Patent Publication Number: US-2009218328-A1

Title: Nozzle

Description:
The present invention relates to a nozzle for emitting solid carbon dioxide particles, and to apparatus comprising such a nozzle for cooling a heated weld zone produced in a workpiece by a welding process. 
     During the thermal welding of metallic workpieces a high heat input is required to generate an acceptable weld. However, this high heat input has the disadvantage that the thermal stresses generated by the welding process can cause significant levels of distortion of the workpieces being welded. Sheet workpieces of relatively soft metals and alloys such as mild steel, titanium, titanium alloys and stainless steels are particularly prone to distortion and reduced tensile strength. 
     It is known to use gases to provide forced cooling during arc welding processes. However, the results of using cooling gases are limited as the cooling ability of the gas streams is relatively low. Cryogens, particularly liquid nitrogen and solid carbon dioxide, have been used to provide enhanced cooling to arc welding processes, but if the weld zone is cooled excessively this can lead to distortion of the workpieces in the opposite direction. It is therefore desirable to balance the amount of cooling of the weld zone with the heat input from the welding process to result in little or no distortion of the workpieces. 
     In a first aspect, the invention provides apparatus for cooling a heated weld zone formed in a workpiece by a welding process, the apparatus comprising a nozzle for emitting solid carbon dioxide particles towards the weld zone, the nozzle comprising an inlet for receiving liquid carbon dioxide, a nozzle aperture from which solid carbon dioxide particles flow from the nozzle, a duct extending between the inlet and the nozzle aperture, the duct having a convergent section and a divergent section, and an axially displaceable valve member located within the duct, the valve member having first and second convergent portions and a substantially cylindrical portion located between the first and second convergent portions. 
     A high velocity gas stream containing solid carbon dioxide particles can be produced by the adiabatic expansion of liquid carbon dioxide as it passes through a nozzle. Solid CO 2  particles collide with the hot surface of the weld zone and are rapidly sublimated as heat is extracted from the weld zone. 
     The apparatus preferably comprises a control system for controlling the position of the valve member within the duct. The rate at which the weld zone is cooled is dependent upon the rate at which solid CO 2  particles impact the surface of the weld zone. By controlling the position of the valve member within the duct, the rate at which solid CO 2  particles flow from the nozzle, and therefore the cooling rate of the weld zone, can be controlled so that distortion of the workpiece can be inhibited. 
     The flow rate of solid CO 2  particles from the nozzle, and therefore the position of the valve member within the duct, may be controlled in dependence on one or more of the following parameters:
         the temperature of the weld zone;   one or more operational parameter of the welding process, such as welding voltage, welding current, the speed of the welding tool used to form the weld zone and the time taken to form the weld zone;   the thickness of the workpiece;   the material from which the workpiece is formed;   the nature of the welding process (for example, MIG, TIG or submerged arc welding); and   any strain measurement technique used to measure the residual stresses within the workpiece.       

     Information relating to the thickness and material of the workpiece, and the welding process, may be input into a control interface by an operator. Information relating to the various operational parameters of the welding process may be similarly input by the operator, or supplied to the control system directly by the welding tool. A non-contact temperature detector, such as an infrared temperature sensor, may be positioned in close proximity to the weld zone so that information relating to the temperature of the weld zone can be supplied to the control system. Similarly, strain gauges may also be positioned in close proximity to the weld zone so that information relating to deformation of the workpiece during the welding process can be supplied to the control system. This can enable the control system to rapidly and automatically adjust the ejection rate of the solid CO 2  particles in response to variation in the temperature of the weld zone and/or the operational parameters of the welding process and/or deformation of the workpiece so as to minimise residual stresses generated in the workpiece. Use of such a control system can also enable detailed cooling procedures to be created and performed in a controlled and reproducible manner. 
     The nozzle is preferably spaced from the workpiece by a distance in the range from 30 to 50 mm. Liquid carbon dioxide is preferably supplied to the nozzle at a pressure in the range from 16 to 18 bar. 
