Abstract:
A gas metal arc welding system that includes leading and trailing welding torch assemblies arranged to operate in tandem. The leading welding torch assembly further includes a torch body; a contact tip connected to one end of the rotatable torch body, wherein the contact tip further includes a passage formed therethrough; a consumable wire electrode passing through the body and exiting the contact tip through an exit hole; and a source of shielding gas. The trailing welding torch assembly further includes a non-rotatable torch body; a contact tip connected to one end of the non-rotatable torch body; a consumable wire electrode passing through the non-rotatable torch body and exiting through the contact tip; and a source of shielding gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation of U.S. patent application Ser. No. 13/233,366 filed on Sep. 15, 2011 and entitled “Tandem Gas Metal Arc Welding System”, which claimed the benefit of U.S. Provisional Patent Application Ser. No. 61/382,971 filed on Sep. 15, 2010 and entitled “Tandem Gas Metal Arc Welding System,” the disclosures of which are hereby incorporated by reference herein in its entirety and made part of the present U.S. utility patent application for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The described invention relates in general to welding systems and devices, and more specifically to an arc welding system that includes tandem welding torches for use in narrow-groove applications. Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding apparatus. GMAW produces high-quality welds and yields high productivity in many applications. In GMAW, the welding heat source is an arc maintained between the consumable wire electrode and the workpiece. The weld is formed by melting and solidification of the joint edges together with filler material transferred from the electrode. The positive (+) lead is connected to the torch and the negative (−) lead is connected to the work piece for providing a relatively consistent voltage to the arc. Arc voltage is the voltage between the end of the wire and the work piece. The purpose of shielding gas is to protect the weld area from the contaminants in the atmosphere and the gas can be inert, reactive, or mixtures of both. Argon, helium, and carbon dioxide are the main three gases used in GMAW. GMAW process variables include welding current (electrode melting rate), polarity, arc voltage (length), travel speed, electrode extension, electrode size, and shielding gas composition. 
         [0003]    Many manufacturers of thick-section components such as pressure vessels, heavy equipment, ship hulls, thick-wall pipe, and the like join parts together using high-deposition-rate welding processes such as GMAW and/or Submerged Arc Welding (SAW) with conventional open-groove designs. Although these processes may be considered to be high-deposition-rate processes, they are not necessarily high-productivity processes for this application due to the large number of welds that are required to fill conventional open-groove weld joints. Other manufacturers of these components join parts by using low-deposition-rate welding processes such as gas tungsten arc welding (GTAW) with narrow-groove designs. Although the narrow-groove design drastically reduces overall volume of the weld joint, the lower deposition-rate processes used with these joint configurations prevent them from being high productivity processes. As the market for these components continuously drives the need to reduce cost while maintaining a high level of quality, innovative methods of joining these components together at much higher productivity levels must be created. 
       SUMMARY OF THE INVENTION 
       [0004]    The following provides a summary of certain exemplary embodiments of the present invention. This summary is not an extensive overview and is not intended to identify key or critical aspects or elements of the present invention or to delineate its scope. 
         [0005]    In accordance with one aspect of the present invention, a first gas metal arc welding system is provided. This system includes leading and trailing welding torch assemblies arranged to operate in tandem. The leading welding torch assembly includes a fluid-cooled first torch body, wherein the first torch body further includes at least one cooling channel formed therein; a first contact tip connected to one end of the first torch body, wherein the first contact tip further includes a passage formed therethrough, and wherein the passage defines an exit hole in one of the end of the first contact tip; a first consumable wire electrode passing through the first torch body and the first contact tip and exiting the first contact tip through the offset exit hole; and a first apparatus for providing at least one shielding gas located in close proximity to the portion of the electrode exiting the first contact tip, wherein the first apparatus for providing at least one shielding gas further includes at least one gas delivery tube terminating in a gas delivery nozzle positioned in close proximity to the terminus of the first wire electrode. The trailing welding torch assembly further includes a fluid-cooled non-rotatable second torch body, wherein the second torch body further includes a body containing internal fluid passages and a spatter guard formed one end thereof; a second contact tip connected to one end of the non-rotatable second torch body; a second consumable wire electrode passing through the non-rotatable second torch body and the second contact tip and exiting the second contact tip; and a second apparatus for providing at least one shielding gas located in close proximity to the portion of the electrode exiting the second contact tip, wherein the second apparatus for providing at least one shielding gas further includes at least one gas delivery tube terminating in a gas delivery nozzle positioned in close proximity to the terminus of the second wire electrode. An optional adjustment mechanism for adjusting the height of the non-rotatable second torch body relative to the rotatable first torch body and for adjusting the horizontal distance between the operative tips of the two consumable wire electrodes is also provided. The tandem gas metal arc welding system is adapted for use in narrow-groove joints of up to about six inches in thickness. 
