Consumable Double-Electrode GMAW — Part 1 The Process

1 Consumable Double-Electrode GMAW Part I: The Process by Kehai Li and Y.M Zhang Kehai Li (kehai.li@uky.edu, PhD student,) and Y. M. Zhang (ymzhang@engr.uky.edu, Professor of Electrical Engineering and Corresponding Author) are with Center for Manufacturing and Department of Electrical and Computer Engineering, University of Kentucky, Lexington, KY 40506. Abstract Double-Electrode Gas Metal Arc Welding (DE-GMAW) is a novel welding process recently developed to increase welding productivity while maintaining the base metal heat input at a desired low level. In this paper, the DE-GMAW process was improved by replacing its non-consumable tungsten electrode with a consumable welding wire electrode resulting in a new process called consumable DE-GMAW. To understand this new process and its prospects as an effective manufacturing process, the authors have studied fundamental issues and proposed solutions to resolve the problems encountered. These fundamental issues include the stability of the process, the adjustability of the bypass current, the effects of the bypass arc on the total current and the melting rate, and the mode of metal transfer of the bypass welding wire. 2 Keywords: Double-Electrode, Arc Welding, Welding Productivity, Deposition Rate, Dual Wire Welding, Welding Controls, Heat Input 1. Introduction Novel processes are one key to maintaining the manufacturing industry’s competitiveness and technological leadership by increasing productivity or reducing cost. With this in mind, several new welding technologies have been developed. For example, laser welding can deliver very dense energy, thus the weld pool is small but the penetration is deep. The filler metal is reduced resulting in high productivity. The combination of laser welding and gas metal arc welding (GMAW) has created hybrid laser-arc processes (Refs.1-8) to further improve productivity. As the most widely used process, GMAW has been modified to obtain faster deposition. Because the welding current I in conventional GMAW is the same for the anode and cathode, increasing the welding current to improve deposition rate will also cause an increase in the base metal heat input (Refs.9-11). In addition, arc pressure is considered to be proportional to 2I (Ref.12). A large arc pressure blows metal away from the weld pool and generates undesired undercuts. In certain applications, the allowable base metal heat input and arc pressure are limited, therefore the allowable welding current is capped. Thus, to increase the deposition rate, conventional GMAW must be modified. Two technologies have been developed to modify GMAW for faster deposition: Tandem GMAW (Refs.13, 14) and Variable-Polarity GMAW (VP-GMAW) (Refs.15-19). In 3 Tandem GMAW, two torches have been integrated into one bigger torch, and two close parallel arcs are adjusted by two GMAW power supplies independently. In essence, Tandem GMAW is still considered two parallel GMAW processes, but Tandem GMAW can alternate the maximum welding current to each torch. In that way, the arc pressure remains unchanged, and the wire feed speed can be doubled. Hence, if arc pressure is the major concern, Tandem GMAW can double the deposition rate. 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 (Refs.15, 20). 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 (Ref.20). Thus, when the allowed base metal heat input is given, VP-GMAW may also double the deposition rate. Recently, another novel modification to GMAW has been proposed and successfully implemented at the University of Kentucky (Refs.21-25). This modification utilizes a non-consumable tungsten electrode to bypass part of the melting current in a conventional GMAW process as illustrated in Fig. 1. It can be seen that the total melting current is decoupled into base metal current and bypass current: bpbm III += (1) As a result, the melting current can be increased to improve the deposition rate while the base metal current can still be controlled at the desired level. Hence, the DE-GMAW process increases the deposition rate using a mechanism totally different from existing technologies. 