Advances in Automatic Hot Wire GTAW (TIG) Welding
Advances in Automatic Hot Wire GTAW (TIG) Welding
Keywords: Hot wire TIG, automatic GTAW welding, narrow gap torch, welding automation, cladding, flow sensors, nuclear components
The hot wire GTAW (TIG) process is an under-utilized technology that can pay big dividends in the right application and when used in industries requiring high quality, high productivity welding. Although the GTAW process has a much lower deposition rate than GMAW (MIG), the quality is generally superior. When hot wire technology is used in conjunction with GTAW, especially in downhand welding on plate or on larger diameter pipe, deposition rates can approach that of MIG.
One example of the use of hot wire GTAW technology is provided by a manufacturer of differential pressure flow sensors in Colorado. This company has used an advanced Windows-based 400 Amp power suppy/controller to control the weld parameters and to turn on a separate hot wire GTAW power supply as well as a rotary positioner. They have made over 8,300 welds on a variety of corrosion resistant materials in just four years with virtually no rejects. 1
Hot wire GTAW narrow groove welding has been used extensively for the welding of nickel-based alloys, a wide range of steel alloys as well as reactive metals such as titanium. Hot wire GTAW equipment is finding increasing uses in industry applications as varied as valve manufacture, oil and gas industry components, and components used in nuclear power generation.
Recent developments in hot wire GTAW equipment as well as the benefits and limitations of the process will be presented.
Although the hot wire GTAW process, commonly known as hot wire TIG, was introduced in 1966, its development and application have been slow to take hold. However, recently there has been considerable renewed interest in the process which initially promised “MIG speed with TIG quality.” While there are some limitations, the hot wire gas tungsten arc welding process has found increasing use over a wide range of groove welding, buildup and cladding operations. The development of automated and specialized hot wire equipment has driven application of the process in industries including nuclear, power generation, pressure vessel and offshore oil which all require high productivity-high quality welds.
3. Hotwire Gas Tungsten Arc Welding
Figure 1. Plot of deposition rate vs. arc energy for cold wire GTAW and hot wire GTAW. Hot wire GTAW with oscillation provides even higher deposition rates. From Irving, 1966
3.1 The hot wire concept
Hot wire GTAW welding is a further development of the Gas Tungsten Arc Welding or Tungsten Inert Gas process. GTAW was invented by Russell Meredith working at Northrop Aircraft Company in 1939-1941. The GTAW process was initially called "Heliarc" as it used an electric arc to melt the base material and helium to shield the molten puddle. The patents were sold to the Linde Division of Union Carbide who developed a number of torches for different applications and sold them under the brand name Heliarc. Linde also developed procedures for using argon which was more readily available and less expensive than helium. Hot wire GTAW technology was invented in 1964 by A.F. Manz and developed by Linde. Filler metal in the form of wire is commonly added to welds in the GTA welding process, particularly when heavy wall thicknesses are being welded. With conventional GTAW the filler wire is introduced into the leading edge of the weld puddle in the cold state (ambient temperature). Energy from the arc is required to melt the wire reducing the efficiency of the process. In hot wire welding, filler wire is resistance heated until close to the melting point and added to the weld puddle behind the tungsten. This prevents the wire from chilling the weld pool and allows the filler metal to flow out across the weld puddle resulting in a smooth, attractive weld bead. Since nearly all of the full energy of the welding arc is available for penetration or to generate the weld pool and fusion, a two to three times faster travel speed is realized. More wire can be deposited and fill rates are increased with the added benefit of weld quality as good as or in some cases better than with cold wire GTAW (see Figure 1).
Figure 2. illustrates the principle of hot wire GTAW welding. Two separate power supplies are used: the GTAW torch with current from the GTA power supply establishes an arc which melts the area under the arc. The wire heating power supply sends current through a circuit from the contact tube through which the wire is fed to the tip of the wire as it contacts the weld pool. Resistance heating of the wire is provided by the power drop across the wire extension as I2R. The resistance is determined by the length of wire stick out and wire diameter with larger wire diameters providing less resistance.
