 . 
     Then, the controller  310  may use the temperature signal to determine rivet deformation by comparing the temperature of the deformable rivet  130  with a known deformation pattern. The known deformation pattern is the deformation pattern of a typical deformable rivet subjected to a pressure as a function of its temperature. In some examples, the controller  310  may match the temperature of the deformable rivet  130  to a temperature value in the known pattern to predict the deformation of the deformable rivet  130 . Further, the known deformation pattern may, in some cases, account for the size of the void  214  formed by the chamfer  210 , when it is desired that the deformable rivet  130  deform so as to fill the void. Since the dimensions and deformation sequences are consistent among deformable rivets, the prediction of the deformation of the deformable rivet  130  made by the controller  310  may be highly accurate. Thus, the controller  310  uses the temperature signal to regulate the power output from power supply  304  as a function of rivet deformation to ensure the deformable rivet  130  properly deforms. 
     In another example, the joining apparatus  300  may include an electric current sensor  318 . The current sensor  318  may be an inductor and may be operative with one of the power output terminals. Further, the current sensor  318  may generate a power consumption signal proportional to the power output from the power supply  304  during the welding process. The controller  310  may cooperate with the current sensor  318 , as indicated by line  319 , so as to receive the power consumption signal. 
     The controller  310  may use the power consumption signal to determine rivet deformation by comparing the power applied to the deformable rivet  130  with another known deformation pattern. This known deformation pattern is the deformation pattern of a typical deformable rivet subjected to a pressure as a function of the power applied. The controller  310  may match the power applied to the deformable rivet  130  to a power value in the known pattern in order to predict the deformation of deformable rivet  130 . The controller  310  may then use the power consumption signal to regulate the power output of the power supply  304  as a function of rivet deformation to ensure that the deformable rivet  130  properly deforms. 
     Turning now to  FIG.  4   , a detailed cross-sectional view of the axle assembly  102  is shown, with cross-hatching omitted for simplicity. More specifically,  FIG.  4    shows the deformable rivet  130  positioned in the aperture  200  during heating and deformation of the deformable rivet (e.g., during resistance welding by the joining apparatus  300  depicted in  FIG.  3   ). The deformable rivet  130  has been inserted in the aperture  200  and a downward pressure is applied to the deformable rivet  130 , as indicated by arrow  400 . As illustrated, the rivet  130  may have a diameter substantially equal to a diameter  402  of the aperture  200 , and may therefore be in close proximity with the interior surface  201  of the aperture  200  prior to and during deformation. Further, as downward pressure is applied to the deformable rivet  130 , the initial height  200  shown in  FIG.  2    (of the deformable rivet relative to the neck portion  122  of the axle housing  112 ) decreases to a welding height  401  illustrated in  FIG.  4   . The welding height  401  continues to decrease as pressure and heat are applied to the deformable rivet  130 , approaching a final non-zero height. In other words, the final position of the deformable rivet  130  within the aperture  200  may be non-zero so that the deformable rivet is not flush with the neck portion of the axle housing upon completion of the welding process. In one example, when the initial height  200  is at least 5 mm, as previously noted with reference to  FIG.  2   , the welding height  401  shown in  FIG.  4    may approach a final height that is a non-zero value less than 2 mm, or less than 1 mm, such as 0.5 mm, for instance. 
     During operation of the joining apparatus, lower end of the deformable rivet  130  is in contact with the axle tube  116 , and the downward pressure causes the deformable rivet to deform as heat is applied at the aperture  200  (e.g., via the resistance heating power supply  304  shown in  FIG.  3   ). As the rivet  130  deforms, a portion  404  of the deformable rivet flows into the void  214  formed by the chamfer  210 , forming a welded connection at a contact surface  406  between the deformed rivet  130  and the axle tube  116  to join the axle tube  116  to the axle housing  112 . The contact surface  406  of the weld may thus have a diameter  408  that is larger than the diameter  402  of the aperture  200 . In this way, the enlarged contact area provides additional torsional strength and resistance at the welded connection, without sacrificing strength of the axle housing by increasing the diameter  402  of the aperture  200 . In other words, increased weld strength between the axle housing and the axle tube may be realized by deforming a rivet to fill an aperture with a chamfer, as described herein, when compared to an aperture having a straight bore (e.g., without a chamfer). 