         [0006]    In accordance with another aspect of the present invention, a second gas metal arc welding system is provided. This system also includes leading and trailing welding torch assemblies arranged to operate in tandem. The leading welding torch assembly further includes a fluid-cooled first torch body, wherein the first torch body further includes at least one cooling channel formed therein; a first contact tip connected to one end of the first torch body, wherein the first contact tip further includes a passage formed therethrough, and wherein the passage defines an exit hole in one of the end of the first contact tip; a first consumable wire electrode passing through the first torch body and the first contact tip and exiting the first contact tip through the offset exit hole; and a first apparatus for providing at least one shielding gas located in close proximity to the portion of the electrode exiting the first contact tip, wherein the first apparatus for providing at least one shielding gas further includes two gas delivery tubes positioned parallel to one another, each terminating in a gas delivery nozzle positioned in close proximity to the terminus of the first wire electrode. The trailing welding torch assembly further includes a fluid-cooled non-rotatable second torch body, wherein the second torch body further includes a body containing internal fluid passages and a spatter guard formed one end thereof; a second contact tip connected to one end of the non-rotatable second torch body; a second consumable wire electrode passing through the non-rotatable second torch body and the second contact tip and exiting the second contact tip; and a second apparatus for providing at least one shielding gas located in close proximity to the portion of the electrode exiting the second contact tip, wherein the second apparatus for providing at least one shielding gas further includes two gas delivery tubes positioned parallel to one another, each terminating in a gas delivery nozzle positioned in close proximity to the terminus of the second wire electrode. An optional adjustment mechanism for adjusting the height of the non-rotatable second torch body relative to the rotatable first torch body and for adjusting the horizontal distance between the operative tips of the two consumable wire electrodes is also provided. The tandem gas metal arc welding system is adapted for use in narrow-groove joints of up to about six inches in thickness. 
         [0007]    In yet another aspect of this invention, a third gas metal arc welding system is provided. This system also includes leading and trailing welding torch assemblies arranged to operate in tandem. The leading welding torch assembly further includes a fluid-cooled first torch body, wherein the first torch body further includes at least one cooling channel formed therein; a first contact tip connected to one end of the first torch body, wherein the first contact tip further includes a passage formed therethrough, and wherein the passage defines an exit hole in one of the end of the first contact tip that may be offset from the central axis of the contact tip; a first consumable wire electrode passing through the first torch body and the first contact tip and exiting the first contact tip through the offset exit hole; and a first apparatus for providing at least one shielding gas located in close proximity to the portion of the electrode exiting the first contact tip, wherein the first apparatus for providing at least one shielding gas further includes two gas delivery tubes positioned parallel to one another, each terminating in a gas delivery nozzle positioned in close proximity to the terminus of the first wire electrode. The trailing welding torch assembly includes fluid-cooled non-rotatable second torch body, wherein the second torch body further includes a body containing internal fluid passages and a spatter guard formed one end thereof; a second contact tip connected to one end of the non-rotatable second torch body; a second consumable wire electrode passing through the non-rotatable second torch body and the second contact tip and exiting the second contact tip; and a second apparatus for providing at least one shielding gas located in close proximity to the portion of the electrode exiting the second contact tip, wherein the second apparatus for providing at least one shielding gas further includes two gas delivery tubes positioned parallel to one another, each terminating in a gas delivery nozzle positioned in close proximity to the terminus of the second wire electrode. An adjustment mechanism for adjusting the height of the non-rotatable second torch body relative to the rotatable first torch body and for adjusting the horizontal distance between the operative tips of the two consumable wire electrodes is also provided. The tandem gas metal arc welding system is adapted for use in narrow-groove joints of up to about six inches in thickness. 
