Dr. A. RAJA,
Former,Additional General Manager
Welding Research Institute, BHEL, Trichy.
Arc welding occupies the most important position among the fusion welding processes, due to its flexibility and cost effectiveness. It has become an indispensable technology for the construction of steel structures, shipbuilding, automotive vehicle manufacture, power plant equipment manufacture and other industries. The global industrial competition demands shorter cycle time and increased productivity in production operations. The increasing demand for higher productivity and quality in welding has been partly addressed by the development of ―high performance arc welding‖ processes. Most of these high performance arc-welding processes or process variants have been developed with the Gas Metal Arc Welding [GMAW] as the basis. According to the work sheet DVS 0909, all the GMAW processes or process variants which use one or several wire electrodes at feed rates of more than 15 m/min (individually or in total) are regarded as high-performance welding methods . As per above guideline, Dual Wire GMAW and Hybrid Laser-GMAW can be considered as high performance arc welding processes. Development of these advanced arc welding processes has been made possible by the advancements in arc welding power sources and computer technology. Particularly the inverter power sources with software control and digital communication has enabled the process development. The dual-wire GMAW equipment is available under different names e.g., TIME TWIN GMAW [Fronius, Austria], Tandem MIG [Lincoln, USA], Tandem Weld [Carl Cloos, Germany] and others. Tandem GMAW equipment continues to be developed and most of the main manufacturers now have commercially available tandem GMAW systems. The dual-wire GMAW process is referred to as ―Tandem GMAW’ in this paper as a generic name only. The Tandem GMAW is intended for use in robot integrated automatic mode or SPM automated mode of welding only, as manual handling of these processes is difficult due to the too high amperage and welding speed involved in this process. The working principle, process
capability, applications and advantages of Tandem GMAW, a high performance arc welding process are discussed in this paper.
2.0. Tandem GMAW Process Description:
The idea of welding with two wires is not a new one. The tandem and other multi-wirewelding is a technology well established in Submerged Arc Welding process for decades. As mentioned earlier, the availability of high-powered inverter power sources and Waveform Control Technology has created interest and enabled dual-wire or Tandem GMA Welding using the GMAW process as the basis. Tandem GMAW is one of the high performance variant of the GMAW process. In this process typically two 1.2 diameter wires are fed through the torch body designed to accommodate two electrically isolated contact tips. The shielding gas required to protect the common weld-pool created by the two arcs is also supplied through the same torch. The two electric arcs created during welding, are independently powered from two separate GMAW power sources and each of the power sources have its own control and regulation system, an independently controllable wire feeders. In reality, in Tandem GMAW one wire is positioned normal and the other wire is positioned slightly at an angle towards the first wire. While welding any wire can be the leading or lagging, depending on the direction of travel.
Conventional GMAW process operates with single consumable electrode wire and in this method the process can offer a deposition rate of up to 6 kg/hr. This limitation on the deposition rate is imposed by the current carrying capacity limitation of the single wire. It can be reasoned out that when the power is increased, the arc pressure rises very rapidly, making the weld pool difficult to control. For instance with a 1.2 mm diameter AlMg wire electrode, in conventional GMAW the amperage limit beyond which weld-pool control becomes critical will be found between approx. 320 and 350 A at a wire feed speed of 20 to 22 m/min . When the welding current exceeds this limit, the arc becomes somewhat violent accompanied by spatter and humpy bead formation. Tandem GMAW process overcomes the above limitation by having two consumable electrode wires operating in close proximity in the same torch with a common molten pool. In Tandem MIG, with two wires operating simultaneously the deposition rate is increased by more than two times (e.g.15Kg/hour can be achieved at travel speeds of 5m/min). In other words, Tandem GMAW extends the welding productivity range beyond what is normally possible with conventional single-wire GMAW process.
Fig1. Schematic of Tandem GMAW Process.
