Abstract:
A cold metal transfer (CMT) contact tip has a substantially copper-free contact surface defining a wire passage. The CMT contact tip is used with a CMT welding apparatus having a weld wire and a wire feeder for feeding and oscillating the weld wire forward and backward through the wire passage where the weld wire shares a concentric axis with the wire passage.

Description:
BACKGROUND 
       [0001]    The present invention relates to an arc welding apparatus, and in particular, to a cold metal transfer (CMT) welding apparatus and method of operation. 
         [0002]    Gas metal arc welding (GMAW), commonly referred to as metal inert gas (MIG) welding or metal active gas (MAG) welding, is a welding process in which a consumable weld wire and a shielding gas are fed through a welding torch. There are many types of GMAW that can be used in various situations. Typically, the weld wire is fed though a contact tip made of copper or a copper alloy in the welding torch. The contact tip guides the weld wire and also provides a continuing electrical connection from a power supply to the weld wire as the weld wire is fed. The weld wire is held near or in contact with a metal welding surface such that electricity can arc between the weld wire and the surface. The arc causes a tip of the weld wire to liquefy and is subsequently applied to the welding surface. Typically, an inert or semi-inert gas is blown over the weld wire to limit contaminants near the weld. While GMAW is a useful welding technique, it can heat the welding surface to temperatures that cause undesirable material changes, such as hardening and warpage. 
         [0003]    GMAW cold metal transfer (CMT) is a welding technique that reduces heat created on the welding surface. CMT is based on a deliberate and systematic activation and deactivation of the heating arc so as to systematically heat and cool the weld wire while bringing the wire into and out of contact with the weld pool at a rapid frequency. This is performed by axially oscillating the weld wire forward and backward through the copper contact tip with a frequency of up to 70 times per second. Unfortunately, the grinding contact between the weld wire and the contact tip can wear away the contact tip, especially if the weld wire is a harder material than the contact tip. This wearing away can cause material worn from the contact tip to be deposited on the weld wire or blown into the weld. 
       SUMMARY 
       [0004]    According to the present invention, a cold metal transfer (CMT) contact tip has a substantially copper-free contact surface defining a wire passage. The CMT contact tip is used with a CMT welding apparatus having a weld wire and a wire feeder for feeding and oscillating the weld wire forward and backward through the wire passage where the weld wire shares a concentric axis with the wire passage. 
         [0005]    In another embodiment, a CMT welding apparatus includes a CMT contact tip, a weld wire, and a CMT wire feeder. The CMT contact tip has a contact surface that is a substantially copper-free metal. The weld wire is in physical contact with the contact surface. The CMT wire feeder is constructed and arranged to feed the weld wire through the CMT contact tip with an axially oscillating motion with respect to a centerline axis of the CMT contact tip. 
         [0006]    Another embodiment includes a method for welding a workpiece. The method includes the steps of installing a CMT contact tip having a substantially copper-free metal contact surface into a CMT welding device, feeding a weld wire through the CMT contact tip such that the weld wire axially oscillates along a centerline axis of the CMT contact tip and in contact with the contact surface, and depositing material from the weld wire on the workpiece to form a substantially copper-free weld. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic elevation view of a cold metal transfer welding apparatus embodying the present invention. 
           [0008]      FIG. 2  is a side partial cut-away view of a contact tip of in the welding apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring to FIG&#39;s  1  and  2 , cold metal transfer (CMT) welding apparatus  10  of the present invention is useful for welding workpiece  50  that may be made of a cobalt alloy or nickel alloy. Welding apparatus  10  includes weld wire  40  that may be made of a cobalt alloy, and contact tip  32  having a substantially copper-free contact surface  74 , such as an aluminum surface. When weld wire  40  grinds against contact surface  74 , small particles of aluminum can break off instead of small particles of weld-contaminating copper. 
         [0010]    Supply housing  12  of welding apparatus  10  generally supports and surrounds electric power supply  18 , wire feeder  20  and gas supply  22 . A preferably flexible conduit  16  of apparatus  10  is supported by and axially projects outward from supply housing  12  to a distal end that connects to torch  14  of apparatus  10 . Conduit  16  generally surrounds and protects power cable  38  connected electrically to power supply  18 , weld wire  40  fed from wire feeder  20 , and gas hose  42  communicating with gas supply  22 . Both gas supply  22  and wire feeder  20  may be powered via power supply  18 . 