4 The DE-GMAW process shown in Fig. 1 uses non-consumable tungsten as the bypass electrode, and is referred to as non-consumable DE-GMAW. It offers an effective way to reduce the base metal heat input and distortion without compromising productivity. However, if the energy absorbed by the tungsten electrode can be used in wire melting, welding productivity can be further improved. Thus, another variant of the DE-GMAW process has been studied by replacing the non-consumable tungsten electrode with a consumable welding wire provided with a separate GMAW torch. This new DE-GMAW is referred to as consumable DE-GMAW process in order to distinguish it from the non- consumable DE-GMAW. 2. Consumable DE-GMAW Principle and System The proposed consumable DE-GMAW is illustrated in Fig. 2. It can be seen that there are two GMAW torches: one bypass GMAW torch and one main GMAW torch. The main GMAW torch is powered by a constant voltage (CV) welding machine. The bypass GMAW torch is powered by a constant current (CC) welding machine whose current can be adjusted. The non-consumable tungsten electrode has been replaced with a consumable welding wire. The two torches are connected to their corresponding GMAW wire feeders. Both torches are moved together from right to left by a motor. As demonstrated in Fig. 2, the total melting current consists of two parts: the bypass current 2I provided by the CC welding machine and the base metal current 1I provided by the CV welding machine. (Here, the base metal current is denoted as 1I or bmI , and the 5 bypass current is denoted as 2I or bpI . This notation will also apply to other variables or parameters, such as arc voltage and wire feed speed: 1V , 2V , 1WFS , and 2WFS .) Thus, the decoupling principle in the non-consumable DE-GMAW denoted in Eq. (1) still holds. The consumable DE-GMAW illustrated in Fig. 2 was established at the University of Kentucky. Two Hobart 8065 Excel-Arc CV/CC welding machines were utilized to provide the base metal and bypass currents, respectively. This model of welding power supply has an interface to control its output, either welding voltage or welding current. Current sensors and voltage sensors were added to monitor the currents (base metal current and bypass current) and voltages (main arc voltage and bypass arc voltage), respectively. A Pentium PC computer equipped with a 12-bit A/D D/A transformation board was used to collect the data sample and run the control program. Two controllable GMAW wire feeders (Miller R115 and Hobart UltraFeed 1000) were used to feed in the welding wires, which are 1.2mm (0.045in.) diameter low carbon steel shielded with pure argon at a flow of 18.9 liter/min (40CFH). During experiments, the torches were moved by a motor while the workpiece remained stationary. The workpiece was 12.7mm (0.5 in.) mild carbon steel, and the travel speed was set to 0.64m/min (25IPM). At the same time, an Olympus i-speed high speed camera with a narrow-banded optical filter (central wavelength 940nm, bandwidth 20nm) was used to study the arc behavior and metal transfer. 6 3. Torch Configuration To establish a stable DE-GMAW process, both torches must be appropriately designed and originated. For the proposed consumable DE-GMAW, the bypass GMAW torch is aligned before the main GMAW torch as illustrated in Fig. 3. Because the shielding gas is provided by the main torch, the nozzle of the bypass torch is not necessary. Hence, the bypass torch is used without a nozzle to allow for a relatively small angle between the main torch and the bypass torch. In addition, it does not require any shielding gas although a small flow of argon might be beneficial for preventing the torch from overheating. The contact tip of the bypass torch is arranged slightly above the nozzle of the main torch to further reduce the distance between the two wires. Because the nozzle is isolated from the contact tip in the GMAW torch, the contact tip of the bypass torch is electrically insulated from that of the main torch. Preliminary experiments suggest that the following parameters in Fig. 4 can be used to arrange the torches: (1) less than 40 degrees for the angle between the two torches; (2) approximately 5 mm for the distance from the tip of the bypass welding wire to the workpiece; (3) approximately 25 mm for the distance from the contact tip of the main torch to the workpiece; and (4) approximately 10 mm for the length of the main arc established between the tip of the main welding wire and the workpiece. 7 4. Experiments and Analysis 4.1 Need for Two Power Supplies The non-consumable DE-GMAW was successfully implemented using a single power supply, although the bypass current energy was not effectively used. However, the preliminary experiments with the consumable DE-GMAW showed that one power supply was not sufficient. In the preliminary experiments, a temporary consumable DE-GMAW system, illustrated in Fig. 5, was established with a single power supply. This system was derived from the non-consumable DE-GMAW system simply by replacing the tungsten electrode with the bypass welding wire. In this temporary system, the bypass welding wire has the same electrical potential as the workpiece. Experiments showed that the current tended to flow through the workpiece