Title of Invention	SHORT CIRCUIT WELDER
Abstract	An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defmes a gap by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said apparatus comprising a power supply that supplies current to said wire, said power supply creating a series of current pulses that constitute a welding cycle, sad current pulses in said cycle each having a given electrical polarity with respect to said workpieces; and a polarity selector that selects the polarity of said pulses in said welding cycle between a first polarity with said electrode being positive and a second polarity with said electrode being negative to control the temperature of said weld pool in said gap, said selector regulating the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire.

Title of Invention

SHORT CIRCUIT WELDER

Abstract

An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defmes a gap by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said apparatus comprising a power supply that supplies current to said wire, said power supply creating a series of current pulses that constitute a welding cycle, sad current pulses in said cycle each having a given electrical polarity with respect to said workpieces; and a polarity selector that selects the polarity of said pulses in said welding cycle between a first polarity with said electrode being positive and a second polarity with said electrode being negative to control the temperature of said weld pool in said gap, said selector regulating the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire.

Full Text	This application is a continuation-in-part of prior copending application S.N. 09/004,707 filed January 9,1998. The invention relates to the art of welding with an electric arc and more particularly to an improved method and apparatus for short circuit welding, especially for the welding together of two steel plates, such as two pipe sections. INCORPORATION BY REFERENCE The present invention relates to an improvement in spatter controlled systems and heat control systems of the general type described in United States Letters Patent No. 4,972,064. This prior issued patent is incorporated by reference herein and background information and for their discussion of concepts in the spatter control area to which the present invention is specifically directed. Also incorporated by reference is United States Letters Patent No. 5,676,857. This prior issue patent is incorporated by reference herein as background information and for its discussion of welding sections of pipe together. BACKGROUND OF THE INVENTTON— This invention relates to the field of arc welding using a consumable electrode and more particularly to an improved apparatus and method of short circuiting arc welding two steel plates, such as two pipe sections, together by use of a cored electrode. In the art of welding the ends of large diameter pipe, it is conventional to machine the ends of each pipe to provide an external bevel and a narrow flat land; and to bring the machined ends into axle alignment with the lands in close but usually spaced relationship to form a weld groove which includes a gap between the two ends of the pipe. Once the pipes are in position, one or more welding heads are moved around the pipe so as to effect a 360° weld. The weld is usually made in several steps. First, a root pass is made where at least the inner edges or lands of the pipes are fused and the gap between the lands filled with weld metal. Thereafter, several filler passes are made wherein the space formed by the bevel is filled so that the weld metal is at least flush with the outer surface of the pipe. Just before and during the root pass, the ends of the pipe must be in accurate alignment with one another so as to form a quality weld between the two pipe ends. The root pass is a very critical part of the welding operation. Once the root pass is completed, the alignment of the pipes is assured and the welding of the next joint down the line can be commenced. Thus, during the root pass, a 100% sound weld bead must be laid. Soundness of the weld bead means the complete fusion of both the lands clear through to the inner surface of the pipes and the complete filling of the gap between the lands with the weld metal Depositing of the weld metal in the gap is difficult because the weld must be made by moving the weld heads around the pipe such that the welding position varies from down-hand welding, vertical up or down welding, to overhead weld as the root pass is formed around the pipe. During the root pass, the pipe sections must be maintained in alignment throughout the root pass so as to form a quality weld. Typically, the pipes are clamped together to maintain the pipe alignment during welding. In addition to weld position complications and pipe alignment concerns, the weld metal formed during the root pass should fill the gap between the pipe sections, but the weld metal should not be allowed to pass through the gap and accumulate on the interior surface of the pipe. The weld bead should form a relatively smooth surface with respect to the interior of the pipe which has very little, if any, protrusion into the interior of the pipe. Excessive protrusion of the weld bead in the pipe can: 1) create problems with apparatuses running inside the pipes to detect the soundness of the pipe system, and 2) cause unwanted fluid mixing and turbulence as the fluids are transported through the pipe system. In order to overcome the problem of the weld bead protruding into the interior of the pipe, it is common practice to make a root pass from the interior of the pipe. Such a welding method insures that the land of the weld bead during the root pass is controlled so as to prevent unwanted protrusion into the interior of the pipeline. However, such a welding method requires specially designed and costly equipment. In addition, such a welding method is very time-consuming and cost-preventative in various types of applications. Furthermore, such a welding method can only be used for large diameter pipes. Smaller diameter pipes cannot accommodate the welding apparatus inside the pipe. Another method of welding which prevents protrusion of the weld bead into the interior of the pipe is the use of backplates or back-up shoes positioned on the interior of the pipe and over the gap between the pipe sections. The backplate prevents the weld bead from protruding into the interior of the pipe sections during the root pass of the weld bead. However, the use of backplates is also very time-consuming and is limited to relatively large diameter pipes. In addition, the backplate, in many instances, becomes welded to the interior of the pipe section during the laying of the root pass. Consequently, the backplate must later be removed which can be both costly and can adversely damage the weld bead during the removal of the welding plate. A welding apparatus which overcomes many of the problems associated with past pipe welding methods is disclosed in United States Letters Patent No, 5,676,857. This patent discloses an improvement in the welding of the end of two pipe sections by the use of a welding apparatus having two welding bugs which continuously move on a track around the periphery of the pipe. The welding bugs include a special short circuiting power source to apply a root bead between the two ends of a pipe. The patent discloses that by using this system of welding with the proper bug speed and welding wire speed, only a slight bum through each edge of the bevel occurs and a small flat weld is formed on the interior of the pipe, thus eliminating the need to run an initial root pass from the interior of the pipe or to use other types of equipment inside the pipe such as a backplate. Although the method of welding disclosed in United States Letters Patent No. 5,676,857 overcomes several of the problems associated with the welding of pipe sections together, problems associated with the alloy composition of the weld metal of the root bead and the shielding of the weld metal of the root bead from the adverse affects of the atmosphere remain a problem. The short circuiting power source is designed to be used with a solid wire electrode and, during welding, the weld metal is protected from the atmosphere by use of various types and blends of shielding gas. Although the use of a solid wire and shielding gas produces an excellent weld bead in many environments, the short circuiting welding method has several limitations when used to weld together the pipe sections. Because solid wire electrodes are used, the composition of the weld bead is limited to the available alloy compositions of electrodes for use in short circuit welding. The composition of the weld metal should closely match the composition of the metal pipe to form a strong and durable weld bead. Because the composition of the pipe will vary depending on the appUcation of use for the pipe, problems could arise with the obtaining of a solid wire electrode which forms a weld metal that bonds with the pipe sections with optimum characteristics. Another limitation of the short circuiting welding process is that a shielding gas must be used to protect the weld bead from the adverse effects of the environment. The welding apparatus must include an arrangement for storing and directing shielding gas to the area of welding. Such an arrangement must include a mounting arrangement for the containers of shielding gas, regulators, flow meters, hoses, and other materials necessary to direct the shielding gas to the welding area during welding The shielding gas prevents oxygen, nitrogen, hydrogen and other compounds in the atmosphere from reacting with the molten metal and/or being trapped in the molten metal. These elements can cause porosity in the weld bead, cracking of the welding bead, spattering of the weld metal, etc., which can significantly compromise the strength and quality of the weld bead. The use of a shielding gas in a controlled indoor environment is effective in preventing the adverse effects on the weld bead from the environment; however, the use of shielding gases in an outdoor environment are highly susceptible to the effects of wind during the welding process. In order to minimize these effects, special shields must be erected around the perimeter of the electrode to shield the shielding gas from the wind during welding. The use of such shields or other welding configurations to minimize the effects of the atmosphere on the weld bead is both costly and significantly complicates the weld apparatus arrangement. In view of the problems associated with welding of pipe sections in various types of environments and for producing a high quality weld bead having a composition substantially similar to the composition of the pipes that are being welded together, there is a need for an improved welding method and apparatus which can overcome such problems during the welding of pipe, SUMMARY OF THE INVENTION The present invention relates to a method and apparatus of short circuit arc welding together two steel plates, preferably on one side of the plates. Preferably, the method and apparatus of short circuiting arc welding is used to weld together pipe sections together and form a root bead between the two pipe sections; however, the invention has broader applications and can be used to weld together a variety of metal objects, in a variety of ways, in a variety of environments. includes a flux system within the cored electrode to provide a shielding gas during the welding process. In accordance with another aspect of the present invention, the consumable cored electrode includes alloy metals in the core so as to obtain a weld bead composition which is substantially similar to the composition of the pipes which are being welded together. A weld bead having a composition which closely matches the composition of the pipe section forms a strong, durable, high quality weld bead. Some cored electrodes require shielding gas especially when used for alloying. In accordance with another aspect of the present invention, the second circuit of the welding current circuit provides a high energy boost during the initial portion of the arcing condition. The high current boost preferably has a preselected I(t) area or energy for melting, a relatively constant volume of metal on the end of the consumable wire when the wore is spaced from the welding pool The energy created during the plasma boost is prcrcrahlv sufficient to create a spherical metal ball having a diameter of no more than twice the diameter of the welding wire. Preferably after the initial high current plasma boost current the high current is maintained for a preselected period of time and then subsequently decayed over a period of time until the desired amount of energy or wattage is applied to the electrode to melt the desired volume of the electrode. In accordance with still another aspect of the present invention, the welding current circuit limits the amount of energy directed to the electrode so as to prevent the unneecssary melting of the ends of the pipe sections during the application of the weld bead and/or to maintain too hot of a weld bead during welding to thereby prevent molten metal from passing through the gap between the ends of the pipe sections and into the interior of the pipe sections. In accordance with another aspect of the present invention, the welding current circuit includes a circuit to produce a background current. The background current is a low level current which is maintained just above the level necessary to sustain an arc after the termination of a short circuit condition. The background current is preferably mainitained throughout the welding cycle to insure that the arc is not inadvertently extinguished during welding. In accordance with another aspect of the invention the welder includes a controller tor shifting between polarity during the welding process to obtain a desired weld puddle heat By using the STT welder of The Lincoln Electric Company or STT short circuit welding process practiced by this welder with a core electrode, a gasless welding process is which forms a weld bead without the need of external shielding gases. A further object of the present invention is the provision of a system and method as claimed above which produces a weld bead having: a composition which is substantially similar to the composition of the pipe being welded. Yet another object of the present invention is the use of a cored electrode in a short circuiting weld apparatus to form a high quality weld bead. A further object of the invention is the provision of an apparatus and method of short circuit welding, which apparatus and method involves changing the polarity of the weld current during a welding process. o Another object of the present invention is the provision of an apparatus and method, which apparatus and method controls the heat of the weld puddle by adjusting the ratio of electrode positive current to electrode negative CUJ rent, either during ; cycle or from one cycle to the next cycle. Yet a further object of the invention is the operation of an STT welder without the need for a shielding gas and with the control of the weld puddle temperature by polarity ratio adjustment. Other objects and advantages will become apparent from the folowing description taken together with the accompanied drawings. Accordingly the present invention provides an apparatus for short circuit and welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said apparatus comprising a power supply that supplies current to said wire, said power supply creating a series of current pulses that constitute a welding cycle, sad current pulses in said cycle each having a given electrical polarity with respect to said workpieces; and a polarity selector that selects the polarity of said pulses in said welding cycle between a first polarity with said electrode being positive and a second polarity with said electrode being negative to control the temperature of said weld pool in said gap, said selector regulating the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire. Accordingly the present invention also provides a method of short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said groove to join said two spaced ends. said method comprising the steps of a) providing a metal wire; b) moving said wire toward said groove as said wire is moved along said groove; c) melting said wire by an electric wave generated by a power supply, said electric wave including a scries of current pulses that constitute a welding cycle, said current pulses in said cycle eaeh having a given electrical polarity with respect to said workpieces; d) selecting the polarity of said pulses in said welding cycle between a first polarity with said wire being positive and a second polarity with said wire being negative to control the temperature of said weld pool in said groove; and, e) selecting the polarity of s aid pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a wire diagram illustrating the broad aspect of the welding control circuit of the present invention; FIGURE 2 is a partial view of the groove between two pipe section ends and the electrode position in the groove to form a root bead; FIGURE 3 is a cross sectional view of the electrode nozzle and a cored electrode therein; FIGURE 4 is a current graph illustrating a representative curve profile of a single welding cycle employed in a system or method used