Technical paperDouble-electrode arc welding process: Principle, variants, control and developments
Introduction
Two technologies have been developed to modify GMAW for faster deposition: Tandem GMAW [1], [2] and Variable-Polarity GMAW (VP-GMAW) [3], [4], [5], [6], [7]. In Tandem GMAW, two torches have been integrated into one bigger torch, and two close arcs are independently established between their own wire and work-piece in parallel and are adjusted by their own GMAW power supply. In essence, Tandem GMAW is still considered two parallel conventional GMAW processes. It allows the deposition rate be doubled without increasing the arc pressure. For VP-GMAW, liquid droplets are still detached during the reverse polarity (wire positive) period, but the welding wire can be melted faster during the straight polarity (wire negative) period [3], [8]. It was found that to melt the welding wire at the same rate, the base metal heat input could be “up to 47 percent less” than the conventional pulsed GMAW [8]. Thus, when the allowed base metal heat input is given, VP-GMAW may also double the deposition rate. Modifications by adding a laser to form hybrid laser-arc processes [9], [10], [11], [12], [13], [14], [15], [16] can penetrate deeper to reduce the needed deposition. However, the resultant process is no longer a pure arc welding process and many advantages associated with arc welding may be compromised.
The double-electrode GMAW [17], [18] and its variants are introduced to increase the deposition rate without increasing the heat input, reduce the heat input without compromising the deposition rate, or freely provide the needed heat input and deposition rate as desired by different applications which typically use GMAW or its variants. For conventional GMAW and its variants, the base metal current is exactly the same as the wire current, i.e., the current flows through the wire. This is the fundamental principle not only for GMAW but also for all other conventional arc welding processes in which an arc must be established between an electrode and the work-piece. Because of this fundamental principle, while the wire current needs to be increased to increase the deposition rate, the base metal current increases exactly the same regardless of the actual need of the work-piece. The DE-GMAW changes this principle by introducing a bypass channel such that the deposition rate no longer needs to be proportional to the heat input applied into the work-piece.
In this tutorial, the principle, developments and extension of the DE-GMAW are discussed and outlined to help further develop/extend this process for manufacturing applications.
Section snippets
Double-electrode GMAW principle
Fig. 1 demonstrates the principle of the DE-GMAW process and its variants where the main electrode is a consumable wire. The main power supply, main torch/electrode and work-piece form the conventional GMAW process and the main loop. The bypass torch added provides an additional electrode to form an additional arc, i.e., the bypass arc, with the main electrode and closes the bypass loop. In Fig. 1, the bypass arc is powered by an added second power supply but it may also be powered by the same
Non-consumable DE-GMAW using constrained bypass arc
A non-consumable DE-GMAW uses a non-consumable bypass electrode to realize the general DE-GMAW system in Fig. 1. Its feasibility was first verified using a PAW torch to provide the non-consumable second electrode in 2004 at the University of Kentucky [17], [18] as shown in Fig. 2. The use of PAW torch was to ease the establishment of the bypass arc because the pilot arc can easily provide a reliable channel to bridge the main arc with the tungsten second electrode. In fact, the constrained
Non-consumable DE-GMAW using unconstrained bypass arc
While the pre-existence of a constrained pilot arc can ease the ignition of the bypass arc after the main arc has been established, its associated high cost for the equipment and the inconvenient large size of the bypass torch are all unwanted. In the non-consumable DE-GMAW system shown in Fig. 4, the PAW torch in Fig. 2 is replaced by a GTAW torch. The bypass power supply is replaced by a bypass control circuit which controls the passing bypass current at the desired level. The main GMAW power
Metal transfer in non-consumable DE-GMAW using unconstrained bypass arc
The American Welding Society (AWS) classifies the metal transfer into three primary modes: spray transfer, globular transfer, and short-circuiting transfer. In the spray transfer, the liquid metal droplets transfer into the weld pool across the arc gap with diameters similar to or smaller than that of the wire. The International Institute of Welding (IIW) further classifieds the spray transfer mode into the projected spray (or drop spray), streaming spray, and rotating spray. In the globular
