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DOUBLE-SIDED ARC WELDING INCREASES WELD JOINT PENETRATION Connecting two torches to the two terminals of a single power supply achieves more concentrated arcs that increase weld joint penetration By Y. M. Zhang and S. B. Zhang Joining of components plays a critical role in determining the cost and productivity of manufacturing and the quality of final products. As the primary method for metal joining, welding is practiced in almost all industries. Cost reduction and productivity improvement in welding operation can therefore generate considerable impacts on competitiveness of various manufacturing industries. Plasma Arc Welding Power Supply PA Electrode PA Torch Plasma Arc Work (a) Plasma Jet and Keyhole Current and Arc Zone Current lines Work (b) Plasma Jet Zone (no current and arc) Fig. 1 Conventional plasma arc welding. (a) System configuration. (b) Current flow. Welding time and joint preparation are among the most important factors dominating the cost and productivity of welding. If the desired penetration of the weld pool can be achieved in a single pass, the welding speed will be the major factor that determines the welding time. When the thickness of the part increases, grooves may have to be machined so that the desired penetration may still be obtained in a single pass. In this case, the welding time remains unchanged at the expense of groove preparation. If the thickness of the part further increases, multiple passes may become mandatory in addition to the joint preparation. Both the welding time and the cost for the joint preparation will significantly increase. However, if the penetration capability is increased so that joint preparation and multiple passes be eliminated, the resultant impact on cost and productivity will be quite significant. Laser beam and electron beam welding can produce much deeper penetration than arc welding [1]. However, the costs associated with laser Y.M. Zhang and S.B. Zhang are with the Welding Research and Development Laboratory, Center for Robotics and Manufacturing Systems, University of Kentucky, Lexington, KY. 1 beam and electron beam welding are often unacceptable for many applications. In addition, their requirement on joint machining and fixture is often too high for many industries. In short, compared with the most widely used metal joining method, arc welding, laser beam and electron beam welding achieve deep penetration at the expense of high cost. IMPROVING PENETRATION WITHOUT INCREASING COSTS This work focuses on improving weld penetration without substantial cost increase. Instead of using another welding process, arc welding process will be performed in a novel method. It is known that plasma arc welding (PAW) can achieve deeper penetration than other arc welding because of its more concentrated arc and plasma jet [2]. However, the majority of the welding current in PAW earths through the top surface of the base metal [3] so that only the plasma jet which has been ionized and heated by the arc, rather than the arc itself, can directly penetrate into the keyhole (Fig. 1). If the arc can directly penetrate into the keyhole, the penetration will be significantly increased. Hence, this paper proposes increasing the penetration by placing a second torch from the opposite side of the base metal plate. The two torches are directly connected to the two terminals of the power supply. The resultant welding current loop becomes power supply-torch-work-torch-power supply instead of the conventional power supplytorch-work-power supply loop. As a result, the welding current, thus the arc, is guided into the keyhole (Fig. 2). The weld penetration is significantly increased. In this paper, this arc welding process is referred to as double-sided arc welding (DSAW). It should be pointed out that other Second Electrode approaches have been applied to increase weld penetration during arc welding. One Second Arc approach is to use flux to change the fluid Plasma Arc flow in the weld pool during gas tungsten Work Welding arc (GTA) welding. Significant Power Supply Plasma Arc improvement in penetration has been (Primary Arc) PA observed [4]. Compared with the Torch proposed DSAW, this approach can be (Primary Torch) PA Electrode used without the restriction of back-side accessibility. However, in order to apply Fig. 2 Proposed double-sided arc welding. The second this technology, different fluxes (chemical torch can be either gas tungsten arc, gas metal arc, or composition) must be carefully designed plasma arc, depending on the application. for different materials. Because of the sensitivity of the penetration capability to the amount of the added flux, even coating of the flux is a basic requirement. Another method that has been applied to increase weld penetration involves the use of two independent