in the present invention wherein the electrode is the positive contact and the workpiece is a negative contacl: FIGURE 5 is an inverted curve profile of single welding cycle similar to the one shown in FIGURE 4 wherein the electrode is negative and the workpiece is positive; FIGURE 6 is a wiring diagram of a welder to perform the method of selecting the- polarity of the weld cycles in a welding process using the short circuit procedure; FIGURE 7 is a graph of the weld current with straight polarity; FIGURE 8 is a graph of the weld current with a reverse polarity; FIGURE 9 is a block diagram of the polarity selector used in the illustated embodiment of the present invention; FIGURE 10 is a graph of the weld current showing a modification of the preferred embodiment; and, FIGURE 11 is a block diagram of the polarity selector used to shift from one polairity to the other polarity during a welding cycle. PREFERRED EMBODIMENT OF THE INVENTION Referring now to the drawings wherein the showings are for the purpose of illustrating the preferred embodiment of the invention only and not for the purpose of limiting same, FIGURE 1 illustrates a short circuiting arc welding system connected to the output of a DC power supply. The preferred type of short circuiting welding is SURFACE TENSION TRANSFER or STT type of welding. The welding circuit and control arrangement for such type of welding is disclosed in United States Letters Patent No, 4,972,064, which patent is incorporated herein. Therefore, only a general discussion of the welding circuit will be discussed below. The power supply is a D.C. power supply preferably made up of a motor 10, such as a gas motor, which powers a generator 12 to produce an AC current 13. The AC current 1.5 is then rectified by rectifier 14 to form a DC current 20. A phase controller 16 controls the rectifier 14 to produce a substantially uniform DC current 20. DC current 20 is then duccicci into a pulse with modulator 30. The shapes of the pulse are controlled by shaping circuit 32 to thereby create a desired pulse with the DC current 20 through output terminals 34, 36. As can be appreciated, the power supply need not be a rectified output but can be any other appropriate DC source. The DC current from the pulse width modulator 30 is directed across a welding, area which includes a consumable cored electrode 50 and workpiece 60. Referring to the welding of the workpiece 60, electrode 50 alternates between a short circuit condition when the electrode 50 engages workpiece 60 and an arcing condition where the electrode 50 is spaced from the workpiece 60. During the arcing condition, an electric ar e is created between the workpiece 60 and the electrode 50 for purposes of melting and maintaining molten the end of the electrode as it is fed toward workpiece for a subsequent short circuit condition. This type of welding cycle is schematically illustrated in FIGURl ;S 4 and 5, As shown in FIGURES 4 and 5, the welding cycle alternates between a short circuit condition and a plasma condition. During the plasma condition, it is necessary that an arc is created and maintained at all times for the purpose of smooth and effective welding. The welding cycle which is repealed several times per second must be accurately controlled for the purpose of reducing spatter at various times during the welding cycle. Pulse width modulator 30 operates al a high frequency. In the preferred embodiment, the operating frequency of the pulse width modulator controller 30 is 20 kHz with a width of the successive current pulse being, determined by the voltage on line 33 from shape controller 32. As the feedback control system demands more current in the welding cycle, a higher voltage appears on line 33 causing a wider pulse during the next pulse from the pulse width modulator 30. Thus, ihe demanded current for the welding cycle is changing 220,000 times each second. Sinee the highest rate of the welding cycle is generally in the neighborhood of 100 to 400 cycles per second, many update pulses are provided during each welding cycle. In accordance with known practice, the welding current circuit includes a premonition circuit having an output line 40 for controlling switch 42. The welding circuit directs current to work area in accordance with the operation of the pulse width modulator controller 30 until the dr/dt (where r is the electrode resistance), di/dt or dv/dt indicates an upcoming fuse during the pinch cycle. When this detection ccurs. the logic on line 40 changes polarity to open switch 42, This places resistor or snubber 39 in series with main choke 38. Since the main choke has a small inductive reactance, very little energy is stored in the welding current circuit. Consequently, the current flow caused by the welding circuit between the electrode and the workpiece is immediately dropped to a level determined by resistor 39. In accordance with the present invention, there is added to the welding cunent cirtuit from ever falling below the preselected current low current level and allowing the arc to extinguish. The current control circuit is designed to produce all the melting of the electrode during the plasma boost and plasma portion of the welding cycle. Further melting of electrode 50 does not take place when the background current level 100 occurs since the IR necessary for melting the electrode is not obtainable through an arc maintained only by the background current. Thus, the background current only serves to maintain the arc and the ball of molten metal in the molten state. The amount of molten metal at the end of electrode 50 which is formed by the plasma boost and plasma is selected to melt a preselected volume of molten metal at the end of the electrode, and the plasma portion of the current is reduced to the background current once the preselected volume is obtained. The duration of the plasma boost and plasma portion is also selected to prevent unnecessary melting of the metal around gap 74 of pipe ends 70, Such over-melting of the metal can result in the weld metal seeping into the interim of the pipe sections. During the formation of the molten metal ball at the end of the electrode during the plasma portion of the current, the jet forces of the high current repel the melted metal from the welding pool until the preselected amount of molten metal has been melted at the end of the electrode. Once the current is reduced, the molten metal is allowed to form into a ball and the molten metal pool in the groove is allowed to stabilize, thereby allowing for a smooth contact between the substantially spherical ball and the quelled weld metal pool. The desired amount of molten metal at the end of the electrode is controlled by directing a preselected amount of energy or wattage into the electrode during the plasma portion of the welding cycle. All during the time the molten metal ball is being formed at the end of the electrode, the core components are releasing shielding gases to shield the molten ball and the weld metal in gap 74 from the atmosphere. The shield gases continue until the molten ball is transferred into the molten metal in gap 74. Once the molten metal ball is formed during the plasma boost and the plasma portion of the welding cycle, the molten ball is forced into the molten pool by feeding the electrode into the pool, thereby forming a short circuit condition. When the melted metal ball engages the molten metal pool, it is transferred into the pool by surface tension. This action causes an ultimate necking down of the molten metal extending between the pool and the wire in the electrode, and then a rupture and separation of the ball from the wire occurs. Since there is only a low background current during the separation, little if any spatter occurs. Preferably, the current control circuit monitors the necking of the molten metal ball such that when the neck rapidly reduces in diameter by electric pits, the current flow during the pinch curve 110 increases more gradually until a detection of an impending fuse is obtained. Once the detection of an impending fuse occurs, the current is reduced to the background current until the molten metal at the end of the electrode transfers into the weld pool. FIGURES 4 and 5 show a standard STT short circuit welding cycle with the cycle in FIGURE 4 applying the positive terminal to the advancing wire 50. With this polarity, the molten metal puddle is relatively cool compared to the standard STT welding cycle shown in FIGURE 5 wherein the electrode is negative. The short circuit welding process utilizing the standard negative polarity, or electrode negative condition shown in FIGURE 5, has a tendency to increase the temperature of the molten metal. This condition is normally alleviated by reducing the background current of the STT welder. By employing another aspect of the present invention, the temperature of the molten metal puddle is controlled by welder 200 operated in accordance with the present invention and shown in FIGURE 6. Welder 200 applies a welding pulse across the advancing cord metal electrode 50 as the electrode or wire moves toward workpiece 60. The primary aspect of the present invention is the use of a cord electrode. The additional aspect of the invention can use a solid wire electrode. Welder 200 includes a switching type power supply 202 in the form of an inverter having switching pulses controlled by pulse width modulator 204 with the width of the successive current pulses determined by the voltage at the output of error amplifier 206. This amplifier receives a voltage from current shunt 208 that is proportional to the actual arc current. Input line 210 directs the current voltage to the amplifier with the desired current signal in line 212 from standard controller 220. Controller 220 creates a voltage in line 212 that sets the width of the individual rapidly occurring current pulses at the output of inverter or power supply 202. Output stage 230 of inverter 202 includes a transformer 232 having center tap 234 connected to a positive rectifier 236 and a negative rectifier 238. A polarity selector in controller 220 provides a logic in output 240 when the output of the power supply is to have a positive polarity and a logic in output line 242 when the power supply is to be shifted into a negative polarity. Switches Ql, Q2, each having a standard snubber 244, are used to control current in inductor LI having a positive polarity portion 250 controlled by switch Ql and a negative polarity portion 252 controlled by switch Q2. A logic in line 240 closes switch Ql causing current flow through inductor portion 250, A logic in line 242 changes the polarity causing current flow in negative polarity portion 252 of inductor LL So long as the polarity is positive by the logic in line 240, the STT welder produces positive current pulses to give the positive polarity current cycle shown in FIGURE 7, Weld cycle 300 is shown as a positive cycle with all current having a positive polarity. Cycle 300 has a starting point at tj which is at the time a short occurs. The background current is reduced toward zero. Thereafter, the pinch current 302 causes the shorted metal ball to transfer by a tension transfer and an electrical pinch until a neck is created, as indicated at 304. The current is again plunged as indicated at portion 306 to reduce spatter. After the metal has been transferred by the electric pinch action, the' plasma condition is reestablished by a plasma boost pulse 310 having a maximum arc current. The area of the plasma boost pulse 310 determines the general size of the molten metal ball at the end of the advancing wire electrode 50. After the boost pulse, the current has a time constant tailout 312 terminating at the background current 314. At 316 the next short circuit occurs. As long as a logic 1 appears in output 240 the rapidly created current pulses have a positive polarity, as shown in FIGURE 7. Upon receipt of a logic one in output line 242, the polarity of the welding operation is reversed. A reversed or negative polarity cycle 320 is created, as shown in FIGURE 8. In accordance with this aspect of the invention, the number of positive polarity current cycles 300 and negative polarity current cycles 320 are controlled to obtain the desired heat in the molten metal puddle of the welding operation. If the puddle is too cold, the number of negative polarity cycles 320 is increased with respect to the number of positive current cycles 300. The desired ratio is obtained by appropriate selector circuit in controller 220, which selector circuit is illustrated schematically in FIGURE 9 wherein the selector circuit is a flip-flop 350, software implemented, having a non-inverted output 240 and an inverted output 242. The output is selected by anti-coincident circuit 352 having a set input line 352a and a reset input line 352b controlled by a digital decoder 354. Input 360 receives an input initiation pulse at time t, when a cycle is started by a short circuit. The adjusting inputs 362, 364 of decoder 354 set the ratio of a number of positive current cycles at input 362 and the number of negative current cycles at input 364. By adjusting these two inputs, the ratio of positive current cycles 300, two negative current cycles 320 is selected to control the heat of the welding process. To change the heat, the ratio is manipulated by changing the data at inputs 362 and 364. Although the preferred embodiment of this aspect of the invention involves the selection of the ratio between the positive current cycles 300 and the negative current cycles 320 during a welding process, an alternative control concept has been devised wherein each cycle 302 is initiated as a standard negative polarity cycle and is then shifted to a positive polarity cycle at a preselected point in the cycle. This aspect of the invention is illustrated in FIGURE 10 wherein current cycle 400 is started as a negative polarity cycle with the pinch current portion 402 followed by a standard plasma boost current portion 404, In accordance with this aspect of the invention, the polarity of the rapidly created current pulses is shifted after the termination 410 of plasma boost portion 404. The shift at point x is after time delay TD. Thus, tailout portion 420 is divided into a negative portion 422 and a positive portion 424 with an instantaneous shift in polarity at point x. Thereafter, the current cycle is a positive polarity until the end 430 of the cycle. Flip-flop 350 shifts logic state to await the next output of trailing edge detector 454 as shown in selector S' in FIGURE 11, At the end of a plasma boost portion, detector 454 reads the trailing edge at input 452 to start time delay 456 which has a manual adjusted time at input 460. In this manner, the heat of the weld puddle is determined by the selection of the time delay for reversing the polarity of weld cycle 400. Other modifications could be made to alternate between a positive polarity and a negative polarity for the current pulses fi-om the STT welder for controlling the heat of the welding operation. The aspects of the invention have been described with reference to preferred and altemative embodiments. Other modifications are apparent and are within the scope of the present invention. L-2169 FOREIGN 1. An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said apparatus comprising a power supply that supplies current to said wire, said power supply creating a series of current pulses that constitute a welding cycle, said current pulses in said cycle each having a given electrical polarity with respect to said workpieces; and a polarity selector that selects the polarity of said pulses in said welding cycle between a first polarity with said electrode being positive and a second polarity with said electrode being negative to control the temperature of said weld pool in said gap, said selector regulating the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire. 2. The apparatus as defined in claim 1, wherein said selector causing a shift between said first polarity and said second polarity at the beginning of at least one welding cycle. 3. The apparatus as defined in claim 2 or 3, wherein said selector causing a shift between said first polarity and said second polarity during at least one welding cycle. 4. The apparatus as defined in claim 3, wherein said selector causing a shift between said first polarity and said second polarity during said plasma arc melting portion. 5. The apparatus as defined in claims 1 -4, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second portion creating said second polarity, a first switch for connecting said first portion of said inductor between said wire and said plates, a second switch for connecting said second portion of said inductor between said wire and said plate, said selector causing either said first switch or said second switch to close during a given weld cycle. 6. The apparatus as defined in claims 1 -5, wherein said selector includes a decoder with a first condition to select one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles and controller to alternate between said first and second conditions during a welding operation. 7. The apparatus as defined in claim 6, wherein said first number is different from said second number. 8. The apparatus as defined in claims l-V, including a shunt to sense the actual arc current and an error amplifier to compare an actual arc current with a desired arc current to control a width of said current pulses. 