Abstract:
The invention recites a plasma-arc spray gun comprising a cathode and an anode defining a longitudinal axis. The anode further includes an external surface and an internal chamber, the internal chamber extending from a first end to a second end. At least a portion of the internal chamber is defined by revolving a non-linear curve about the longitudinal axis. The plasma-arc spray gun also includes a gun body supporting the cathode and the anode.

Description:
RELATED APPLICATION DATA  
       [0001]    This application claims the benefit of the priority date under 35 U.S.C. Section 119(e) of U.S. Provisional Application No. 60/375,268 filed Apr. 24, 2002, which is hereby fully incorporated by reference. 
     
    
     
       BACKGROUND AND SUMMARY OF THE INVENTION  
         [0002]    The present invention relates to thermal spraying, and particularly to improved guns for spraying metallic and ceramic particles onto a substrate. More particularly, the present invention relates to water-cooled thermal spray guns having an anode.  
           [0003]    Plasma-arc spray guns use a power supply and a cathode disposed within an anode to generate a plasma for use in depositing a material onto a substrate. A gas supplied to the chamber between the anode and the cathode converts to high-temperature plasma as it passes through an arc that extends between the anode and cathode. To provide for stable and controllable plasma, it is important to control the location of the arc between the anode and cathode. To that end, other anodes contain a series of cylindrical and frustoconical sections designed to position the arc at the desired point. However, these contours produce undesirable turbulence behind the arc attachment point and reduce the performance of the gun.  
           [0004]    The large currents of electricity flowing between the anode and the cathode cause the anode to heat significantly, thereby reducing its performance and operating life. To control the heating and reduce anode damage, a cooling-water flow passes around and within the anode. Present plasma-arc spray guns employ water channels that have multiple chambers and flow paths with differing flow areas. Rapid increases in flow area cause sudden pressure drops that can be detrimental to the cooling efficiency of the water flow. More specifically, the pressure drop allows the water to boil and greatly reduces its cooling effectiveness.  
           [0005]    Another factor in the determination of anode life is the wall thickness of the anode. Large changes in wall thickness in adjacent sections can result in significant thermal stress and component failure. In addition, varying wall thickness can result in significantly different heat transfer characteristics causing hot spots or cold spots on the surface of the anode.  
           [0006]    Thus, the plasma-arc spray gun of the present invention provides a cathode and an anode defining a longitudinal axis. The anode further includes an external surface and an internal chamber, the internal chamber extending from a first end to a second end. At least a portion of the internal chamber is defined by revolving a non-linear curve about the longitudinal axis. The plasma-arc spray gun also includes a gun body supporting the cathode and the anode.  
           [0007]    In another construction of the plasma-arc spray gun the gun is powered by an external power source having a first lead and a second lead. The gun provides a gun body and an anode supported by the gun body and electrically connected to the first lead of the power source. The anode also has a longitudinal axis and includes an external surface and an internal chamber. The internal chamber has a first open end receiving a flow of gas and a second open end discharging a flow of plasma. The internal chamber also includes a portion defined by revolving a non-linear curve about the longitudinal axis. The plasma-arc spray gun further includes a cathode supported by the gun body and electrically connected to the second lead of the power source and a gas injector providing the flow of gas through the first open end of the anode. The power source initiates an arc between the anode and the cathode, and a portion of the flow of gas passes through the arc to generate the flow of plasma.  
           [0008]    In preferred embodiments, the non-linear curve is defined by a polynomial equation. In addition, the non-linear curve is disposed between the first open end of the anode adjacent the gas injector and the arc attachment area.  
           [0009]    The invention further provides a method of manufacturing a plasma-arc spray gun. The method comprises the steps of forming an inner chamber within an anode having a longitudinal axis. The inner chamber includes a first open end, a second open end, and at least one region disposed therebetween and defined by the revolution of a non-linear curve about the longitudinal axis. The method further includes the steps of positioning the anode and the gas injector within the gun body and positioning the cathode at least partially within the inner chamber of the anode.  
