KEYHOLE DOUBLE-SIDED ARC WELDING PROCESS FOR DEEP NARROW PENETRATION

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1 KEYHOLE DOUBLE-SIDED ARC WELDING PROCESS FOR DEEP NARROW PENETRATION Y. M. Zhang and S. B. Zhang Welding Research and Development Laboratory Center for Robotics and Manufacturing Systems and Department of Electrical Engineering University of Kentucky, Lexington, Kentucky ABSTRACT In regular arc welding, the arc heat is transferred from the surface of the workpiece to melt it. In keyhole plasma arc welding, the workpiece is melted by the plasma jet which heats the wall of the keyhole along the thickness direction. Penetration is achieved with reduced heat input. However, the plasma jet only gains its energy in the arc before reaching the keyhole. In the keyhole where no current/arc is present, the plasma jet consumes its energy without compensation. This heat generation/consumption mechanism limits its penetration capability. In the proposed method, the current/arc is guided through the keyhole so that the energy of the plasma jet is compensated while it is consumed in heating the workpiece along the keyhole. As a result, deep narrow penetration has been achieved on 12.7 mm (1/2") thick stainless steel plates using 70 A welding current. INTRODUCTION Deep narrow penetration often implies reduced heat input, reduced thermal distortion, reduced consumption of filler metal, reduced number of passes, improved productivity, improved weld quality, and an increased window of welding parameters or improved process robustness against variations in welding conditions and welding parameters. To date, laser and electron beam welding have been the primary processes for achieving deep narrow penetration. Their applications have been limited by their high costs and incapability of handling large structures. To find economic alternatives for achieving deep narrow penetration, flux has been added to change the fluid flow in the weld pool during gas tungsten arc (GTAW). Significant improvement has been observed on penetration [1]. Investigations are currently being conducted to develop flux for different materials, study and improve the weld quality, and improve the welding speed [2]. It is known that keyhole plasma arc welding (PAW) achieves deeper penetration than all other arc welding processes because of its concentrated arc and its plasma jet [3]. However, in PAW the majority of the welding current is grounded through the top surface of the base metal [4] so that only the plasma jet which has been ionized and heated by the arc, rather than the arc itself, can directly extend into the keyhole (Fig. 1). Thus, when the plasma jet penetrates the workpiece, it consumes its energy to heat the workpiece without any compensation. If there exists a method to compensate for the energy of the plasma jet consumed to melt the metal during penetration, its penetration capability will be significantly increased. 1

2 Work Work PA Electrode Plasma Jet and Keyhole PA Torch Current and Arc Zone PAW Torch (a) Plasma Arc Welding Power Supply Current lines Plasma Arc Work Plasma Jet Zone (b) (no current, no arc) Fig. 1 Conventional plasma arc welding. (a) System configuration. (b) Current Plasma flow. Arc Welding Power PAW Electrode Supply Arc, Plasma Jet and Keyhole Arc GTAW Torch Fig. 2 Proposed keyhole double-sided arc welding. This paper proposes compensating for the consumed energy of the plasma jet by directing the welding current, thus the arc, through the keyhole. In fact, if the welding current can flow through the keyhole, the arc will directly extend into the keyhole. In this way, the energy consumed to melt the workpiece will be compensated. If the compensation rate can be kept no less than the consumption rate, the weld pool may remain narrow and nearly constant. As a result, deep and narrow penetration will be achieved. To realize the above idea, the authors propose disconnecting the workpiece from the power supply and placing a GTAW torch on the opposite side of the base metal plate as a second torch. The two torches are directly connected to the two terminals of the power supply. In this way, the welding current from the PAW torch is forced to flow through the keyhole to reach the GTAW torch, instead of being grounded through the workpiece surface. As a result, the welding current, thus the arc, can be forced to flow into the keyhole (Fig. 2). The corresponding process is referred to as double-sided arc welding (DSAW). When the keyhole fully penetrates through the whole workpiece along the thickness direction, the process is called keyhole double-sided arc welding. 2