first. This is because the workpiece and bypass welding wire are similar materials and thus have similar values for the electron work function [eV] (the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface), but the welding wire has a smaller size. Experiments also showed that the bypass current can not be increased beyond a limited level with the preliminary system. In the experiment demonstrated in Fig. 6, the main arc voltage and wire feed speed were 36volts and 11.4m/min (450IPM), respectively. Both welding wires were 1.2 mm (0.045 in.) diameter low carbon steel. The bypass current was limited around 106amps (mean 8 value) even though the bypass welding wire was fed at a speed as high as 8.9m/min (350IPM). This wire feed speed was so high that the bypass welding wire contacted the workpiece, thus preventing higher bypass currents from being obtained. The bypass electrode in the non-consumable DE-GMAW has an electron work function of 2.6eV, which is much smaller than iron’s electron work function (4.7eV). This difference creates a need to restrict the bypass current. In the consumable DE-GMAW, the welding wire has material similar to that of the workpiece so that the cathode size becomes the determining factor for the electron emission. As a result, more electrons are emitted from the weld pool, and fewer from the bypass welding wire. The bypass current in consumable DE- GMAW thus must be much smaller than the base metal current. A similar experiment was repeated by increasing the main wire feed speed to 14m/min (550IPM). It can be seen in Fig. 7 that the bypass current (mean value) was also 106amps while the base metal current was increased to approximately 280amps. Thus, the bypass current was not controllable with the preliminary system, in which only one power supply was used. Hence, to obtain the desired bypass and base metal currents, the use of two power supplies to provide the two currents separately appears an effective solution. The preliminary experiments (Figs. 6 and 7) showed that the total melting current increased a little bit when the bypass welding wire was introduced. High speed videos show that the wire extension of the main wire became shorter after the new current path was established. Thus, the resistive heat produced in the main welding wire was 9 decreased. To maintain a constant arc voltage, the total current had to increase to compensate for the decrease in resistive heat. 4.2 Adjustability of Base Metal Current The experiment in Fig. 8 was conducted using the proposed system shown in Fig. 2 with a set of carefully selected constant welding parameters. As can be seen, the bypass current remained constant during the experiment. This implies that the bypass arc was present during the whole experiment period. Both the base metal current and bypass current waveforms were smooth indicating a stable process. Hence, a stable consumable DE-GMAW process may be obtained if the parameters are carefully selected. The total current varied slightly because of the changes in welding conditions. Fig. 9 and 10 illustrate similar experiments repeated with different welding parameters selected carefully to obtain a stable bypass arc. In Fig. 8, the bypass current was 100amps while the base metal current was over 250amps. When the bypass wire feed speed was increased to 10.2m/min (400IPM) in Fig. 9, the bypass current had to be increased to 165amps to maintain the balance between the wire melting speed and feeding speed. While the bypass current was increased, the base metal current was decreased to approximately 200amps. In that way, the total melting current, which is the sum of the bypass current and base metal current, was not changed. In Fig. 10, the bypass current was increased to 240amps and the base metal current was decreased to approximately 145amps. But the bypass wire feed speed had to increase up to 16.5m/min (650IPM), 10 which is the maximum bypass wire feed speed available for the experimental system used, to maintain the balance between the wire melting speed and feeding speed. The above experiments (Figs. 8 - 10) suggest that a stable consumable DE-GMAW process may be achieved for applications with different base metal currents. 