Initially, hot wire technology was used with manual GTAW and the advantages of hot wire were more theoretical than practical. Advances in technology that paved the way for practical applications of hot wire GTAW technology include the development of automated GTA welding power supplies that provide accurate, repeatable control of weld parameters.
These advances in technology have increased the capability and application of welding systems in general. For example, early automated GTA welding power supplies were programmed by digit switches. While this provided more accurate control of parameters than was possible by foot controls used by manual welding, the digit switches had to be changed manually each time a new parameter or program was required. The application of microprocessors and memory devices allow users to store a large number of weld programs in the memory of today’s machines and to easily call them up for a specific application. Running changes in a pre-programmed weld schedule are also easily made by the operator via a remote operator’s pendant.
Different influences have come together over several decades to increase the application of hot wire technology. The need for more efficient manufacturing with fewer defects, and the increased use of corrosion resistant materials are just two that come to mind. Increased technology leads to more adaptable and user-friendly machines. Increased use of these machines leads to communication between users and equipment suppliers which results in machines more suited to specific production needs. Hot wire power supplies used in conjunction with multiple axis positioning equipment combined with narrow gap torches is an example of this progression.
3.1 Other processes tried the hot wire concept
Although the hot wire concept has been applied to other processes, the majority of usage is with the GTAW process. Submerged arc, plasma arc, gas metal arc, laser and electron beam have all been used to some extent with hot wire, but these applications have been very limited.
3.2 Advantages and Limitations of Hot Wire GTAW
3.2.1 Advantages Today hot wire GTAW has recognized benefits in welding. Hot wire technology is readily mechanized and automated and increased deposition rates are realized. The independent control of the arc and wire feed variables allow for flexibility in setting parameters. Skills required for operation of cold wire GTA apply to hot wire so additional training is minimized.
The hot wire GTAW process is applicable for most materials. The GTAW process is metallurgically “simple” in that the electrode is not consumed and filler metals added to the weld may be precisely designed to achieve a particular deposit chemistry and metallurgical results. The hot wire GTAW process produces a very clean weld with a very low incidence of porosity. Thus, the hot wire GTAW process is particularly applicable to 300 series stainless steel, as well as to engineered materials such as duplex stainless steels, or high-nickel alloys, and titanium where the metallurgical properties of the base metal may be adversely affected by welding unless high-quality welding procedures are employed. The proper use of the GTAW process with the addition of hot wire can optimize the corrosion resistance and/or mechanical properties of the weldment and have a beneficial effect on the material performance in service.
3.2.2 Limitations One limitation of hot wire GTA welding is that it is not normally used for manual welding. There are also additional equipment costs for the hot wire power supply in addition to the cold wire GTAW equipment. The power supplies are not especially compact and portable for moving around in the field. In going from cold wire to hot wire GTAW, it may be necessary to change the groove design to one more suitable to hot wire and the increased deposition rate. There are some consumable costs such as the hot wire feed guide tube contact tips and tungsten electrodes. The cost of helium or helium argon mixes used as shield gas for some applications are greater than for straight argon and more gas is consumed. And there may be increased costs for training welding personnel.
3.2.3 Orbital hot wire Perhaps the biggest limitation in the application of hot wire technology is that it is not suitable for welding small pieces and it is limited with respect to welding position. The most practical welding position is 1G, or downhand welding on plate or welding of pipes when the pipe can be rotated, followed by 2G. While it would be tempting to apply hot wire technology to true out-of-position orbital welding, the significant increase in deposition that can be achieved in downhand hot wire welding will not be seen in orbital applications. There are often difficulties in welding at the 6 to 8 o’clock positions. When welding downhill, the puddle tends to run into the tungsten electrode unless the travel speed is very slow. When welding uphill the puddle tends to sag. However, some metallurical benefit may be achieved in orbital applications at the expense of greater complexity.
Regardless of the application, techniques and equipment configuration, hot wire must be controlled with the principals similar to cold wire welding.
