Abstract:
The present invention provides a plasma transferred wire arc torch assembly that includes a monolithic block assembly that combines into a single component several features that have previously been separate components. The monolithic block of the present invention combines the functions of a wire guide, an air baffle, and a nozzle. Integration of this components allows for a reduction in size of the plasma transferred wire arc torch assembly thereby making it possible to coat smaller diameter bores with metal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a thermal spray apparatus and method of thermally materials and, in particular, to a thermal spray apparatus with a spray gun capable of coating relatively small bores. 
     2. Background Art 
     A particularly useful high pressure plasma coating process is the Plasma Transferred Wire Arc (“PTWA”) process. The PTWA process is 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 where the wire is melted and atomized by the plasma. High speed gas emerging from the plasma torch directs the molten metal towards the surface to be coated. PTWA systems are high pressure plasma systems. 
     Specifically, the PTWA thermal spray process melts a feedstock material, usually in the form of a metal wire or rod, by using a constricted plasma arc to melt the tip of the wire or rod, removing the molten material with a high-velocity jet of partially ionized gas issuing from a constricting orifice. The ionized gas is a plasma and hence the name of the process. Plasma arcs operate typically at temperatures of 18,000°-25,000° F. (10,000°-14,000° C.). A plasma arc is a gas which has been heated by an electric arc to at least a partially ionized condition, enabling it to conduct an electric current. A plasma exists in any electric arc, but the term plasma arc is associated with plasma generators which utilize a constricted arc. One of the features which distinguishes plasma arc devices from other types of arc generators is that, for a given electrical current and plasma gas flow rate, the arc voltage is significantly higher in the constricted arc device. In addition, a constricted arc device is one which causes all of the gas flow with its added energy to be directed through the constricted orifice resulting in very high exiting gas velocities, generally in the supersonic range. There are two modes of operation of constricted plasma torches—non-transferred mode and transferred mode. The non-transferred plasma torch has a cathode and an anode in the form of a nozzle. In general, practical considerations make it desirable to keep the plasma arc within the nozzle with the arc terminating on the inner nozzle wall. However, under certain operating conditions, it is possible to cause the arc to extend outside the nozzle bore and then fold back, establishing a terminal point for the arc on the outside face of the anode constricting nozzle. In the transferred arc mode, the plasma arc column extends from the cathode through a constricting nozzle. The plasma arc extends out of the torch and is terminated on a workpiece anode which is electrically spaced and isolated from the plasma torch assembly. 
     In the plasma transferred wire arc thermal spray process, the plasma arc is constricted by passing it through an orifice downstream of the cathode electrode. As plasma gas passes through the arc, it is heated to a very high temperature, expands and is accelerated as it passes through the constricting orifice often achieving supersonic velocity on exiting the orifice, towards the tip of the wire feedstock. Typically, the different plasma gases used for the plasma transferred wire arc thermal spray process are air, nitrogen, or an admixture of argon and hydrogen. The intensity and velocity of the plasma is determined by several variables including the type of gas, its pressure, the flow pattern, the electric current, the size and shape of the orifice and the distance from the cathode to the wire feedstock. 
     The prior art plasma transferred wire arc processes operate on direct current from a constant current type power supply. A cathode electrode is connected to the negative terminal of a power supply through a high frequency generator which is employed to initiate an electrical arc between the cathode and a constricting nozzle. The high frequency arc initiating circuit is completed by the momentary closure of a pilot arc relay contact allowing direct current to flow from the positive terminal of power supply through a pilot resistor to the constricting nozzle, through the high frequency arc formed between the cathode and the constricting nozzle, through the high frequency generator to the negative terminal of the power supply. The high frequency circuit is completed through the bypass capacitor. This action heats the plasma gas which flows through the orifice. The orifice directs the heated plasma stream from the cathode electrode towards the tip of the wire feedstock which is connected to the positive terminal of the power supply. The plasma arc attaches to or “transfers” to the wire tip and is thus referred to as a transferred arc. The wire feedstock is advanced forward by means of the wire feed rolls, which are driven by a motor. When the arc melts the tip of the wire, the high-velocity plasma jet impinges on the wire tip and carries away the molten metal, simultaneously atomizing the melted metal into fine particles and accelerating the thus formed molten particles to form a high-velocity spray stream entraining the fine molten particles. 