3 Fig. 3 Keyhole Double-sided Arc Welds on 9.5 mm (3/8 inch) Stainless Steel Made Using Moderate Heat Input. Welding position: flat, material: stainless steel (304), thickness: 9.5 mm (3/8 inch), welding current: 67A, travel speed: 1.3 mm/s (3.1 inch/min). EXPERIMENTATION To verify the effectiveness of the proposed method in achieving deep narrow penetration, an experimental setup as shown in Fig. 2 has been developed to implement the keyhole DASW process. In particular, a DC welding power supply was modified to meet the requirement of the keyhole DSAW on voltage. (Higher voltage is needed because of the long arc along the keyhole.) The welding current was kept constant during experiments. A PAW torch and GTAW torch were connected to the negative terminal and positive terminal of the power supply, respectively. The workpiece was disconnected from the power supply. Table 1 Invariant Welding Parameters Orifice diameter 1.57 mm (0.062 inch) Electrode diameter: PAW torch 4.8 mm (3/16 inch) Electrode diameter: GTAW torch 4.8 mm (3/16 inch) Flow rate of plasma gas 1.15 L/min (2.5 ft 3 /h ) Flow rate of shielding gas (Plasma torch) 13.8 L/min (30 ft 3 /h ) Flow rate of shielding gas (GTAW torch) 23 L/min (50 ft 3 /h ) Stand-off (PAW electrode) 6 mm (0.24 inch) Stand-off (GTAW electrode) 10 mm (0.38 inch) Welding Voltage Approx. 45 V 3

4 Stainless steel plates were butt welded at both flat position and vertical-up position. Argon was used as shielding gas for both the PAW torch and the GTAW torch as well the plasma gas. Plates up to 12.7 mm (1/2 inch) thick were keyhole double-sided arc welded. Because of the use of DC power supply, the welding current has been limited to 70 A in order to avoid consuming the tungsten electrode of the GTAW torch. Table 1 lists the welding parameters which were invariant in the experiments. Other welding parameters, including the welding current and the travel speed, varied from experiment to experiment. RESULTS AND DISCUSSION Flat Position Fig. 4 Excessive Depression on Thick Plate using flat position. Method: keyhole double-sided arc welding, material: stainless steel (304), thickness: 12.7 mm (1/2 inch), welding position: flat, welding current: 69A, travel speed: 0.83 mm/s (2.0 in/min). Fig. 3 shows two keyhole double-sided arc welds on 9.5 mm (3/8 inch) thick stainless steel. The welding speed was 1.3 mm/s (3.1 inch/min). It can be seen that deep narrow penetration has been achieved. In fact, the width of the weld zone in the middle portion along the thickness direction remains in the range of 2 mm to 3 mm. Because of this very narrow width, the Fig. 5 Vertical-Up Keyhole Double-sided Arc Welds on 12.7 (1/2 inch). Position: vertical-up material: stainless steel (304), thickness: 12.7 mm (1/2 inch), welding current: 70A, travel speed: 0.83 mm/s (2.0 in/min). melted metal was maintained at a low level and sustained from generating burn-through despite the great thickness and the impact of the plasma jet. It is known that deep narrow penetration is the most distinguishing characteristic of laser and electron beam welding. For 9.5 mm (3/8 inch) thick steel, a 10 KW laser beam achieves penetration as narrow as 2 mm [5]. Hence, laser welds are narrower than the keyhole double-sided arc welds such as those shown in Fig. 3, but keyhole DSAW does achieve deep narrow penetration close to that obtained by laser welding. Fig. 4 shows the weld made on a 12.7 mm (1/2 inch) thick stainless steel. It can be seen that in comparison with those on 9.5 mm (3/8 inch) thick plates, the depression of the weld 4