4. Hot wire power supplies
Several manufacturers have developed power supplies for hot wire GTAW welding. Some use sine wave AC for heating the wire, some use square wave and even DC. The Arc Machines, Inc. Hot Wire Power Supply Model 501 has controls for (heating) current and (hot wire) voltage. The standard mode of operation for the Model 501 is Constant AC Voltage (0 - 10 Volts), but Constant Current mode (Max output 250 Amps) is also available. The Model 501 implements advanced technology. AC current is used to minimize magnetic interference (arc blow). The AC current of the Model 501 is very close to a true sine wave. To control the hot wire power the voltage is sensed where the power cable is attached to the guide tube. By measuring the voltage at the guide tube, the effects of cable length (voltage drop) are controlled.
The accuracy of true RMS provided by these measurements make it possible to duplicate production conditions from one set-up to the next with excellent repeatability of process.
4.1 Equipment Configurations
4.1.1 Stand alone hot wire power supply. The Model 501 can be operated by an Arc Machines’, Model 415 Power Supply, an external weld process controller or program, or can stand alone and be operated from its own programming pendant. A wire feeder is used to deliver the wire to the weld pool and a power supply (or controller) controls the rate of wire entry.
4.1.2 Use with GTAW power source/controller When the hot wire power supply is used with the Model 415 GTAW Power supply/controller, the M415 initiates the command to operate the hot wire power supply as part of the program. The Model 415 is Windows™-based with built-in digital data acquisition and multi-servo control capabilities. It is a flexible system for which almost all function ranges and modes can be defined by the user to exactly match the unique torch configuration being used. Included among the many possible torch configurations are the AMI Model 2 end effector and the narrow gap torch. The closed-loop motor servos and optional open-loop motor manipulator controls make these systems ideal for work-cell manufacturing environments.
Figure 3. Left:Model 415 Power supply/controller used in a system with a 501 hot wire power supply and Model 2 torch for downhand groove welding. Photo courtesy of Veris, Inc1. Right: Standard torch with hot wire set up on a Model 2 for cladding.
5. Hot wire set up parameters
5.1 Wire entry
The location of wire entry into the weld pool is critical. For hot wire GTA welding, wire entry is normally at the rear of the puddle with the wire deposited in the depression behind the arc at the rise of the weld pool. This permits faster travel speed than with cold wire GTAW where wire entry is at the leading edge of the puddle. Hot wire travel speeds of 8-9 IPM (203-228 mm/min) are typical compared to about 4 IPM (101 mm/min) for cold wire GTAW. Wire extension from the contact tip of the wire guide tube to the inside edge of the electrode is set to nominally 0.750 to 1.062 inches (19-26.9 mm). A manipulator is used to adjust the wire to tungsten distance which is the space between the tungsten tip and the wire which, depending on the application, is in the range of 0.125 inches (3.1 mm) and 0.156 inches (3.9 mm). The wire entry angle is nominally 45° or higher, to control the wire entry into the weld pool.
5.2 Wire geometry
The length of wire stick out from the contact tip of the guide tube to the weld pool as well as the wire diameter and wire material affects the resistance for I2R heating of the wire: the longer the stick out and the smaller the diameter of the wire, the higher the resistance. Since the current across wire extension is squared, the current has a larger effect on heating than voltage. Settings are adjusted so that the wire is heated to just below melting temperature as it enters the weld pool.
Figure 4. Wire entry is at the back of the weld pool. A trailing shield is necessary to provide gas coverage to the entire liquid weld pool..
Courtesy of Arc Applications, Inc.
Wire diameters are typically from 0.035 to 0.062 inches (0.889-1.65 mm). I
While increasing the wire diameter provides increased surface contact, at a set deposition rate increasing the wire diameter requires increased heating due to reduced resistance. An increase in wire heating may affect arc deflection. In general, the larger the wire diameter, the lower the cost will be. The wire must be very clean to avoid problems with contamination and residue buildup in the contact tip.