           [0010]    In other embodiments, the method further comprises the step of forming an external anode surface defined by the revolution of a second non-linear curve about the longitudinal axis. The second non-linear curve is substantially parallel to and spaced apart from the first non-linear curve. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The detailed description particularly refers to the accompanying figures in which:  
         [0012]    [0012]FIG. 1 is a longitudinal cross-sectional view of a plasma-arc spray gun including a contoured anode in accordance with the present invention;  
         [0013]    [0013]FIG. 2 is a longitudinal cross-sectional view of the anode of FIG. 1;  
         [0014]    [0014]FIG. 3 is a longitudinal cross-sectional view of another embodiment of an anode in accordance with the present invention;  
         [0015]    [0015]FIG. 4 is an x-y plot illustrating one possible polynomial curve that defines a section of the anode.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 is a longitudinal sectional view of a plasma gun  10  capable of producing a plasma for the application of metallic or ceramic particles on a substrate. A similar plasma gun is described in U.S. Pat. No. 5,444,209 issued to Crawmer, which is hereby fully incorporated by reference. The gun  10  of FIG. 1 includes a front housing  15 , a middle housing  20 , a rear housing  25 , a cathode holder  30  supporting the cathode  35 , a gas injector  40 , and an anode  45 . The front, middle, and rear housings  15 ,  20 ,  25  are generally tubular and define a common longitudinal axis  11 - 11 . The housings  15 ,  20 ,  25  may be connected by bolts, screws or any attachment mechanism capable of firmly holding and aligning the components. In addition, the housings  15 ,  20 ,  25  support the cathode holder  30 , anode  45 , and gas injector  40  forcing their proper alignment relative to one another. The housings  15 ,  20 ,  25  also provide coolant passages  50  (described below), arc gas passages  51 , and electrical circuits.  
         [0017]    The cathode holder  30  supports the cathode  35  in the proper position within the anode  45  and provides a convenient point to connect an electrical power supply and a water inlet  55  to the gun  10 . In some constructions, the cathode holder  30  includes a threaded hole sized to receive a threaded portion extending from the cathode  35 . In other constructions, the cathode holder  30  includes a projection that threads into the cathode  35 . The actual method used to attach the cathode  35  to the cathode holder  30  is not important to the function of the present invention. The anode  45  and cathode  35  cooperate with one another to define an annular flow chamber  56  for the flow of gas therebetween. The desired position of the cathode  35  within the anode  45  is determined based on the shape of the cathode  35  and the anode  45  as well as their sizes relative to one another. Accordingly, a wide variety of positions are possible depending upon the particular arrangements and sizes of the anode  45  and cathode  35 . FIG. 1 illustrates one possible configuration of a cathode  35  disposed in the desired position within the anode  45 .  
         [0018]    The anode  45  is an elongated substantially tubular member having a large opening  60  near its rear and a smaller opening  65  near its front. Between the large opening  60  and the small opening  65  is a contoured section  70 . The structure of the anode  45  is discussed in more detail below with respect to FIGS. 2 and 3. In operation, an arc  76  between the anode  45  and cathode  35  attaches to the anode  45  at an arc attachment area  77 . In one preferred embodiment, the inner radius  78  of the anode  45  at the arc attachment area  77  is approximately 0.0938 inches (2.38 mm), however larger and smaller openings will also function. For example, inner radii  78  that are 0.005 inches larger or smaller than the radius described above will allow the gun  10  to function properly. In many instances, still larger or smaller radii may be employed in the anode  45 .  
         [0019]    [0019]FIG. 2 shows a cross section of an anode  45  in accordance with the present invention. The anode  45  has an outer surface  80  and an inner surface  82 . The shape of the outer surface  80  of the anode  45  allows it to engage the front housing  15  to prevent movement of the anode  45  relative to the cathode  35 . At least a portion of the inner surface  82  of the anode  45  is defined by a non-linear curve. More particularly, at least a portion of the inner surface  82  of the anode  45  is defined by a curve characterized by a second order or higher polynomial equation. In FIGS.  1 - 3 , this portion that is defined by a polynomial equation has been identified as contoured section  70  or  70 ′. To form the contoured section or internal portion of the anode defined by a polynomial equation or non-linear curve, the non-linear curve is rotated about the longitudinal axis  11 - 11  of the anode  45 . For example, in FIGS.  1 - 3 , the non-linear curve is rotated around axis  11 - 11 .  
         [0020]    In one embodiment, the inner surface  82  of the anode  45  may be divided into multiple sections. FIG. 1 shows an anode having four sections: the contoured section  70 , a straight section  83 , a transitional section  85 , and an exit section  90 . The exit section  90  is sized to provide the desired exit velocity and flow out of the gun  10 . Similarly, the transitional section  85  provides a smooth transition between the exit section  90  and the straight section  83 . In other constructions (not shown), the straight section  83  is combined with the contoured section  70 , thus eliminating the straight section  83 . In other embodiments, the entire inner surface  82  of the anode  45  may be defined by a non-linear curve.  