         [0011]    Torch  14  has gas nozzle  24  that has and extends axially along centerline axis CL between opposite ends with the first end connected structurally to the distal end of conduit  16  and the opposite second end carrying nozzle output face  28 . Contact tip  32  of torch  14  is disposed in cavity  30  defined by and radially inward from nozzle  24 . Contact tip  32  as well as weld wire  40  are also oriented with centerline axis C L . 
         [0012]    Power cable  38  electrically connects power supply  18  to contact tip  32 . Power supply  18  provides electrical power to contact tip  32  for welding. Power supply  18  also provides power to, and controls operation of, wire feeder  20  and gas supply  22 . Wire feeder  20  feeds weld wire  40  from a spool (not shown) through conduit  16  and through contact tip  32  such that weld wire tip  48  extends past nozzle output face  28 . Weld wire  40  is in contact with contact tip  32  such that electrical power transmitted to contact tip  32  is subsequently transmitted to weld wire  40 . Weld wire  40  can be made of cobalt or cobalt alloys. Gas supply  22  is connected to gas nozzle  24  via gas hose  42 . Gas supplied from gas supply  22  flows into nozzle cavity  30  and out nozzle output face  28 . Gas supply  22  can supply inert and semi-inert gasses such as argon, helium, carbon dioxide, or mixtures thereof. The gas used can be selected so as to adequately shield weld wire  40  from undesirable contaminants during welding. 
         [0013]    In operation, power supply  18  signals wire feeder  20  to feed weld wire  40  toward workpiece  50 . Power supply  18  also signals gas supply  22  to supply gas to and out of gas nozzle  24 . Power supply  18  further supplies electrical power to contact tip  32 , which transmits that power to weld wire  40 . When weld wire tip  48  becomes sufficiently close to, or in contact with, workpiece  50 , electricity can flow from weld wire  40  to workpiece  50 . Workpiece  50  may be a high strength metal such as cobalt, cobalt alloys, or nickel alloys. In one embodiment, workpiece  50  can be a damaged turbine blade for a gas turbine engine. When power supply  18  senses a short circuit, it signals wire feeder  20  to retract weld wire  40 . After a brief retraction, power supply  18  signals wire feeder  20  to feed weld wire  40  toward workpiece  50  again. This repeats such that weld wire  40  is effectively oscillating axially with respect to contact tip  32 . With each oscillation, electricity arcs from weld wire tip  48  to workpiece  50  and causes a portion of weld wire  40  (also called weld filler) to melt and become deposited as weld metal  52  (also called weld pool) on workpiece  50 . As oscillating frequency increases, heat imparted to workpiece  50  decreases. In one embodiment, oscillations may occur about 70 times per second. In other embodiments, oscillating frequency can be less than about  70  times per second so long as heat imparted to workpiece  50  does not exceed an acceptable threshold. During these oscillations, weld wire  40  maintains electrical, and physical, contact with contact tip  32 . 
         [0014]      FIG. 2  is a side partial cut-away view of contact tip  32 . Contact tip  32  includes electrically conductive metal body  60 , connection portion  62 , proximal end  64 , tip or distal end  66 , passage  68 , passage inlet  70 , passage outlet  72 , and contact surface  74 . Shape of contact tip  32  is defined by a substantially solid metal body  60 . Metal body  60  may be a substantially copper-free metal such as aluminum, aluminum alloys, titanium, titanium alloys, nickel, nickel alloys, cobalt, cobalt alloys, and stainless steel. Contact tip  32  extends from proximal end  64  to tip or distal end  66 . Connection portion  62  is configured to connect to a welding torch of a CMT welding apparatus such as torch  14  as illustrated in  FIG. 1 . In the illustrated embodiment, connection portion  62  carries a threaded surface for threading into torch  14  (shown in  FIG. 1 ). 
