Abstract:
The invention relates to a method for producing an electrically conductive connection between a copper component  23, 23   a  and an aluminum component  24  by means of cold metal transfer welding, in which a welding wire is periodically moved back and forth from the material of a component to be welded.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for producing an electrically conductive connection between a copper component and an aluminum component. 
         [0002]    Electric starter motors are known by way of example from WO 02/16763 A1 and said starter motors are used to start internal combustion engines and are provided with a commutating device so as to transfer and reverse current to an armature that is mounted in such a manner as to be able to rotate in the starter. The commutating device comprises an armature side commutator or collector and multiple carbon brushes that lie on the collector, said carbon brushes in each case being influenced with a force by a brush spring radially onto the peripheral surface of the collector. The collector comprises multiple lamellae that are distributed over the periphery, said lamellae conducting current to armature windings in the case of contact with the brushes. The lamellae are typically embodied from copper, the connection to the armature windings is produced by way of connecting stranded wires that are connected to the lamellae. 
       SUMMARY OF THE INVENTION 
       [0003]    The object of the invention is to produce a permanent, current-conducting connection between a copper component and an aluminum component of an electric machine. 
         [0004]    The method in accordance with the invention is implemented so as to produce an electrically conductive connection between a copper component and an aluminum component. The electrically conductive connection can be used in different electric machines, for example in motors or generators or in electromagnetic relays. By way of example, a connection of this type is produced between copper lamellae of a collector in a commutating device and an aluminum wire that is connected to an armature winding or part of the armature winding. For example, the connection in a generator between an aluminum wire and a copper current rail or the connection between a copper crimp and one or multiple aluminum wires that are encompassed by the copper crimp is also possible. Furthermore, a connection between a copper component and an aluminum component is possible in electric machines that are part of a hybrid system, by way of example in combination with an internal combustion engine. The aluminum-copper connection can be used in the case of cell connectors, connecting pieces, current rail connections, in the case of battery systems and when integrating battery systems by way of current-conducting systems. 
         [0005]    In the case of the method in accordance with the invention, the connection between the copper component and the aluminum component is produced by means of cold metal transfer welding (CMT) in which a welding wire is periodically moved in the direction of and away from the basic material of one of the components that is to be welded. The CMT welding method has the advantage that when producing the aluminum-copper connection it is possible to avoid a brittle intermetallic phase. The CMT method is characterized by means of a precise procedure control in the case of a relatively low input of energy into the joining zone between the joining partners aluminum and copper. Accordingly, the temperature—compared with other welding methods—is relatively low. The CMT method provides a permanent, reliable connection between the joining partners aluminum and copper with a high electrical conductivity of the connection. 
         [0006]    In the case of the CMT method, the welding wire or supplementary wire periodically moves forward and backward in the direction of the joining site or welding site. During the welding procedure, the welding wire moves in the electric arc for as long as it takes to produce a short circuit and for the electric arc to be interrupted, at which point the welding wire is drawn back. In the burning phase, the electric arc merely introduces heat over a relatively short period of time. After the welding wire has been drawn back, the short circuit is eliminated and the electric arc is regenerated and directed at the joining site. During the backwards movement of the welding wire, the droplet release procedure is supported, which leads to a splash-free welding procedure. 
         [0007]    The forward and backward movement of the welding wire occurs with a relatively high frequency of by way of example at least 50 Hz, for example 70 Hz, wherein where appropriate lower or higher frequencies are also possible, by way of example 130 Hz. 
         [0008]    The aluminum component is embodied at least predominantly from aluminum, said component therefore comprising an aluminum proportion of at least 50%, advantageously at least 80% or at least 90%. Accordingly, alloys are also possible if the proportion of aluminum in the basic material is at least 50%. 
         [0009]    The copper component is likewise at least predominantly embodied from copper so that the copper proportion is at least 50%, preferably at least 80% or at least 90%. In this case, alloys are also possible if the copper proportion is at least 50%. 
         [0010]    The welding wire in a preferred embodiment is likewise embodied from aluminum or comprises an aluminum proportion of at least 50%. 
         [0011]    The aluminum component is by way of example an aluminum wire that can be provided where appropriate with an insulating layer, by way of example an insulating paint, wherein in the region of the welding site the insulating layer has been advantageously removed. The aluminum wire can be used for a winding, by way of example for an armature winding in an electric machine, wherein the welded connection to the copper is produced in the region of the free ends of the aluminum wire. 