Deposition rates for applications average 4-8 lbs/hr but deposition rates of greater than 10 lbs/hr can be achieved without difficulty. This makes it practical for overlays, seam welds and other similar type welds in the 1G (downhand) welding position. For example, hot wire GTAW would be suitable for welding high-quality stainless steel tanks and vessels that are rotated during welding. Attempting to increase deposition rates by increasing hot wire current beyond certain levels or by maxing out the wire feed speed can result in oxides on the weld bead, increased porosity and generally poor results. If deposition rates are increased, travel speed must increase and torch oscillation is required.
5.3 Heat input is difficult to calculate
The only formula used to calculate heat input is the commonly used:
I X V X 60/s = KJ/m
This formula certainly does not take into account all of the variables that affect heat input in hot wire GTAW applications. For GTAW, normal energy losses occur due to water cooling, convection and radiation. Increased arc efficiency can be gained with higher travel speeds and trailing wire entry although a trailing wire can still provide a slight chilling effect. There is no standardized formula and some consider the energy (I X V) used to heat the filler wire to be part of the heat input calculation. Process variables such as gas type, deposition rate, material characteristics, etc. which may affect heat input make it difficult to evaluate the true heat input.
Figure 6. A trailing shield is recommended for use with hot wire GTAW in order to provide coverage to the entire weld zone. Photo from Arc Applications, Inc.
5.4. Significance of Shield Gas
5.4.1 Gas coverage Shielding with inert gas is particularly important for hot wire GTAW applications because the larger weld pool and length of solidifying/cooling weld passes demand greater gas coverage. Effective design of gas cups is essential for hot wire GTAW. Lamellar gas coverage is critical. There must be adequate screening in the torch and the gas lens must seal snugly around the tungsten.
5.4.2 Trailing shield. The use of trailing shields to provide gas coverage to the entire weld zone is highly recommended for hot wire GTA welding, especially for cladding operations and at the top of the groove. This helps the solidifying and cooling of the weld and is critical for higher currents and welding speeds to minimize the formation of oxides and the potential for porosity. In addition, a protected weld pool will have a lower surface tension and low viscosity that will wet and tie-in better. The extent of coverage required depends on the material being welded and the temperature.
5.4.2 Effect of shield gas composition on weld bead shape
The shield gas type and composition has been found to have a major effect on the shape of the weld bead as shown in Figure 7. Gas mixtures can be adjusted to suit particular applications. The bead shape produced with helium shielding is flat with good wetting and is ideal for cladding and overlays. Argon produces a more rounded profile and deeper penetration which is suitable for groove welding, while the addition of small amounts of hydrogen can be used to increase penetration. Argon/helium mixes are generally preferred for hot wire welding in grooves, and have been used in cladding and buttering. On some materials such as 300 series stainless steels, argon with small amounts of hydrogen have been used to increase penetration and reduce the formation of oxides. Hydrogen mixtures should not be used in applications where hydrogen-induced cracking could occur.
Figure 7. Shield gas type has a profound effect on weld bead shape. Left: Weld bead done with 100% helium, increasing amounts of argon to 75% argon/25% helium and the two beads on the right were done with a tri-mix of helium-argon with the addition of 1% and 5% hydrogen. Courtesy of Arc Applications, Inc.
6. Narrow gap torch
6.1 Rotating tungsten narrow gap torch
One of the special applications of the Model 415 is the control of Arc Machines’ design narrow-gap torch. On this torch, both the tungsten electrode and the filler wire rotate from side to side which provides superior side wall fusion in very narrow-gap joint preparations. While originally produced to weld thick-wall pipe, this torch is very effective for repairing large turbines and for manufacturing large, heavy-walled vessels which may require welding in grooves about 9/16 inch wide and up to 12 inches deep (12.8 mm wide by up to 304.8 mm deep).
Although the concept of narrow groove welding has been known for a long time as a technique that offers lower overall heat input, a smaller HAZ, a lower amount of consumables and reduced weld time, the main problem with previous narrow gap welding had been the lack of side wall fusion. Arc Machines, Inc. narrow gap torch with oscillating tungsten effectively provides for repeatable sidewall fusion. For cold wire TIG, both the wire and the tungsten electrode oscillate. For some narrow gap applications the root and hot passes may be done with cold wire and the fill passes done with hot wire. The narrow gap torch configured with hot wire GTAW has yielded deposition rates greater than 6 lbs per hour while achieving GTAW (TIG) metallurgical weld quality with low shrinkage and residual stress.