9. The apparatus as defined in claims 1 -8, including a pulse width modulator for creating said current pulses at a frequency greater than about 10 kHz. 10. The apparatus as defined in claims 1 -9, wherein said power supply is an inverter with an output transformer driving a rectifier. 11. The apparatus as defined in claims 1-10, wherein said wire is a cored metal electrode. 12. The apparatus as defined in claims 1-11, wherein said wire is a cored electrode includes alloying components in the core. 13. The apparatus as defined in claims 1-12, wherein said wire does not require extemal gas shielding. 14. The apparatus as defined in claims 1-13, wherein said wire is a self-shielding electrode. 15. The apparatus as defined in claims 1-14, wherein said selector regulates the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire. 16. The apparatus as defined in claims 1-15, wherein said selector alternates said polarity of said pulses as a function of the cumulative amount of energy applied to said wire and said melted wire in said groove in one polarity and the cumulative amount of energy applied to said wire and said melted wire in said groove in the other polarity. 17. The apparatus as defined in claims 1-16, wherein said power supply creates a background current, said background current having a low level that is just above the level necessary to sustain an arc throughout each welding cycle. 18. The apparatus as defined in claims 1-17, wherein said power supply controls a preselected amount of energy to said wire to melt a substantially constant volume of said wire during each welding cycle. 19. The apparatus as defined in claims 1-18, wherein said power supply limits the amount of energy directed to said wire to inhibit said molten metal from passing through said spaced ends of said workpieces. 20. The apparatus as defined in claims 1-19, wherein said power supply reduces the amount of current to said wire prior to said molten metal on said wire forming a short circuit condition with said groove, said reduced current having a decaying current profile, 21. The apparatus as defined in claim 1-20, wherein said power supply forms a high current pulse at the end of a short circuit condition and terminates said pulse just prior to a predicted termination of said short circuit condition. 22. The apparatus as defined in claims 1-21, wherein said wire is moved about the outer peripheral surface of said workpieces and substantially along said groove. 23. The apparatus as defined in claims 1-22, including a welding carriage that continuously moves around said workpieces. 24. The apparatus as defined in claims 23, said welding carriage moves at a variable speed. 25. The apparatus as defined in claims 1-24, wherein said two workpieces are two pipe sections. 26. The apparatus as defined in claims 1 -25, wherein said power supply is an STT power supply. 27. An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece forming a gap there between by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said welding wire being a cored metal electrode; said apparatus including a power supply that supplies current to said wire, said power supply including a first circuit to create a transfer current and a second circuit to create a melting current, said second circuit supplying a sufficient amount of current to said wire to form a root bead in said groove which bridges said gap. 28. The apparatus as defined in claim 27, including a welding carriage positioned closely adjacent to said gap, said carriage adapted to move said wire about the outer peripheral surface of said workpieces and along said gap. 29. The apparatus as defined in claim 28, wherein said welding carriage continuously moves around said workpieces and wherein the speed of said welding carriage can be varied. 30. The apparatus as defined in claims 28 or 29, including a mechanism to varying the speed of said welding carriage and/or the feed rate of said wire as the welding carriage moves about the periphery of said workpieces. 31. The apparatus as defined in claims 27-30, wherein said wire is a self-shielding electrode 32. The apparatus as defined in claims 27-31, wherein said wire is a cored electrode which includes alloying components in the core to form said root bead having a substantially similar composition as the composition of said workpieces. 33. The apparatus as defined in claims 27-32, wherein said second circuit directing a preselected amount of energy to said electrode to melt a relatively constant volume of said electrode during each welding cycle. 34. The apparatus as defined in claims 27-33, wherein said power supply limiting the amount of energy directed to said wire to inhibit molten metal from passing through said gap, 35. The apparatus as defined in claims 27-34, wherein said second circuit reduces the amount of current to said wire prior to said molten metal on said wire forming a short circuit condition with said gap, said current having a decaying current profile when said current is reduced. 36. The apparatus as defined in claims 27-35, wherein said first circuit directs a high current pulse at the end of a short circuit condition and applies said pulse until just prior to a predicted termination of said short circuit condition. 37. The apparatus as defined in claims 27-36, wherein said power supply includes a polarity controller to control the polarity of at least a portion of said current. 38. The apparatus as defined in claim 37, wherein said polarity controller maintains a positive polarity during at least a portion of said melting current. 39. The apparatus as defined in claim 37 or 38, wherein said polarity controller maintains a positive polarity during at least a portion of said transfer current. 40. The apparatus as defined in claims 37-39, wherein said polarity controller maintains a negative polarity during at least a portion of said melting current. 41. The apparatus as defined in claims 37-40, wherein said polarity controller maintains a negative polarity during at least a portion of said transfer current. 42. The apparatus as defined in claims 27-41, wherein said two workpieces are two pipe sections. 43. The apparatus as defined in claims 27-42, wherein said power supply is an STT power supply, 44. A method of short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said groove to join said two spaced ends, said method comprising the steps of: a) providing a metal wire; b) moving said wire toward said groove as said wire is moved along said groove; c) melting said wire by an electric wave generated by a power supply, said electric wave including a series of current pulses that constitute a welding cycle, said current pulses in said cycle each having a given electrical polarity with respect to said workpieces; d) selecting the polarity of said pulses in said welding cycle between a first polarity with said wire being positive and a second polarity with said wire being negative to control the temperature of said weld pool in said groove; and, e) selecting the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire, 45. The method as defined in claim 44, including the step of alternating between said first and said second polarity during at least one welding cycle. 46. The method as defined in claim 45, wherein step of alternating between said first and said second polarity during at least one welding cycle occurs during said plasma arc melting portion, 47. The method as defined in claim 44-46, including the step of alternating between said first and said second polarity at the beginning of at least one welding cycle. 48. The method as defined in claims 44-47, including the additional step of selecting one of said first or second polarity for a first number of consecutive welding cycles and a second condifion for selecting the other of said polarity for a second number of consecutive cycles, 49. The method as defined in claim 48, wherein said first number is different from said second number. 50. The method as defined in claims 44-49, wherein said wire is a self-shielding electrode, 51. The method as defined in claims 44-50, wherein said wire is a cored electrode. 52. The method as defined in claim 51, wherein said cored electrode includes alloying components in the core, 53. The method as defined in claims 44-52, including the step of providing a welding carriage which moves said wire about an outer peripheral surface of said workpieces. 54. The method as defined in claim 53, wherein the speed of said welding carriage is varied as said carriage moves about said workpieces. 55. The method as defined in claims 44-54, wherein said electric wave includes a background current, said background current having a low level just above the level necessary to sustain an arc throughout each welding cycle. 56. The method as defined in claims 44-55, wherein said two workpieces are two pipe sections, 57. The method as defined in claims 44-56, wherein said electric wave includes a transfer portion and a controlled melting portion, said melting portion having a preselected amount of energy for melting a relatively constant volume of said electrode during each welding cycle. 58. The method as defined in claims 44-57, wherein said step of melting said electrode including the application of a preselected amount of energy to said electrode during at least one welding cycle and reducing current to said electrode prior to molten metal on said electrode forming a short circuit condition vdth said groove. 59. The method as defined in claims 44-58, wherein said electric wave limiting energy to said electrode to prevent molten metal from passing through said gap. 60. The method as defined in claim 44-60, including the steps of sensing the actual arc .current and comparing said actual arc current with a desired arc current of a welding cycle to control the width of said current pulses. 61. The method as defined in claims 44-60, wherein said power supply creates an alternating current. 62. The method as defined in claims 44-61, wherein said power supply is an STT power supply. 63. The method as defined in claims 44-62, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second portion creating said second polarity, a first switch for connecting said first portion of said inductor between said wire and said plates, a second switch for connecting said second portion of said inductor between said wire and said plate, said selector causing either said first switch or said second switch to close during a given weld cycle. 64. The method as defined in claims 44-63, wherein said power supply including a selector that regulates the polarity of said pulses in said welding cycle between said first polarity and said second polarity, said selector includes a decoder with a first condition to select one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles and controller to alternate between said first and second conditions during a welding operation. 65. The method as defined in claim 64, wherein said selector regulates the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire. 66. The method as defined in claim 64 or 65, wherein said selector alternates said polarity of said pulses as a function of the cumulative amount of energy applied to said wire and said melted ,wire in said groove in one polarity and the cumulative amount of energy applied to said wire and said melted wire in said groove in the other polarity. 67. An apparatus for short circuit electric arc welding two spaced ends of two metal plates at a groove between said two metal plates by melting an advancing welding wire and depositing said melted wire into said groove to join said spaced ends, said apparatus comprising: a switching power supply for creating a series of small width current pulses constituting a welding cycle with a short circuit transfer portion and a plasma arc melting portion, said current pulses in said cycle each having a given electrical polarity of said advancing wire with respect to said two metal plates; and, selector means for selecting the polarity of said pulses in said cycle between a first polarity with said wire being positive and a second polarity with said wire being negative. 68. An apparatus as defined in claim 67, wherein said selector means includes shifts between said first polarity and said second polarity at the beginning of a welding cycle. 69. An apparatus as defined in claim 67 or 68, wherein said selector means includes a decoder with a first condition for selecting one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles and means for alternating between said first and second conditions during a welding operation. 70. An apparatus as defined in claim 69, wherein said first number is different ft-om said second number. 71. An apparatus as defmed in claims 67-70, wherein said cycles each have desired arc current and including a shunt to sense the actual arc current and an error amplifier to compare said actual arc current with said desired arc current to control the width of said current pulses. 72. An apparatus as defined in claims 67-71, including a pulse width modulator for creating said current pulses at a frequency greater than about 10 kHz. 73. An apparatus as defined in claims 67-72, wherein said power supply is an inverter with an output transformer driving a rectifier. 74. .An apparatus as defined in claims 67-73, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second portion creating said second polarity, a first switch for connecting said first portion of said inductor between said wire and said plates, a second switch for connecting said second portion of said inductor between said wire and said plate, and said selector means including means for closing either said first switch or said second switch during a given weld cycle, 75. An apparatus as defined in claim 74, wherein said closing occurs at the beginning of a weld cycle, 76. An apparatus as defined in claims 67-75, wherein said wire is a cored metal electrode. 77. An apparatus as defined in claims 67-76, wherein said power supply is an inverter with an output transformer driving a rectifier. 78. An apparatus as defined in claims 67-77, wherein said cycles each have desired arc current and including a shunt to sense the actual arc current and an error amplifier to compare said actual arc current with said desired arc current to control the width of said current pulses. 79. An apparatus as defined in claims 67-78, including a pulse width modulator for creating said current pulses at a frequency greater than about 10 kHz. 80. A method for short circuit electric arc welding two spaced ends of two metal plates at a groove between said two metal plates by melting an advancing welding wire and depositing said melted wire into said groove to join said spaced ends, said method comprising the steps of: (a) creating a series of small width current pulses constituting a welding cycle with a short circuit transfer portion and a plasma arc melting portion, said current pulses in said cycle each having a given electrical polarity of said advancing wire with respect to said two metal plates; and^ (b) selecting the polarity of said pulses in said cycle between a first polarity with said wire being positive and a second polarity with said wire being negative. 81. The method as defined in claim 80, including the additional step of: (c) shifting between said first polarity and said second polarity at the beginning of a welding cycle. 82. The method as defined in claim 80 or 81, including the additional steps of: (c) selecting one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles; and, (d) alternating between said first and second conditions during a welding operation. 83. The method as defined in claim 82, wherein said first number is different from said .second number. 84. The method as defined in claims 80-83, wherein said cycles each have desired arc cunent and including the steps of sensing the actual arc current and comparing said actual arc current with said desired arc current to control the width of said current pulses. 85. The method as defined in claims 80-84, including the step of creating said cuireni pulses at a irequency greater than about 10 kHz. 86. The method as defined in claims '80-85, including the step of creating said current pulses at a frequency greater than about 10 kHz. 87. The method as defined in claims 80-86, wherein said power supply is an inverter with an output transformer driving a rectifier. 88. The method as defined in claims 80-87, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second.portion creating said second polarity, and including the steps of: (c) connecting said first portion of said inductor across said wire and said plates by closing a first switch; (d) connecting said second portion of said inductor across said wire and said plate by closing a second switch; and, (e) closing either said first switch or said second switch at a selected position in a given weld cycle. 89. The method as defined in claim 88, wherein said selected position is at the beginning of said given weld cycle, 90. A method as defined in claims 80-89, wherein said wire is a cored metal electrode. 91. An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece substantially as here in before described with reference to the accompanying drawings. 92. A method of short circuit arc welding two spaced ends of a first and a second workpiece substantially as here in before described with reference to the accompanying drawings.