     Referring now to  FIG.  5   , the operation of the joining apparatus  300  will be discussed in greater detail, with joint reference to  FIGS.  2 - 4   .  FIG.  5    is a graph  500  having an electrode position curve  502 , a rivet temperature curve  504 , and a pressure curve  506 . The electrode position curve  502  illustrates the position of the hot forging electrode  306  as a function of rivet deformation over time. As indicated above, the position of the hot forging electrode  306  tracks the rivet deformation when hot forging electrode  306  is engaged with the deformable rivet  130 . The rivet temperature curve  504  illustrates the temperature of the deformable rivet  130  over time. The pressure curve  506  illustrates the pressure on the deformable rivet  130  over time, as applied by the actuator  308 . 
     The time axis is divided into five intervals. The first interval “I” is the fit-up interval. During this interval, the axle tube  116  is inserted into the opening  218  of the axle housing  112  (e.g., of the neck portion  122 ) mounted on the assembly fixture  302 . The deformable rivet  130  is placed in the aperture  200  while the actuator  308  holds the hot forging electrode  306  out of engagement with the deformable rivet  130 . 
     The controller  310  then commands the actuator  308  to shift the hot forging electrode  306  into engagement with the deformable rivet  130 . The actuator  308  responds by moving the hot forging electrode  306  until it rests on the deformable rivet  130 . Thus, the deformable rivet  130  is in the initial fit-up position. The controller  310  monitors the initial position of the hot forging electrode  306 . If the initial position of the hot forging electrode  36  falls within a predetermined initial fit-up range  508 , such as initial position point  510  on the position curve  502 , then the controller  310  commands the power supply  304  to apply power as shown by “power on” point  512 , near the end of interval I. The predetermined initial fit-up range  508  is indicative of a proper fit-up of the deformable rivet  130 . 
     The second interval “II” is the resistance welding interval. During this interval, the hot forging electrode  306  applies a constant high pressure on the deformable rivet  130 . Preferably, for the manufacturing of axle assemblies according to the examples described herein, the constant high pressure may be around 3000-5000 pounds per square inch (psi), though other suitable pressure ranges have been considered in other examples, depending upon the application. The hot forging electrode  306  also applies the electrical current from the power supply  304  to the deformable rivet  130 . Due to the resistance of the deformable rivet  130 , the power is dissipated into heat energy. Thus, the current heats the deformable rivet  130  as shown by the rising slope of rivet temperature curve  504  in interval II. 
     The power applied by the power supply  304  may be AC or DC electrical power. It may also take a variety of input patterns such as pulse, ramp, sinusoidal, sawtooth, etc. depending upon the application, the type of power supply, and on the thicknesses and type of materials used. In one example, mid-frequency direct current (MFDC) may be applied by the power supply  304 , where AC power is inverted and converted to an inverted DC output. Using MDFC in the welding process may allow for faster and more reliable heating of the deformable rivet, to ensure adequate weld quality as desired, while also reducing inductive power losses for improved energy efficiency. Further, the use of MFDC may lead to reduced pressure demands (e.g., from the actuator  308 ), which may help to reduce the chance of excessive heat deformation at the axle tube and/or housing. In other examples, for the manufacturing of axle assemblies as described herein, the power output may be pulsed with a frequency of around 60 Hertz, a duty cycle of around 50%, and 18,000 to 25,000 amps RMS of secondary current. Further, during interval II, the power supply  304  may apply power for around 10 to 20 cycles, such as 10 to 15 cycles, for instance, depending on the current level demanded in a particular application. 
     At the beginning of interval II deformable rivet  130  maintains its initial position. The deformable rivet  130  begins to expand and may then start to contract once it reaches a sufficient temperature. The slight expansion and contraction are shown by the electrode position curve  502  in interval II. As the deformable rivet  130  initially expands, the high pressure applied during interval II causes the material of the deformable rivet to begin to flow into the void  214  formed by the chamfer  210  as the temperature of the rivet increases. After the deformable rivet  130  initially expands, it may continue to soften and begin to collapse under the application of pressure and heat. Because of the pressure and the heat generated from the power output, the deformable rivet  130  may then coalesce with the axle tube  116  and become welded to it. This process is the basis of resistance welding. 
