Abstract:
A thermal metal spraying apparatus for applying a metal coating to a target surface. The apparatus provides a cathode, a wire feed stock having a free end, and a wire guide that directs the free end of the wire feedstock to a position for establishing and maintaining a plasma transferred wire arc between the cathode and the free end of the wire feedstock. The wire guide maintains at least three points of contact with the wire feedstock as the wire feedstock is fed through the wire guide.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 62/346,081, filed on Jun. 6, 2016. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to the field of thermal or plasma metal spraying for use in applying thin films and coatings to workpieces, and in particular, wire guide apparatuses that reduce the variation in coatings produced by thermal or plasma metal spraying. 
       BACKGROUND 
       [0003]    The plasma transferred wire arc (“PTWA”) process is a particularly useful high-pressure plasma coating process capable of producing high-quality metallic coatings for a variety of applications, such as the coating of engine cylinder bores. In the PTWA process, a high-pressure plasma is generated in a small region of space at the exit of a plasma torch. A continuously-fed metallic wire impinges upon this region, wherein the wire is melted and atomized by the plasma. High-speed gas emerging from the plasma torch directs the molten metal toward the surface to be coated. 
         [0004]    When feeding the wire during the PTWA process, a cylindrical wire guide on the torch head directs the wire by feeding the wire through the wire guide immediately prior to the wire being fed into the plasma jet. The positioning of the wire relative to the plasma jet is critical to the thermal spray process. Thus, the wire guide has an extremely tight tolerance relative to the outer diameter of the wire so as to strictly control the positioning of the wire relative to the plasma jet. However, even with the tight tolerance established between the wire guide and the wire, the wire guide and the wire establish a coaxial relationship which still allows the wire to float to a certain degree since there must be a sufficient amount of space between the wire and the wire guide to allow the wire to pass through the wire guide. This floating of the wire may allow the wire to move from its optimal position when entering the thermal jet of the PTWA process. Since the positioning of the wire in the thermal jet spray is critical to the quality of the PTWA process, such floating can affect the quality of the PTWA process. 
       SUMMARY 
       [0005]    Disclosed herein are thermal metal spraying apparatuses and methods for applying a metal coating to a target surface. In one implementation, a thermal metal spraying apparatus includes a cathode, a wire feed stock, and a wire guide. The wire guide directs a free end of the wire feedstock to a position for establishing and maintaining a plasma transferred wire arc between the cathode and the free end of the wire feedstock. The wire guide maintains at least three points of contact with the wire feedstock as the wire feedstock is fed through the wire guide. 
         [0006]    The at least three points of contact can comprise a first point, a second point, and a third point. The first and second points can be on a first side of the wire guide, and the third point can be on a second side of the wire guide. The second side of the wire guide can be radially opposite the first side of the wire guide relative to an axis of the wire guide. The wire feedstock can be fed through an inner bore of the wire guide. 
         [0007]    The wire guide can include an aperture extending through a wall of the wire guide and a member that extends through the aperture into the inner bore of the wire guide. The wall can define the inner bore of the wire guide, and the aperture can be in communication with the inner bore of the wire guide. The member can bias the wire feed stock into engagement with an inner surface of the inner bore of the wire guide. The aperture can extend through a first side of the wire guide, and the wire feedstock can be biased into engagement against a second side of the wire guide. The second side of the wire guide can be radially opposite the first side of the wire guide relative to an axis of the wire guide. The member can be a leaf spring extending axially across the aperture. The member can be a ball partially disposed within the aperture. The wire guide can include a spring disposed outside of the inner bore of the wire guide. The spring can bias the ball toward the inner bore of the wire guide. The aperture can extend through a first side of the wire guide. Two of the at least three points of contact can be on a second side of wire guide, and the second side of the wire guide is radially opposite the first side of the wire guide relative to an axis of the wire guide. 
         [0008]    The inner bore of the wire guide can have a slight curvature formed therein that extends axially. Two of the at least three points of contact can be on a first side of the wire guide, and one of the at least three points of contact can be on a second side of the wire guide. The second side of the wire guide can be radially opposite the first side of the wire guide relative to an axis of the wire guide. 
         [0009]    The wire guide can include a first section and a second section adjacent to and axially misaligned with the first section. The first section can be forced into engagement with the wire feedstock along a first side of the wire guide, and the second section can be forced into engagement with the wire feedstock along a second side of the wire guide. The second side of the wire guide can be radially opposite the first side of the wire guide relative to an axis of the wire guide. The wire guide can include a cutaway section that collapses around the wire feedstock to engage the wire feedstock. The cutaway section can be retained in a collapsed position by a ring. 