sets of torch and power supply for simultaneously welding the work from the opposite two sides. Two conventional power supply-torch-work-power supply loops parallelly function and approximately twice penetration is achieved. Although it does not change the mechanism of conventional arc welding where the power supply-torch-work-power supply loop is essential, this approach has found successful applications when the both sides of the work are accessible. Second Torch 2 SETTING UP THE EXPERIMENTS GTA Electrode GTA Torch Gas Tungsten Arc Work Plasma Arc (Primary Arc) Variable Polarity Plasma Arc Welding Power Supply PA Torch PA Electrode Fig. 3 Experimental set-up for feasibility studies. An experimental system has been developed (Fig. 3) to verify the proposed DSAW approach. This system, with gas tungsten arc as the second arc, successfully conducted the proposed DSAW. This verified the feasibility of the proposed DSAW approach. To confirm the effectiveness of the proposed DSAW in improving the penetration, extensive experiments, in comparison with regular variable polarity plasma arc (VPPA) welding, have been conducted using the developed DSAW experimental system. Comparative experiments are conducted on 5052 Al at flat position. Both the shielding gas and plasma gas are pure argon. The flow rate of the plasma gas, (a) (b) Fig. 4 Penetration comparative experiments on 6.5 mm thick plates. the flow rate of the shielding gas for plasma arc, and the diameter No. filler. Bead-on-plate. (a) DSA weld. (95A, 47V, 4.7 mm/s). of the orifice are 2.6 sfch, 30 (b) Regular VPPA weld (100A, 30V, 1.1mm/s). Bead-on-plate. sfch, and 2.57 mm, respectively for both DSAW and regular VPPA welding. The flow rate of the shielding gas for GTA in DSAW is 20 sfch. The duration of positive and negative current in each pulse are all 15 ms, and the amplitudes in positive and negative duration are equal. The arc voltage in DSAW is higher than it is in regular VPPA welding due to the second arc. Fig. 4 illustrates the penetration status of a DSA weld in comparison with a regular VPPA weld. The thickness of the plate is 6.5 mm. For DSAW, the current is 95A and the voltage is 47 V. For VPPA, the current is 100 A and the arc voltage is 30 V. The Fig. 5 DSA butt weld on 9.5 mm thick plate (110 A, 49V, 2.7 welding speed for DSAW is 4.7 mm/s, while the welding speed is 1.1 mm/s for VPPAW. It can be seen that the joint has been well mm/s). No filler. penetrated by DSAW. However, although the power input during VPPAW is approximately 3 times as it is during DSAW, the joint is just penetrated approximately 37 percent by regular VPPAW. 3 (a) (b) Fig. 6 Behaviors of plasma arc in DSAW. Welding current: 90 A. (a)-EN period. (b)-EP period. Fig. 5 shows a DSA weld made on 9.5 mm thick plate. In this case, the welding current is 110 A and the welding speed is 2.7 mm/s. It can be seen that the joint has been well penetrated. However, for regular VPPAW, when the welding speed is 2.5 mm/s, 220 A current is needed in order to achieve a full penetration [5]. Extensive comparative experiments have also been done for stainless steel, mild steel, and other Aluminum alloys. Similar results have always been observed as in Figs. 4 and 5. Experiments confirmed that the proposed DSAW did dramatically improve the penetration in all the cases. PRINCIPLES OF THE PROCESS As it has been stated earlier, by placing the second torch, welding current flows through the keyhole, instead of earthing through the work surface, in the proposed DSAW. As a result, the plasma arc is guided into the keyhole and becomes more concentrated. Hence, the penetration capability is dramatically increased. It is apparent that the welding current must flow through the work during DSAW. Thus, in order to verify the principle, proposed above, of DSAW in improving penetration, only two questions should be answered: does the welding current flow through the keyhole, and does the plasma arc really become more concentrated? Direct observation of the existence of the welding current inside the keyhole is not easy. More work will be directed to resolve this issue. To answer the second question, the plasma arc has been monitored during welding. The images in Fig. 6 shows the behaviors of the plasma arc during electrode-positive (EP) and electrode-negative (EN) periods. It can be (a) (b) Fig. 7 Behaviors of plasma arc in regular VPPAW. Welding current: 90 seen that during EN period, the plasma arc is A. (a)-EN period. (b)-EP period. much more concentrated than it is in a regular VPPAW (Fig. 6(a) and Fig. 7(a)). However, during EP period, the concentration of the plasma arc in DSAW and regular VPPAW is similar (Fig. 6(b) and Fig. 7(b)). It is known that 4 penetration is primarily due to the EN period. Hence, the proposed DSAW is characterized by the very high concentration of its plasma