         [0021]    The use of a continuous curve to define the contoured section  70  improves the functionality of the gun  10 . More particularly, the improved streamlined configuration of the anode inner surface  82  improves the flow characteristics of the gas within the annular flow chamber  56 , thereby improving the cooling of the cathode  35 . In addition, the non-linear contour of the anode  45  minimizes turbulence behind the point of arc attachment, namely, between the gas injector and the arc attachment area  77 . The use of a high order polynomial to define the contoured section  70  improves the gas flow characteristics by eliminating sudden section transitions, reduces the break in period of the anode  45 , and promotes longer anode life by providing better resistance to erosion induced by multiple starts and stops. Sudden section transitions induce turbulence and pressure loss in the flow of gas.  
         [0022]    The contoured section  70  follows a curve characterized by a high-order polynomial function of the form y=A 0 +A 1 x+A 2 x 2 +A 3 x 3 +A 4 x 4 +A 5 x 5 +. . . A n x n . More particularly, the high-order polynomial may be a second-order polynomial or higher. The following table characterizes two embodiments of the contoured section  70 :  
                                                                                                 A 0     A 1     A 2     A 3     A 4     A 5                                      1   0.365835   0.446148   −2.13431   3.009243   −1.72739   0.343881       2   −0.015814973   0.30758798   −1.259815399   1.764776317   −0.903923652   0.155297451                  
 
         [0023]    Any number of polynomials can accurately describe the desired curve or a similar curve within the required tolerances of the anode  45 . In addition, one or many of the coefficients (A 0 , A 1  . . . A n ) could be zero so long as one of the higher order coefficients (A 2  . . . A n ) is not zero. In other embodiments, the coefficients A 0  . . . A n  are between −10 and 10, while in still other embodiments x-values between 0 and 3 yield y-values between −1 and 10. It should be understood that many contours defined by many high order curves are available that will function with the present invention, and therefore, the invention should not be limited to the two curves described above. FIG. 3 illustrates another construction of the anode  45  having a contoured section  70 ′ different from that illustrated in FIGS. 1 and 2.  
         [0024]    [0024]FIG. 4 illustrates a curve  96  generated by a high-order polynomial. To arrive at the contour section  70  of the anode  45 , the curve  96  shown in FIG. 4 is revolved around the x-axis  97  which corresponds to the longitudinal axis  11 - 11  in FIG. 1. Again, any shaped contoured section desired can be defined by a non-linear curve characterized by a polynomial equation. Thus, the y-value represents the radius of the inner chamber of the anode  45  in the contour section  70 , while the x-value represents the axial position along the anode  45 . In other constructions, the curve  96  is revolved around an axis other than the x-axis  97  to arrive at the desired internal contour.  
         [0025]    To further improve the performance of the gun  10 , the wall thickness of at least a portion of the anode  45  is substantially uniform as shown in FIG. 1. This improves the overall performance of the gun  10 , particularly at high power levels and high total arc gas flows, which increase pressure in the anode  45 , thereby increasing the heat load in the rear section of the anode  45 . To maintain the consistent wall thickness, the outer wall of the anode  45  in the anode throat area  75  follows a substantially similar curve  96 ′ as the contoured section  70 . By using similar parallel curves  96 ,  96 ′ for the inner wall and outer wall respectively, the parallel relationship of the walls is maintained, eliminating sudden wall thickness changes and corresponding hot and cold spots. Hot and cold spots reduce the effectiveness of the gun in several ways. By providing unequal heat transfer, hot and cold spots may produce plasma of differing temperatures exiting the gun. The unequal plasma temperatures may result in a variation of the quality of the material being deposited on the substrate, which is undesirable. In addition, hot and cold spots can produce unequal thermal expansion of the anode  45  resulting in misalignment between the anode  45  and the cathode  35 . The misalignment may result in varying arc lengths and an inconsistent plasma. Again, this is undesirable. Further, hot and cold spots can result in significant thermal stress within the anode  45 . The stress may result in rapid arc erosion and/or permanent distortion of the anode  45 , thereby shortening its useful life.  
         [0026]    The gas injector  40  is sandwiched between the anode  45  and the cathode holder  30 . The outer diameter of the gas injector  40  and a portion of the inner surface of the middle housing  20  cooperate to form an annular passage  98 . Another passage (not shown) in the middle housing  20  leads between the annular passage  98  and a mating passage (not shown) in the rear housing  25  to supply a source of inert primary gas, such as, but not limited to, argon or helium. A series of bores  99  extend through the gas injector  40  in a generally radial direction to direct the gas to the inner diameter of the gas injector  40  where it is redirected by an annular gap  100  into the annular flow chamber  56  defined by the anode  45  and the cathode  35 .  