     The nozzle comprises a duct through which liquid carbon dioxide is expanded, a nozzle aperture through solid CO 2  particles are emitted from the nozzle, and a valve member moveable within the duct to vary the flow rate of solid CO 2  particles from the nozzle. The valve member is preferably in the form of an elongate valve member that is axially displaceable within the duct to control the cross-section of the flow of CO 2  through the duct, and thereby control the flow rate of solid CO 2  particles from the nozzle. 
     The duct comprises a convergent section in which liquid carbon dioxide is accelerated towards a sonic speed. The second convergent portion of the valve member is preferably located at least partially within the convergent section such that axial displacement of the valve member relative to the convergent section varies the cross-section of the flow of CO 2  through the duct. The second convergent portion preferably has a taper angle that is substantially the same as that of the convergent section of the duct, so that in a closed position of the valve member the second convergent portion of the valve member engages the convergent section of the duct to prevent flow of CO 2  from the nozzle. 
     The duct and valve member are profiled to control the location of the minimum cross-section of the CO 2  flow when the valve member is subsequently moved to an open position. For example, in the preferred embodiment the duct has a throat located between the convergent section and the nozzle aperture. The valve member has a substantially cylindrical portion located between the convergent portions of the valve member, and having a diameter slightly smaller than, preferably between 5 and 15 μm smaller than, the diameter of the throat so that, in the closed position, the cylindrical portion is located just beneath or adjacent the throat. As the valve member is axially displaced from the closed position to an open position, the cylindrical portion of the valve member can be rapidly positioned within the throat, in the preferred embodiment within an axial displacement of between 0 and 100 μm, for example around 50 μm, from the closed position. This can rapidly establish the minimum cross-section of the CO 2  flow through the duct at the throat, inhibiting solid CO 2  formation upstream from the throat. 
     The cylindrical portion of the valve member preferably extends between 0.4 and 0.6 mm between these convergent portions of the valve member. The first convergent portion, located between the cylindrical portion and tip of the valve member, preferably has a taper angle that is smaller than that of the first convergent portion. The taper angle of the first convergent section of the valve member is between 5 and 15°, and the taper angle of the second convergent section of the valve member is between 15 and 25°. 
     The duct comprises a divergent section located between the convergent section and the nozzle aperture so that as the liquid carbon dioxide passes through the throat it expands, resulting in a phase change from liquid carbon dioxide to solid and gaseous carbon dioxide. As the gaseous carbon dioxide flows through the divergent section of the duct, it further expands and therefore increases in speed. Consequently, the solid carbon dioxide particles entrained within the gaseous carbon dioxide are accelerated towards the weld zone to impact with the weld zone at a high velocity. This can enable the solid CO 2  particles to impact the surface of the weld zone with sufficient velocity to allow a good heat transfer rate to occur without a gas insulant layer being created. 
     Movement of the valve member within the duct may be actuated by any suitable electro-mechanical device, such as a linear or stepper motor. An encoder or other linear position sensor may be provided for tracking motion of the valve member, and for providing data to a controller of the control system for use in controlling the motor drive. 
     In a second aspect, the present invention provides welding apparatus comprising a welding tool for forming a weld zone in a workpiece and cooling apparatus as aforementioned for cooling the weld zone. The cooling apparatus may be arranged relative to the welding tool such that the nozzle is spaced from the welding tool by a fixed distance, for example in the range from 50 to 100 mm, during relative movement between the welding tool and the workpiece. The cooling apparatus and the welding tool may be stationary, with the workpiece moved relative to the tool and cooling apparatus to form a weld line in the workpiece. Alternatively, the workpiece may be stationary, with the welding tool and cooling apparatus moved relative to the workpiece. 