         [0010]    In another implementation, a thermal metal spraying apparatus for thermally depositing molten metal from a free end of a consumable wire onto a target surface is disclosed. The thermal metal spraying apparatus includes a cathode and a wire guide that directs the free end of the consumable wire into a position for establishing and maintaining a plasma transferred wire arc between the cathode and the free end of the consumable wire. The wire guide can bias the consumable wire toward one side of the wire guide as the consumable wire is fed through the wire guide. 
         [0011]    In yet another implementation, a method of thermally depositing molten metal onto a target surface using a thermal metal spraying apparatus is disclosed. The thermal metal spraying apparatus includes a cathode and a wire guide directing a free end of the a consumable wire to a position for establishing and maintaining a plasma transferred wire arc between the cathode and the free end of the consumable wire. The method includes biasing the consumable wire toward a first side of the wire guide as the consumable wire is fed through the wire guide. At least three points of contact are maintained on the first side and the second side of the wire guide. The second side is radially opposite the first side of the wire guide relative to an axis of the wire guide. 
         [0012]    An aperture can extend through a wall of the wire guide. The wall can define an inner bore of the wire guide, and the aperture can be in communication with the inner bore of the wire guide. At least one of a flange, a leaf spring, or a ball can extend through the aperture into the inner bore of the wire guide to bias the consumable wire into engagement with the first side of the wire guide. The consumable wire can be fed through an inner bore of the wire guide, and the inner bore of the wire guide can have a slight curvature formed therein that extends axially. A portion of the wire guide collapses or axially misaligns to bias the consumable wire into contact with the first side of the wire guide. 
         [0013]    These and other aspects of the present disclosure are disclosed in the following detailed description of the implementations, the appended claims and the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to-scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. 
           [0015]      FIG. 1  is a schematic drawing showing a PTWA assembly; 
           [0016]      FIGS. 2A-B  are schematic drawings showing a first alternative embodiment of a wire guide for the PTWA assembly; 
           [0017]      FIGS. 3A-B  are schematic drawings showings a second alternative embodiment of the wire guide for the PTWA assembly; 
           [0018]      FIGS. 4A-B  are schematic drawings showing a third alternative embodiment of the wire guide for the PTWA assembly; 
           [0019]      FIGS. 5A-B  are schematic drawings showing a fourth alternative embodiment of the wire guide for the PTWA assembly; 
           [0020]      FIGS. 6A-B  are schematic drawings showings a fifth alternative embodiment of the wire guide for the PTWA assembly; and 
           [0021]      FIGS. 7A-B  are schematic drawings showings a sixth alternative embodiment of the wire guide for the PTWA assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  shows a schematic representation of a PTWA torch assembly  10  consisting of a torch body  11  containing a plasma gas port  12  and a secondary gas port  18 . The torch body  11  is formed of an electrically conductive metal. A supply of plasma gas  60  is connected by means of the plasma gas port  12  to a cathode holder  13  through which the plasma gas  60  flows into the inside of the cathode assembly  14  and exits through tangential ports  15  located in the cathode holder  13 . The plasma gas  60  forms a vortex flow between the outside of the cathode assembly  14  and the internal surface of a pilot plasma nozzle  16  and then exits through the constricting orifice  17 . The plasma gas vortex provides substantial cooling of the heat being dissipated by the cathode function. 
         [0023]    A supply of secondary gas  58  enters the PTWA torch assembly  10  through the secondary gas port  18 , which directs the secondary gas  58  to a gas manifold  19 . (The gas manifold  19  is a cavity formed between baffle plate  20  and the torch body  11 , and then through bores  20 a into another manifold  21  containing bores  22 .) The secondary gas flow is uniformly distributed through the equi-angularly spaced bores  22  concentrically surrounding the outside of the constricting orifice  17 . The flow of the secondary gas  58  through the equi-angularly spaced bores  22  within the pilot plasma nozzle  16  provides atomization to the molten particles, a carrier gas for the particles, cooling to the pilot plasma nozzle  16 , and a minimum disturbance to the plasma arc, which limits turbulence. 
         [0024]    A wire feedstock  23  is fed (by wire pushing and pulling feed rollers  42 , driven by a speed controlled motor  43 ) uniformly and constantly through a wire contact tip  24 , the purpose of which is to make firm electrical contact to the wire feedstock  23  as it slides through the wire contact tip  24  along a longitudinal axis  55  of the wire feedstock  23 . As shown, the wire contact tip  24  is composed of two pieces  24 A,  24 B held in spring or pressure load contact with the wire feedstock  23  by means of a rubber ring  26  or other suitable means. The wire contact tip  24  is fabricated from a high electrical conducting material. 