arc. Magnetic force The concentration of the plasma arc can be explained by the welding current direction and its induced magnetic field. In DSAW, the welding current flows approximately normally through the work. Welding current in such a direction generates a magnetic filed (Fig. 8). Under the effect of this magnetic field, the current lines (welding current) are driven towards the axis (Fig. 8). The welding current is concentrated both in the arcs and in the work. Because of the spatial correlation between the welding current and the arc distribution, the arcs are therefore converged. Magnetic force Work Magnetic line Current lines Fig. 8 Current and induced magnetic field. DEPTH-TO-WIDTH RATIO Penetration, width (mm) 9 8 Width (VPPAW) 7 Width (DSAW) 6 5 Penetration (DSAW) 4 Penetration (VPPAW) 3 2 90 92 94 96 98 100 102 104 106 Current (A) Fig. 9 Comparative experiments in penetration depth and width. Material: 5052 Al. Thickness: 6.5 mm. Welding speed for DSAW: 4.7 mm/s, for VPPA: 1.1 mm/s. The width of the weld pool is the primary factor determining the width of the heat-affected-zone (HAZ), thus the material degradation and mechanical properties of the resultant welds. For a particular application, the required depth of the weld pool is given. Thus, the depth-to-width ratio plays the most important role in determining the width of the HAZ. Extensive experiments have been conducted to investigate the depth-towidth distinctions of DSAW in comparison with regular VPPAW. The results are documented in Fig. 9. It can be seen that for regular plasma arc welding, when the welding current increases, the depth and width of the weld pool increase simultaneously. With Filler No Filler The depth-to-width ratio Fig. 10 DSA welds under different conditions. No undercuts are does not significantly observed. change. Consequently, an increase in the penetration 5 can only be achieved at the expense of increasing the width of the weld pool. Such a property is not preferred. For DSAW, when the welding current increases, the penetration depth increases rapidly, while the resultant increase in the width of the weld pool is insignificant. This implies that the increase in the depth of penetration in DSAW is primarily caused by an increased depth-towidth ratio. Hence, increasing penetration will not subject a substantial increase in the width of the weld pool, (a) (b) thus the width of the Fig. 11 Microstructure of DSA weld (2024 Al). HAZ. (a) Weld-HAZ interface. (b) HAZ. DISTORTION Thermal distortion is caused by uneven heat input during welding. It is undesired and should be minimized. It is evident that the DSA heats the work from both sides of the work and tends to reduce the thermal distortion. DISCONTINUATION Undercut and porosity are the two major weld discontinuations associated with PAW. Although porosity may successfully be eliminated by using high pure gas [6], the prevention of the undercut has not been an easy issue [6, 7]. In fact, in regular plasma arc welding, both the plasma pressure and the gravity serve as detaching forces which tend to separate the liquid pool from the solid material. The retaining force, the surface tension between the solid material and the melted metal, tends to prevent the liquid metal from being separated from the solid material. If the retaining force is not sufficient, undercut occurs. In the proposed DSAW, the plasma arc welds underneath. The gravity serves as a retaining force, instead of a detaching force. In addition, the pressure of the second arc adds another retaining force. Hence, the undercut can be prevented. Extensive experiments have been conducted in the feasibility study and preliminary investigation phase of this project. No undercut has been produced despite the arc-blow and electrode erosion (Fig. 10). (a) (b) Fig. 12 Microstructure of VPPA weld (2024 Al). (a) Weld-HAZ interface. (b) HAZ. 6 However, when a regular plasma arc welding is performed, a great care has to be taken in order to prevent the undercut. TENSILE STRENGTH AND DUCTILITY To test the mechanical properties of DSA welds, extensive experiments have been done. The table below summaries the test results for 5454 Al butt welds. Weldment (#) 1 2 3 4 5 Average Yield Strength Ksi MPa 17.2 118.2 18.1 124.4 18.3 126.1 18.9 130.2 18.5 127.6 18.2 125.3 Ultimate Strength Ksi Mpa 36.3 250 35.9 247.7 34.6 238.1 37 255 34 233.8 35.6 244.9 Elongation Percentage for 2 in (50mm) 21.3 18.9 16 17.4 16.8 18.1 Failure Location Base Metal Base Metal HAZ HAZ HAZ It can be seen that there is no significant distinction in the ultimate strength between DSA welds and annealed base metal. (The ultimate strength is 36 Ksi for the annealed 5454 Al [8]). The average ultimate strength is slightly better than the ultimate strength in gas-shielding arc welding which is 34 Ksi [9]. The final failure occurs either in base