     In order to initiate the transferred plasma arc a pilot arc must be established. A pilot arc is an arc between the cathode electrode and the constricting nozzle. This arc is sometimes referred to as a non-transferred arc because it does not transfer or attach to the wire feedstock as compared to the transferred arc which does. A pilot arc provides an electrically conductive path between the cathode electrode within the plasma transferred wire arc torch and the tip of the wire feedstock so that the main plasma transferred arc current can be initiated. The most common technique for starting the pilot arc is to strike a high frequency or a high voltage direct voltage (DC) spark between the cathode electrode and the constricting nozzle. A pilot arc is established across the resulting ionized path generating a plasma plume. When the plasma plume of the pilot arc touches the wire tip, an electrically conductive path from the cathode electrode to the anode wire tip is established. The constricted transferred plasma arc will follow this path to the wire tip. 
     U.S. Pat. No. 5,808,270 addresses a number of problems in the prior arc related to plasma torch operation. U.S. Pat. No. 5,808,270 is hereby incorporated by reference. Such problems include double arcing, electrical shorting due to metallic dust being attracted to the cathode, and the buildup of coating material on the outer surface of the torch which faces the surface being coated. Furthermore, problems associated with the starting of spraying often cause a “spit” or large molten globule to be formed and propelled to the substrate. This globule may cause an imperfection by being included into the coating as the coating builds up on the substrate. However, because of the complexity and the number of individual components of the plasma torch of U.S. Pat. No. 5,808,270, this torch is somewhat limited by how small a bore may be coated. Accordingly, there exists a need for an improved plasma spray torch that can coat smaller diameter bores. 
     SUMMARY OF INVENTION 
     The present invention overcomes the problems encountered in the prior art by providing a plasma transferred wire arc torch assembly that includes a monolithic block assembly combining into a single component several features that have previously been separate components. The monolithic block of the present invention combines the functions of a wire guide, an air baffle and a nozzle. Integration of this components into one component allows for a reduction in size of the plasma transferred wire arc torch assembly. Accordingly, the plasma transferred wire arc torch assembly of the present invention is able to coat the inside of smaller diameter bores than the assemblies of the prior art specifically, the assembly of the present invention is able to coat bores of diameter equal to or greater than about 1.3 inches. Furthermore, the monolithic block assembly of the present invention is simple to fabricate thereby resulting in a reduction in the cost of fabricating a plasma transferred wire arc torch assembly. 
     In another embodiment of the present invention, a method of coating a surface with a metallic coating utilizing the plasma transferred wire arc torch assembly and the monolithic block assembly of the present invention is provided. The method of the invention comprises initiating and sustaining a plasma in a plasma gun which incorporates the plasma transferred wire arc torch assembly and the monolithic block assembly of the present invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic of a preferred embodiment of the present invention showing the integration of the monolithic block assembly in a plasma spray torch; 
     FIG. 2 is a cross-section of the monolithic block assembly of the present invention; 
     FIG. 3 a  is a perspective view of the plasma gun of the present invention as viewed from the side with the back plate; 
     FIG. 3 b  is a perspective view of the plasma gun of the present invention as viewed from the side with the monolithic block assembly of the present invention; 
     FIG. 4 is a perspective view of the assembled plasma gun of the present invention; 
     FIG. 5 is an exploded view of the plasma gun including the monolithic block assembly of the present invention; 
     FIG. 6 is a longitudinal cross-section of a bore to be coated with a metallic coating with the plasma gun of the present invention; 
     FIG. 7 is a transverse cross-section of a bore to be coated with a metallic coating with the plasma gun of the present invention; and 
     FIG. 8 is a schematic of a variation of the plasma gun of the present invention in which a plasma emerges at an acute angle relative to a substrate to be coated. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to presently preferred compositions or embodiments and methods of the invention, which constitute the best modes of practicing the invention presently known to the inventors. 
