Abstract:
A portable gas dynamic cold spray gun eliminates many of the inherent limitations of the prior art by minimizing a scatter of operating parameters and improving its efficiency. According to one feature of the present invention, the powder flow rate is continuously measured so that the powder flow rate and/or the flow rate of the pressurized gas can be adjusted accordingly in order to control the deposition efficiency of the spray gun.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a portable gas dynamic spray gun for cold gas dynamic spraying of a metal, alloy, polymer or mechanical mixtures thereof.  
         [0002]     Gas dynamic spray guns coat substrates by conveying powder particles in a carrier gas at high velocities and impacting the substrate to form the coating. The gas and particles are formed into a supersonic jet having a temperature below the fusing temperature of the powder material, and the jet is directed against an article to be coated.  
         [0003]     One difficulty associated with some of the prior art spray systems is that the powder is injected into the heated main gas stream prior to passage through the nozzle. The powder has a tendency to plug a throat of nozzle to result in backpressure and attendant malfunction of the gun. This requires a complete shutdown of the system and cleaning of the nozzle. Larger particles tend to plug the nozzle even more.  
         [0004]     The second difficulty is associated with low durability of the convergent and throat portions of nozzle. Because the heated main gas stream is under high-pressure, the injection of the powder also requires high-pressure powder delivery systems, which are quite expensive and would be difficult to use in a portable cold spray gun.  
         [0005]     Some known spray guns use a powder feeding system having an enclosed hopper for containing powder in loose particulate form. A carrier gas conduit connected to a carrier gas supply extends through the hopper in its lower portion and continues to a point of powder-carrier gas utilization. Fluidizing gas in a regulated amount is supplied to the hopper and the flow of the fluidizing gas is regulated by sensing the pressure at a point in a carrier gas line, which pressure is responsive to the mass flow rate of solids, and then using the change in the pressure in the conveying gas line, if any, to regulate the flow of the fluidizing gas. This type of system has certain problems with control and uniformity of the powder feed rate. One such problem is pulsation, apparently due to a pressure oscillation, resulting in uneven coating layers.  
         [0006]     Another problem with some of the known spray guns relates to the heating unit for heating the carrier gas prior to the nozzle. Generally, the heating unit is either too large to be used in a portable spray gun, or it is too small to heat the carrier gas sufficiently.  
       SUMMARY OF THE INVENTION  
       [0007]     A portable gas dynamic cold spray gun according to the present invention eliminates many of the inherent limitations of the prior art spray guns by minimizing the scatter of operating parameters and improving its efficiency. According to one feature of the present invention, the powder flow rate is continuously measured so that the powder flow rate and/or the flow rate of the pressurized gas can be adjusted accordingly in order to control and improve the deposition efficiency of the spray gun.  
         [0008]     The spray gun generally includes a gas passageway through the spray gun. A gas supply port supplies pressurized air (or other gas) to the inlet of the passageway. A nozzle in the passageway forms the pressurized air into a supersonic jet stream. A powder feed passage leads to the passageway and supplies powder at a controlled rate to the passageway, where it is entrained in the gas and exits the spray gun in the supersonic jet stream.  
         [0009]     The spray gun further includes a powder flow rate sensor that measures the powder flow rate of the powder. In the example spray gun described herein, the powder flow rate sensor includes a light emitter transmitting light across a duct through which the powder travels. A light receiver mounted opposite the light emitter determines the flow rate of the powder based upon the amount of light received from the light emitter. A controller adjusts the gas flow rate and/or the powder flow rate based upon the measured powder flow rate and based upon a set powder flow rate or a stored desired powder flow rate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Other advantages of the present invention can be understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
         [0011]      FIG. 1  is the front and side views, partially in cross-section, of a portable gas-dynamic spray gun;  
         [0012]      FIG. 2A  is a front view, shown in partial cross-section, of a powder pickup device used in the spray gun shown in  FIG. 1 .  
         [0013]      FIG. 2B  is a side view of the powder pickup device of  FIG. 2A .  
         [0014]      FIG. 3A  is a fragmentary longitudinal cross-section view of a portion of a powder supply vibrating bowl of  FIGS. 2A and 2B .  
         [0015]      FIG. 3B  is a bottom view of the bowl nose of  FIG. 3A .  
         [0016]      FIG. 4A  is a cross-section of an alternative heating chamber that could be used in the spray gun of  FIG. 1 .  
         [0017]      FIG. 4B  is a perspective view of another alternative heating unit that could be used in the spray gun of  FIG. 1 .  