         [0027]    Referring again to FIG. 1, the cooling water flow paths  50  allow cooling water to enter through the cathode holder  30  and flow to an annular chamber  101  defined between the anode  45  and the housings  15 ,  20 ,  25 . The cooling water then enters one of a plurality of cooling bores  102  within the anode  45 . The cooling bores  102  improve the cooling efficiency in the hotter region of the anode  45  adjacent the arc attachment area  77  and the areas of the anode  45  exposed to the plasma flow. The cooling water then circulates around a cover piece (not shown), through outlet bores  103  in the anode  45 , and out the cooling water outlet  105  illustrated at the top of FIG. 1. To further improve heat transfer, the flow areas of the different flow paths are carefully sized to prevent sudden increases or decreases in pressure. A sudden increase in flow area can reduce the pressure to a point that allows the water within the chamber to boil and change to steam. If boiling begins, heat transfer is hampered reducing the performance and the capabilities of the gun  10 . Boiling water and steam do not perform well as coolants and are thus undesirable. If, on the other hand, the water has boiled or begun to boil, and the flow area is drastically reduced, the steam could condense, also hampering heat transfer. As shown in FIG. 1, the water flow paths  50  provide for gradual area transitions and generally consistent diameters throughout the gun  10  to minimize pressure loss and enhance the cooling effect of the water.  
         [0028]    The area most susceptible to pressure drops and boiling is the annular chamber  101  defined by the inner surface of the front housing  15  and the outer surface  80  of the anode  45 . The annular chamber  101  acts as a manifold, receiving the coolant flow from the cathode holder coolant bores  104  and distributing it through the cooling bores  102  of the anode  45 . The annular chamber  101  has a large volume compared to the cooling bores  102  and the cathode holder coolant bores  104 . To reduce the likelihood of boiling, the flow area and the volume of the annular chamber  101  are minimized. In preferred constructions, the largest flow area is less than about 0.5 in 2 . Guns having larger flow areas are susceptible to coolant boiling.  
         [0029]    In other constructions (not shown), the flow direction described above may be reversed. The flow enters at the previous water outlet  105  and exits through the cathode holder  30 . Cooling water enters the front housing  15  through the cooling water outlet  105  and flows through the outlet bores  103  in the anode  45  to the cover (not shown). The cover connects to the cooling bores  102  in the anode  45  to direct coolant near the inner bore of the anode  45 . The coolant then flows into the annular chamber  101 , out the cathode holder coolant bores  104 , and out the water inlet  55 .  
         [0030]    In operation, the gun functions as follows: Cooling water is introduced into the plasma-arc spray gun  10  through a fitting (not shown) attached to the cathode holder  30 . The water flows through the various internal passages in the spray gun  10  and out front housing  15 . The cathode  35  is connected to the negative lead of a power supply (not shown) while the anode  45  is electrically connected to the positive lead. An electrical arc  76  is established between the anode  45  and the cathode  35 . Primary gas is supplied to the plasma-arc spray gun  10  through passages (not shown) to the annular space  98 . The gas, which is injected into the gun  10  at the rear of the anode  45  by the gas injector  40 , flows into the anode  45  and through the arc attachment area  77  where it is heated by the arc  76 . The gas changes to a plasma state and flows out the small opening  65  of the anode  45 . In many constructions, the annular gap  100  is configured to induce a swirl in the gas flow. The swirl forces the arc  76  to rotate around the anode  45 , thereby increasing the life of the anode  45 . The coating powder, introduced into the interior of the anode  45  through the holes  106 , is entrained in the plasma stream and is accelerated out the plasma-arc spray gun  10  with the plasma stream. The plasma gun  10  is therefore capable of producing a plasma for the application of metallic or ceramic particles on a substrate. The holes  106  are shown in one possible position within the anode. Other constructions inject the coating powder upstream of the arc  76 , while still others inject the coating downstream of the arc  76  as shown in FIG. 1. For purposes of the present invention, the actual point at which the powder is introduced into the flow stream is not important.  
         [0031]    It should be noted that throughout the description of the drawings, water was described as the cooling fluid. This should not be read to limit the invention to plasma-arc spray guns  10  that employ water as a coolant. The present invention will function using coolants other than water and therefore should be interpreted as such.  
         [0032]    Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.