     The nozzle may be used for purposes other than cooling a heated weld zone, and so in a third aspect the present invention provides a nozzle for emitting solid carbon dioxide particles, the nozzle comprising an inlet for receiving liquid carbon dioxide, a nozzle aperture from which solid carbon dioxide particles flow from the nozzle, a duct extending between the inlet and the nozzle aperture, the duct having a convergent section and a divergent section, and an axially displaceable valve member located within the duct, the valve member having first and second convergent portions and a substantially cylindrical portion located between the first and second convergent portions. 
     Features described above in relation to the first aspect of the invention are equally applicable to the second and third aspects of the invention, and vice versa. 
    
    
     
       Preferred features of the present invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates apparatus for welding a workpiece; 
         FIG. 2  is a cross-sectional view of a nozzle from which solid carbon dioxide particles flow toward the weld zone formed in the workpiece; 
         FIG. 3  illustrates the tapered head of a valve member of the nozzle of  FIG. 2 ; 
         FIG. 4  illustrates the tapered head of the valve member when the valve member is in a closed position; and 
         FIG. 5  illustrates the tapered head of the valve member when the valve member is in an open position. 
     
    
    
       FIG. 1  illustrates a welding tool  10  for welding a workpiece  12 , in this example in the form of a sheet metal plate. The welding tool  10  may be any form of welding torch, such as a MIG welding torch. As is known, such a welding torch feeds a consumable electrode to a weld zone of the workpiece  12 . An electric arc  14  is struck between the tip of the electrode and the workpiece  12  in the vicinity of the weld zone. Molten metal is transferred from the electrode to the weld zone through the arc  14 . A shielding gas, typically consisting of argon, optionally with relatively small quantities of oxygen and carbon dioxide added, is supplied from the welding torch around the consumable electrode so as to inhibit oxidation of the weld metal. The workpiece  12  may be moved relative to the welding tool  10  to cause the weld zone to move along the workpiece, or the welding tool  10  may be moved relative to the workpiece  12 . 
     In order to cool the weld zone formed in the workpiece  12 , a nozzle  16  is provided for emitting a supersonic stream  18  of solid CO 2  particles towards the heated weld zone. The nozzle  16  is spaced from the welding tool  10 , preferably by a distance in the range from 50 to 100 mm in the direction (indicated at X in  FIG. 1 ) of relative movement between the welding tool  10  and the workpiece  12 , so that the weld zone formed in the workpiece  12  by the welding tool  10  is impacted by the stream  18  of solid CO 2  particles flowing from the nozzle  16  without the CO 2  stream impinging upon the shielding gas surrounding the melt pool formed in the workpiece  12  during the welding process. 
     The nozzle  16  receives liquid CO 2  from a supply line  20  connected between the nozzle  16  and a supply tank  22  storing liquid CO 2  at a pressure in the range from 16 to 18 bar. A valve (not shown) may be provided in the supply line  20  for closing the supply of CO 2  to the nozzle. A phase separator (not shown) is also provided in the supply line  20  for separating gaseous CO 2  from liquid CO 2 . As described in more detail below, the stream  18  of solid CO 2  particles is formed through adiabatic expansion of the liquid CO 2  within the nozzle  16 . This causes the pressure of the liquid CO 2  to fall below the triple point pressure of CO 2 , resulting in a phase change from liquid CO 2  to a mixture of solid CO 2  particles and gaseous CO 2 . 
       FIG. 2  illustrates the nozzle  16  in more detail. The nozzle  16  has an elongate tubular body  24  housing a CO 2  flow duct  26 . A CO 2  inlet  28  supplies liquid CO 2  radially into the duct  26 , a connector  30  being provided for connecting the inlet  28  to the supply line  20 . A nozzle aperture  32  co-axial with the longitudinal axis  34  of the duct  26  emits a jet stream of gaseous CO 2  and solid CO 2  particles from the duct  26 . The duct  26  has a convergent section  36 , and a divergent section  38  located between the convergent section  36  and the nozzle aperture  32 , the intersection of the convergent and divergent sections  36 ,  38  of the duct  26  defining a throat  40  at which the cross-section of the duct  26  is at a minimum. The nozzle aperture  32  is preferably spaced from the workpiece  12  by a distance in the range from 25 to 125 mm, most preferably in the range from 30 to 50 mm. 
