Patent Publication Number: US-7717358-B2

Title: Nozzle for use with thermal spray apparatus

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to thermal spray apparatus for spraying molten or heat softened material onto a workpiece at high velocities by means of high temperature carrier mediums. In particular, the present invention is directed to an improved nozzle assembly for use with a plasma generator for directing heated coating materials at high velocity against a workpiece. 
   BACKGROUND OF THE INVENTION 
   In general, thermal spraying techniques and apparatus for coating metallic substrates are well known in the art. Thermal spray processes normally include the generation of a jet of high temperature carrier medium and the injection of a coating material into the carrier medium. The coating materials are usually powders which become heat-softened or melted by the carrier medium and propelled thereby at high velocities against the surface of a substrate to be coated. 
   Plasma flame spray apparatus typically include a plasma generator coupled to a nozzle. A plasma stream generated by the plasma generator is passed through the nozzle and towards a workpiece. A coating material is injected into the nozzle and the plasma stream and heat-softened or melted by the plasma stream and propelled thereby towards a workpiece applying the coating material thereto. 
   For example, U.S. Pat. No. 4,256,779 to Sokol et al. (hereinafter referred to as “Sokol”) discloses a plasma spray method and apparatus which is known in the industry as the Gator-Gard System manufactured by Sermatech International, Inc. of Limerick, Pennsylvania. Generally, the Gator-Gard System includes a plasma generator coupled with a nozzle for providing a plasma stream having improved characteristics. Upon entering the nozzle, a plasma stream is passed through a plasma cooling zone defined by a plasma cooling passageway, to a plasma accelerating zone defined by a narrowed passageway that expands into a plasma/particle confining zone. The narrowed passageway is cooled and the powder material to be applied to a substrate is introduced into the plasma stream along the cooled narrow passageway. Thereafter the particles and the plasma stream are discharged from the apparatus. 
   Similarly, in U.S. Pat. No. 5,858,469 to Sahoo et al. (hereinafter “Sahoo”), a modified thermal spray apparatus and method is disclosed. Generally, Sahoo discloses that plasma spray coatings of increased hardness can be applied to a workpiece by extending the distance at which the apparatus can spray the plasma/particle stream towards the workpiece. In the Sahoo apparatus, this is achieved by lengthening the passageway which defines the plasma/particle confining zone of the nozzle of the thermal spray apparatus. Additionally, Sahoo discloses that improved coatings can be obtained by increasing the ratio of the length to the diameter of the passageway defining the confinement zone. 
   One known problem associated with the plasma/particle confinement zones present in the nozzles of both of the Sokol and Sahoo apparatuses is that the particles of coating materials passing through these confinement zones can become cooled too much prior to exiting the nozzle. This can cause the particles of coating material to become clumped together resulting in imperfections in the applied coatings. 
   Further, the confinement zone present in the above-identified prior art nozzles necessitates a nozzle portion downstream of the material feed tube. This increases the minimum distance between the outlet of the nozzle and a workpiece during use of the nozzle in a coating process. 
   Another disadvantage of the above-identified prior art nozzle assemblies is that the nozzle assemblies do not include means for cooling the workpiece prior to applying a coating material. 
   Further, the above-identified prior art nozzles do not include means for enveloping the plasma stream exiting the nozzle with pressurized air for reducing contamination of the plasma stream discharged from the nozzle. 
   Based on the foregoing, it is the general object of the present invention to provide a nozzle assembly for use with known thermal spray apparatus that improves upon, or overcomes the problems and drawbacks associated with prior art nozzles. 