Consumable DE-GMAW and analysis [23,24]
In non-consumable DE-GMAW, although extra heat input and arc force have been reduced, the energy absorbed by the bypass electrode is wasted. If the bypass electrode is a consumable wire, the waste can be eliminated while still providing the advantages associated with DE-GMAW. The resultant process is the consumable DE-GMAW shown in Fig. 10 and its heat input controllability as represented by the range of the deposition efficiency p has been discussed earlier in Section 2 and especially
Control of consumable DE-GMAW
A method to produce desired welds is to control the base metal current and bypass current at desired levels such that the heat input determined by the total current (their sum) and penetration capability determined by the base metal current and heat input are accurately controlled. When CV power supplies are used, these two currents may be adjusted by their corresponding wire feed speeds in large ranges. The adjustments on the wire feed speeds affect the total mass input but it may be
Double-electrode submerged arc welding (DE-SAW) [27]
Submerged arc welding (SAW) is a variant of GMAW which allows the use of extra high currents to deposit metals at high speeds. The major issue associated with high currents and high deposition rates is the associated large heat input which causes large distortion whose correction is highly costly. An extension of the DE-GMAW into SAW may result in desirable heat input and distortion reductions. The resultant variant is referred to as DE-SAW.
Fig. 12 shows a DE-GMAW system developed at the
Penetration depth control [27]
A number of studies have been devoted to modeling the SAW process [31], [32], [33], [34], [35]. Based on these studies, a comprehensive model has been proposed to correlate the depth of the weld penetration to a number of welding parameters as the regression variables:
Here P is the depth of partial penetration weld (in.), Ibm the base metal current (A), Wt the total deposition rate (lb/h), G the gap of the joint (in.), CTWD the contact-tip-to-work distance (in.), and S the
Pulsed DE-GMAW for aluminum and galvanized steel welding
Aluminum alloy and galvanized steel hybrid structures can effectively reduce vehicle weights. However, aluminum and steel have very different physical characteristics including the melting temperature and thermal expansion that make the corresponding dissimilar metal joining to be challenging. Laser and cold metal transfer (CMT) welding are few fusion welding methods that have found success for this type of dissimilar metals joining due to their low heat input [36], [37]. Since the DE-GMAW
Other variants of DE-GMAW and double-electrode arc welding
A few variants have been proposed to extend the DE-GMAW concept or beyond the exact definition of DE-GMAW. The indirect arc method [43] has been independently proposed and developed at Shandong University, which establishes an arc between two consumable rods without the work-piece to be a part of the arc, either anode or cathode, will not be discussed below. It reduces the heat input to a minimum and shares a certain similarity with DE-DMAW but lacks the mechanism to adjust the heat input as
Summary and future work
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Double-electrode GMAW changes the principle of conventional arc welding where the arc is established between the electrode and work-piece, and the electrode current equals the base metal current.
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Double-electrode GMAW reduces the base metal current from the main wire melting current by means of a bypass loop.
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Both non-consumable and consumable DE-GMAW, including their variants, increase and adjust/control the deposition efficiency from that of the conventional GMAW, resulting in a reduced and
Acknowledgements
The Adaptive Intelligent Systems LLC thanks the support from the Navy under contracts N00024-09-C-4140, N65538-08-M-0049, and N65538-10-M-0110 and Kentucky Cabinet for Economic Development (CED) Office of Commercialization and Innovation through Kentucky Science and Engineering Corp. under agreements KSTC-184-512-08-038 and KSTC-184-512-09-067. The Adaptive Intelligent Systems LLC also thanks the approvals for public release from the Navy (5720/00DT 2013-0033, 5720/00DT 2012-0854, 5720/00DT
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