     The nozzle  16  includes a valve member  42  that is moveable within the duct  26  to vary the flow rate of solid carbon dioxide particles from the nozzle. In this example, the valve member  42  is in the form of an elongate valve member  42  that is axially displaceable along the longitudinal axis  34  of the duct  26  and aligned co-axially therewith. The valve member  42  has a shaft  44  that projects outwardly from the body  24  of the nozzle  16  and is coupled to an electro-mechanical device  46 , such as a linear or stepper motor, for axially displacing the valve member  42  within the duct  26  to vary the flow of CO 2  through the duct  26 . The shaft  44  is supported within the duct  26  by a support spider  48 , and also by a guide bushing  50  that closes the end of the duct  26  opposite the nozzle aperture  32 . 
     The valve member  42  also has a tapered head  52 , the profile of which is illustrated in  FIG. 3 . From the tip  54  of the valve member  42 , the head  52  comprises a first convergent section  56  having a taper angle α, in this example between 5 and 10°, a first substantially cylindrical portion  58 , a second convergent section  60  having a taper angle β, where β&gt;α, in this example between 15 and 25°, a third convergent section  62  having a taper angle γ, where γ≈α, and a second substantially cylindrical portion  64 . The first cylindrical portion  58  has a length in the range from 0.4 to 0.6 mm, in this example 0.5 mm, and a diameter that is in the range from 1.5 to 1.7 mm, in this example approximately 1.59 mm, and is slightly less than the diameter of the throat  40  of the duct  26 , which in this example is approximately 1.60 mm. 
       FIG. 4  illustrates the position of the head  52  relative to the throat  40  of the duct when the valve member  42  is in a closed position. In this position, the outer surface of the second convergent portion  60  of the head  52  engages the inner surface  66  of the convergent section  36  of the duct  26  to form a seal that prevents flow of CO 2  into the divergent section  38  of the duct  26 . In the closed position, the first cylindrical portion  58  of the head  52  is located just beneath (as illustrated) the throat  40  of the duct  26 , and is preferably no more than 100 μm beneath the throat  40 . In this example, the cylindrical portion  58  is less than 50 μm beneath the throat  40  when the valve member  42  is in the closed position. 
     As the valve member  42  is axially displaced from the closed position to an open position by actuation of the electro-mechanical device  46 , an annular flow channel  68  is created between the inner surface of the duct  26  and the outer surface of the head  52 , as illustrated in  FIG. 5 . The duct  26  and valve member  42  are profiled so that when the valve member  42  is in an open position, the cross-section of the annular flow channel  68  narrows between the inlet  28  and the throat  40  so that the speed of liquid CO 2  increases towards a supersonic speed, and then widens from the throat  40  to the nozzle aperture  32  so that the liquid CO 2  expands whilst gathering further speed to reach a supersonic speed. As mentioned above, expansion of the liquid CO 2  causes the pressure of the liquid CO 2  to fall below the triple point pressure of CO 2 , resulting in a phase change from liquid CO 2  to a mixture of solid CO 2  particles and gaseous CO 2 . 
     In order to inhibit formation of solid CO 2  particles upstream from the throat  40  of the duct  26 , the first cylindrical portion  58  of the head  52  is preferably located less than 50 μm beneath the throat  40  when the valve member  42  is in the closed position, and has a diameter that is preferably around 0.1 mm less than that of the duct  26 . Consequently, within the first 50 μm axial displacement of the valve member  42 , the first cylindrical portion  58  is located within the throat  40 , where it remains, in this embodiment, for the next 0.5 mm axial displacement of the valve member, this being the length of the first cylindrical portion  58 . Due to the narrow annular gap established between the first cylindrical portion  58  and the throat  40  of the duct  26  during this period, as the wall  70  of the second convergent portion  60  of the head  52  continues to move away from the wall  66  of the convergent section  36  of the duct  26  the minimum cross-section of the annular flow channel at the throat  40  of the duct  26  is rapidly established at the throat  40 . 