     In one embodiment of the present invention, an improved PTWA spray gun is proved. The spray gun of the present invention is a component in a plasma transferred wire arc thermal spray apparatus that may be used to coat a surface with a dense metallic coating. The spray gun of the present invention includes a monolithic block assembly that has a wire feed guide section for introducing wire feedstock into a plasma torch, an air baffle section for introducing a secondary gas around the plasma formed by the plasma torch, and a nozzle section for confining a plasma formed by the plasma torch. Furthermore, the monolithic block assembly is made from a single piece of metal such that heat is removed from the thermal spray apparatus during operation. 
     With reference to FIG. 1, a preferred embodiment of the present invention is shown in schematic form. Plasma transferred wire arc thermal spray apparatus  2  is shown to include plasma torch gun  4 . During operation as set forth below, extended plasma arc  6  and metal spray  8  emerge from plasma torch gun  4 . Monolithic block assembly  10  includes constricting nozzle section  12  which has a cup-shaped form with a constricting orifice  14  located at the center of the cup-shaped form. Preferably, monolithic block assembly  10  is made of a conductive material such as copper. Cathode electrode  16 , which may be constructed from 2% thoriated tungsten, is located coaxial with the constricting nozzle section  12  and has cathode free end  18 . Cathode electrode  16  is attached to cathode plate  20  which is preferably made from a metal such as brass. Cathode plate  20  is electrically insulated from constricting nozzle section  12  by insulating body  22 , forming an annular plasma gas chamber  24  internally between the cathode electrode  16  and the inner walls of the constricting nozzle section  12  and insulating body  22 . In addition, a separate chamber  26  is formed within the outer section of the monolithic block assembly  10 . Chamber  26  is in turn connected to chamber  28  by a plurality of bores  30  formed within the monolithic block assembly  10  at air baffle section  32 . Wire guide section  34  is connected to constricting nozzle section  12  and formed within monolithic block assembly  10 . Wire feed guide  36  is contained within bore  38  of wire guide section  34 . Wire feedstock  40  is constantly fed by means of wire feed rolls  42  and  44  through wire feed guide  36 . Wire feed rolls are driven by means of a conventional motor (not shown). Free wire end  46  emerges from wire feed guide  36  and contacts extended plasma  6  at position  48  located opposite to constricting orifice  14  to form metal spray  8 . In operation, metal spray  8  is directed towards a substrate to be coated. The present invention advantageously combines constricting nozzle section  12 , air baffle section  32 , and wire guide section  34  into one component made from a single piece of metal. 
     Still referring to FIG. 1 the electrical circuitry that is associated with the operation of the plasma transferred wire arc thermal spray apparatus  2  consists of a pilot power supply assembly  50 , the negative terminal of which is connected through electrical leads  52  and  54  to the cathode electrode  16 . The pilot power supply assembly consists of a direct current (DC) constant current pilot power supply  56  and a high voltage DC power supply  58  which is in an electrically parallel connection across the pilot power supply. A suitable pilot power supply assembly is Model “PowerPro 55” available from Smith Equipment Division of TESCOM Corp. A high voltage DC blocking filter  60  is located in the negative leg of the high voltage power supply  58  which prevents any high voltage from feeding back into the pilot power supply  56 . Positive terminal  61  of pilot power supply assembly  50  is connected through lead  62  to a pilot relay contact  64  which is connected through lead  66  to the wire contact tip or feedstock free end and wire guide section  34  which is in electrical contact with the constricting nozzle  12 . A separate main plasma transferred wire arc power supply  68  is also employed, the positive terminal being connected by means of lead  70  to the lead  66  which in turn is connected to the wire guide section  34  and by means of electrical contact connected to the constricting nozzle as well as the wire feedstock  40 . The negative terminal of power supply  68  is connected through lead  72  to the contacts of an isolation contactor  74  through lead  76  and lead  54  to the cathode electrode  16 . A suitable main power supply is Model PCM-100 available from ESAB Welding and Cutting Products. 