         [0015]    Passage  68  is disposed concentrically to and extending along an axial length of contact tip  32  from passage inlet  70  (at proximal end  64 ) to passage outlet  72  (at distal end  66 ). In the illustrated embodiment, passage inlet  70  has a substantially frusto-conical shape for facilitating insertion of weld wire  40  (shown in  FIG. 1 ). The remainder of passage  68  has a substantially cylindrical shape and is defined by contact surface  74  carried by body  60 . Contact surface  74  may comprise a substantially copper-free metal such as aluminum, aluminum alloys, titanium, titanium alloys, nickel, nickel alloys, cobalt, cobalt alloys, and stainless steel. In one embodiment, contact surface  74  may be an aluminum alloy with a relatively high conductivity. In the illustrated embodiment, contact tip  32  is a single-homogeneous piece made of a substantially uniform metal. Consequently, the metal used for connection portion  62  is preferably the same metal as that used for contact surface  74  and is a single, continuous piece there between. Using a single continuous piece, as opposed to multiple pieces, may reduce electrical resistance of contact tip  32 . In an alternative embodiment, a portion of contact tip  32  may be made of virtually any suitable material so long as contact surface  74  is a substantially copper-free metal. The substantially copper-free metal has a sufficiently small quantity of copper (or zero copper) such that performance of weld metal  52  will not be measurably altered due to copper contamination. The substantially copper-free metal can contain essentially no copper. 
         [0016]    As weld wire  40  (shown in  FIG. 1 ) is fed through, and oscillates axially in, passage  68 , weld wire  40  can rub or grind against contact surface  74 . This grinding can wear away contact surface  74 , causing particles from contact surface  74  to become deposited in weld metal  52  (shown in  FIG. 1 ). In certain circumstances, it may be undesirable for certain metals to contaminate weld metal  52 . For example, certain portions of gas turbine engines, such as turbine blades, can be made from high-strength, heat-tolerant materials such as cobalt, cobalt alloys, and nickel alloys. Using a weld wire of cobalt or cobalt alloys to repair the turbine blade can create a relatively strong, creep-resistant weld. However, if weld metal  52  is contaminated with copper, weld metal  52  can have a greater susceptibility to creep. This can be especially problematic when using CMT welding apparatus  10  with a cobalt weld wire  40  and a copper contact surface  74 . The relatively fast oscillation of the relatively hard cobalt weld wire  40  can grind a relatively large quantity of copper off contact surface  74 . By using contact tip  32  configured for use with CMT welding apparatus  10  and made from a substantially copper-free metal, such as aluminum, the amount of copper in weld metal  52  can be reduced or eliminated. Many alloys, including cobalt alloys, already have aluminum in the alloy. Such alloys can tolerate trace additions of aluminum. In some applications, contamination of weld metal  52  can weaken weld metal  52  in ways other than creep. For example, contaminating a relatively ductile alloy with an incompatible metal can make the ductile alloy more brittle. 
         [0017]    Selection of the substantially copper-free metal can depend on the application and on design preferences. In one embodiment, where weld wire  40  comprises cobalt or a cobalt alloy and contact surface  74  is desired to have a relatively low electrical resistance, contact surface  74  may comprise aluminum or an aluminum alloy. In another embodiment, where weld wire  40  is cobalt or a cobalt alloy and contact surface  74  is desired to be relatively wear-resistant, contact surface  74  can be cobalt, a cobalt alloy, or stainless steel. In another embodiment, where weld wire  40  is nickel or a nickel alloy and contact surface  74  is desired to be relatively wear-resistant, contact surface  74  can be nickel, a nickel alloy, or stainless steel. In yet another embodiment, where weld wire  40  is titanium or a titanium alloy, contact surface  74  can be titanium or a titanium alloy. Weld wire  40  of a particular titanium alloy can be paired with contact tip  32  of substantially the same titanium alloy. In still other embodiments, contact surface  74  can be other non-copper metals to suit the needs of particular applications. 
         [0018]    It will be recognized that the present invention provides numerous benefits and advantages. For example, the invention allows for welding using CMT technology while limiting copper contamination in weld metal  52 . This can allow use of CMT in various applications that would not otherwise be practical. This can be particularly useful in assembly and repair of gas turbine engines when utilizing a cobalt or cobalt alloy weld wire  40  in a repair welding process. Using a substantially copper-free contact surface  74  allows one to create welds with a reduced propensity to creep. Using CMT allows for welding with less warpage in workpiece  50  do to welding heat. 
         [0019]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. For example, use of welding apparatus  10  is not limited to assembly and repair of gas turbine engines, rather, can be used in virtually any suitable welding project that can benefit from its use. Additionally, shape of contact tip  32  need not be limited to that illustrated in  FIG. 2  so long as the shape is suitable for use in a cold metal transfer welding apparatus.