metal or HAZ. The yield strength is also higher than it is in the annealed base metal (17ksi) [8]. It is clear that the tensile strength of DSA welds can be regarded as the same as the annealed base metal and is slightly better than regular arc welds. Although the ductility test reading (18.1%) is slightly lower than the similar reading in annealed base metal, it is higher than the ductility of regular arc welds (17%) [9]. Hence, the mechanical properties of DSA welds are quite satisfactory. MICROSTRUCTURE Fig. 13 Microstructure of 2024 Al base metal. The mechanical properties are primarily determined by the microstructure. Figs. 11 and 12 give the microstructures at the weld-HAZ interface and in the HAZ for the DSA weld in Fig. 4(a) and for the regular VPPA weld in Fig. 4(b), respectively. The microstructure in the base metal is shown in Fig. 13. Although the grains have increased after welding (Figs. 11 and 13), finer grains, in both the HAZ and the weld zone, have been observed than in regular VPPA weld. Such finer grains are mainly due to the narrowed weld and HAZ, thus shorter heat-treatment period. Hence, the microstructure explained the satisfactory mechanical properties of DSAW. 7 APPLICATION ISSUES There are a number of issues related to applying the DSAW to production. The first issue is the accessibility of both sides of the work. Although many applications do not meet the requirement of accessibility, the cases where both sides of the work are accessible are still not limited due to the diversity of applications. For example, the external tank of the space shuttle and many pipes can be accessed from both sides during welding. Because the arcs exist on the both sides, the arc voltage is increased in comparison with regular arc welding. As a result, the nominal maximum current of the used power supply may not be achieved. Our experiments were conducted using a 200 A variable polarity plasma arc welding power supply. However, because of the higher arc voltage, the actually achievable welding current is only approximately 110 A during DASW. The authors plan obtaining a more powerful power supply to weld thicker materials. Other application related issue is the alignment of the two torches during welding. To ensure an adequate joining, the joint must be throughoutly fused by the weld pool. For thin material joining where the size of the weld is small, the two torches must be well aligned. However, for thick material, a larger error in the alignment of the two torches may be permitted. Basically, the required accuracy for the alignment is close to the accuracy required on seam tracking. CONCLUSIONS • • • • • An experimental set-up has been established to verify the proposed DSAW approach. It is shown that the proposed DSAW configuration works. Experiments demonstrated the effectiveness of the proposed DSAW in improving depth-to-width ratio and penetration. Preliminary studies found that the plasma arc during DSAW becomes much more concentrated. Although more studies should be conducted to better understand the principle of DSAW in improving penetration, this finding has provided a convincing support to explain the experimental results of DSAW in improving penetration. Preliminary studies have also demonstrated the merits of DSAW in decreasing HAZ, eliminating undercuts, reducing thermal distortion, and achieving satisfactory microstructure and mechanical properties. A significant restriction of DSAW is the mandatory requirement on the double side accessibility of the work. Because of the wide application of arc welding in manufacturing and application diversity, the cases where both sides of the work are accessible are still not limited. REFERENCES 1. Laser Welding Handbook. 8th edition, Vol. 2: Welding Processes, AWS, 1991. 2. E. Craig, 1988. “The plasma arc welding-a review,” Welding Journal, 67(2): 19-25. 8 3. J. Dowden, P. Kapadia, and B. Fenn, 1993. “Space charge in plasma arc welding and cutting,” Journal of Physics (D): Applied Physics, Vol. 26: 1215-1223. 4. T. Paskell, C. Lundin, and H. Castner, 1997. “GTAW flux increases weld joint penetration,” Welding Journal, 76(4): 57-62. 5. M. Tomsic and Barhorst, 1984. “Keyhole plasma arc welding of aluminum with variable polarity power,” Welding Journal, 63(2): 25-32. 6. M. R. Torres, J. C. McClure, A. C. Nunes, and A. C. Gurevitch, 1992. “Gas contamination effects in variable polarity plasma arc welded aluminum,” Welding Journal, 71(4): 123s-131s. 7. R. Hou, D. M. Evans, J. C. McClure, A. C. Nunes, and G. Garcia, 1996. “Shielding gas and heat transfer efficiency in plasma arc welding,” Welding Journal, 75(10): 305s-1310s. 8. B. Irving, 1994. “Welding the four most popular aluminum alloys,” Welding Journal, 73(2): 51-55. 9. J. R. Davis (Editor), 1994. ASM Specialty Handbook: Aluminum and Aluminum Alloys, ASM International, 376-415. 9