         [0025]    As the wire feedstock  23  exits the wire contact tip  24 , it enters a wire guide  25  for guiding the wire feedstock  23  into precise alignment with an axial centerline  41  of the constricting orifice  17 . The wire guide  25  is supported by a wire guide block  27  contained within an insulating block  28 , which provides electrical insulation between the torch body  11  (held at a negative electrical potential) and the wire contact tip  24  (held at a positive electrical potential). A small port  29  in the insulator block  28  allows a small amount of secondary gas  58  to be diverted through the wire guide block  27  in order to provide heat removal from the wire guide block  27 . This can also be done by bleeding gas around or through the pilot plasma nozzle  16 . 
         [0026]    The wire guide block  27  is maintained in pressure contact with the pilot plasma nozzle  16  to provide an electrical connection between the pilot plasma nozzle  16  and the wire guide block  27 . The electrical connection is made with the torch body  11 , and thereby to the cathode assembly  14  (having a cathode  59 ), through the cathode holder  13  from the negative terminal of a power supply  40 . The power supply  40  may contain both a pilot power supply and a main power supply operated through isolation contactors (not shown). A positive electrical connection is made to the wire contact tip  24  and the insulating block  28  of the PTWA torch assembly  10  from the positive terminal of the power supply  40 . 
         [0027]    The wire feedstock  23  is fed toward the axial centerline  41  of the constricting orifice  17 , which is also the axis of a transferred arc  46 . Concurrently, the cathode assembly  14  is electrically energized with a negative charge, and the wire feedstock  23 , as well as the pilot plasma nozzle  16 , although the pilot plasma nozzle  16  can be isolated, is electrically charged with a positive charge. The wire guide  25  and the wire feedstock  23  can be positioned relative to the pilot plasma nozzle  16  by many different methods, including the pilot plasma nozzle  16  having the features for holding and positioning the wire guide  25 . 
         [0028]    To initiate operation of the PTWA torch assembly  10 , plasma gas  60  at an inlet gas pressure of between 50 and 140 psig is caused to flow through the plasma gas port  12 , creating a vortex flow of the plasma gas  60  about an inner surface of the pilot plasma nozzle  16 , and after an initial period of time of typically two seconds, high-voltage dc power or high frequency power is connected to the electrodes causing a pilot arc and pilot plasma to be momentarily activated. Additional energy is then added to the pilot arc and plasma by means of increasing the plasma arc current to the electrodes to typically between 60 and 85 amps to extend the plasma arc providing an electrical path  45  for the plasma arc to transfer from the pilot plasma nozzle  16  to the wire tip or free end  57  of the wire feedstock  23  (as shown in  FIG. 2 ). The wire feedstock  23  is fed by means of the feed rollers  42  into the extended transferred plasma arc wherein the free end  57  of the wire feedstock  23  is melted by the intense heat of the transferred arc  46  and associated plasma  47  that surrounds the transferred arc  46 . Molten metal particles  48  are formed on the free end  57  of the wire feedstock  23  and are atomized into fine particles  50  by the viscous shear force established between the high velocity, supersonic plasma jet and the initially stationary molten droplets. The molten metal particles  48  are further atomized and accelerated by the much larger mass flow of secondary gas  58  through bores  22  that converge at a location or zone  49  beyond the melting of the free end  57  of the wire feedstock  23 . The fine particles  50  created from the wire feedstock  23  are propelled to a substrate surface  51  to form a deposit  52 . 
         [0029]    It has been observed that the positioning of the free end  57  of the wire feedstock  23  relative to the plasma arc is critical to the quality of the deposit  52  formed on the substrate surface  51  by the PTWA process. Previous designs have strictly controlled the positioning of the wire feedstock  23  by maintaining an extremely tight tolerance between the outer diameter of the wire feedstock  23  and the inner diameter of the wire guide  25 . However, even with a tight tolerance established between the wire feedstock  23  and the wire guide  25 , the wire feedstock  23  is still allowed to float to a certain degree within the wire guide  25 . The floating of the wire feedstock  23  occurs because there must still be a sufficient amount of space between the wire feedstock  23  and the wire guide  25  to allow the wire feedstock  23  to be coaxially fed through the wire guide  25 . The floating of the wire feedstock  23  can allow the wire feedstock  23  to move from its optimal position when entering the plasma arc. 