     Once the deformable rivet  130  contracts to a sufficient level such as forge point  514  on the position curve  502 , the controller  310  commands the actuator  308  to step (e.g., instantaneously) the constant high pressure applied on hot forging electrode  306  to a constant lower pressure. Preferably, for the manufacturing of axle assemblies described herein, the constant low pressure may be around 2000 psi, depending on the size of the deformable rivet  130 , though other suitable pressure ranges have been considered in other examples, depending upon the application. In turn, the hot forging electrode  306  applies the constant low pressure on the deformable rivet  130 . The pressure changing instantaneously from high to low on a sufficiently heated deformable rivet  130  is the basis of the hot forging process described herein. However, it is not required for the high and low pressures to be constant. The instantaneous change of pressure is shown by the pressure curve  506  at the beginning of the forging interval “III”. Thus, an advantage of the present invention is that the controller  310  monitors the deformation of the deformable rivet  130  during the resistance welding interval to determine precisely when to command the actuator  308  to step down the pressure. 
     Depending on the application, an increase in power output from the power supply  304  may be required during the forging interval, particularly when it is desired that the deformable rivet  130  deform so as to substantially fill the void  214  formed by the chamfer  210 . As such, the controller  310  may command the power supply  304  to adjust the power output accordingly. Also, a delay of about one to five cycles may be required to allow the deformable rivet  130  a chance to cool to properly coalesce with the axle tube  116  before forging. If so, the controller  310  will command the power supply  304  accordingly. 
     Once the actuator  308  applies the constant high pressure (at interval II), the deformable rivet  130  contracts rapidly. The rapid contraction is shown by the drastic change of the position curve  502  at the beginning of the interval III. The controller  310  receives the output of the transducer  312 , which monitors the contraction of the deformable rivet  130 . The controller  310  then commands the power supply  304  to terminate the power once the deformable rivet  130  contracts to a sufficient level, such as calculated position point  516  on the position curve  502 . The calculated position point  516  is within a predetermined position range  518 . The rivet temperature starts to decrease after the power output is terminated, as shown by the rivet temperature curve  504  in interval III. 
     After the deformable rivet  130  has rapidly contracted to the calculated position point  516  within the predetermined position range  518 , it gradually contracts under the low constant pressure of the hot forging electrode  306  until it reaches the final position point  520 . The final position point  520  is within the final position “envelope”  522  on the electrode position curve  502 . Further, the position of the hot forging electrode  306  falling within the final position envelope  522  is indicative of proper rivet deformation. The controller  310  will identify the axle assembly  102  as defective if the position of the hot forging electrode  306  does not fall within the final position envelope  522 . In this way, the joining apparatus  300  may reliably determine proper welding of the axle assembly components. 
     When the position of the hot forging electrode  306  reaches the final position point  520 , or any other point within the final position envelope  522 , the cooling interval “IV” commences. During the cooling interval, the temperature of the deformable rivet  130  continues to decrease, as shown by the rivet temperature curve  504  in interval IV. The controller  310  may command the actuator  308  to keep continuing to apply the constant low pressure on the hot forging electrode  306 . In turn, this allows the deformable rivet  130  a chance to cool. Upon cooling, the deformable rivet  130  becomes forged to the axle tube  116 . Thus, the axle tube  116  may be permanently joined to the axle housing  112 . 
     The controller  310  then commands the actuator  308  to retract the hot forging electrode  306 . During this retraction interval “V”, the hot forging electrode  306  is shifted out of engagement with the deformable rivet  130 . The controller  310  may continue to monitor the position of the hot forging electrode  306  to ensure that it actually has retracted to avoid any “stuck gun” conditions (e.g., where the hot forging electrode  306  becomes fused to the deformable rivet  130 ). In other words, the controller  310  makes sure that hot forging electrode  306  has retracted before transfer of the axle assembly  102 , for guarding the operator and the equipment against harm or damage. This allows the axle assembly  102  to be removed from the assembly fixture  302 , and the operation of the joining apparatus  300  can be repeated on a new axle assembly as desired. 
     The operation of a preferred embodiment of the joining apparatus  300  having the transducer  312  to monitor the position of the hot forging electrode  306  by measuring the movement of the hot forging electrode  306  has just been described. In an alternate example, the temperature sensor  316  may be used instead of the transducer  312  to monitor the position of the hot forging electrode  306 . In yet another alternate example, the current sensor  318  may be used instead of the transducer  312  to monitor the position of the hot forging electrode  306 . The difference among all of the examples is how the position of hot forging electrode  306  is obtained. Thus, the graph  500  would still be applicable no matter which embodiments are used. 