     In operation, plasma gas enters through port  80  into the internal chamber  24  formed by constricting nozzle section  12  and insulating body  22 . The plasma gases flow into chamber  24  and form a vortex flow being forced out of the constricting orifice  14 . A suitable plasma gas is a gas mixture consisting of 65% argon and 35% hydrogen. Other gases have also been used, such as nitrogen. In order to start the operation of the plasma transferred wire arc process, it is necessary to initiate a pilot plasma. To initiate a pilot plasma, the pilot plasma power supply  56  is activated and the positive terminal is connected through the pilot relay contactor  64  to the constricting nozzle section  12  and the negative terminal is connected to the cathode electrode  16 . Simultaneously, the high voltage power supply  58  is pulsed “on” for sufficient time to strike a high voltage arc between the cathode electrode  16  and the constricting nozzle section  12 . The high voltage arc thus formed provides a conductive path for the DC current from the pilot plasma power supply to flow from the cathode electrode  16  to the constricting nozzle section  12 . As a result of this added electrical energy, the plasma gas is intensely heated which causes the gas, which is in a vortex flow regime, to exit the constricting orifice at very high velocity, generally forming a supersonic plasma jet extending from the constricting orifice. The plasma arc thus formed is an extended plasma arc which initially extends from the cathode through the core of the vortex flowing plasma jet to the maximum extension point and the “hairpins” back to the face of the constricting nozzle. The high velocity plasma jet, extending beyond the maximum arc extension point provides an electrically conductive path between the cathode electrode  16  and free end  46  of the wire feedstock  40 . As soon as the pilot plasma is established, the isolation contactor  74  is closed, adding additional power to the pilot plasma. A plasma is formed first between cathode  16  and nozzle section  12  which subsequently transfers to wire feedstock  40  causing the wire tip to melt as it is being continuously fed into the plasma jet. A secondary gas entering through port  82 , such as air, is introduced under high pressure into chamber  26 . Chamber  26  acts as a plenum to distribute this secondary gas to the series of spaced bores  30 . The secondary gas then flows into chamber  28  and then through a plurality of angularly spaced bores  84 . The flow of this secondary gas provides a means of cooling wire guide section  34 , constricting nozzle section  12 , and air baffle section  32 , as well as providing an essentially conically shaped flow of gas surrounding extended plasma arc  6 . This conically shaped flow of high velocity gas intersects with the extended plasma jet downstream of the tip (free end  46 ) of wire feedstock  40 , thus providing addition means of atomizing and accelerating the molten particles formed by the melting of wire feedstock  40 . 
     Reference is made to FIG. 2, which is a cross-section of the monolithic block assembly, and to FIGS. 3 a  and  3   b  which are, respectively, a top and bottom exploded view illustrating the placement of the monolithic block assembly in the plasma gun. Monolithic block assembly  10  is bolted to plasma gun  86  through bolt holes  90 ,  92 ,  94  and threaded receiving holes  96 ,  98 ,  100 . Monolithic block assembly  10  integrates wire guide section  34 , air baffle section  32 , and nozzle section  12  into a monolithic block of metal. Nozzle section  12  defines constricting orifice  14  through which the plasma emerges. Air baffle section  32  includes a series of annularly spaced bores  84 . Angularly spaced bores  84  direct the flow of the secondary gas such that the plasma is shaped by the flow action of this secondary gas. Wire guide section  34  provides support for wire guide  36  through which wire feedstock is directed. Furthermore, wire guide section  34  functions as a heat sink that cools the feedstock as it is introduced in the plasma. 
