Abstract:
Methods and apparatuses for plasma MIG welding or TIG MIG welding are disclosed. They include a plasma or TIG torch for following along a weld path by a MIG torch (or the order may be reversed). A constant distance may be maintained between the torches, and the angle of the torches, relative to the workpiece, may vary. The MIG process is performed EP or EN in various embodiments.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the art of welding and welding power supplies. More specifically, it relates to welding with a plasma process or a TIG process and a MIG process. 
     BACKGROUND OF THE INVENTION 
     There are a wide number of known welding processes used for a variety of welding applications. Various processes have strengths and weaknesses with respect to characteristics such as speed, precision, workpiece composition, cost, flexibility, etc. 
     For example, MIG welding (metal inert gas welding) is relatively fast, but somewhat imprecise. The process is fast because, in part, a consumable wire electrode is used as a filler metal. However, for some applications, such as welding galvanized steel, MIG does not perform well, at least in part because the MIG process, which is typically DC, does not, effectively prepare for welding (or remove) the zinc on the steel. If not properly prepared the zinc can vaporize during the welding process and cause bubbles in the weld. Also, for some applications an even faster MIG process is desired. 
     Another process, TIG welding, is precise and can work with galvanized steel, but TIG is a relatively slow process. Thus, it is often used for high-quality, low speed applications. 
     Plasma arc welding (PAW) is a welding process that also does not lend itself readily to high speed welding. For example, PAW is best performed at under 100 amps, and it is particularly useful for welding under 20 amps and as low as 0.1 amp. If higher current is needed, PAW is performed in a keyhole process, where the plasma gas creates a hole in the workpiece, and molten metal flowing behind the moving hole creates the weld bead. 
     TIG welding has been combined with plasma welding in plasma TIG welding. Plasma TIG welding has been performed using a TIG torch, followed by a plasma torch, followed by a TIG torch. Plasma TIG welding is not well suited for galvanized steel, and TIG can be slow. 
     A weld process that can be fast and precise is laser MIG welding. This entails the simultaneous application of a laser beam and a MIG arc on the weld. While the process may be fast, precise, and useful on galvanized steel, it is expensive and may be difficult to use. 
     Accordingly, a welding process that provides for relative high speed, acceptable precision, without excess cost is desirable. Preferably the process will weld galvanized steel. 
     SUMMARY OF THE PRESENT INVENTION 
     According to a first aspect of the invention a method of plasma MIG welding includes creating a plasma arc and a MIG arc between torches and a workpiece. There is relative movement between the torches and a weld path. 
     A constant distance is maintained between the plasma torch and the MIG torch in one embodiment. 
     According to a second aspect of the invention a system of plasma MIG welding includes a plasma torch and a MIG torch. The MIG torch and the plasma torch are mounted such that they are a fixed distance from one another. 
     The angle of the plasma arc is between +10 degrees and −10° degrees, or between +5 degrees and perpendicular, and the angle of the MIG torch is preferably between +10 degrees and −45°, or between 0 degrees and −30 degrees, in various embodiments. 
     The distance between the plasma torch and the MIG torch is greater for faster movement along weld path in another embodiment. 
     According to a third aspect of the invention a system for plasma MIG welding includes at least one power source having a plasma power output and a MIG power output. The power source also has a control input and a controller is operatively connected to the control input. 
     The power source includes a plasma power source and a MIG power source, and the controller includes a plasma controller and a MIG controller in various embodiments. 
     According to a fourth aspect of the invention a product is formed by the process of plasma MIG welding a plurality of workpieces. At least one of the workpieces is comprised of galvanized steel, such as G-60 or G-90. 
     According to a fifth aspect of the invention a method of TIG MIG welding includes creating a TIG arc and a MIG arc between torches and a workpiece. There is relevant movement between the torches and a weld path. 
     A constant distance is maintained between the TIG torch and the MIG torch in one embodiment. 
     According to another aspect of the invention a system of TIG MIG welding includes a TIG torch and a MIG torch. The MIG torch and the TIG torch are mounted such that they are a fixed distance from one another. 
     The angle of the TIG arc is between +10 degrees and −10° degrees, or between +5 degrees and perpendicular, and the angle of the MIG torch is between +10 degrees and 45°, or between 0 degrees and −30 degrees, in various embodiments. 
     The distance between the TIG torch and the MIG torch is greater for faster movement along weld path in another embodiment. 
     According to yet another aspect of the invention a system for TIG MIG welding includes at least one power source having a TIG power output and a MIG power output. The power source also has a control input and a controller is operatively connected to the control input. 
     The power source includes a TIG power source and a MIG power source, and the controller includes a TIG controller and a MIG controller in other embodiments. 
     According to an eighth aspect of the invention a product is formed by the process of TIG MIG welding a plurality of work pieces. At least one of the workpieces is comprised of galvanized steel, such as G-60 or G-90. 
     The MIG and/or TIG process is performed EP or EN in various embodiments. 
     Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a plasma MIG system in accordance with the present invention; and 
     FIG. 2 is a diagram of a plasma torch mounted with a MIG torch. 
     Before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. Like reference numerals are used to indicate like components. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention will be illustrated with reference to a particular system and method using particular power supplies, it should be understood at the outset that the invention may be implemented using other embodiments, including other components and other methods. 
     Generally, the invention is a method and apparatus for plasma MIG welding. Plasma MIG welding, as used herein, includes a welding process performed with a plasma arc and a MIG arc acting on a common weld path, either sequentially in any order, or simultaneously. 