         [0018]      FIG. 4C  is an end view of the heating unit of  FIG. 4B . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     A portable gas dynamic spray (GDS) gun  100  according to the present invention is shown in  FIG. 1 . The GDS gun  100  generally includes a pressurized gas source  102  supplying high-pressure air or other gas to a heat chamber  16 . A ceramic insert  7  leads from the heat chamber  16  and forms the throat and part of the converging portion of a nozzle. A steel tube  9  leading from the ceramic insert  7  forms the diverging portion of the nozzle. The tube  9  extends through an outer housing  2  from which it is supplied with powder  17  from a container  18 . Generally, as the pressurized air or other gas passes through the nozzle, it reaches supersonic velocities and draws powder  17  from the container  18  into the tube  9 .  
         [0020]     The outer housing  2  has multiple passages  4  therethrough each leading to axially-spaced orifices  10  on the tube  9 . A rotatable switch  3  selectively supplies powder to one of the multiple passages  4  in the outer housing  2  based upon the value of negative pressure at certain points of the air jet. The rotatable switch  3  may be set manually, or automatically by the controller  22  based upon expected negative pressure points along the tube  9 . Depending upon the pressure from pressurized gas source  102 , the location along the tube  9  of a negative pressure point may vary. The rotatable switch  3  should be set so that the selected orifice  10  coincides with the negative pressure point.  
         [0021]     The powder container  18  feeds powder  17  to the switch  3  through a vibrating bowl  19 , funnel  20  and a powder-aspirating duct  6  into the partial-vacuum powder passages  4  of the outer housing  2 . The powder  17  then mixes with the jet of conveyance air and then jointly with it flows through the duct  1  of the nozzle to impart supersonic velocities to the air and entrained powder.  
         [0022]     A jet of conveyance air  13  from pressurized air supply  102  is supplied via a compressed-air line  14  through a guide vane  15  to be heated in the heat chamber  16 . The compressed-air line  14  contains a variable throttle  21  by which the flow impedance (e.g. the flow cross-section) is regulated from a controller  22  as a function of a setpoint value of the volumetric flow of conveyance air and/or of a setpoint value for the volume concentration of the particles in powder laden jet. The controller  22  may be a computer having a processor, memory and other storage, and being suitably programmed to perform the operations described herein.  
         [0023]     The heat chamber  16  includes a serpentine or helical coil heating element  23  mounted on a ceramic support  24  and an insulation chamber  25 , which is located in an internal chamber housing  26 . The second insulation sleeve  27  with insulation cup  28  is arranged in outer chamber housing  29 . The air  13  flows along the helical path defined by the helical coil heating element  23 , the ceramic support  24  and the insulation chamber  25 . The heated air exits the heater via tapered chamber  30 , which together with ceramic insert  7  forms the convergent portion of the nozzle.  
         [0024]     The powder supply system is shown in more detail in  FIGS. 2A and 2B . The powder supply system includes the powder container  18  enclosing a powder  17  to be sprayed in loose particulate form, a bowl vibration unit  31  (such as a motorized vibration unit) for control of the powder flow rate, and the funnel  20  connected to the powder aspirating duct  6  and a flexible hose  12 . Additionally, a powder container vibration unit  32  is incorporated into the upper portion of powder supply system. The vibration unit  32  is installed on a baffle plate  34  supporting the container  18 . Simultaneous control of the two vibration units  31 ,  32  provides precise and constant control of the powder feeding rate.  
         [0025]     Powder is fed into the powder container  18  through a port  35  so that a certain level of powder  17  is maintained by a sensor  36  which controls an operation of a main powder hopper (not shown). Referring to  FIGS. 3A and 3B , the rate of dispensing powder (powder flow rate) is additionally controlled by the removable bowl  19  nose  37  with a diameter d of hole and size a of slots. The rate of dispensing powder  17  is defined by flowability of the powder  17 . The hole has a diameter d with slots of width a creating channels along the hole. The diameter d of the hole is preferably approximately three times the width a of the slots. The diameter d is preferably approximately ten to twenty times the particle diameter. The shape and dimensions of the opening in the bowl nose  37  make the flow more controllable based upon adjustments in the vibration. The bowl nose  37  can be replaced with holes and slots of different sizes when used with different particle sizes.  
         [0026]     The partial vacuum existing in the partial-vacuum zone in the lower portion of pick-up housing  38  aspirates air from the atmosphere while being strongly throttled by the flow throttle  39  when passing into the partial-vacuum zone of chamber  38 . The chamber  38  is fitted with a flow sensor  40  generating a measurement signal in the signal line  49  as a function of the air flowing from the atmosphere through the throttle  39  into the partial-vacuum zone of chamber  38 , i.e. the quantity per unit time, or rate, of air passing through the throttle  39  and passage  41  and hence also being a control of the rate of powder passing through the powder passage  4 .  
         [0027]     The pick-up device comprises a powder metering unit  42  detecting a flow of powder particles in a measurement duct, which in the embodiment shown is a glass powder transportation tube  43  connecting the funnel  20  to the powder aspirating duct  6  attached to the powder switch  3 . The powder-metering unit  42  includes an infrared sensor  44  and an infrared emitter or light source  45  disposed within the channel made in pick-up bottom plate  46 . The infrared sensor  44  can determine the mass flow of powder  17  through the glass tube  43  based upon the amount of light from light source  45  that is able to pass through the glass tube  43  to the infrared sensor  44 . Although an infrared light source  45  and infrared sensor  44  are preferred, other wavelengths of light or other waves could also be used.  