     The assembly of the plasma gun of the present invention is illustrated by reference to FIGS. 4,  5 , and  6 . FIG. 4 is a perspective view of the assembled plasma gun  101 , while FIG. 5 is an exploded view of the plasma gun including the monolithic block assembly. FIG. 6 is a longitudinal cross-section of a bore to be coated with a metallic coating with the plasma gun of the present invention. Monolithic block assembly  10  is bolted to plasma gun body  102  with bolt hole  90 ,  92 ,  94 . A seal is made to plasma gun body  102  with by O-ring  103  placed in groove  104 . Wire guide  36  extends slightly out of monolithic block assembly  10 . Cathode  106  is attached to brass back plate  108  with nozzle nut  110 . Insulating plate  112  is attached to plasma gun body  102  and is held in place by bolting brass back plate  108  to plasma gun body  102 . Plasma gun body  102  is preferably made from an insulting material with a high dielectric strength. Torlon is a suitable material for plasma gun body  102 . Spacer block  114  contacts plasma gun body  102  at indentation  116 . Spacer block  114  is also made from an insulting material such as torlon. Furthermore, spacer block also fits into rear body  118  which is held against plasma gun body  102  by retaining ring  120  which is bolted onto rear body  118 . Preferably, rear body  118  is made from a conductive material such as brass. Wire guide  36  telescopes through bore  38 . Upper contact electrode  122  and lower contact electrode  123  are joined and held together by O-rings  132  and  134  to form a tube-like structure. Accordingly, wire feedstock  40  make electrical contact with either contact electrode  122  and lower contact electrode  123 . Insulating tube  124  slides over metal tube  126 . Insulating tube  124  is made from an insulating material such as glassed filled Teflon. Metal tube  126  screws into upper contact electrode  122 . Collectively, metal tube  126 , upper contactor  122 , lower contactor  123 , and wire guide  36  form a conduit for wire feed stop to be fed into the plasma torch. This conduit goes through cavity  128  in spacer block  114  and opening  130  in plasma gun body  102  where it proceeds to extend through bore  38  in monolithic assembly  10 . O-rings  132 ,  134  sit in grooves  136 ,  138  and provide a seal with an internal cavity (not shown) in plasma gun body  102 . Rear body  118  has channels  146 ,  148  that allow introduction of the secondary gas into the plasma torch. Channels  146 ,  148  line up with and fed into channels  140 ,  142  in spacer block  114 . Channels  140 ,  142  in turn fed into channels  150 ,  152  which introduce the secondary gases to monolithic block assembly  10 . Gas inlet tube  154  slips into rear body  118 . Plasma gas is introduced into gas tube inlet  154  flows through channel  156 . The gas proceeds through a channel (not shown) in plasma gun body  102  and into channel  158  in back plate  108  through opening  160 . A seal with plasma gun body  102  is made with an O-ring placed in groove  162 . The plasma gas then flows into the chamber created by the attachment of monolithic block assembly  10  and back plate  108  to plasma gun body  102 . Back plate  108  is bolted to plasma gun body  102  through bolt holes  164 ,  166 ,  168 ,  170 . A seal is made to plasma gun body  102  with an O-ring placed into O-ring groove  171 . 
     The operation of the plasma torch assembly of the present invention in coating the interior surface of a bore with a metallic coating is best understood by reference to FIGS. 5,  6  and  7 . Plasma gas introduced into plasma gun  101  through gas inlet tube  154  flow through rear body  118 , spacer block  114 , and plasma gun body  102  into channel  158  of back plate  108 . The plasma gas next proceeds into annular plasma gas chamber  24  where the high gas pressure and flow create a vortex flow. The plasma gas then emerge from restricting orifice  14 . When a plasma is initiated as set forth above, extended plasma arc  6  extends out of restricting orifice  14 . Wire feedstock  40  is continually fed into extended into extended plasma  6  where wire feedstock  40  is melted and atomized into metal spray  8 . FIG. 6 is a longitudinal cross-section of bore  180  and the plasma transferred wire arc torch assembly illustrating metal spray  8  being sprayed onto the inner surface of bore  180 . Bore  180  may be the cylinder bores of an internal combustion engine. FIG. 7 is a transverse cross section through bore  180  illustrating the rotation of plasma transferred wire arc torch assembly within bore  180  about an axis through wire feedstock  40  to coat the interior of bore  180  with metal coating  184 . Plasma transferred wire arc torch assembly may be moved along the length of bore  180  and rotated in direction  182  about an axis through wire feedstock  40 . Both of these movements of plasma transferred wire arc torch assembly allows for a substantial area of the interior surface of bore  180  to be coated with a metallic coating. 