     With continuing reference to  FIG.  5   ,  FIG.  6    shows a graph  600  illustrating examples of how the pressure applied to the deformable rivet  130  is adjusted, specifically showing a relationship between the maximum pressure applied during the high pressure resistance welding interval II and the chamfer angle (e.g., angle  212  of chamfer  210 ). The chamfer angle is indicated on the abscissa and increases in the direction of the arrow, and the pressure is indicated on the ordinate and increases in the direction of the arrow. 
     In one example, as indicated by plot  602 , the high (e.g., maximum) pressure level applied during resistance welding interval II may increase proportionally with an increase in the chamfer angle. In another example, as indicated by plot  604 , the high pressure level applied during resistance welding interval II may increase exponentially with an increase in the chamfer angle. In these and other exemplary pressure and angle relationships, it will be understood that a larger chamfer angle may require a larger pressure to allow the deformable rivet to sufficiently (e.g., substantially) fill the chamfer during the resistance welding interval. 
     Referring now to  FIG.  7   , a flow chart illustrating a hot forging method  700 , according to one example, is shown. The hot forging method  700  includes the steps of providing a first workpiece (e.g., axle tube  116 , shown in  FIGS.  1 - 4   ) with a deformable rivet (e.g., deformable rivet  130 , shown in  FIGS.  2 - 4   ), as shown in block  702 , and providing a second workpiece with an aperture having a chamfer (e.g, neck portion  122  of axle housing  112 , with the aperture  200  having chamfer  210 , shown in  FIGS.  2 - 4   ) sized to receive the deformable rivet, as shown in block  704 . Next, at block  706 , the first and second workpieces are placed together with the deformable rivet of the first workpiece extending through the aperture in the second workpiece. 
     A resistance heating power supply having a pair of power output terminals is then provided, as shown in block  708 . One of the power output terminals is connected in electrical communication to the first workpiece, as shown in block  710 . Next, a hot forging electrode movable relative to the deformable rivet is connected to the other one of the power output terminals as shown in block  712 . 
     Next, at block  714 , an actuator is affixed to the hot forging electrode. The actuator shifts the hot forging electrode into and out of engagement with the deformable rivet. The position of the hot forging electrode is then monitored to determine rivet deformation, as shown in block  716 . Then, the power output of the resistance heating power supply is regulated as a function of rivet deformation to ensure that the deformable rivet properly deforms, as shown in block  718 . 
     The actuator is controlled to regulate a pressure (e.g., a downward pressure) on the deformable rivet, as shown in block  720 . In some examples, the pressure applied and/or the change in the pressure applied, may be dependent on the size of the chamfer in the aperture between the first and second workpieces. In one example, the pressure applied may be proportional to an angle of the chamfer (as denoted by angle  212  shown in  FIGS.  2  and  4   ), as shown by plot  602  in  FIG.  6   . For instance, a higher pressure may be applied to the deformable rivet (e.g., during interval II shown in  FIG.  5   ) when the chamfer angle is larger. Conversely, when the chamfer angle is smaller, a lower pressure may be desired, wherein the pressure should be sufficient to encourage a flow of heated (e.g., melted) deformable rivet into the chamfer area (e.g., void  214 ). In this way, the pressure on the deformable rivet can be controlled during the joining process (in which the first and second workpieces are joined via the deformable rivet) to force material of the deformable rivet, during deformation thereof, into the void defined by the chamfer. Thus, the contact area of the rivet weld between the first and second workpieces can be increased without increasing the general size of the aperture. In this way, the contact area of the weld joining the first and second workpieces may be wider than the area of the aperture formed in the first workpiece, thereby increasing resistance strength of the weld joint. Further, by allowing for a reduced area of the majority of the aperture (e.g., without the chamfer area), the potential for premature degradation and/or separation of components may be reduced. 
     The present invention may also be applied to join a first workpiece having an integral deformable rivet to a second workpiece having a corresponding aperture for receiving the integral deformable rivet. Here, no separate deformable rivet is used. However, in other examples, the deformable rivet may be provided as a separate component inserted into the aperture of the second workpiece so as to contact the first workpiece. Further, while the examples described herein have focused on joining an axle tube to an axle housing, other applications of the joining process described herein have been contemplated. For instance, a similarly welded deformable rivet may be utilized in the joining of an axle tube to a carrier of a differential. However, it will be understood that the joining processes and weld couplings discussed herein may be implemented for joining components in a variety of assemblies, not limited to axle assemblies and/or vehicle systems. 