         [0008]    Additional features and aspects of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the exemplary embodiments. As will be appreciated by the skilled artisan, further embodiments of the invention are possible without departing from the scope and spirit of the invention. Accordingly, the drawings and associated descriptions are to be regarded as illustrative and not restrictive in nature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The accompanying drawings, which are incorporated into and form a part of the specification, schematically illustrate one or more exemplary embodiments of the invention and, together with the general description given above and detailed description given below, serve to explain the principles of the invention, and wherein: 
           [0010]      FIG. 1  is a front view of an exemplary embodiment of the tandem gas metal arc welding system of the present invention; 
           [0011]      FIG. 2  is a side view of the tandem gas metal arc welding system of  FIG. 1 ; 
           [0012]      FIG. 3  is a bottom view of a portion of an alternate embodiment of the tandem gas metal arc welding system of the present invention showing the lead torch drive mechanism in greater detail; 
           [0013]      FIG. 4  is a front view of a portion of the tandem gas metal arc welding system of  FIG. 1  showing the water rotary union and drive belt in greater detail; 
           [0014]      FIG. 5  includes a side view of the water-cooled lead-torch contact tube as well as multiple cross-sectional views of the construction of the interior of the water-cooled lead-torch contact tube of the tandem gas metal arc welding system of  FIG. 1 ; 
           [0015]      FIGS. 6A-D  are side ( 6 A), cross-sectional ( 6 B), rear ( 6 C), and front views ( 6 D) respectively of the lead electrode contact tip of the tandem gas metal arc welding system of  FIG. 1 ; 
           [0016]      FIG. 7  is a front view of the lead torch gas delivery assembly of the tandem gas metal arc welding system of  FIG. 1 ; 
           [0017]      FIG. 8  is a front view of the trailing torch of the tandem gas metal arc welding system of  FIG. 1  shown with the top portion removed such that the construction of the interior of the trailing torch is visible; and 
           [0018]      FIG. 9  is a front view of the trailing torch gas delivery assembly of the tandem gas metal arc welding system of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0019]    Exemplary embodiments of the present invention are now described with reference to the Figures. Although the following detailed description contains many specifics for purposes of illustration, a person of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following embodiments of the invention are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
         [0020]    This invention addresses the market need for increased productivity in thick-section welding applications, such as pressure vessels, heavy equipment, ship hulls, heavy-wall pipe, and other thick-plate structures. This invention allows the production of high-quality welds at much higher productivity levels than can be achieved with either high deposition rate processes using conventional open-groove designs, or conventional narrow-groove gas tungsten arc welding (NG-GTAW). The use of a narrow-groove joint configuration can also reduce distortion and residual stress in the completed weldment. In general terms, the present invention provides a narrow-groove tandem gas metal arc welding system having two electrodes arranged in a “lead” and “trail” configuration capable of creating a high-frequency weave. This invention may also be referred to as narrow-groove tandem GMAW with oscillating lead arc welding. Gas-delivery nozzles provide shielding of the molten weld. The system is designed for use in narrow-groove joints up to about 6 inches thick; however modifications can be made to adapt the torch to thicker joints. Adjustments can be made to the horizontal spacing between the electrodes, the relative height of the contact tips, and the included angle between the electrodes. By properly adjusting the horizontal spacing between the electrodes and using the spin-arc capability of the lead torch to produce a high frequency weave, stable arcs, smooth puddle flow, and excellent sidewall fusion can be achieved. With reference now to the Figures, one or more specific embodiments of this invention shall be described in greater detail. 
         [0021]    As shown in  FIGS. 1-9 , tandem gas metal arc welding system  100  includes first torch assembly  120  and second torch assembly  220 , which further include lead torch  121  and trailing torch  221 , respectively. When system  100  is in operation, lead torch  121  rotates at a high rate of speed. Lead torch drive motor  122  is mechanically coupled to lead torch  121  by belt  124 , the tension of which may be adjusted by belt tensioner  126  and adjustment knobs  127 . Power is provided to lead torch  121  by power rotary union  128 , which includes positive terminal  129 , to which positive lead  112  is connected. Wire conduit  114  enters the top portion of power rotary union  128  and provides the pathway through which the consumable wire electrode enters lead torch  121 . Fluid (e.g., water) rotary union  130  is positioned beneath power rotary union  128  and includes fluid inlet  132  and fluid outlet  134 . Mounting block  119  supports power rotary union  128  and fluid rotary union  130 . 