     With continued axial displacement of the valve member  42  from the closed position, the first convergent portion  56  of the head  52  becomes located within the throat  40  of the duct  26 . Due to the narrow taper of this portion  56  of the head  52 , in this example α≈8°, the minimum cross-section of the annular flow channel  68  remains located at the throat  40 , and the size of the minimum cross-section increases gradually with continued axial displacement of the valve member  42 . Furthermore, the shape of the first convergent portion  56  of the head  52  enables a concentrated, controlled stream of solid carbon dioxide particles to flow from the nozzle towards the weld zone, thereby optimising the efficiency of the cooling of the weld zone. 
     As the valve member  42  is axially displaced from the closed position, the size of the annular flow channel  68  within the convergent section  38  of the duct  26  increases, and so both the flow rate of CO 2  through the duct  26  and the amount of solid CO 2  particles flowing from the nozzle  16  increases with movement of the valve member  42  from the closed position. Consequently, the flow rate of solid CO 2  particles towards the heated weld zone, and therefore the rate of cooling of the weld zone, can be controlled through control of the position of the valve member  42  within the duct  26 . Returning to  FIG. 1 , a control system is provided for controlling the position of the valve member  42 . The control system includes the electro-mechanical drive  46  for actuating the axial displacement of the valve member  42 , and a controller  80  for controlling actuation of the drive  46  and thereby control the position of the valve member  42  relative to the duct  26 . The controller  80  may be configured to control the drive  46  in dependence on one or more parameters, including, but not limited to:
         the temperature of the weld zone;   one or more operational parameter of the welding process, such as welding voltage, welding current, the speed of the welding tool  10  and the time taken to form the weld zone;   the thickness of the workpiece  12 ;   the material from which the workpiece  12  is formed;   the nature of the welding process (for example, MIG, TIG or submerged arc welding); and   any strain measurement technique used to measure the residual stresses within the workpiece  12 .       

     Information relating to the temperature of the weld zone may be provided by an infrared temperature sensor  82  located adjacent the nozzle  16 . The infrared temperature sensor  82  absorbs ambient infrared radiation given off by the heated weld zone. The incoming light is converted to an electric signal, which corresponds to a particular temperature, and is supplied to the controller  80 . The controller  80  can then rapidly and automatically adjust the flow rate of the solid CO 2  particles from the nozzle  16  in response to variation in the temperature of the weld zone. 
     In addition to, or as an alternative to, using a temperature sensor  82  to provide information regarding the temperature of the weld zone, a strain measurement technique may be used to provide information regarding deformation of the workpiece during the welding process, with the controller  80  rapidly and automatically adjusting the flow rate of the solid CO 2  particles from the nozzle  16  in response to the deformation of the workpiece  12 . 
     Information relating to the thickness and material of the workpiece may be input into a control interface  84  by an operator and supplied to the controller  80  for controlling the flow rate of the solid CO 2  particles from the nozzle  16 . The interface may be physically separate from the controller  80 , or it may be integral with the controller  80 . Information relating to the various operational parameters of the welding process, and relating to the nature of the welding process itself, may be similarly input by the operator using the control interface  84 , or supplied to the controller directly by the welding tool. 
     The control system may also include an encoder (not shown) for monitoring the position of the valve member  42 , and for supplying signals indicative of the current position of the valve member  42  to the controller  80  for use in controlling the drive  46 . 
     The current rate at which CO 2  is flowing from the nozzle can be used to modulate the extraction rate of gases from the weld zone in order to capture the spent coolant whilst both preventing the arc from being blown out, and preventing extraction of the shielding gases surrounding the melt pool during the welding process.