       FIGS.  1 - 4    show example configurations with relative positioning of the various components. If shown directly contacting each other, or directly coupled, then such elements may be referred to as directly contacting or directly coupled, respectively, at least in one example. Similarly, elements shown contiguous or adjacent to one another may be contiguous or adjacent to each other, respectively, at least in one example. As an example, components laying in face-sharing contact with each other may be referred to as in face-sharing contact. As another example, elements positioned apart from each other with only a space there-between and no other components may be referred to as such, in at least one example. As yet another example, elements shown above/below one another, at opposite sides to one another, or to the left/right of one another may be referred to as such, relative to one another. Further, as shown in the figures, a topmost element or point of element may be referred to as a “top” of the component and a bottommost element or point of the element may be referred to as a “bottom” of the component, in at least one example. As used herein, top/bottom, upper/lower, above/below, may be relative to a vertical axis of the figures and used to describe positioning of elements of the figures relative to one another. As such, elements shown above other elements are positioned vertically above the other elements, in one example. As yet another example, shapes of the elements depicted within the figures may be referred to as having those shapes (e.g., such as being circular, straight, planar, curved, rounded, chamfered, angled, or the like). Further, elements shown intersecting one another may be referred to as intersecting elements or intersecting one another, in at least one example. Further still, an element shown within another element or shown outside of another element may be referred as such, in one example. 
     The invention will be further described in the following paragraphs. In one aspect, a method is provided that comprises: forming an aperture in a first axle component, the first axle component mounted to a second axle component, the aperture having a chamfer on one end of the aperture; inserting an object into the aperture, the object sized to fit into the aperture; and resistance welding at the aperture while pressing on the object with a variable pressure, the pressure adjusted to be at a first, higher level during a first portion of the welding and then at a second, lower level during a second later portion of the welding as the object fills the chamfer. In one example, the one end of the aperture may be closer to the second component than another end of the aperture. In another example, the object may be a slug. In yet another example, the pressure may be adjusted based on an angle of the chamfer. In a further example, the higher the angle, the higher the first pressure level may be. In another example, the first pressure level may be proportionally increased for an increase in the angle. In another example, the method may further comprise powering on a resistance heating power supply during the first portion and powering off the power supply during the second later portion of the welding. In another example, the method may further comprise completing the resistance welding after the second portion. In another example, the first axle component may be a differential carrier. In another example, the second axle component may be an axle tube. In yet another example, one of the first and second axle components may be coupled with an electric drive. 
     In another aspect, a method is provided that comprises forming a cylindrical aperture in a first axle component, the first axle component mounted to a second axle component, the aperture having a chamfer on an end of the aperture adjacent to the second component, the chamfer having a side wall at a fixed angle relative to a central axis of the aperture; inserting a cylindrical slug into the aperture, the slug sized to fit into the aperture; and resistance welding at the aperture while pressing on the slug with a variable pressure, the pressure adjusted to be at a first, higher level during a first portion of the welding and then at a second, lower level during a second later portion of the welding as the slug fills the chamfer. In one example, the pressure may be adjusted based on an angle of the chamfer. In another example, the higher the angle, the higher the first pressure level may be. In yet another example, the first pressure level may be proportionally increased for an increase in the angle. In another example, the method may further comprise powering on a resistance heating power supply during the first portion and powering off the resistance heating power supply during the second later portion of the welding. 
     In yet another aspect, an electric axle is provided that comprises an electric drive; an axle having a first component and a second component joined by a slug weld formed via a slug in an aperture, the aperture having a chamfer on an end of the aperture adjacent to the second component, the chamfer having a side wall at a fixed angle relative to a central axis of the aperture, the slug filling the aperture including the chamfer. 
     In any of the aspects or combinations of aspects, the aperture may be cylindrical. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. Moreover, unless explicitly stated to the contrary, the terms “first,” “second,” “third,” and the like are not intended to denote any order, position, quantity, or importance, but rather are used merely as labels to distinguish one element from another. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     As used herein, the term “approximately” is construed to mean plus or minus five percent of the range unless otherwise specified. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.