5 pool has been significantly increased. Also, the weld zone on the bottom (GTAW torch) side has become much larger. This was due to the increase in the thickness of the workpiece, thus the increase in the amount of melted metal in the weld pool. If the shape of the weld bead on the bottom surface is acceptable for particular applications, a second pass must follow to fill the depression on the top side of the workpiece in order to achieve acceptable reinforcement. Vertical-Up Position It can be seen that, although keyhole DSAW achieves deep narrow penetration, the depression of the weld pool does increases as the thickness of the workpiece increases. This is of course unavoidable because of gravity. To minimize the depression, vertical-up position may be used. As can be seen in Fig. 5, the use of vertical-up position has significantly changed the shape of the weld bead. In particular, the weld depression on the plasma torch side is much smaller than it is at flat position. In fact, at vertical-up position, the effect of gravity on weld depression is insignificant. Hence, only the impact of the plasma jet remains as a major factor determining the weld depression. As a result, the depression was reduced in comparison with flat position. The undesirable effect of thickness on the weld depression is eliminated. Such an elimination helps increase the thickness of material at which the keyhole DSAW process may apply. Robustness Fig. 6 shows another keyhole double-sided arc weld on 9.5 mm (3/8 inch) thick stainless steel made with a lower travel speed (1 mm/s). In comparison with the welds in Fig. 3, the heat input was increased by 30 percent. As a result, the melted metal in the weld pool and the depression of the weld pool on the bottom surface both increased. Of course, the larger depression of the weld pool on both the top and bottom surface observed in Fig. 6 is in general less desirable than those in Fig. 3. However, no burn-through occurred despite a 30 percent increase in heat input. This implies that the keyhole double-sided process is less sensitive to the heat input than are most other arc welding processes. For example, for 3 mm thick stainless steel plates, when the welding speed is fixed at 2 mm/s, the range of the welding current which can achieve a full penetration without burn-through for butt welding without filler metal is approximately from 115 A to 130 A. The window of the heat Fig. 6 Keyhole Double-sided Arc Welds on 9.5 mm (3/8 inch) Stainless Steel Made Using Excessive heat input. Welding position: flat, material: stainless steel (304), thickness: 9.5 mm (3/8 inch), welding current: 67A, travel speed: 1 mm/s (2.3 in/min). input is much smaller than it is in the keyhole DSAW process. A large window of welding parameters offers great advantages in practical applications. The robustness of the double-sided process with respect to welding parameters has also been observed for vertical-up position on 12.7 mm (1/2 5

6 inch) thick plates. In particular, the weld in Fig. 7 was made by using a speed 20 percent slower than used in Fig. 5. As expected, although the width of the weld zone increased from approximately 3 mm to 4.5 mm, the geometry of the welds in both cases appears acceptable for root pass. Hence, the process is robust with respect to variation in heat input. Fig. 7 Vertical-Up Keyhole Double-sided Arc Weld Made with Excessive Heat Input. Position: vertical-up material: stainless steel (304), thickness: 12.7 mm (1/2 inch), welding current: 70A, travel speed: 0.67 mm/s (1.6 in/min). The keyhole DSAW is also expected to be insensitive to the variation in minor elements in the base metal as can be seen in other keyhole processes. In fact, in nonkeyhole arc welding processes such as GTAW and GMAW, the arc heats the surface of the base metal. The heat from the arc is transferred to the workpiece. In addition to the amount of the heat input, heat transfer inside the weld pool plays a critical role in determining the penetration. It has been found that some minor elements significantly change the fluid flow which dominates the heat transfer in arc welding [6-8]. Hence, for non-keyhole arc welding processes, certain casts, which meet the required material specifications, have been found to produce joints which differ significantly from the norm with respect to weld penetration [9]. In keyhole DSAW, the arc heats the workpiece along the keyhole. The fluid flow plays much less critical role in transferring heat and determining the penetration. The possible effect of minor elements is thus minimized. Keyhole Mode The proposed keyhole DSAW is based on the assumption that the welding current flows through the keyhole to ensure the presence of the arc in the keyhole. Although the configuration shown in Fig. 2 can direct the welding current through the workpiece, it is not ensured that the keyhole be the only pass for the current to flow. A portion of current may flow through the metal around the keyhole. When the current flows through the metal, it generates no arc and no heat compensation. It is desired that all the current flux be directed through the keyhole to generate arc for heat compensation. Analysis shows that if the keyhole only partially extends into the workpiece, the two arcs from the two sides will be separated by metal which has not been penetrated by the keyhole. In this case, a portion of current may flow through the metal surrounding the keyhole from one torch to another. When the keyhole fully penetrates through the workpiece, a continuous arc will be established between the two torches. As a result, two shorter arcs become one longer arc. Two cathodes and two anodes reduce to one cathode and one anode. In most cases, the sum of the voltage drops in the cathode and anode is approximately 10 V. On the other hand, the ratio between the voltage drop 6