4.3 Local Stability In the experiment shown in Fig. 11, parameters 1V , 1WFS , and 2WFS were set constant but 2I increased gradually. As can be seen, when the bypass current was increasing, the base metal current was decreasing at the same speed, but the total current remained constant. It can also be observed that the bypass voltage 2V increased linearly with a slope of 0.1271 V/A. As was verified by the high speed video, the increase in the bypass arc voltage reflected a backward movement of the tip of the bypass welding wire because of the imbalance between the bypass melting speed and feeding speed. In Fig. 11, the process was stable before time t = 52seconds. After that, the bypass current frequently oscillated to zero. The bypass voltage oscillated to the open-circuit voltage, which was about 64volts. This implies that the bypass arc extinguished from time to time and the process became unstable. Apparently, the instability was because the high cathode heat burned the bypass welding wire at a speed higher than its feeding speed. Because of the continuous feeding of the bypass welding wire, the extinguished bypass arc was re-ignited when the bypass welding wire touched the main arc again. 11 In the experiment demonstrated in Fig. 12, parameters 1V , 1WFS , and 2I were set at constant but 2WFS decreased gradually. It can be seen from the waveforms that the bypass arc voltage 2V was greater than the main arc voltage when the bypass wire feed speed became smaller. In this case, the bypass arc was in the outside region of the main arc and the CC power supply maintained the desired bypass current despite the increased voltage. If 2WFS decreased further, the bypass arc voltage increased abruptly (at t = 45seconds) then the bypass arc became unstable and could be extinguished at any time. Experiments in Figs. 11 and 12 suggest that a stable consumable DE-GMAW process can be achieved when welding parameters are carefully selected. Hence, the process possesses local stability. This makes it possible to perform consumable DE-GMAW without feedback control if the welding parameters are appropriately selected and do not change abruptly. It appears that this local stability is a result of the self-regulation of the bypass arc, considering the resistive heat from the wire extension or stickout (Ref.10). In consumable DE-GMAW, the bypass welding wire is primarily melted by the cathode heat and resistive heat (Ref.26). The resistive heat Q for an extended welding wire with diameter d and extension l can be calculated as 2Q I R= where 2( ) 0.25 lR T d ρ π= (2) where ( )Tρ [ohm x m] is the resistivity at temperature T. At room temperature 20T C= ° , iron has a resistivity of 81071.9)20( −×=°Cρ [ohm x m], but when the welding wire 12 temperature is near to its melting point, the resistivity will increase 16 times (Refs.26, 27) so that the resistive heat becomes significant in the melting of the welding wire. If the welding conditions change slightly and the cathode heat associated with the given bypass current decreases, the extension of the bypass welding wire may increase until the decrease in the cathode heat is compensated for by the increased resistive heat. Hence, this self-regulation capability can allow a stable bypass arc be achieved if the welding parameters are carefully selected and do not change abruptly. However, self-regulation would not be sufficient to compensate for the change in the cathode heat to maintain a stable process. The stability due to self-regulation is thus local and not sufficient. To guarantee a stable process, feedback control is needed to adjust the welding parameters. 4.4 Effects on Total Current and Melting Rate Another observation in Fig. 8 - 10 is that the total melting current with the bypass arc is larger than that without the bypass arc. For example, in Fig. 10, the total melting current without the bypass arc was 335amps, but after the bypass arc was ignited, the total melting current was increased to 385amps, resulting in an increase of 50amps. High speed video reveals that this increase is a result of the increase in the main arc length after the bypass arc was ignited as can be seen in Fig. 13. But even though the arc length and base metal current increased, the arc voltage was not changed because of the CV power supply. With the same CTWD, the wire extension would decrease when the welding arc became longer. Thus, the decrease in resistive heat resulted in a need to increase the melting current to maintain the constant welding voltage. (Recall that the 13 welding wire is melted by both the arc heat and resistive heat.) As a result, the total melting current was increased after the bypass arc was ignited. However, the decoupling principle denoted in Eq. (1) still holds and can be used to develop an algorithm to control the base metal current by adjusting the bypass current. Once the process becomes stable, the total current will remain approximately constant. It should also be noted that consumable DE-GMAW can use energy much more effectively than conventional GMAW, even though there is a small increase in the total melting current. Thus, consumable DE-GMAW also has the advantage of achieving a higher melting rate (mass/c