   SUMMARY OF THE INVENTION 
   The present invention provides an improved nozzle assembly for use with a thermal spray apparatus for applying a coating to a workpiece. The nozzle assembly having a housing defining a nozzle opening through a length thereof and centered about an axis defined by the housing. A nozzle is disposed in the nozzle opening and extends substantially through the length of the housing. The nozzle defining a gas conduit extending therethrough and having an inlet for receiving a carrier medium from a thermal spray apparatus and an outlet for discharging the carrier medium towards a workpiece. A material feed opening in communication with the gas conduit is provided proximate the outlet end of the gas conduit. The housing and the nozzle cooperating to define the material feed opening extending through a sidewall of both of the housing and the nozzle, the material feed opening being angularly disposed relative to the gas conduit. The outlet end of the nozzle being proximate a juncture of the gas conduit and the material feed opening such that coating material injected through the material feed opening into the gas conduit is heat-softened and propelled towards a workpiece to be coated by the carrier medium flowing out of the nozzle. Thus, the present invention nozzle assembly does not include a plasma/particle confinement zone downstream of a material feed tube such as disclosed in the above-identified prior art nozzles for use with thermal spray apparatus. 
   A primary object of the present invention is to provide an improved nozzle assembly for use with a thermal spray apparatus for improving the consistency and quality of the coating materials applied therewith. 
   Another object of the present invention is to provide a nozzle assembly for use with a thermal spray apparatus including a housing having a tapered portion at an outlet end of the housing that converges towards the outlet of the nozzle for facilitating use of the nozzle assembly in close proximity to a workpiece and angularly disposed with respect thereto. 
   It is also an object of the present invention to provide a nozzle assembly including means for conveying pressurized air towards a workpiece adjacent a carrier medium and coating material flowing out of the nozzle. The pressurized air for preventing dust and other contaminates from infiltrating the plasma stream and coating material discharged from the nozzle prior to contacting the workpiece. The pressurized air can be used for cooling the workpiece prior to and after applying the coating material. 
   It is also an object of the present invention to provide a nozzle assembly including an improved tip portion of increased hardness for reducing the wear and erosion of the tip portion due to the coating materials and plasma stream passing therethrough. 
   A further object of the present invention includes a method for thermal spray application of a coating material to a workpiece including providing a nozzle assembly for spraying a plasma stream carrying heat-softened particles of a coating material towards a workpiece, as well as a jet of pressurized air adjacent to the plasma stream on at least two opposing sides thereof. The jets of pressurized air being directed substantially parallel to the plasma stream for preventing dust and other contaminants from infiltrating the plasma stream containing the coating material prior to contacting the workpiece. 
   The foregoing and still other objects and advantages of the present invention will be more apparent from the following detailed explanation of the preferred embodiments of the invention in connection with the accompanying drawings wherein throughout the figures, like reference numerals describe like elements of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of a nozzle assembly for use with a thermal spray apparatus in accordance with the present invention. 
       FIG. 2  is a cross-sectional view of the nozzle assembly of  FIG. 1 . 
       FIG. 3  is an exploded perspective view of the nozzle assembly of  FIG. 1 . 
       FIG. 4  is a perspective view of a nozzle and material feed tube in accordance with the present invention. 
       FIG. 5  is a front view of the outlet end of the nozzle assembly of  FIG. 1 . 
       FIGS. 6 and 7  are cross-sectional views of the nozzle assembly of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the Figures, the present invention nozzle assembly for use with a thermal spray apparatus is generally referred to with the reference numeral  10 . The nozzle assembly  10  is attachable to known thermal spray apparatus such as a plasma generator (not shown) and used in conjunction therewith for applying a heat-fusible coating material to a workpiece. The nozzle assembly  10  includes a housing  12  having an inlet end  14  and outlet end  16 . The housing  12  is generally cylindrically shaped and includes a threaded portion  18  at the inlet end  14  for coupling the housing and nozzle assembly  10  to a plasma generator. 