7 and the arc length in the arc column is approximately 0.5 V/mm. (Assume that the shielding gas is argon and the current is 200 A). When the thickness is 12.7 mm (1/2 inch), the voltage drop of the keyhole is approximately 6.5 V, lower than the sum of the voltage drops in one cathode and one anode. Hence, the current will tend to flow only through the keyhole. Assume that the thickness further increases so that the voltage drop along the keyhole is higher than the sum of the voltage drops in the cathode and the anode. In this case, the electrons emitted from the tungsten electrode of the PAW torch which hit the metal (either melted or unmelted) on the top of the workpiece will flow through the metal and generate an anode and cathode when they enter into the base metal and emit from the base metal respectively. However, for the electrons which hit the metal below the top surface of the workpiece, the equivalent thickness of material is reduced. For example, for a 25.4 mm (1 inch) thick workpiece, if some electrons hit the base metal 5.4 mm below the top surface, the equivalent thickness of the workpiece corresponding to these electrons will be 20 mm. If the voltage drop in the 20 mm long arc column is lower than the sum of an anode voltage drop and a cathode voltage drop, these electrons will flow only through the Fig. 8 Non-keyhole double-sided arc weld. Method: double-sided arc welding, material: stainless steel (304), thickness: 12.7 mm (1/2 inch), welding position: flat, welding current: 69A, travel speed: 0.83 mm/s (2.0 in/min). keyhole. It is known that both the plasma gas and the direction of the current, from one torch to another, tend to concentrate the current (electrons). The electrons thus tend to hit the metal of the workpiece below the top surface. Hence, the equivalent thickness is typically smaller than that of the workpiece. As a result, the actual thickness which allows the current flow through the keyhole to maintain the keyhole mode increases. It is predicted that the keyhole mode may be achieved on 19 mm (3/4 inch) to 25.4 mm (1 inch) thick plates. If the keyhole does not fully penetrate the workpiece, the current will have to go through the metal on its way from one torch to another. It was found that, although the keyhole does not fully penetrate the workpiece to establish a keyhole mode in double-sided arc welding, the plasma jet is significantly concentrated because the current flows through the workpiece approximately normally [10-12]. As a result, both the arc and the plasma jet become more concentrated than they are in regular plasma arc welding. Hence, as reported earlier [11, 12], non-keyhole DSAW does improve 7

8 (A) (B) Fig. 9 Arc Behavior on GTAW Torch Side in Double-Sided Arc Welding. (A) Keyhole Mode (B) Non-keyhole Mode penetration in comparison with regular plasma arc welding. However, the improvement in the penetration is not as significant as it is in keyhole DSAW. Fig. 8 shows a weld made using non-keyhole DSAW. The parameters were exactly the same as they were for the weld in Fig. 4. It can be seen that despite the use of the same heat input for the welds in Fig. 8 and Fig. 4, the mode of the penetration has made a significant difference in penetration depth and weld shape. In fact, the area of the weld zone in the two welds does not have a significant difference, but the difference in penetration is significant. In addition, porosity is often observed in welds made using non-keyhole mode as the one shown in Fig. 8. (The keyhole is the primary channel for bubbles to escape [13].) Hence, in order to achieve deep narrow penetration and to minimize the porosity, keyhole mode must be ensured. 8