         [0012]    The copper component is by way of example a copper wire or a copper current rail. In the case of a commutating device, the copper component is a lamella of the collector that is connected by means of the CMT method to the aluminum wire of the armature winding or an aluminum stranded wire by way of which the electrical connection to the armature winding is produced. 
         [0013]    In accordance with a further advantageous embodiment, the copper component is coated at least in the region of the welding site with a tin coating. The tin coating prevents a contact corrosion between the aluminum and the copper after the welding procedure. An intermetallic phase growth that is produced as a result of the effect of temperature is avoided by means of the tin coating. 
         [0014]    In accordance with a further expedient embodiment, it is also possible for aluminum components that are arranged in a multi-layered manner to be connected to one another using the CMT method. It is thus by way of example possible to weld two aluminum conductors to one another with the aid of the CMT method. It is possible chronologically prior to or after welding the aluminum components to weld said aluminum components to the copper component using the CMT method. 
         [0015]    In accordance with a further expedient embodiment, the joining partners are mechanically connected to one another, by way of example by means of crimping, prior to implementing the CMT method. For example, two aluminum conductors can be held together with the aid of a crimp that is preferably embodied from copper, wherein after the mechanical connection has been produced, the CMT method is implemented so as to weld the crimp to the aluminum conductors. It is consequently expedient for the mechanical connection of two aluminum components to use a copper component that is connected to one or to the two aluminum components using the CMT method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Further advantages and expedient embodiments are evident in the further claims, the description of the figures and the drawings. In the drawings: 
           [0017]      FIG. 1  illustrates a starting device for an internal combustion engine in a longitudinal section, 
           [0018]      FIG. 2  illustrates in a perspective view two aluminum conductors that are connected using the CMT welding method to a tin-coated commutator lamella that is embodied from copper, 
           [0019]      FIG. 3  illustrates schematically a sectional view with the welded connection between two aluminum conductors and a commutator lamella, 
           [0020]      FIG. 4  illustrates a commutator lamella in a side view with a bevel in the region of a section that is bent radially outwards and the welded connection to the aluminum conductor connects to said bent section, 
           [0021]      FIG. 5  is an illustration corresponding to  FIG. 4  however with a rounded transition on the section of the lamella that extends radially outwards, 
           [0022]      FIG. 6  illustrates a lamella having a U-shaped recess in the region of the part, said recess being used to connect to the aluminum conductor, wherein the entire lamella is projected in one plane, 
           [0023]      FIG. 7  illustrates in a perspective view two aluminum wires in connection with a copper current rail of a generator, 
           [0024]      FIG. 8  illustrates the connection in accordance with  FIG. 7  in a schematic plan view, 
           [0025]      FIG. 9  illustrates in a perspective view two aluminum conductors that are connected with the aid of a copper crimp, 
           [0026]      FIG. 10  illustrates a welding tool for the cold metal transfer method. 
       
    
    
       [0027]    Identical components in the figures are provided with identical reference numerals. 
       DETAILED DESCRIPTION 
       [0028]      FIG. 1  illustrates a starting device  1  for an internal combustion engine, said starting device receiving an electric starter motor in a housing  2  that comprises a forward-lying bearing plate  3 . The motor shaft  5  of the starter motor  4  drives a drive shaft  11  by way of a planetary gear  6 , and a carrier  8  of a freewheel device  7  is arranged on said drive shaft and is coupled in an axially-displaceable manner yet in the direction of rotation to the drive shaft  11 . The carrier  8  supports itself by way of supporting rollers  9  on a roller bundle  10  that is embodied as a single part with a starter sprocket  12 . In the case of an axial feed motion, the starter sprocket  12  moves from a retracted out-of-operation position into an advanced engagement position with the sprocket wheel of an internal combustion engine. 
         [0029]    The axial feed motion of the starter sprocket  12  is performed with the aid of an electromagnetic starter relay  13  that comprises an axially-adjustable lifting armature  14  that is coupled to a fork lever. In the case of an axial adjusting movement of the lifting armature  14 , the fork lever  15  that is mounted on the housing is pivoted as a result of which the carrier  8  including the starter sprocket  12  is adjusted in the axial direction. 
         [0030]    The electric starter motor  4  is embodied as an internal rotor motor and comprises an armature  16  that is connected to the motor shaft  5  in such a manner that said armature cannot rotate with respect to said motor shaft and said armature includes armature coils or armature windings that can be electrically excited. The armature windings of the armature  16  are energized by way of a commutating device  17 . The electromagnetic field that is generated by the electric armature windings interacts with the magnetic field of permanent magnets  18  that are arranged on the inner side of the stator that surrounds the armature. 