Figure Left: Narrow gap torch for 12 inch groove. Note cameras for viewing welding in groove. Right: Twelve inch NGT in mock-up groove. Photos from Arc Applications, Inc. and Arc Machines, Inc.
6.2 Vision system
A remote vision system is used with the narrow gap torch to view the electrode in the joint and to monitor the progression of the weld. The vision system can be used as a stand alone feature or it can be adapted to other GTAW installations including the Model 2. The vision system permits a clear view of the weld pool and wire entry which can reduce or eliminate operator error and fatigue. It can serve as a visual quality manager and may be recorded on video tape for a permanent QC record.
Figure Left: Images of welding with a narrow gap torch displayed on an AMI Model 415 DV. Left: Closeup of images from cameras at leading and trailing edges of the weld pool. Photos from Arc Machines, Inc.
7.1 Downhand groove welding with a positioner
Veris, Inc., manufacturers of flow sensors in Colorado, used automated hot wire GTA welding system for welding advanced technology differential pressure flow sensors. The system consisted of a Model 2 end effector with a standard torch, a Model 415 GTAW power supply/ controller with a Model 501 hot wire power supply and two rotary positioners.
Figure 8. Downhand welding of flow sensors with an automated hot wire GTAW system.1
7.1.1 Model 2 effector The Model 2 end effector was used in conjunction with the Model 415 Power Supply and 501 hot wire power supply at Veris, Inc.1 The effector uses slides as axis positioners for which slides of a particular length can be made specifically for a given application. Each individual slide can be made to perform arc gap control (AVC) or oscillation or travel or jog-only-positioning. The effector can feature up to 8 motions.
The weld was done downhand with the pipe rotated by a positioner (Figure 8). By adding a second positioner Veris was able to double their productivity as a second weld could be done while the first weld was cooling to the specified interpass temperature. The joint sizes on their units ranged in size from 2 to 14 inch schedule 80 pipe. The weld joint on these assemblies was a 37 1/2° angle with a 0.060 inch weld land which would normally have been done with MIG because of the higher deposition rate, but the GTAW process was specified because the corrosion resistant materials comprising their flow sensors required high quality welds.
7.1.2 Pipe diameter vs. hot wire GTAW productivity Veris, Inc. found that the efficacy of the hot wire GTAW was highly dependent on the pipe diameter, with the largest diameter being the most productive. Hot wire was not used on the smallest diameter pipe or on the root of larger pipe as they used a consumable insert of the appropriate chemistry as filler. On smaller diameter pipe the hot wire deposition rates were reduced to approximately 2.5 lbs/hr while the larger 12 inch pipes were run with approximately 5 lbs/hr. As of November, 2009, they had completed a total of 8,300 welds over four years with virtually no rejects.
7.2 Hot wire GTAW for heavy nuclear components
Babcock & Wilcox designs and manufactures large, heavy components for industry including manufactured components for commercial power reactors. Jeff Kikel of Babcock & Wilcox states that in order for U.S. companies to be competitive in the next generation of nuclear power plants, “high productivity processes and more efficient joining technologies must be developed and deployed.” The nuclear power generation industry in the United States has been at a virtual standstill since the Three Mile Island incident. Existing U.S. nuclear power plants were constructed using 1970’s and 1980’s technologies, and since new construction of nuclear power plants have continued overseas, the technology in those countries have advanced while that in the U.S. has not. Unless more efficient manufacturing methods are adapted, the near term deployment of GEN III+ units would likely not be manufactured by U.S. facilities, but rather by Japanese, Korean, and European manufacturers.
Although welding technology for the nuclear industry in the United States has fallen behind since the incident at Three Mile Island, Babcock and Wilcox have been using advanced automated welding processes in the fabrication of heavy nuclear components. This technology includes hot wire GTAW for cladding and buttering operations and hot wire GTAW with a narrow gap torch.