9 Keyhole mode can be monitored by observing the arc behavior on the GTAW torch side. As can been seen in Fig. 9, after the keyhole fully penetrates the whole section of the base metal, the plasma jet can be observed from the GTAW torch side. If the workpiece is not fully penetrated, the plasma jet is invisible from the GTAW torch side. In addition, if the plasma jet fully penetrates the workpiece, the arc on the GTAW torch side is subject to the impact of the plasma jet as can be seen in Fig. 9. Hence, it is possible to monitor and control the process to ensure the keyhole mode for DSAW. Future Work and Perspectives Although keyhole DSAW does demonstrate certain advantages in deep penetration, this process is currently not well understood. Further research is needed to understand this process and determine its characteristics. Certain issues such as penetration limitation, process modeling, and grain structure are currently being investigated. Most importantly, this study has been conducted using a DC power supply. The tungsten electrode of the GTAW torch, 4.8 mm (3/16 inch) in diameter, has been positively connected. To avoid melting of this tungsten electrode, the current used in this study has been limited to 70 A. Using this current, 6.4 mm (1/4 inch) thick plates can be penetrated at a travel speed of 2 mm/s (4.3 inch/min) which is typical for GTAW. However, for 12.7 mm (1/2 inch) plates, the travel speed reduces to 0.83 mm/s (2 inch/min). To increase the welding current without melting the tungsten electrode of the GTAW, the authors are currently investigating the use of large diameter electrode and modified GTAW torch. An AC power supply is also being developed to meet the requirements of the keyhole DSAW on voltage. It is expected that with a modified torch and a larger electrode or an AC power supply, the permitted welding current will increase to up to 200 A without melting the electrode. The welding speed is expected to increase by 200 to 250 percent. That is, the welding speed would increase to 6 mm/s (14 inch/min) for 6.4 mm (1/4 inch) thick stainless steel, and to 2.5 mm/s (6 inch/min) for 12.7 mm (1/2 inch) thick stainless steel. CONCLUSIONS The proposed keyhole DSAW process is capable of achieving deep narrow penetration on thick plates. In particular, 9.5 mm (3/8 inch) and 12.7 mm (1/2 inch) thick stainless steel plates have been fully penetrated using the keyhole DSAW with relatively small amperage, less than 70A. Also, as observed from in the experiments, the keyhole DSAW process is robust with respect to variations in heat input, thus welding conditions. Preliminary analysis suggests that the presence of the arc in the keyhole which results in a heat compensation mechanism is the key to achieving such deep narrow penetration. However, to truly understand the principles of the process such as the penetration mechanism and the conditions for keyhole mode, further research is needed. In addition, efforts should also be directed to practical issues such as welding speed improvement, power supply development, and torch alignment. 9

10 ACKNOLEDGMENT This work is funded by the National Science Foundation under Grant DMI and the Center for Robotics and Manufacturing Systems at the University of Kentucky. The authors would like to also thank The Lincoln Electric Company, Thermal Arc, Inc., and NASA Marshall Space Flight Center for financial, equipment, and material support. Finally, the authors wish to thank Mr. Warren Mayott at The Electric Boat and Dr. Arthur Nunes at Marshall Space Flight Center for fruitful technical discussions. REFERENCES 1. T. Paskell, C. Lundin, and H. Castner, GTAW flux increases weld joint penetration, Welding Journal, 76(4): M. Johnson, C. Fountain, and H. Castner, GTAW fluxes for Nickel based alloys, Abstracts of Papers, American Welding Society, pp E. Craig, The plasma arc welding: A review, Welding Journal, 67(2): J. Dowden, P. Kapadia, and B. Fenn, Space charge in plasma arc welding and cutting, Journal of Physics (D): Applied Physics, Vol. 26: Welding Handbook. 8 th edition, Vol. 2: Welding Processes, AWS, 1991, P A. Matsunawa, and T. Ohji, Role of surface tension in fusion welding, Transactions of Japan Welding Research Institute. Part 1, Vol. 11(2): , 1982; Part 2, Vol. 12(1): , 1983; Part 3, Vol. 13(1): , B. Pollard, Effects of minor elements on the welding characteristics of stainless steel, Welding Journal, Vol. 67: 202s-213s. 8. W. F. Savage and D. W. Walsh, Technical note: autogenous GTA weldments-bead geometry variation due to minor elements, Vol. 64(2): 59s. 9. K. C. Mills and B. J. Keene, Factors affecting variable weld penetration, International Materials Reviews, 35(4): Y. M. Zhang and S. B. Zhang, Method of arc welding using dual serial opposed torches, approved for U.S. Patent. 11. Y. M. Zhang and S. B. Zhang. "Welding aluminum alloy 6061 with opposing dual torch GTAW process," Welding Journal, 78(6): 202s-206s, Y. M. Zhang, S. B. Zhang, Double-sided arc welding for increasing weld joint penetration, Welding Journal, 77(6): 57-61, M. R. Torres, J. C. McClure, A. C. Nunes, and A. C. Gurevitch, Gas contamination effects in variable polarity plasma arc welded aluminum, Welding Journal, 71(4): 123s-131s. 10