The tandem welding process comprises two completely independent welding circuits, each with its own power source, torch cable, wire drive, and contact tip (Figure 1). Optionally, a push-pull system is available for aluminum applications. Tandem welding systems use independent circuits to run the operation. Some manufacturers provide the option of pulse synchronization between the two power supplies. Pulse synchronization can be used to stabilize the arc by reducing interference between the two welding circuits. Whether pulse synchronization is necessary depends on the specific application. Operators should not assume that pulse synchronization is always necessary
The Tandem GMAW process employs two electrically isolated wire electrodes positioned in line, one behind the other, in the direction of welding. The first electrode is referred to as the lead electrode and the second electrode in line is referred to as the trail electrode. The spacing between the two wires is usually less than 12 mm so that both welding arcs are delivering to a common weld puddle. The function of the lead wire is to generate the majority of the base plate penetration, while the trail wire performs the function of controlling the weld puddle for bead contour, edge wetting and adding to the overall weld metal deposit rate.
The process works best with a large diameter lead wire and a small diameter trail wire. The larger lead wire may represent as much as 65% of the total deposition rate, while providing greater penetration . The smaller, trail welding wire is focused on the trail edge of the weld puddle. The trail wire is typically smaller in diameter and therefore draws less current. This helps to control the shared weld puddle and aids in keeping it cool.
A common compromise is to specify the lead and trail welding wires to be the same diameter typically 1.2 mm diameter to satisfy inventory constraints or because the direction of welding must be reversed somewhere on the weld seam. Satisfactory operation may be achieved with this compromise but the maximum travel speed is limited and the robustness of the process is reduced.
Fig.2. Tandem GMAW Torch [Courtesy: Carl Cloos]
The Tandem GMAW equipment comprises of two GMAW power sources, two wire feeders with a special torch for accommodating the two wires coming from the two wire feeders. The torch houses two contact tips which are electrically isolated from each other (Fig.2). The two contact tips that are contained within a common torch body is surrounded by a common gas nozzle. The two contact tips are angled in such a way that during welding, the two wires actually share the same arc, and a single molten puddle is formed. Although the tandem welding torch is larger than a standard single- wire torch, equipment suppliers have developed slim, compact torch designs. This enables good access into many types of weld joints. Generally, the tandem welding process is used in such a way that one welding wire follows directly behind the other in the welding joint, although this is not always necessary. The tandem welding process is effective on carbon steels, stainless steels, and aluminum. For aluminum applications, push-pull torches are available to ensure reliable wire feeding.
4.0. Tandem GMAW Operating Modes:
In conventional GMAW process, three different modes of metal transfers viz. Short circuit transfer, Globular Transfer and Spray Transfer- can be set-up by proper selection of process parameters and shielding gas combination. Since in Tandem two arcs are operating simultaneously several combinations of metal transfer modes are possible. Besides the natural transfer modes, the pulsed spray mode is another option to choose. Thus using the spray and pulsed spray mode, the possible combinations of modes of metal transfers that can be employed in Tandem GMAW are as follows Axial Spray Transfer Lead Arc + Axial Spray Transfer Trail Arc
- Axial Spray Transfer Lead Arc + Axial Spray Transfer Trail Arc
- Axial Spray Transfer Lead Arc + Pulsed Spray Transfer Trail Arc
- Pulsed Spray Transfer Lead Arc + Pulsed Spray Transfer Trail Arc
- In pulsed arc mode, Tandem delivers short-circuit-free, low-spatter metal transfer in which one droplet of weld-filler per pulse is shed from the wire electrode and into the weld pool . This pulsing technology enables a virtually constant size of droplet to be set, regardless of the arc power. This greatly improves the weld quality. The lead arc, in all cases, determines the penetration level of the weld, and the trail arc provides the final bead shape and weld bead reinforcement. The development of tandem GMAW had higher welding speed as its core objective in sheet metal welding. The welding speed on sheet metal applications is usually 1.5 to 1.9 times the travel speed for a single arc, and it is not uncommon to find travel speeds of more than 2.5 m/min. For thicker sections of material where multiple pass welding is necessary, the deposition rate for tandem GMAW can vary from 9 to 21 kg/hr. The arc travel speeds range from 0.6–1.2 m/min. To achieve anticipated mechanical properties, Tandem GMAW may require the use of special welding techniques [5,6].