Full Text

This application is a continuation-in-part of prior copending application S.N. 09/004,707 filed January 9,1998.
The invention relates to the art of welding with an electric arc and more particularly to an improved method and apparatus for short circuit welding, especially for the welding together of two steel plates, such as two pipe sections.
INCORPORATION BY REFERENCE
The present invention relates to an improvement in spatter controlled systems and heat control systems of the general type described in United States Letters Patent No. 4,972,064. This prior issued patent is incorporated by reference herein and background information and for their discussion of concepts in the spatter control area to which the present invention is
specifically directed.
Also incorporated by reference is United States Letters Patent No. 5,676,857. This prior issue patent is incorporated by reference herein as background information and for its discussion of welding sections of pipe together.
BACKGROUND OF THE INVENTTON—
This invention relates to the field of arc welding using a consumable electrode and more particularly to an improved apparatus and method of short circuiting arc welding two steel plates, such as two pipe sections, together by use of a cored electrode.
In the art of welding the ends of large diameter pipe, it is conventional to machine the ends of each pipe to provide an external bevel and a narrow flat land; and to bring the machined ends into axle alignment with the lands in close but usually spaced relationship to form a weld groove which includes a gap between the two ends of the pipe. Once the pipes are in position, one or more welding heads are moved around the pipe so as to effect a 360° weld. The weld is usually made in several steps. First, a root pass is made where at least the inner edges or lands of the pipes are fused and the gap between the lands filled with weld metal. Thereafter, several filler passes are made wherein the space formed by the bevel is filled so that the weld metal is at least flush with the outer surface of the pipe.
Just before and during the root pass, the ends of the pipe must be in accurate alignment with one another so as to form a quality weld between the two pipe ends. The root