         [0022]    The topmost portion of contact tube  146  is connected to rotary power union  128 . The center portion of contact tube  146  is connected to the bottom portion of rotary fluid union  130 , where it is rotated by belt  124  (see  FIGS. 3-4 ). In the embodiments of the present invention described herein, contact tube  146  is adapted to receive cooling (e.g., water) fluid introduced into the system through fluid inlet  132 . As best shown in  FIG. 5 , manifold block  136 , which may be copper or another suitable metal, is welded or otherwise attached to contact tube  146  and includes fluid inlet  138  into which tube  140  is inserted and secured for providing a fluid path from fluid inlet  132  in fluid rotary union  130 . Compression fitting  141  is used to connect tube  140  to the bottom portion of fluid rotary union  130  (see  FIG. 4 ). Fluid inlet  138  is in fluid communication with fluid inlet  152 , which is formed in lead torch contact tube  146 . Cooling fluid flows into contact tube  146  at inlet  152  and passes through internal conduit  154  and exits contact tube  146  at fluid outlet  156 . Fluid exiting contact tube  146  passes through fluid outlet  142  in manifold block  136 , through tube  144  and ultimately out of fluid rotary union  130  through outlet  134 . Compression fitting  145  is used to connect tube  144  to the bottom portion of fluid rotary union  130  (see  FIG. 4 ). Lead-torch contact tube  146  is constructed by machining channels into a section of extruded copper and sliding a section of copper tubing over the machined section to create fluid passages. The copper tubing is welded circumferentially and additional fittings are brazed in place. As shown in  FIG. 5 , internal conduit  154  is formed by sliding outer portion  150  over inner portion  148 , which has been machined to include a channel. The introduction of water or other cooling fluid into contact tube  146  ultimately results in higher weld material deposition rates. 
         [0023]    As shown in  FIGS. 6A-D , contact tip  158  is the portion of lead torch  121  through which lead electrode  159  exits and interacts with a workpiece. In this exemplary embodiment, contact tip  158  includes cylindrical body  160  which has a threaded rear portion  162  and a flattened front portion  164 . As shown in  FIG. 6B , an angled bore is formed through cylindrical body  160 , thereby creating an exit point for lead electrode  159  that, in some embodiments, is offset from the central axis of contact tip  158  (see  FIG. 6D ). An offset exit point allows lead electrode  159  to oscillate when lead torch  121  is rotated at a high rate of speed, thereby creating a weave pattern in the weld area. As will be appreciated by one of ordinary skill in the art, alternate mechanical or structural means for inducing desired oscillation in lead electrode  159  are possible and these alternate means are contemplated by and considered part of this invention. 
         [0024]    As previously indicated, the GMAW process includes at least one shielding gas for protecting the weld area from the contaminants in the atmosphere. As best shown in  FIG. 7 , lead torch gas delivery assembly  170  includes support  172  and an electrical isolation sleeve  174  which is connected to manifold block  176 . Gas hose fitting  178  is connected to a gas line (not shown) and plug  180  is inserted into manifold block  176 . Compression fitting  183  is used to connect first lead gas delivery tube  182  to manifold block  176  and first lead gas delivery nozzle  184  is connected to or formed integrally with one end of first lead gas delivery tube  182 . Compression fitting  187  is used to connect second lead gas delivery tube  186  to manifold block  176  and second lead gas delivery nozzle  188  is connected to or formed integrally with one end of second lead gas delivery tube  186 . Alternate configurations of gas delivery assembly  170  are possible with regard to the placement and geometry of the delivery tubes and nozzles as well as the other components of the assembly (see  FIG. 3 ). 
         [0025]    The GMAW system of the present invention includes two welding torches, the first of which has been previously described. As best shown in  FIGS. 1 and 8 , trailing torch assembly  220  includes trailing torch  221 , which trails or follows lead torch  121  through a weld area. In the exemplary embodiment shown in the Figures, torch height adjustment knob  222 , torch height adjustment lock  224 , torch height adjustment slide  226 , torch height adjustment mechanism  228 , and wire spacing adjustment knob  230  permit the position of trailing torch  221  to be changed or adjusted relative to the position of lead torch  121 . An important parameter with regard to this system is the spacing of wire electrodes  159  and  256 . A preferred spacing is about 4 to 8 mm as measured at the end of the wire electrodes at a contact-tip-to-work distance of 19 mm. Trailing torch  221  includes a fluid cooled torch body  232  that further includes bottom portion  234  and top portion  236  as well as positive terminal  238  to which positive lead  116  is attached. Wire conduit  118  enters the top portion of torch body  232  and provides the pathway through which the consumable wire electrode enters trailing torch  221 . Clamp  252  is used to connect conduit  118  to torch body  232 . Top portion  236  includes fluid inlet  240  and fluid outlet  244 , while bottom portion  234  includes fluid passage or channel  242  formed therein. O-ring  246  is provided to seal the connection between bottom portion  234  and top portion  236 . Torch body  232  further includes spatter guard  248  and slot for vertical adjustment  250 . Contact tip  254  is attached to the bottom of torch body  232  and trailing electrode  256  extends therefrom. 