Figure 9. Hot wire GTA Welding of a heavy nuclear component by Babcock and Wilcox.2
7.2.1 Torch considerations. Babcock & Wilcox worked with the Arc Machines, Inc. Narrow Gap Torch, but found they needed some custom designed torches for their special applications. They found several design features to be advantageous for narrow gap welding. These included having a remote control wire feed manipulator with motorized tilt and wire positioning. Since direct viewing of the narrow groove is not possible during welding, leading and trailing cameras are needed for most applications. Torches should also be designed to survive long time exposure to UV and high temperatures.
7.2.2 Arc Voltage control. In narrow gap welding, oscillation of the tungsten electrode allows the weld bead to wet the side wall while automatic arc voltage control (AVC) is used to control the arc gap. Initially, there were problems with the AVC response and the ability to trace the weld contour. This was fixed by the development of an improved AVC circuit. In preparation for welding, a gage is used to set the distance between the tungsten electrode and the side wall.
7.2.3 Joint design and weld bead deposit. With more standard narrow groove torches welding of nickel-based materials a groove angle of 8° or greater is generally used to insure side wall fusion. A root width of about 0.500 inches (12.7 mm) depending on torch design is required. Rather than welding with a single bead per layer extending from side wall to side wall, a split bead technique has been found to promote competitive grain growth resulting in more favorable metallurgical properties.
7.2.4 Consumables Babcock & Wilcox found that selection of consumables is important for the success of hot wire GTAW applications. The quality of filler wire, tungsten electrodes and hot wire guide tube tip material make a significant difference in the quality of the weld deposit. Processing is important for filler metal with AOD/VIM melt preferred. In helium-rich shielding gas environments, lanthanated tungsten were preferred for longevity, with a blunt tip and larger diameter, such as 5/32 inch (3.96 mm), providing increased penetration and longevity. Consumables from different vendors should be evaluated for performance as differences may occur from vendor to vendor.
For the hot wire tip material, copper tungsten alloys were found to provide superior tip life to pure copper and other copper alloys.
7.2.5 Gas selection. Torch oscillation is used to stir the weld deposit and spread it over a wider area. Oscillation is required in order to get high deposition rates. The choice of shielding gas is important as it is difficult to get the deposit to spread out and flow using straight argon gas. Weld beads done with argon are rounded much like beads of water on a waxed surface. The crown makes tie-in difficult and may result in lack of fusion between beads or on subsequent layers. A more satisfactory deposit is achieved by using helium or helium/argon mixtures. Weld beads done with helium tend to spread out and result in faster deposition.
The GTAW process has always offered high quality weld deposits suitable for higher alloys but the low deposition rate put it at a disadvantage in comparison to other processes. Hot wire GTAW goes a long way towards closing the gap in productivity while maintaining the quality inherent in the process.
New developments in GTAW equipment and hot wire technology have resulted in the number of new applications increasing in a wide range of industries. Industry now enjoys increased productivity with GTAW quality, but successful implementation is still limited compared to other processes and equipment.
The author would like to thank the following individuals for their technical expertise and helpful discussions in the preparation of this paper:
Jonathan T. Salkin, President Arc Applications, Inc. York, Pennsylvania,
has done extensive development of automated hot wire GTAW systems and the Arc Machines, Inc. narrow gap torch. The author visited his facility in April, 2010 and had comprehensive discussions and demonstrations of hot wire GTAW set-up and procedures.
Mike Myers, Welding Engineer, Arc Applications, Inc. York Pennsylvania
Pat Kemp, Engineering Manager, Veris, Inc. Niwot, Colorado. The author visited the Veris site in October, 2009 to observe their welding operations.
Jeff Kikel, Manager, Welding Engineering, Nuclear Operations Group Barberton Facility, Babcock & Wilcox Company
John Cushing, Vice President Product Management, Arc Machines, Inc., Pacoima, CA 91331
Alan Harrison, Welding Applications Engineer, Arc Machines, Inc.
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