pass is a very critical part of the welding operation. Once the root pass is completed, the alignment of the pipes is assured and the welding of the next joint down the line can be commenced. Thus, during the root pass, a 100% sound weld bead must be laid. Soundness of the weld bead means the complete fusion of both the lands clear through to the inner surface of the pipes and the complete filling of the gap between the lands with the weld metal Depositing of the weld metal in the gap is difficult because the weld must be made by moving the weld heads around the pipe such that the welding position varies from down-hand welding, vertical up or down welding, to overhead weld as the root pass is formed around the pipe. During the root pass, the pipe sections must be maintained in alignment throughout the root pass so as to form a quality weld. Typically, the pipes are clamped together to maintain the pipe alignment during welding. In addition to weld position complications and pipe alignment concerns, the weld metal formed during the root pass should fill the gap between the pipe sections, but the weld metal should not be allowed to pass through the gap and accumulate on the interior surface of the pipe. The weld bead should form a relatively smooth surface with respect to the interior of the pipe which has very little, if any, protrusion into the interior of the pipe. Excessive protrusion of the weld bead in the pipe can: 1) create problems with apparatuses running inside the pipes to detect the soundness of the pipe system, and 2) cause unwanted fluid mixing and turbulence as the fluids are transported through the pipe system.
In order to overcome the problem of the weld bead protruding into the interior of the pipe, it is common practice to make a root pass from the interior of the pipe. Such a welding method insures that the land of the weld bead during the root pass is controlled so as to prevent unwanted protrusion into the interior of the pipeline. However, such a welding method requires specially designed and costly equipment. In addition, such a welding method is very time-consuming and cost-preventative in various types of applications. Furthermore, such a welding method can only be used for large diameter pipes. Smaller diameter pipes cannot accommodate the welding apparatus inside the pipe. Another method of welding which prevents protrusion of the weld bead into the interior of the pipe is the use of backplates or back-up shoes positioned on the interior of the pipe and over the gap between the pipe sections. The backplate prevents the weld bead from protruding into the interior of

the pipe sections during the root pass of the weld bead. However, the use of backplates is also very time-consuming and is limited to relatively large diameter pipes. In addition, the backplate, in many instances, becomes welded to the interior of the pipe section during the laying of the root pass. Consequently, the backplate must later be removed which can be both costly and can adversely damage the weld bead during the removal of the welding plate.
A welding apparatus which overcomes many of the problems associated with past pipe welding methods is disclosed in United States Letters Patent No, 5,676,857. This patent discloses an improvement in the welding of the end of two pipe sections by the use of a welding apparatus having two welding bugs which continuously move on a track around the periphery of the pipe. The welding bugs include a special short circuiting power source to apply a root bead between the two ends of a pipe. The patent discloses that by using this system of welding with the proper bug speed and welding wire speed, only a slight bum through each edge of the bevel occurs and a small flat weld is formed on the interior of the pipe, thus eliminating the need to run an initial root pass from the interior of the pipe or to use other types of equipment inside the pipe such as a backplate. Although the method of welding disclosed in United States Letters Patent No. 5,676,857 overcomes several of the problems associated with the welding of pipe sections together, problems associated with the alloy composition of the weld metal of the root bead and the shielding of the weld metal of the root bead from the adverse affects of the atmosphere remain a problem.
The short circuiting power source is designed to be used with a solid wire electrode and, during welding, the weld metal is protected from the atmosphere by use of various types and blends of shielding gas. Although the use of a solid wire and shielding gas produces an excellent weld bead in many environments, the short circuiting welding method has several limitations when used to weld together the pipe sections. Because solid wire electrodes are used, the composition of the weld bead is limited to the available alloy compositions of electrodes for use in short circuit welding. The composition of the weld metal should closely match the composition of the metal pipe to form a strong and durable weld bead. Because the composition of the pipe will vary depending on the appUcation of use for the pipe, problems could arise with the obtaining of a solid wire electrode which forms a weld metal that bonds with the pipe sections with optimum characteristics.

Another limitation of the short circuiting welding process is that a shielding gas must be used to protect the weld bead from the adverse effects of the environment. The welding apparatus must include an arrangement for storing and directing shielding gas to the area of welding. Such an arrangement must include a mounting arrangement for the containers of shielding gas, regulators, flow meters, hoses, and other materials necessary to direct the shielding gas to the welding area during welding The shielding gas prevents oxygen, nitrogen, hydrogen and other compounds in the atmosphere from reacting with the molten metal and/or being trapped in the molten metal. These elements can cause porosity in the weld bead, cracking of the welding bead, spattering of the weld metal, etc., which can significantly compromise the strength and quality of the weld bead. The use of a shielding gas in a controlled indoor environment is effective in preventing the adverse effects on the weld bead from the environment; however, the use of shielding gases in an outdoor environment are highly susceptible to the effects of wind during the welding process. In order to minimize these effects, special shields must be erected around the perimeter of the electrode to shield the shielding gas from the wind during welding. The use of such shields or other welding configurations to minimize the effects of the atmosphere on the weld bead is both costly and significantly complicates the weld apparatus arrangement.
In view of the problems associated with welding of pipe sections in various types of environments and for producing a high quality weld bead having a composition substantially similar to the composition of the pipes that are being welded together, there is a need for an improved welding method and apparatus which can overcome such problems during the welding of pipe,
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus of short circuit arc welding together two steel plates, preferably on one side of the plates. Preferably, the method and apparatus of short circuiting arc welding is used to weld together pipe sections together and form a root bead between the two pipe sections; however, the invention has broader applications and can be used to weld together a variety of metal objects, in a variety of ways, in a variety of environments.

includes a flux system within the cored electrode to provide a shielding gas during the welding process.
In accordance with another aspect of the present invention, the consumable cored electrode includes alloy metals in the core so as to obtain a weld bead composition which is substantially similar to the composition of the pipes which are being welded together. A weld bead having a composition which closely matches the composition of the pipe section forms a strong, durable, high quality weld bead. Some cored electrodes require shielding gas especially when used for alloying.
In accordance with another aspect of the present invention, the second circuit of the welding current circuit provides a high energy boost during the initial portion of the arcing condition. The high current boost preferably has a preselected I(t) area or energy for melting, a relatively constant volume of metal on the end of the consumable wire when the wore is spaced from the welding pool The energy created during the plasma boost is prcrcrahlv sufficient to create a spherical metal ball having a diameter of no more than twice the diameter of the welding wire. Preferably after the initial high current plasma boost current the high current is maintained for a preselected period of time and then subsequently decayed over a period of time until the desired amount of energy or wattage is applied to the electrode to melt the desired volume of the electrode.
In accordance with still another aspect of the present invention, the welding current circuit limits the amount of energy directed to the electrode so as to prevent the unneecssary melting of the ends of the pipe sections during the application of the weld bead and/or to maintain too hot of a weld bead during welding to thereby prevent molten metal from passing through the gap between the ends of the pipe sections and into the interior of the pipe sections.
In accordance with another aspect of the present invention, the welding current circuit includes a circuit to produce a background current. The background current is a low level current which is maintained just above the level necessary to sustain an arc after the termination of a short circuit condition. The background current is preferably mainitained throughout the welding cycle to insure that the arc is not inadvertently extinguished during welding.

In accordance with another aspect of the invention the welder includes a controller tor shifting between polarity during the welding process to obtain a desired weld puddle heat
By using the STT welder of The Lincoln Electric Company or STT short circuit welding process practiced by this welder with a core electrode, a gasless welding process is
which forms a weld bead without the need of external shielding gases.

A further object of the present invention is the provision of a system and method as claimed above which produces a weld bead having: a composition which is substantially similar to the composition of the pipe being welded.
Yet another object of the present invention is the use of a cored electrode in a short circuiting weld apparatus to form a high quality weld bead.
A further object of the invention is the provision of an apparatus and method of short circuit welding, which apparatus and method involves changing the polarity of the weld current during a welding process.

o
Another object of the present invention is the provision of an apparatus and method, which apparatus and method controls the heat of the weld puddle by adjusting the ratio of electrode positive current to electrode negative CUJ rent, either during ; cycle or from one cycle to the next cycle.
Yet a further object of the invention is the operation of an STT welder without the need for a shielding gas and with the control of the weld puddle temperature by polarity ratio adjustment.
Other objects and advantages will become apparent from the folowing description taken together with the accompanied drawings.
Accordingly the present invention provides an apparatus for short circuit and welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said apparatus

comprising a power supply that supplies current to said wire, said power supply creating a series of current pulses that constitute a welding cycle, sad current pulses in said cycle each having a given electrical polarity with respect to said workpieces; and a polarity selector that selects the polarity of said pulses in said welding cycle between a first polarity with said electrode being positive and a second polarity with said electrode being negative to control the temperature of said weld pool in said gap, said selector regulating the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire.
Accordingly the present invention also provides a method of short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said groove to join said two spaced ends. said method comprising the steps of a) providing a metal wire; b) moving said wire toward said groove as said wire is moved along said groove; c) melting said wire by an electric wave generated by a power supply, said electric wave including a scries of current pulses that constitute a welding cycle, said current pulses in said cycle eaeh having a given electrical polarity with respect to said workpieces; d) selecting the polarity of said pulses in said welding cycle between a first polarity with said wire being positive and a second polarity with said wire being negative to control the temperature of said weld pool in said groove; and, e) selecting the polarity of s aid pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire.

BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a wire diagram illustrating the broad aspect of the welding control circuit of the present invention;
FIGURE 2 is a partial view of the groove between two pipe section ends and the electrode position in the groove to form a root bead;
FIGURE 3 is a cross sectional view of the electrode nozzle and a cored electrode therein;
FIGURE 4 is a current graph illustrating a representative curve profile of a single welding cycle employed in a system or method used in the present invention wherein the electrode is the positive contact and the workpiece is a negative contacl:
FIGURE 5 is an inverted curve profile of single welding cycle similar to the one shown in FIGURE 4 wherein the electrode is negative and the workpiece is positive;
FIGURE 6 is a wiring diagram of a welder to perform the method of selecting the-

polarity of the weld cycles in a welding process using the short circuit procedure;
FIGURE 7 is a graph of the weld current with straight polarity;
FIGURE 8 is a graph of the weld current with a reverse polarity;
FIGURE 9 is a block diagram of the polarity selector used in the illustated embodiment of the present invention;
FIGURE 10 is a graph of the weld current showing a modification of the preferred embodiment; and,
FIGURE 11 is a block diagram of the polarity selector used to shift from one polairity to the other polarity during a welding cycle.
PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the drawings wherein the showings are for the purpose of illustrating the preferred embodiment of the invention only and not for the purpose of limiting same, FIGURE 1 illustrates a short circuiting arc welding system connected to the output of a DC power supply. The preferred type of short circuiting welding is SURFACE TENSION TRANSFER or STT type of welding. The welding circuit and control arrangement for such type of welding is disclosed in United States Letters Patent No, 4,972,064, which patent is incorporated herein. Therefore, only a general discussion of the welding circuit will be discussed below.
The power supply is a D.C. power supply preferably made up of a motor 10, such as a gas motor, which powers a generator 12 to produce an AC current 13. The AC current 1.5 is then rectified by rectifier 14 to form a DC current 20. A phase controller 16 controls the rectifier 14 to produce a substantially uniform DC current 20. DC current 20 is then duccicci into a pulse with modulator 30. The shapes of the pulse are controlled by shaping circuit 32 to thereby create a desired pulse with the DC current 20 through output terminals 34, 36. As can be appreciated, the power supply need not be a rectified output but can be any other appropriate DC source.
The DC current from the pulse width modulator 30 is directed across a welding, area which includes a consumable cored electrode 50 and workpiece 60.
Referring to the welding of the workpiece 60, electrode 50 alternates between a short

circuit condition when the electrode 50 engages workpiece 60 and an arcing condition where the electrode 50 is spaced from the workpiece 60. During the arcing condition, an electric ar e is created between the workpiece 60 and the electrode 50 for purposes of melting and maintaining molten the end of the electrode as it is fed toward workpiece for a subsequent short circuit condition. This type of welding cycle is schematically illustrated in FIGURl ;S 4 and 5, As shown in FIGURES 4 and 5, the welding cycle alternates between a short circuit condition and a plasma condition.
During the plasma condition, it is necessary that an arc is created and maintained at all times for the purpose of smooth and effective welding. The welding cycle which is repealed several times per second must be accurately controlled for the purpose of reducing spatter at various times during the welding cycle. Pulse width modulator 30 operates al a high frequency. In the preferred embodiment, the operating frequency of the pulse width
modulator controller 30 is 20 kHz with a width of the successive current pulse being, determined by the voltage on line 33 from shape controller 32. As the feedback control system demands more current in the welding cycle, a higher voltage appears on line 33 causing a wider pulse during the next pulse from the pulse width modulator 30. Thus, ihe demanded current for the welding cycle is changing 220,000 times each second. Sinee the highest rate of the welding cycle is generally in the neighborhood of 100 to 400 cycles per second, many update pulses are provided during each welding cycle. In accordance with known practice, the welding current circuit includes a premonition circuit having an output line 40 for controlling switch 42.
The welding circuit directs current to work area in accordance with the operation of the pulse width modulator controller 30 until the dr/dt (where r is the electrode resistance), di/dt or dv/dt indicates an upcoming fuse during the pinch cycle. When this detection ccurs. the logic on line 40 changes polarity to open switch 42, This places resistor or snubber 39 in series with main choke 38. Since the main choke has a small inductive reactance, very little energy is stored in the welding current circuit. Consequently, the current flow caused by the welding circuit between the electrode and the workpiece is immediately dropped to a level determined by resistor 39.
In accordance with the present invention, there is added to the welding cunent cirtuit
from ever falling below the preselected current low current level and allowing the arc to extinguish.
The current control circuit is designed to produce all the melting of the electrode during the plasma boost and plasma portion of the welding cycle. Further melting of electrode 50 does not take place when the background current level 100 occurs since the IR necessary for melting the electrode is not obtainable through an arc maintained only by the background current. Thus, the background current only serves to maintain the arc and the ball of molten metal in the molten state. The amount of molten metal at the end of electrode 50 which is formed by the plasma boost and plasma is selected to melt a preselected volume of molten metal at the end of the electrode, and the plasma portion of the current is reduced to the background current once the preselected volume is obtained. The duration of the plasma boost and plasma portion is also selected to prevent unnecessary melting of the metal around gap 74 of pipe ends 70, Such over-melting of the metal can result in the weld metal seeping into the interim of the pipe sections. During the formation of the molten metal ball at the end of the electrode during the plasma portion of the current, the jet forces of the high current repel the melted metal from the welding pool until the preselected amount of molten metal has been melted at the end of the electrode. Once the current is reduced, the molten metal is allowed to form into a ball and the molten metal pool in the groove is allowed to stabilize, thereby allowing for a smooth contact between the substantially spherical ball and the quelled weld metal pool. The desired amount of molten metal at the end of the electrode is controlled by directing a preselected amount of energy or wattage into the electrode during the plasma portion of the welding cycle. All during the time the molten metal ball is being formed at the end of the electrode, the core components are releasing shielding gases to shield the molten ball and the weld metal in gap 74 from the atmosphere. The shield gases continue until the molten ball is transferred into the molten metal in gap 74.
Once the molten metal ball is formed during the plasma boost and the plasma portion of the welding cycle, the molten ball is forced into the molten pool by feeding the electrode into the pool, thereby forming a short circuit condition. When the melted metal ball engages the molten metal pool, it is transferred into the pool by surface tension. This action causes an ultimate necking down of the molten metal extending between the pool and the wire in the

electrode, and then a rupture and separation of the ball from the wire occurs. Since there is only a low background current during the separation, little if any spatter occurs. Preferably, the current control circuit monitors the necking of the molten metal ball such that when the neck rapidly reduces in diameter by electric pits, the current flow during the pinch curve 110 increases more gradually until a detection of an impending fuse is obtained. Once the detection of an impending fuse occurs, the current is reduced to the background current until the molten metal at the end of the electrode transfers into the weld pool.
FIGURES 4 and 5 show a standard STT short circuit welding cycle with the cycle in FIGURE 4 applying the positive terminal to the advancing wire 50. With this polarity, the molten metal puddle is relatively cool compared to the standard STT welding cycle shown in FIGURE 5 wherein the electrode is negative. The short circuit welding process utilizing the standard negative polarity, or electrode negative condition shown in FIGURE 5, has a tendency to increase the temperature of the molten metal. This condition is normally alleviated by reducing the background current of the STT welder. By employing another aspect of the present invention, the temperature of the molten metal puddle is controlled by welder 200 operated in accordance with the present invention and shown in FIGURE 6. Welder 200 applies a welding pulse across the advancing cord metal electrode 50 as the electrode or wire moves toward workpiece 60. The primary aspect of the present invention is the use of a cord electrode. The additional aspect of the invention can use a solid wire electrode. Welder 200 includes a switching type power supply 202 in the form of an inverter having switching pulses controlled by pulse width modulator 204 with the width of the successive current pulses determined by the voltage at the output of error amplifier 206. This amplifier receives a voltage from current shunt 208 that is proportional to the actual arc current. Input line 210 directs the current voltage to the amplifier with the desired current signal in line 212 from standard controller 220. Controller 220 creates a voltage in line 212 that sets the width of the individual rapidly occurring current pulses at the output of inverter or power supply 202. Output stage 230 of inverter 202 includes a transformer 232 having center tap 234 connected to a positive rectifier 236 and a negative rectifier 238. A polarity selector in controller 220 provides a logic in output 240 when the output of the power supply is to have a positive polarity and a logic in output line 242 when the power supply is to be