5.0. Applications of Tandem GMAW process:
The Tandem GMAW with two wires and separately controllable metal transfers enables welding at higher amperages and thus higher welding speeds. As a rule, a slightly higher power is set for the leading arc. The cold base metal is thus thoroughly melted and exact fusion takes place into the root. The weld-metal deposit from the second electrode fills the weld pool. Also, the trailing arc prolongs the weld-pool degasification time, lessening the pore-formation sensitivity. The main process benefits of Tandem GMAW based on the author’s experience and claims made in literatures are increased welding speed, deposition rate, penetration, reduced porosity and better tolerance to variations in joint fit-up compared to conventional single wire GMA welding. Travel speeds and deposition rates are typically 2-3 times higher than for conventional single wire GMA welding. The process has been chiefly applied to steels, and some aluminium alloys with thicknesses in the range 1.5 to 25mm for steel, and 2 to 6mm for aluminium.
Tandem GMAW process finds application in automobile, shipbuilding, boiler manufacturing, construction and other areas. In automobile sector, Tandem GMAW is employed for welding of car wheel rim made of AlMg alloy. In this application, Tandem GMAW provides higher welding speed; low spatter, good fusion and fine weld shape [7-9]. Tandem GMAW found advantageous for welding of LPG cylinders. This application benefits from the good gap bridging capability of Tandem GMAW besides the higher welding speed. Tandem GMAW when applied for the circumferential seam welding of the LPG cylinder, the welding speed could be more than doubled while meeting all the requirements of mechanical properties. The welding speed that was 0.85 rpm with conventional GMAW could be increased to 2.0 rpm with Tandem GMAW . Besides the Tandem GMAW process exhibited its wider tolerance to fit-up variations.
When installing a tandem GMA welding process on robots, it also is important to have the ability to weld in either direction. In other words, either wire should be able to act as the lead wire. This provides great flexibility when writing a robotic welding program and allows a programmer to take full advantage of the robot’s working range. It also is helpful in some circumstances to turn one wire off and weld with a single wire only. Through-arc tracking always should be done with the lead wire, so the system must have the ability to switch tracking capability freely from one wire to the other. Trying to perform through-arc tracking with the trailing wire can lead to inconsistent tracking results.
Remember that the same limitations apply to through-arc tracking when welding with tandem wires as when welding with a single wire—the robot must use a weave pattern during welding to read both sides of the weld joint. If the application is sheet metal or some other joint in which weaving is not feasible, then laser tracking or some other option must be considered [3,4].
Fig.3. LPG Cylinder Welding by TIME Twin Process. [Courtesy: WRI, BHEL, Trichy]
6.0. Narrow Groove Tandem GMAW:
Thick-section components are often joined using high deposition rate welding processes such as GMAW and SAW with conventional open-groove designs. Although these processes are considered “high deposition rate” processes, they are not necessarily
“high productivity” processes due to the large number of weld beads that are required to fill a conventional single or double-V-groove joint. Narrow-groove joint configurations are advantageous as they reduce the overall volume of the weld joint; however, lack of fusion into the sidewall is a common concern in Narrow-groove welding. This can prevent the successful application of many conventional high deposition rate arc welding processes. While mechanized GTAW is used successfully in narrow grooves, its relatively low deposition rate limits overall productivity.
Applying Tandem GMAW in narrow-groove welding can significantly increase the productivity. EWI has developed a prototype Tandem GMAW welding torch and demonstrated its benefits. They could achieve deposition rates surpassing 9 kg/hr on HSLA-100 base materials at a travel speed of 381 mm/min. This is over 900% increase of the deposition rate of the baseline, narrow-groove cold-wire GTAW process typically used with these joint configurations and materials .
Tandem GMAW is truly a high performance welding process due to its higher deposition rate and welding speed associated with it. Besides, the Tandem GMAW has a wider tolerance to fit-up variations a feature facilitated by the two arcs operating in close proximity. Tandem GMAW can only be employed as a robot integrated system or SPM integrated equipment and that is the way to gain the full potential of the process. Though several Tandem GMAW equipment are already in use in the Indian industry and many more applications in Shipbuilding, Heavy engineering, Earth Moving equipment are still to be explored. Matching the right applications for Tandem GMAW would be the key to success.
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2. Karin Himmelbauer, TIME Twin digital–top deposition rate thanks to two wire electrodes, Fronius, Austria.
3. Jeff Nadzam: Tandem GMAW: The Flexibility of Pulsed Spray Transfe Welding Innovation, Vol. XIX, No. 2, 2002.
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