         [0030]    As the result of experimentation, alternative embodiments of the wire guide  25  have been developed to address the problems created by the floating of the wire feedstock  23  within the wire guide  25 . The alternative embodiments of the wire guide  25  shown in  FIGS. 2-7  are designed to bias the wire feedstock  23  toward one side of the wire guide  25  to create at least three points of contact between the wire feedstock  23  and the wire guide  25 , which result in decreasing or eliminating the floating that is traditionally present as described above. First and second points  101 ,  102  of the at least three points of contact can be on a first side  201  of the wire feedstock  23  and can be part of a continuous surface. A third point  103  of the at least three points of contact can be along a second side  202  that is radially or circumferentially opposite the first side  201  of the wire feedstock  23  relative to an axis of the wire feedstock  23 . 
         [0031]      FIGS. 2A-2B  show a first alternative embodiment  251  of the wire guide  25 , wherein the wire guide  25  includes an aperture  260  in a wall of the wire guide  25  and a retainer  301  having a free end that complementary to the aperture  260 . The aperture  260  can be in communication with the inner bore  303  of the wire guide  25 . The inner bore  303  of the wire guide  25  can be substantially straight.  FIG. 2A  shows no load applied to the retainer  301 , and  FIG. 2B  shows the retainer  301  applying a load to the wire feedstock  23 . The free end of the retainer  301  can have a flange  300  that is configured to fit within the aperture  260  of the wire guide  25  so that the retainer  301  may apply a load to the wire feedstock  23 . The retainer  301  may be pivotally supported (not shown) by the wire guide  25  or an additional structure of the PTWA torch assembly  10 , and the load from the retainer  301  can be applied either pneumatically or by a spring force. The retainer  301  can be either conductive or non-conductive. 
         [0032]    As shown, the flange  300  of the retainer  301  extends far enough into the aperture  260  of the wire guide  25  so that the flange  300  can engage the wire feedstock  23  and bias the wire feedstock  23  against an inner surface of the wire guide  25  along the first side  201 . The first and second points  101 ,  102  of the at least three points of contact can be along any point where the wire feedstock  23  contacts the wire guide  25  along the first side  201  of the wire guide  25 . The third point  103  of the at least three points of contact can be along any point where the flange  300  of the retainer  301  of the wire guide  25  contacts the wire feedstock  23 . 
         [0033]      FIGS. 3A-3B  show a second alternative embodiment  252  of the wire guide  25  wherein the inner bore  303  of the wire guide  25  has a slight curvature formed therein. The slight curvature can be less than 100 degrees of curvature.  FIG. 3A  shows the second alternative embodiment  252  of the wire guide  25  without the wire feedstock  23  extending therethrough, and  FIG. 3B  shows the second alternative embodiment  252  of the wire guide  25  with the wire feedstock  23  extending therethrough. The slight curvature of the inner bore  303  extends axially. As  FIG. 3B  illustrates, the wire feedstock  23  does not bend as the wire feedstock  23  travels through the slightly curved inner bore  303  of the wire guide  25 . As a result, the first and second points  101 ,  102  of the at least three points of contact can be where the wire feedstock  23  comes into contact with the first side  201  of the wire guide  25 . The third point  103  of the at least three points of contact can be where the wire feedstock  23  comes into contact with the second side  202  of the wire guide  25 . 
         [0034]      FIGS. 4A-4B  show a third alternative embodiment  253  of the wire guide  25 , wherein the wire guide  25  includes an aperture  304  formed in the wall of the wire guide  25  with a leaf spring  305  extending axially across the aperture  304 . The aperture  304  can be in communication with the inner bore  303  of the wire guide  25 . Similar to the first alternative embodiment  251  of the wire guide  25 , the leaf spring  305  applies a load to the wire feedstock  23  to bias the wire feedstock  23  into engagement with the first side  201  of the inner bore  303  of the wire guide  25 .  FIG. 4A  shows the third alternative embodiment  253  without the wire feedstock  23 , and  FIG. 4B  shows the third alternative embodiment  253  with the wire feedstock  23  extending through the wire guide  25 . The aperture  304  is configured so that the leaf spring  305  can fit within the aperture  304  and allow the leaf spring  305  to engage the wire feedstock  23 . The ends of the leaf spring  305  may be connected to the wire guide  25  at each end of the aperture  304 . The first and second points  101 ,  102  of the at least three points of contact can be along any point where the wire feedstock  23  contacts the wire guide  25  along the first side  201  of the wire guide  25 . The third point  103  of the at least three points of contact can be along any point where the leaf spring  305  of the wire guide  25  contacts the wire feedstock  23 . 