     The preferred embodiment provides that a plasma torch is mounted with a MIG torch, such that as the plasma torch is moved along the weld path, the MIG,torch trails by a small distance (½ inch, e.g.). Two power supplies are provided, one for the plasma torch, and one for the MIG torch. The torches move relative to the weld path on the workpiece, by moving the torches, or moving the workpiece, or moving both torches and the workpiece. Relative movement of a torch along a weld path, as used herein, includes movement of the torch relative to the workpiece, and either the workpiece can be moved, or the torch can be moved. 
     The present invention performs particularly well with galvanized steel because the plasma arc prepares the zinc in the steel for welding in advance of the MIG arc, and the MIG arc provides the welding energy and filler metal. The speed of plasma MIG can be 3-4 times the speed of MIG alone. Also the disadvantage of MIG welding—difficulty in welding a workpiece that needs cleaning or preparing, such as galvanized steel, is overcome. 
     A plasma MIG system  100  in accordance with the present invention is shown in FIG.  1  and includes a power source  101 , a controller  106 , a wire feeder  109 , a plasma torch  111 , and a MIG torch  112 . Power source  101  provides power to the torches, and wire feeder  109  provides wire to MIG torch  112  (through the power source). Controller  106  controls the process. The torches are moved along a weld path on a workpiece  113  (in the direction of arrow  115 ) in the plasma MIG process. 
     The preferred embodiment provides that power source  101  includes a plasma power source  102 , having a plasma power output, and a MIG power source  101 , having a MIG power output. Plasma power source  102  is controlled by a plasma controller  107 , and MIG power source  103  is controlled by a MIG controller  108 . Power source, or source of power, as used herein, includes the power circuitry such as rectifiers, switches, transformers, SCRs, etc. that process and provide the output power. Plasma power output, as used herein, includes an output having sufficient power for use in a plasma process (it may require transformation before being used in the plasma process). MIG power output, as used herein, includes an output having sufficient power for use in a MIG process (it may require transformation before being used in the MIG process). 
     In various embodiments the components are housed separately, or in various combinations. For example, in the preferred embodiments plasma power source  102  and plasma controller  107  are implemented with a Miller® Dynasty power supply (operated in a dc mode), which provides a single housing for the plasma controller and plasma power supply. Also, MIG power source  103  and MIG controller  108  are implemented with a Miller Invision® power supply (operated in a dc mode), which also provides a single housing. Other embodiments entail a single power source that provides power for both MIG and plasma, that may include two output circuits. 
     The controllers may be combined on a single board, and the entire system disposed in a single housing. Wire feeder  109  may be part of the housing, or outside the housing. Also, controller  106  may directly control wire feeder  109 , and wire feeder  109  provides control signals to a control input on power source  103  (thus controller  106  also controls power source  103 ). Controller, as used herein, includes digital and analog circuitry, discrete or integrated circuitry, microprocessors, DSPs, etc., and software, hardware and firmware, located on one or more boards, used to control a device such as a power supply. Control input, as used herein, includes an input received that controls a power supply or other component, such as a setpoint, gate signals, phase control signals, etc. 
     The invention is performed with the MIG process as EN (electrode negative) or EP (electrode positive) in various alternatives. Speed or quality of the weld for various applications may be improved by appropriately selecting EP or EN. Using EP can result in shunting (or partial shunting) of the arc from the workpiece to the plasma torch. Therefore in at least some embodiments EN will be preferred. 
     Referring now to FIG. 2, plasma, torch  111  and MIG torch  112  are mounted on a bracket  201 , which hold them a fixed distance apart. As the welder or robot moves MIG torch  112  in the direction of arrow  115 , plasma torch  111  precedes it in the direction of travel. Plasma torch  111  is shown perpendicular to workpiece  113  in this embodiment. It is angled between +10 and −10 degrees, and between +5 degrees and perpendicular, or at any other angle in various embodiments. Angles are measured from the perpendicular, and the angle is negative when the arc is angled in the direction of travel. 
     MIG torch  112  is disposed at an angle a, which as negative 30 degrees in this embodiment. Other embodiments provide for MIG torch  112  to be angled between +10 and −45 degrees, or at any other angle. 
     When choosing the particular angle of plasma torch  111  and MIG torch  112  the distance between the arcs, the interaction of the arcs, and physical limitations of mounting the torches should be considered. Generally, perpendicular arcs will have less interaction, and should be able to weld at a higher speed. However, the diameter of the torches (near bracket  201 ) may result in the arcs being to far apart for high speed welding if the torches are perpendicular. 
     Bracket  201  is chosen with the desired distance between arcs in mind. Higher speed welding may be performed with the separation greater, although it is generally useful to have the arcs as close as possible without interaction. The distance is about ½ inch in the preferred embodiment, between ¼ and 1 inch in another embodiment, and any distance in other embodiments. 
     The output current is selected based on type of material, speed, desired precision, etc. Generally, the plasma cone is larger than the MIG cone for a given current. It may be desirable to control the plasma cone to keep it narrow so that the current density (and resultant heating and preparation of the zinc) is greater. 
     The present invention is particularly well suited for welding galvanized steel such as G-90 or G-60, because the plasma arc prepares the zinc in the steel, allowing the MIG arc to weld at a higher rate. G-60 and G-90 galvanized steel is steel wherein the weight of zinc on the steel is 0.60 or 0.90 oz. per square foot, respectively. 
     One alternative embodiment is a TIG-MIG system where TIG torch is used to prepare the zinc, followed by a MIG torch that weld the workpiece. Thus, in FIGS. 1 and 2 torch  111  maybe a TIG torch, power source  102  maybe a TIG power source, and controller  107  maybe a TIG controller. Other alternatives include various combinations of EP and EN, as desired for particular applications. 
     Numerous modifications may be made to the present invention which still fall within the intended scope hereof. Thus, it should be apparent that there has been provided in accordance with the present invention a method and apparatus for plasma MIG welding that fully satisfies the objectives and advantages set forth above. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.