     A variation of the plasma transferred wire arc torch assembly of the present invention is provided by reference to FIG.  8 . This embodiment allows for extended plasma arc  6  and metal spray  8  to be directed at an angle 190 relative to an axis through feedstock  40 , i.e., the included angle between direction  192  of extended plasma arc  6  and surface  194  of the object to be coated. Preferably, this included angle is less than 90 degrees. Angling extended plasma  6  in such a manner minimizes particle buildups on the plasma torch. In this embodiment, the shape of monolithic block assembly  10  has been altered to allow the feedstock  40  to be fed at an angle to extended plasma arc  6 . 
     In another embodiment of the present invention, a method of coating a surface with a dense metallic coating using a plasma transferred wire arc thermal spray apparatus is provided. The method of the present invention utilizes the plasma spray torch integrated with the monolithic block assembly as described above. As set forth above, the plasma transferred wire arc torch assembly comprises: 
     a cathode having a free end and biased at a first negative electrical potential; 
     a monolithic block assembly that includes a wire feed guide section, an air baffle section, and a nozzle section; 
     a source of plasma gas directing plasma gas into the nozzle surrounding the cathode and exiting the restricted nozzle orifice; and 
     a wire feed continuously directing a free end of wire feedstock opposite the restricted nozzle orifice and the wire feedstock having the same second positive electrical potential as the nozzle, wherein nozzle section is biased at a second positive electrical potential and generally surrounding the free end of the cathode in spaced relation wherein the nozzle has a restricted orifice opposite the free end of the nozzle section and the thermal spray apparatus establishes a plasma transferred arc between the wire feedstock free end and the cathode melting the wire feedstock free end and the plasma gas exiting the restricted nozzle orifice atomizing melted feedstock and propelling atomized melted wire feedstock toward the surface, thereby coating the surface. 
     Accordingly, the method of the present invention comprises: 
     a) directing a plasma gas into the nozzle surrounding the cathode and exiting the restricted nozzle orifice; 
     b) initiating an electrical pilot arc between the cathode and the nozzle by creating an electrical potential differential there between, wherein the cathode has a negative electrical potential and nozzle has a positive electrical potential; 
     c) extending the electric arc through the restricted nozzle orifice by increasing electrical energy to the electric arc and forming a constricted extended plasma arc; and 
     d) transferring the constricted extended plasma arc from the nozzle to the free end of the wire feedstock by creating an electrical potential differential between the wire feedstock free end and the cathode, wherein the wire feedstock and the nozzle have the same positive electrical potential and the cathode has a negative electrical potential, thereby melting the wire feedstock free end, the plasma gas atomizing molten feedstock and propelling atomized molten feedstock onto the surface and forming the dense metal coating on the surface. In general, a plasma gas under pressure will be introduced tangentially into the nozzle and creating a vortex flow around the cathode and exiting the restricted nozzle orifice. Furthermore, the method optionally includes directing a second gas stream towards the wire feedstock free end in the form of an annular conical gas stream surrounding the wire feedstock free end and having a point of intersection spaced downstream of the wire feedstock free end. 
     When an interior concave surface such as a cylinder bore of a piston engine is to be coated, the method will include rotating and translating the nozzle and the cathode as an assembly about a longitudinal axis of the wire feedstock while maintaining an electrical connection and the same electrical potential between the nozzle and the wire feedstock, thereby directing the atomized molten feedstock rotationally and coating an internal arcuate surface with the dense metal Moreover, the assembly and method of the present invention are able to coat bores of diameter equal to or greater than about 1.3 inches. More preferably, the torch assembly of the present invention is useful in coating bore with a diameter from about 1.3 inches to about 4.0 inches. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.