         [0031]    The commutating device  17  comprises multiple spring-brush units  19  that comprise in each case on the housing side a carbon brush  20  and a brush spring  21 , and also an armature-side collector  22  that comprises lamellae that are distributed over the periphery, said lamellae being electrically separated from one another and connected to the armature windings. The carbon brushes  20  are influenced with a force by the brush springs  21  radially against the peripheral surface of the collector  22 . Carbon brushes  20  and brush springs  21  are expediently received in brush holders that are fixedly connected to the housing of the starter motor. Altogether six spring-brush units  19  are provided distributed over the periphery in a uniform manner. Where appropriate, it is also possible to arrange only four spring-brush units  19  distributed over the periphery. 
         [0032]      FIG. 2  illustrates the connection between a lamella  23  and two aluminum wires  24  that are associated with in each case one armature winding. The lamella  23  is embodied from copper and comprises a section that extends in the axial direction of the motor longitudinal axis and the carbon brushes lie on the outer side of said section. A radial section  23   a  is located as one piece with the axial section on an end face and the connection to the aluminum wires  24  is produced with the aid of the cold metal transfer welding method (CMT) on the radially outer-lying end face of said section. 
         [0033]      FIG. 3  illustrates the various phases during the CMT welding method. The additional material that originates from a welding wire initially connects the end face of the aluminum wire  24 , which is placed closest on the end face of the radial section  23   a  of the lamella  23 , to the section  23   a.  Further additional material  25   b  is subsequently accumulated on the first additional material  25   a  that is connected on the side of the lamella  23  to the beveled end face of the section  23   a  of the lamella. Finally, further additional material  25   c  is applied between the end face of the second aluminum wire  24  that lies further away and the two first accumulations of additional material  25   a  and  25   b.    
         [0034]    As is furthermore evident in  FIGS. 2 and 3 , the aluminum wire  24  is provided with an insulating coating, in particular an insulating paint, wherein merely the end sections that are connected by means of the CMT welding method to the lamellae are free from insulating coating. 
         [0035]      FIGS. 4 to 6  illustrate schematically various geometric embodiments of the radial section  23   a  of a lamella  23 . The radial section  23   a  of the lamella  23  forms a lamellae lug. In accordance with  FIG. 4  the radial section  23   a  comprises a bevel  26  between the radially outer-lying end face and the inner-lying side surface that extends adjacent to the axial section of the lamella. In  FIG. 5 , the transition between the radially outer-lying end face of the radial section  23   a  and the side surface that is facing the axial section is embodied as rounded. In  FIG. 6 , a U-shaped recess  28  is introduced into the radial section  23   a  and the end of an aluminum wire can be inserted into said recess. 
         [0036]      FIGS. 7 and 8  illustrate a further exemplary embodiment for a connection between a copper current rail  29  in a generator and two aluminum wires  24 . The copper current rail  29  is coated with a tin coating  30  ( FIG. 8 ). The two aluminum wires  24  and the copper current rail  29  are connected to one another using the CMT welding method. 
         [0037]      FIG. 9  illustrates two illustrations of the connection of two aluminum wires  24  that extend parallel to one another with the aid of a copper crimp  31 . The crimp  31  is initially placed mechanically around the end sections of the two aluminum wires  24  that lie parallel to one another and are joined together so that a mechanical connection is produced between the crimp  31  and the two aluminum wires  24 . A welded connection is subsequently produced with the aid of the CMT method in which the copper crimp  31  and the aluminum wires  24  are welded to one another. 
         [0038]      FIG. 10  illustrates a welding tool  32  that is used for the CMT welding method. A welding wire  33  is periodically moved in the welding tool  32  in the direction of the welding site  35  or moved away from the welding site  35  as indicated by the double arrow. The frequency of the movement of the welding wire  33  is by way of example 70 Hz. 
         [0039]    The welding tool  32  generates an electric arc  34  that makes contact with the welding site  35  on the workpiece. As the welding wire  33  approaches the welding site  35 , said wire being embodied by way of example from aluminum, a short circuit is produced and as a result of which the electric arc  34  is interrupted. During the subsequent rearwards movement of the welding wire  33  a droplet release procedure occurs, the short circuit is simultaneously eliminated and the electric arc  34  is regenerated. Owing to this periodically-repeating procedure, the introduction of heat into the work piece is relatively low.