         [0035]      FIGS. 5A-5B  show a fourth alternative embodiment  254  of the wire guide  25 , where the wire guide  25  is split into a first section  312  and a second section  313  to allow a misalignment in the wire guide  25 .  FIG. 5A  shows the fourth alternative embodiment  254  aligned without the wire feedstock  23 , and  FIG. 5B  shows the fourth alternative embodiment  254  misaligned with the wire feedstock  23  extending through the wire guide  25 . The first and second sections  312 ,  313  of the wire guide  25  can be angled linearly as shown or in any other configuration. When the first and second sections  312 ,  313  are axially misaligned, as shown in  FIG. 5B , the first section  312  is forced into engagement with one side of the wire feedstock  23 , and the second section  313  is forced into engagement with the other side of the wire feedstock  23 . The misalignment can be performed either statically or dynamically. The misalignment of the first and second sections  312 ,  313  allows the wire feedstock  23  to engage several contact points on the inner bore  303  of the wire guide  25 . 
         [0036]    In the illustrated, non-limiting example, the first and second points  101 ,  102  of the at least three points of contact are along the second side  202  of the wire guide  25 , and the third point  103  of the at least three points of contact is along the first side  201  of the wire guide  25 . However, there are limitless possibilities for the points of contact along the first and second sides  201 ,  202  of the wire guide  25 . For example, the first and second points  101 ,  102  of the at least three points of contact could be along the first side  201  of the wire guide  25 , and the third point  103  of the at least three points of contact could be along the second side  202  of the wire guide  25 . 
         [0037]      FIGS. 6A-6B  show a fifth alternative embodiment  255  of the wire guide  25 , wherein the wire guide  25  includes a cutaway section  318  that is cut-away from the wire guide  25 . The cutaway section  318  collapses around the wire feedstock  23 , which forces or biases the wire guide  25  into engagement with the wire feedstock  23 .  FIG. 6A  shows the fifth alternative embodiment  255  uncollapsed without the wire feedstock  23 , and  FIG. 6B  shows the fifth alternative embodiment  255  with the cutaway section  318  of the wire guide  25  collapsed around the wire feedstock  23  to create several points of contact between the wire feedstock  23  and the wire guide  25 . The cutaway section  318  can be retained to the wire guide  25  by an elastomeric ring  317  or other similar mechanisms. 
         [0038]    In the illustrated, non-limiting example, the first and second points  101 ,  102  of the at least three points of contact are along the first side  201  of the wire guide  25 , and the third point  103  of the at least three points of contact is along the second side  202  of the wire guide  25 . However, there are limitless possibilities for the points of contact along the first and second sides  201 ,  202  of the wire guide  25 . For example, the first and second points  101 ,  102  of the at least three points of contact could be along the second side  202  of the wire guide  25 , and the third point  103  of the at least three points of contact could be along the first side  201  of the wire guide  25 . 
         [0039]      FIGS. 7A-7B  show a sixth alternative embodiment  256  of the wire guide  25 , wherein the wire guide  25  includes an aperture  316  extending through the wall of the wire guide  25  for receiving a spring-biased ball  314 . The aperture  316  can be in communication with the inner bore  303  of the wire guide  25 . The ball  314  is spring biased toward the inner bore  303  of the wire guide  25  through the use of a compression spring  315 .  FIG. 7A  shows the sixth alternative embodiment  256  without the wire feedstock  23 , and  FIG. 7B  shows the sixth alternative embodiment  256  with the wire feedstock  23  extending through the wire guide  25 . 
         [0040]    The aperture  316  is configured to retain the spring-biased ball  314  while also allowing the spring-biased ball  314  to engage the wire feedstock  23  when a force is applied to the ball  314  by the compression spring  315 . For this to occur, the diameter of the ball  314  is slightly larger than the width or diameter of the aperture  316 . When a force is applied to the ball  314  by the compression spring  315 , the ball  314  emerges from the aperture  316  just far enough into the inner bore  303  of the wire guide  25  that the ball  314  engages the wire feedstock  23  to create the third point  103  of the at least three points of contact and bias the wire feedstock  23  into engagement with the first side  201  of the wire guide  25 , which creates the first and second points  101 ,  102  of the at least three points of contact. 
         [0041]    While the invention has been described in connection with certain embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.