   Referring to  FIG. 2 , the housing  12  defines a generally cylindrical nozzle opening  20  extending throughout a length of the housing and centered about a central axis X-X defined by the housing. An annular recess  22  is defined by the housing  12  surrounding an inlet end of the nozzle opening  20 . The housing  12  further defines a tapered end portion  24  that converges toward the central axis X-X of the housing  12  at the outlet end  16  thereof. The tapered end portion  24  allows the nozzle assembly  10  to be used in close proximity to a workpiece and angularly disposed with respect to the workpiece. In certain applications, the tapered end portion  24  of the present invention nozzle assembly  10  allows a coating material to be discharged from the nozzle much closer to, or at a sharper angle relative to a workpiece than prior art nozzle assemblies. 
   Still referring to  FIG. 2 , a nozzle  26  is disposed in the nozzle opening  20 . The nozzle  26  is removably insertable in the nozzle opening  20  via the inlet end  14  of the housing  12  and extends throughout the length of the housing. A shoulder  28  extending outwardly the body of the nozzle  26  near the inlet end thereof is disposed in and engages the recess  22  thereby limiting the insertion of the nozzle into the housing  12 . The angular relationship between the nozzle  26  and housing  12  is fixed by an alignment pin  29  disposed in corresponding openings in the nozzle  26  and housing  12 . 
   The nozzle  26  defines a gas conduit  30  through the length thereof for conveying a plasma stream generated by a plasma generator (not shown) through the nozzle. A flange  32  at the inlet end of the nozzle  26  is provided for receiving a plasma generator and a plasma stream therefrom. In operation, a plasma stream produced by the plasma generator, enters the gas conduit  30  at the flange  32  and passes through the nozzle via the gas conduit. The gas conduit  30  is surrounded by a cooling medium circulated through the housing  12  and in contact with an outer surface of the nozzle  26 . 
   A nozzle tip  34  is positioned at the outlet end of the nozzle  26  and defines a gas passageway  35  in communication with the gas conduit  30 . In a preferred embodiment, the nozzle tip  34  is press fit in the end of the gas conduit  30  and positioned in alignment therewith along the central axis X-X so as to receive the plasma stream traveling through the gas conduit. The nozzle tip  34  further defines a material port  36  that is angularly disposed with respect to the gas passageway  35  and in fluid communication therewith. During use of the nozzle assembly  10 , a coating material is introduced to the plasma stream via the material port  36  in the nozzle tip  34 . Material ports  38  and  40 , defined by each of the nozzle  26  and the housing  12  respectively, are aligned with, and in fluid communication with the material port  36  formed in the nozzle tip  34 . 
   Referring again to  FIGS. 1 and 2 , a material feed tube  42  is coupled to the nozzle assembly  10  for injecting a heat-fusible coating material into the plasma stream. The material feed tube  42  includes a threaded portion  44  threadably engaged with the material port  40  of the housing  12  and an end portion  42  that extends into the material port  38  defined by the nozzle  26 . The material port  38  of the nozzle  26  extends through a sidewall of the nozzle and is in fluid communication with the gas passageway  35  defined by the nozzle tip  34 . 
   In a preferred embodiment, the nozzle tip  34  is formed of a material of greater hardness than the nozzle  26  such that the nozzle tip  34  is less susceptible to wear and abrasion than the nozzle. In one embodiment the nozzle tip  34  of the present invention is formed from carbide material. Alternatively, the nozzle tip  34  could be manufactured from an air-hardened material. 
   The nozzle tip  34  of the present invention is much shorter than the outlet portions of the nozzles of prior art nozzle assemblies. As set forth above, both of the Sokol and Sahoo references disclose nozzles having plasma/particle confinement zones downstream of a material feed tube for allowing the plasma stream to cool following the introduction of the coating material. In contrast, the present invention nozzle tip  34  is designed to eliminate any such plasma/particle confinement zone and provides a juncture of the material port  38  of the nozzle proximate the outlet end of the nozzle tip  34  such that the coating material injected into a plasma stream traveling through the nozzle is heat-softened by the plasma stream substantially beyond the nozzle tip  34 . Thus, the coating material is not cooled within the nozzle  26  of the present invention and no dumping of the coating material due to overcooling will occur. In a preferred embodiment, the material port  36  defined by the nozzle tip  34  is approximately 0.300 inches from the outlet end thereof. 