shifted into a negative polarity. Switches Ql, Q2, each having a standard snubber 244, are used to control current in inductor LI having a positive polarity portion 250 controlled by switch Ql and a negative polarity portion 252 controlled by switch Q2. A logic in line 240 closes switch Ql causing current flow through inductor portion 250, A logic in line 242 changes the polarity causing current flow in negative polarity portion 252 of inductor LL So long as the polarity is positive by the logic in line 240, the STT welder produces positive current pulses to give the positive polarity current cycle shown in FIGURE 7, Weld cycle 300 is shown as a positive cycle with all current having a positive polarity. Cycle 300 has a starting point at tj which is at the time a short occurs. The background current is reduced toward zero. Thereafter, the pinch current 302 causes the shorted metal ball to transfer by a tension transfer and an electrical pinch until a neck is created, as indicated at 304. The current is again plunged as indicated at portion 306 to reduce spatter. After the metal has been transferred by the electric pinch action, the' plasma condition is reestablished by a plasma boost pulse 310 having a maximum arc current. The area of the plasma boost pulse 310 determines the general size of the molten metal ball at the end of the advancing wire electrode 50. After the boost pulse, the current has a time constant tailout 312 terminating at the background current 314. At 316 the next short circuit occurs. As long as a logic 1 appears in output 240 the rapidly created current pulses have a positive polarity, as shown in FIGURE 7. Upon receipt of a logic one in output line 242, the polarity of the welding operation is reversed. A reversed or negative polarity cycle 320 is created, as shown in FIGURE 8. In accordance with this aspect of the invention, the number of positive polarity current cycles 300 and negative polarity current cycles 320 are controlled to obtain the desired heat in the molten metal puddle of the welding operation. If the puddle is too cold, the number of negative polarity cycles 320 is increased with respect to the number of positive current cycles 300. The desired ratio is obtained by appropriate selector circuit in controller 220, which selector circuit is illustrated schematically in FIGURE 9 wherein the selector circuit is a flip-flop 350, software implemented, having a non-inverted output 240 and an inverted output 242. The output is selected by anti-coincident circuit 352 having a set input line 352a and a reset input line 352b controlled by a digital decoder 354. Input 360 receives an input initiation pulse at time t, when a cycle is started by a short circuit. The adjusting

inputs 362, 364 of decoder 354 set the ratio of a number of positive current cycles at input 362 and the number of negative current cycles at input 364. By adjusting these two inputs, the ratio of positive current cycles 300, two negative current cycles 320 is selected to control the heat of the welding process. To change the heat, the ratio is manipulated by changing the data at inputs 362 and 364.
Although the preferred embodiment of this aspect of the invention involves the selection of the ratio between the positive current cycles 300 and the negative current cycles 320 during a welding process, an alternative control concept has been devised wherein each cycle 302 is initiated as a standard negative polarity cycle and is then shifted to a positive polarity cycle at a preselected point in the cycle. This aspect of the invention is illustrated in FIGURE 10 wherein current cycle 400 is started as a negative polarity cycle with the pinch current portion 402 followed by a standard plasma boost current portion 404, In accordance with this aspect of the invention, the polarity of the rapidly created current pulses is shifted after the termination 410 of plasma boost portion 404. The shift at point x is after time delay TD. Thus, tailout portion 420 is divided into a negative portion 422 and a positive portion 424 with an instantaneous shift in polarity at point x. Thereafter, the current cycle is a positive polarity until the end 430 of the cycle. Flip-flop 350 shifts logic state to await the next output of trailing edge detector 454 as shown in selector S' in FIGURE 11, At the end of a plasma boost portion, detector 454 reads the trailing edge at input 452 to start time delay 456 which has a manual adjusted time at input 460. In this manner, the heat of the weld puddle is determined by the selection of the time delay for reversing the polarity of weld cycle 400. Other modifications could be made to alternate between a positive polarity and a negative polarity for the current pulses fi-om the STT welder for controlling the heat of the welding operation.
The aspects of the invention have been described with reference to preferred and altemative embodiments. Other modifications are apparent and are within the scope of the present invention.

L-2169 FOREIGN
1. An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said apparatus comprising a power supply that supplies current to said wire, said power supply creating a series of current pulses that constitute a welding cycle, said current pulses in said cycle each having a given electrical polarity with respect to said workpieces; and a polarity selector that selects the polarity of said pulses in said welding cycle between a first polarity with said electrode being positive and a second polarity with said electrode being negative to control the temperature of said weld pool in said gap, said selector regulating the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire.
2. The apparatus as defined in claim 1, wherein said selector causing a shift between said first polarity and said second polarity at the beginning of at least one welding cycle.
3. The apparatus as defined in claim 2 or 3, wherein said selector causing a shift between said first polarity and said second polarity during at least one welding cycle.
4. The apparatus as defined in claim 3, wherein said selector causing a shift between said first polarity and said second polarity during said plasma arc melting portion.
5. The apparatus as defined in claims 1 -4, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second portion creating said second polarity, a first switch for connecting said first portion of said inductor between said wire and said plates, a second switch for connecting said second portion of said inductor between said wire and said plate, said selector causing either said first switch or said second switch to close during a given weld cycle.
6. The apparatus as defined in claims 1 -5, wherein said selector includes a decoder with

a first condition to select one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles and controller to alternate between said first and second conditions during a welding operation.
7. The apparatus as defined in claim 6, wherein said first number is different from said second number.
8. The apparatus as defined in claims l-V, including a shunt to sense the actual arc current and an error amplifier to compare an actual arc current with a desired arc current to control a width of said current pulses.
9. The apparatus as defined in claims 1 -8, including a pulse width modulator for creating said current pulses at a frequency greater than about 10 kHz.
10. The apparatus as defined in claims 1 -9, wherein said power supply is an inverter with an output transformer driving a rectifier.
11. The apparatus as defined in claims 1-10, wherein said wire is a cored metal electrode.
12. The apparatus as defined in claims 1-11, wherein said wire is a cored electrode includes alloying components in the core.
13. The apparatus as defined in claims 1-12, wherein said wire does not require extemal gas shielding.
14. The apparatus as defined in claims 1-13, wherein said wire is a self-shielding electrode.

15. The apparatus as defined in claims 1-14, wherein said selector regulates the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire.
16. The apparatus as defined in claims 1-15, wherein said selector alternates said polarity of said pulses as a function of the cumulative amount of energy applied to said wire and said melted wire in said groove in one polarity and the cumulative amount of energy applied to said wire and said melted wire in said groove in the other polarity.
17. The apparatus as defined in claims 1-16, wherein said power supply creates a background current, said background current having a low level that is just above the level necessary to sustain an arc throughout each welding cycle.
18. The apparatus as defined in claims 1-17, wherein said power supply controls a preselected amount of energy to said wire to melt a substantially constant volume of said wire during each welding cycle.
19. The apparatus as defined in claims 1-18, wherein said power supply limits the amount of energy directed to said wire to inhibit said molten metal from passing through said spaced ends of said workpieces.
20. The apparatus as defined in claims 1-19, wherein said power supply reduces the amount of current to said wire prior to said molten metal on said wire forming a short circuit condition with said groove, said reduced current having a decaying current profile,
21. The apparatus as defined in claim 1-20, wherein said power supply forms a high current pulse at the end of a short circuit condition and terminates said pulse just prior to a predicted termination of said short circuit condition.

22. The apparatus as defined in claims 1-21, wherein said wire is moved about the outer peripheral surface of said workpieces and substantially along said groove.
23. The apparatus as defined in claims 1-22, including a welding carriage that continuously moves around said workpieces.
24. The apparatus as defined in claims 23, said welding carriage moves at a variable speed.
25. The apparatus as defined in claims 1-24, wherein said two workpieces are two pipe sections.
26. The apparatus as defined in claims 1 -25, wherein said power supply is an STT power supply.
27. An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece forming a gap there between by melting an advancing welding wire and depositing said melted wire into said gap to join said two spaced ends, said welding wire being a cored metal electrode; said apparatus including a power supply that supplies current to said wire, said power supply including a first circuit to create a transfer current and a second circuit to create a melting current, said second circuit supplying a sufficient amount of current to said wire to form a root bead in said groove which bridges said gap.
28. The apparatus as defined in claim 27, including a welding carriage positioned closely adjacent to said gap, said carriage adapted to move said wire about the outer peripheral surface of said workpieces and along said gap.
29. The apparatus as defined in claim 28, wherein said welding carriage continuously
moves around said workpieces and wherein the speed of said welding carriage can be varied.

30. The apparatus as defined in claims 28 or 29, including a mechanism to varying the speed of said welding carriage and/or the feed rate of said wire as the welding carriage moves about the periphery of said workpieces.
31. The apparatus as defined in claims 27-30, wherein said wire is a self-shielding electrode
32. The apparatus as defined in claims 27-31, wherein said wire is a cored electrode
which includes alloying components in the core to form said root bead having a substantially similar
composition as the composition of said workpieces.
33. The apparatus as defined in claims 27-32, wherein said second circuit directing a preselected amount of energy to said electrode to melt a relatively constant volume of said electrode during each welding cycle.
34. The apparatus as defined in claims 27-33, wherein said power supply limiting the amount of energy directed to said wire to inhibit molten metal from passing through said gap,
35. The apparatus as defined in claims 27-34, wherein said second circuit reduces the amount of current to said wire prior to said molten metal on said wire forming a short circuit condition with said gap, said current having a decaying current profile when said current is reduced.
36. The apparatus as defined in claims 27-35, wherein said first circuit directs a high current pulse at the end of a short circuit condition and applies said pulse until just prior to a predicted termination of said short circuit condition.
37. The apparatus as defined in claims 27-36, wherein said power supply includes a polarity controller to control the polarity of at least a portion of said current.