         [0026]    As previously indicated, the GMAW process includes at least one shielding gas for protecting the weld area from the contaminants in the atmosphere. As best shown in  FIG. 9 , trailing torch gas delivery assembly  270  includes support  272  and an electrical isolation sleeve  274  which is connected to manifold block  276 . Gas hose fitting  278  is connected to a gas line (not shown) and plug  280  is inserted into manifold block  276 . Compression fitting  283  is used to connect first trailing gas delivery tube  282  to manifold block  276  and first trailing gas delivery nozzle  284  is connected to or formed integrally with one end of first trailing gas delivery tube  282 . Compression fitting  287  is used to connect second trailing gas delivery tube  286  to manifold block  276  and second trailing gas delivery nozzle  288  is connected to or formed integrally with one end of second trailing gas delivery tube  286 . Alternate configurations of gas delivery assembly  270  are possible with regard to the placement and geometry of the delivery tubes and nozzles as well as the other components of the assembly (see  FIG. 3 ). 
         [0027]    As previously indicated, by using a contact tip with an offset exit hole, a circular oscillating motion is created. The present invention includes two exemplary versions of these specialized tips; one with a 1.5 mm diameter of rotation as measured at the center of the wire at a 19 mm contact tip to work distance and one with a 2.5 mm diameter of rotation. The addition of rotation in the lead arc results in improved sidewall wetting and penetration. Furthermore, larger gaps can be welded when rotating the lead electrode. While rotating the 1.5 mm tip resulted in improved gap-bridging over the non-rotated lead electrode, trials also revealed a further improvement to gap-bridging by using the 2.5 mm tip. Preliminary trials with the non-rotating lead electrode resulted in lack-of-fusion defects at a gap of 16 mm. Welds made with a rotating 2.5 mm tip and a 16 mm gap had good sidewall fusion, consistent bead profiles, and improved bead surface appearance. Welds made with the rotating lead electrode also appeared to be less affected by misalignment of the torch in the welding joint and were less affected by wire cast. Preferred welding parameters include: (i) lead wire feed speed: 450 ipm; (ii) lead voltage: 25-27 Volts; (iii) trail wire feed speed: 350 ipm; (iv) trail voltage: 27-29 Volts; (v) travel speed: 12-15 ipm; (vi) speed of rotation: 400-600 rpm; (vii) wire spacing: about 5-7 mm; (viii) contact tip-to-work distance: 19 mm (both electrodes); (ix) welding mode: synchronized pulsed lead/pulsed trail; and (x) joint preparation: 0.5-inch narrow groove with a 2 degree included angle. 
         [0028]    The present invention combines the high deposition rates of consumable electrode processes, further increased by the addition of a second arc, and the drastically reduced joint volume of narrow groove joint designs to drastically improve productivity. The addition of a second arc allows increased travel speeds and deposition rates resulting in 75 to 100% increases in productivity over single-wire narrow-groove GMAW with a rotating electrode. The rotation of the lead arc as well as the addition of the trailing arc improves sidewall fusion and wetting. Issues with lack of sidewall penetration and fusion that have typically hindered the use of high deposition-rate GMAW in narrow grooves have been solved by the use of an oscillating lead arc while productivity has been significantly increased by the addition of a second electrode contributing to the same weld pool. Another benefit of tandem GMAW is a reduction in calculated heat input when compared to single-wire arc welding processes when operated at the same deposition rate. This results from the use of two separate consumable electrodes which require less energy to melt than a single electrode. If using a single electrode, a higher wire feed speed or a larger diameter electrode is required to achieve the same deposition rate as tandem GMAW, resulting in a higher current and a higher heat input. This is significant because heat input has a direct correlation to the mechanical properties of a completed weldment in certain materials. Better contact tip life is achieved by fluid cooling of the contact tip. 
         [0029]    While the present invention has been illustrated by the description of exemplary embodiments thereof, and while the embodiments have been described in certain detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to any of the specific details, representative devices and methods, and/or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant&#39;s general inventive concept.