   As shown in  FIG. 2 , the nozzle tip  34  of the present invention includes a throat  35  having a reduced interior diameter than that of the gas conduit  30  upstream thereof. An angled opening  37  at an inlet of the nozzle tip  34  provides a smooth transition for the plasma stream entering the nozzle tip  34 . This angular opening  37  reduces the amount of turbulence in the plasma stream entering the nozzle tip  34  and discharged therefrom resulting in increased uniformity and fewer imperfections in the coatings applied thereby. 
   Referring again to  FIGS. 1 and 2 , a coolant inlet  48  and coolant outlet  50  are defined by the housing  12  and are in fluid communication with the outer surface of nozzle  26  at the openings  52  via coolant ports  54  also defined by the housing. Typically, a coolant such as water, is circulated between the nozzle  26  and the housing  12  in the openings  52  defined between an outer surface of the nozzle  26  and the surface of the housing forming a sidewall of the nozzle opening  20 . 
   A pair of seals  58 ,  58  such as O-rings, are disposed in corresponding grooves  60 ,  60  defined by the nozzle  26  near each of the inlet and outlet ends of the nozzle. The seals  58 ,  58  are provided to retain coolant circulated through the housing  12  and around the nozzle  26  from leaking from the nozzle assembly  10  at the joint between the nozzle and the nozzle opening  20 . 
   The nozzle assembly  10  of the present invention provides means for enveloping the plasma stream and coating materials discharged from the nozzle  26  in pressurized air for preventing dust and other contaminants from infiltrating the plasma stream prior to the engagement of the coating material with the workpiece. As shown in  FIGS. 3 and 6 , air inlet ports  45  are provided on opposing sides of the housing  12  for connection with a pressurized air source such as an air compressor (not shown). At least two air passages  62  are defined by the housing  12  and aligned substantially parallel to the gas conduit  30  and spaced apart from and on opposing sides thereof. The air passages  62  each including an inlet  64  in communication with the air inlet ports  45 , and an outlet  66  extending through the housing on opposing sides of the outlet of the nozzle tip  34 . The air passages  62  and outlets  66  for discharging from the nozzle assembly  10  pressurized air towards a workpiece adjacent a carrier medium and coating material flowing out of the nozzle. Additionally, the pressurized air discharged from the nozzle assembly from the air passages  62  and outlets  66  act to cool the workpiece prior to the coating process. 
   Further, the outlets  66  for the pressurized air discharged from the nozzle assembly  10  can be positioned to engage the workpiece forward of the plasma stream and coating material for cooling the workpiece or removing dust and contaminants from the workpiece just prior to the delivery of the coating material. For example, as shown in  FIG. 6 , if the nozzle assembly  10  is used in a coating process wherein the nozzle assembly is moved in sideways motion along the line A to apply a coating material, the pressurized air discharged from the outlets  66  will engage the workpiece prior to and after the coating material contacts the workpiece. 
   Additionally, the present invention provides a method for thermal spray application of a coating material to a workpiece including providing a nozzle assembly including means for enveloping the plasma stream and coating materials discharged from the nozzle in pressurized air for preventing dust and other contaminants from infiltrating the plasma stream prior to the engagement of the coating material with the workpiece. 
   Further, the method includes providing a nozzle assembly including means for discharging a jet of pressurized air for contacting the workpiece prior to and after the coating material contacting the workpiece for cooling the workpiece and/or removing any contaminants such as dust or debris from the workpiece prior to and after the coating material engaging the workpiece. 
   The foregoing description of embodiments of the present invention has been presented for the purpose of illustration and description, it is not intended to be exhaustive or to limit the invention to the form disclosed. Obvious modifications and variations are possible in light of the above disclosure. The embodiments described were chosen to best illustrate the principals of the invention and practical applications thereof to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.