38. The apparatus as defined in claim 37, wherein said polarity controller maintains a positive polarity during at least a portion of said melting current.
39. The apparatus as defined in claim 37 or 38, wherein said polarity controller maintains a positive polarity during at least a portion of said transfer current.
40. The apparatus as defined in claims 37-39, wherein said polarity controller maintains a negative polarity during at least a portion of said melting current.
41. The apparatus as defined in claims 37-40, wherein said polarity controller maintains a negative polarity during at least a portion of said transfer current.
42. The apparatus as defined in claims 27-41, wherein said two workpieces are two pipe sections.
43. The apparatus as defined in claims 27-42, wherein said power supply is an STT power supply,
44. A method of short circuit arc welding two spaced ends of a first and a second workpiece forming an elongated groove there between which defines a gap by melting an advancing welding wire and depositing said melted wire into said groove to join said two spaced ends, said method comprising the steps of:

a) providing a metal wire;
b) moving said wire toward said groove as said wire is moved along said groove;
c) melting said wire by an electric wave generated by a power supply, said electric wave including a series of current pulses that constitute a welding cycle, said current pulses in said cycle each having a given electrical polarity with respect to said workpieces;

d) selecting the polarity of said pulses in said welding cycle between a first polarity with said wire being positive and a second polarity with said wire being negative to control the temperature of said weld pool in said groove; and,
e) selecting the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire,

45. The method as defined in claim 44, including the step of alternating between said first and said second polarity during at least one welding cycle.
46. The method as defined in claim 45, wherein step of alternating between said first and said second polarity during at least one welding cycle occurs during said plasma arc melting portion,
47. The method as defined in claim 44-46, including the step of alternating between said first and said second polarity at the beginning of at least one welding cycle.
48. The method as defined in claims 44-47, including the additional step of selecting one of said first or second polarity for a first number of consecutive welding cycles and a second condifion for selecting the other of said polarity for a second number of consecutive cycles,
49. The method as defined in claim 48, wherein said first number is different from said second number.
50. The method as defined in claims 44-49, wherein said wire is a self-shielding electrode,
51. The method as defined in claims 44-50, wherein said wire is a cored electrode.

52. The method as defined in claim 51, wherein said cored electrode includes alloying components in the core,
53. The method as defined in claims 44-52, including the step of providing a welding carriage which moves said wire about an outer peripheral surface of said workpieces.
54. The method as defined in claim 53, wherein the speed of said welding carriage is varied as said carriage moves about said workpieces.
55. The method as defined in claims 44-54, wherein said electric wave includes a background current, said background current having a low level just above the level necessary to sustain an arc throughout each welding cycle.
56. The method as defined in claims 44-55, wherein said two workpieces are two pipe sections,
57. The method as defined in claims 44-56, wherein said electric wave includes a transfer portion and a controlled melting portion, said melting portion having a preselected amount of energy for melting a relatively constant volume of said electrode during each welding cycle.
58. The method as defined in claims 44-57, wherein said step of melting said electrode including the application of a preselected amount of energy to said electrode during at least one welding cycle and reducing current to said electrode prior to molten metal on said electrode forming a short circuit condition vdth said groove.
59. The method as defined in claims 44-58, wherein said electric wave limiting energy to said electrode to prevent molten metal from passing through said gap.
60. The method as defined in claim 44-60, including the steps of sensing the actual arc

.current and comparing said actual arc current with a desired arc current of a welding cycle to control the width of said current pulses.
61. The method as defined in claims 44-60, wherein said power supply creates an alternating current.
62. The method as defined in claims 44-61, wherein said power supply is an STT power supply.
63. The method as defined in claims 44-62, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second portion creating said second polarity, a first switch for connecting said first portion of said inductor between said wire and said plates, a second switch for connecting said second portion of said inductor between said wire and said plate, said selector causing either said first switch or said second switch to close during a given weld cycle.
64. The method as defined in claims 44-63, wherein said power supply including a selector that regulates the polarity of said pulses in said welding cycle between said first polarity and said second polarity, said selector includes a decoder with a first condition to select one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles and controller to alternate between said first and second conditions during a welding operation.
65. The method as defined in claim 64, wherein said selector regulates the polarity of said pulses in said welding cycle between said first polarity and said second polarity to control the rate of melting of said wire.
66. The method as defined in claim 64 or 65, wherein said selector alternates said polarity of said pulses as a function of the cumulative amount of energy applied to said wire and said melted

,wire in said groove in one polarity and the cumulative amount of energy applied to said wire and said melted wire in said groove in the other polarity.
67. An apparatus for short circuit electric arc welding two spaced ends of two metal plates at a groove between said two metal plates by melting an advancing welding wire and depositing said melted wire into said groove to join said spaced ends, said apparatus comprising: a switching power supply for creating a series of small width current pulses constituting a welding cycle with a short circuit transfer portion and a plasma arc melting portion, said current pulses in said cycle each having a given electrical polarity of said advancing wire with respect to said two metal plates; and, selector means for selecting the polarity of said pulses in said cycle between a first polarity with said wire being positive and a second polarity with said wire being negative.
68. An apparatus as defined in claim 67, wherein said selector means includes shifts between said first polarity and said second polarity at the beginning of a welding cycle.
69. An apparatus as defined in claim 67 or 68, wherein said selector means includes a decoder with a first condition for selecting one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles and means for alternating between said first and second conditions during a welding operation.
70. An apparatus as defined in claim 69, wherein said first number is different ft-om said second number.
71. An apparatus as defmed in claims 67-70, wherein said cycles each have desired arc current and including a shunt to sense the actual arc current and an error amplifier to compare said actual arc current with said desired arc current to control the width of said current pulses.
72. An apparatus as defined in claims 67-71, including a pulse width modulator for

creating said current pulses at a frequency greater than about 10 kHz.
73. An apparatus as defined in claims 67-72, wherein said power supply is an inverter with an output transformer driving a rectifier.
74. .An apparatus as defined in claims 67-73, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second portion creating said second polarity, a first switch for connecting said first portion of said inductor between said wire and said plates, a second switch for connecting said second portion of said inductor between said wire and said plate, and said selector means including means for closing either said first switch or said second switch during a given weld cycle,
75. An apparatus as defined in claim 74, wherein said closing occurs at the beginning of a weld cycle,
76. An apparatus as defined in claims 67-75, wherein said wire is a cored metal electrode.
77. An apparatus as defined in claims 67-76, wherein said power supply is an inverter with an output transformer driving a rectifier.
78. An apparatus as defined in claims 67-77, wherein said cycles each have desired arc current and including a shunt to sense the actual arc current and an error amplifier to compare said actual arc current with said desired arc current to control the width of said current pulses.
79. An apparatus as defined in claims 67-78, including a pulse width modulator for creating said current pulses at a frequency greater than about 10 kHz.
80. A method for short circuit electric arc welding two spaced ends of two metal plates at a groove between said two metal plates by melting an advancing welding wire and depositing said

melted wire into said groove to join said spaced ends, said method comprising the steps of:
(a) creating a series of small width current pulses constituting a welding cycle with a short circuit transfer portion and a plasma arc melting portion, said current pulses in said cycle each having a given electrical polarity of said advancing wire with respect to said two metal plates; and^
(b) selecting the polarity of said pulses in said cycle between a first polarity with said wire being positive and a second polarity with said wire being negative.
81. The method as defined in claim 80, including the additional step of:
(c) shifting between said first polarity and said second polarity at the beginning of a
welding cycle.
82. The method as defined in claim 80 or 81, including the additional steps of:
(c) selecting one of said first or second polarity for a first number of consecutive welding cycles and a second condition for selecting the other of said polarity for a second number of consecutive cycles; and,
(d) alternating between said first and second conditions during a welding operation.

83. The method as defined in claim 82, wherein said first number is different from said .second number.
84. The method as defined in claims 80-83, wherein said cycles each have desired arc cunent and including the steps of sensing the actual arc current and comparing said actual arc current with said desired arc current to control the width of said current pulses.
85. The method as defined in claims 80-84, including the step of creating said cuireni pulses at a irequency greater than about 10 kHz.
86. The method as defined in claims '80-85, including the step of creating said current pulses at a frequency greater than about 10 kHz.

87. The method as defined in claims 80-86, wherein said power supply is an inverter with an output transformer driving a rectifier.
88. The method as defined in claims 80-87, wherein said power supply includes a center tapped inductor with a first portion creating said first polarity and a second.portion creating said second polarity, and including the steps of:

(c) connecting said first portion of said inductor across said wire and said plates by closing a first switch;
(d) connecting said second portion of said inductor across said wire and said plate by closing a second switch; and,
(e) closing either said first switch or said second switch at a selected position in a given weld cycle.

89. The method as defined in claim 88, wherein said selected position is at the beginning of said given weld cycle,
90. A method as defined in claims 80-89, wherein said wire is a cored metal electrode.

91. An apparatus for short circuit arc welding two spaced ends of a first and a second workpiece substantially as here in before described with reference to the accompanying drawings.
92. A method of short circuit arc welding two spaced ends of a first and a second workpiece substantially as here in before described with reference to the accompanying drawings.

Documents:

940-mas-1999-abstract.pdf

940-mas-1999-claims filed.pdf

940-mas-1999-claims granted.pdf

940-mas-1999-correspondnece-others.pdf

940-mas-1999-correspondnece-po.pdf

940-mas-1999-description(complete)filed.pdf

940-mas-1999-description(complete)granted.pdf

940-mas-1999-drawings.pdf

940-mas-1999-form 1.pdf

940-mas-1999-form 13.pdf

940-mas-1999-form 19.pdf

940-mas-1999-form 26.pdf

940-mas-1999-form 3.pdf

940-mas-1999-form 5.pdf

940-mas-1999-other documents.pdf

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Patent Number

214015

Indian Patent Application Number

940/MAS/1999

PG Journal Number

13/2008

Publication Date

31-Mar-2008

Grant Date

23-Jan-2008

Date of Filing

23-Sep-1999

Name of Patentee

LINCOLN GLOBAL INC

Applicant Address

22801 ST. CLAIR AVENUE, CLEVELAND, OHIO 44117-1199,

Inventors:

#	Inventor's Name	Inventor's Address
1	ELLIOTT K. STAVA	8484 EATON DRIVE, SAGAMORE HILLS, OHIO 44067,

PCT International Classification Number

B23 K 09/09

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	09/200, 594	1998-11-27	U.S.A.