         [0028]     Optionally, an additional powder metering unit  62  can be mounted in the pick up housing  38  on opposite sides of the funnel  20 . The powder metering unit  62  is preferably similar to the power metering unit  42  and includes an infrared sensor  64  (or light sensor) and an infrared emitter  65  (or light source). This powder metering unit  62  measures the powder dispensing rate ω d  from the vibrating bowl  19 . The powder dispensing rate ω d  can then be compared to the conveyed powder rate ω p . The amplitudes of the vibration units  31 ,  32  can be adjusted relative to one another in order to ensure that the powder dispensing rate ω d  is equal (over some short period of time) to conveyed powder rate ω p . This prevents clogging of the funnel  20 .  
         [0029]     The particle volume concentration significantly affects the deposition efficiency. The particle volume concentration in a powder laden jet greatly influences the effectiveness of GDS process particularly in the case of radial injection of powder by conveyance air of the partial-vacuum zone. In the preferred embodiment, the control of volume concentration of particles is achieved by regulation of two parameters: a rate of conveyed powder and a rate of conveyance air. The rate of conveyed powder ω p  is substantially dependent on the powder dispensing rate ω d  and the rate of conveyance air. The powder rate is approximately proportional to the rate of conveyance air of the partial-vacuum zone of chamber  38 . Therefore, the conveyance air must be adjusted to adjust a desired particle volume concentration of powder laden jet. Thereupon the controller  22  will automatically set the rate of conveyance air by means of the adjustment motor  47  and the throttle  39  in such a way that the volumetric flow shall remain at the setpoint. From an other side the controller  22  will automatically set the powder dispensing rate ω d  by means of the adjustment of amplitudes of vibration units  31 ,  32  on the basis of measurements of the rate of conveyed powder ω p  in order to achieve the permanent balance ω d =ω p . Additionally the rate of conveyance air is regulated by a change of an injection point location by the switch  3  manually or automatically.  
         [0030]     The controller  22  regulates the powder feeding flow rate, carrier air  13  flow rate and feed of powder conveyance air in the partial-vacuum zone of chamber  38  as a function of the measurement signals of the measurement lines  48 ,  49 , 50  and as a function of the setpoint value of the volume concentration of particles in air-powder jet by means of the vibration units  31 ,  32  and the throttles  21 ,  39 .  
         [0031]     The controller  22  comprises an input  51  for the powder flowability setpoint value receiving a manual or automatic fixed or variable setpoint of the powder dispensing flow rate “ω d ” to be conveyed, for instance in g/sec, and an input  52  for volume concentration of powder setpoint value “C v ” allowing to determine the carrier air flow rate for the air passing through the powder/air duct  1  from an equation  
         C   V     =       ω   p         ρ   p     ·     ω   air             
 
         [0032]     where cop is the particle feeding flow rate from the funnel  20  ( FIG. 2 ), ρ p  is the material density and ω air  is the carrier air flow rate controlled by air pressure and throttle  21  (a graph on controller  22 ).  
         [0033]     An alternative heat chamber  16   a  is shown in  FIG. 4A . The heat chamber  16   a  includes the helical coil-heating element  23  mounted on a ceramic tube  53  within a carrier air transportation pipeline  54 . The carrier air transportation pipeline  54  is mounted inside the internal chamber housing  26  to define a hollow cylindrical passageway therebetween. The air flows in from the line  14  forwardly (to the right in  FIG. 4A ) between the internal chamber housing  26  and the pipeline  54 . The air then enters the forward end of pipeline  54  and flows rearwardly within the helical coil-heating element  23 . At the rearward end of the pipeline  54 , the air enters the ceramic tube  53  and then travels forwardly through the ceramic tube  53  the tapered chamber  30  and the converging ceramic insert  7 . Thus, the air gathers heat from the helical coil-heating element  23  on three serpentine passes. This increase in the heating surface intensifies the heating of the air and increases the temperature of carrier air up to 650-850° C. in the portable heating chamber. The system incorporates safety features for the protection of both the system and the operator. The control system  22  ( FIG. 1 ) switches off the power supply and sends a signal out in case of abnormal increase in the temperature of the gas above a set value.  
         [0034]     An alternative heating element  23   a  is shown in  FIG. 4B , generally including a plurality of coils  123  connected to one another in series and spaced about a passageway by supports  124 .  
         [0035]     In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope. Alphanumeric identifiers on method steps are provided for ease of reference in dependent claims and are not intended to dictate a particular sequence for performance of the method steps unless otherwise indicated in the claims.