MANIPULATING THE SAW VOLTAGE AND CURRENT WAVEFORMS TO CONTROL WELDING PRODUCTIVITY AND QUALITY

Transcription

1 MANIPULATING THE SAW VOLTAGE AND CURRENT WAVEFORMS TO CONTROL WELDING PRODUCTIVITY AND QUALITY

2 MANIPULATING THE SAW VOLTAGE AND CURRENT WAVEFORMS TO CONTROL WELDING PRODUCTIVITY AND QUALITY J. Pepin 1, H. Henein 2, D.G. Ivey 2, M. Yarmuch 3 1 PCL Industrial Constructors Inc Street Nisku, Canada, T9E 7W6 (*Corresponding author: jtpepin@pcl.com, ) 2 University of Alberta, Edmonton, Alberta, Canada 7 th Floor, ECERF Building, Street University of Alberta Edmonton, Alberta, T6G 2V4 3 Weldco Companies St NW Edmonton, Alberta, T5V 1J3 Abstract The submerged arc welding (SAW) process uses relatively high current values and large diameter consumable electrodes to achieve high productivity, good weld quality, and desirable bead profiles. As a result, SAW is used in a wide range of applications, especially with thick-walled components. To achieve even greater deposition rates while maintaining weld quality, it is necessary to better understand the effects of voltage and current waveform manipulation, as well as the wide range of variables that can contribute to the overall waveform shapes. Earlier investigations have shown that when manipulating AC waveforms, traditional heat input is not an ideal parameter for predicting weld deposition rates. However, by separating the heat input into the components supplied during the positive and negative polarity phases of the AC cycle, superior trends are revealed. This paper will further explore the concept of polarity-specific heat input, as well as some of the interdependent relationships that several variables can have on the waveforms, which in turn may obscure their true contributions to overall productivity. As predicted, maximum wire feed speed (WFS) can be achieved by minimizing both balance and offset. However, unlike the previous work, it was determined that maximum base metal penetration was achieved by maximizing balance but minimizing offset.

3 Introduction and Background Information Polarities and Waveforms Due to high productivity, deep penetration and good quality, submerged arc welding (SAW) is an attractive process for fabricating large diameter and/or thick-walled components. During SAW, the welding arc is established beneath a powdered flux; as a result, the process is predominantly limited to the flat welding position, which reduces the possible applications of this productive, high quality process. Furthermore, while the high currents used during welding permit high welding deposition rates and the fusion of relatively thick materials, they can limit the use of SAW to only thicker welded components (due to the risk of burn-through with thinner base materials). SAW is most commonly used with direct current in conjunction with electrode positive polarity * because of its good arc stability and deep penetration. However, to increase the weld deposition rates, electrode negative polarity may be used, though the penetration depth is reduced and specialized wire-flux combinations are required to overcome the polarity s inherent poor arc stability. Alternating current has been investigated [1], but the traditional sinusoidal current and voltage waveforms are not suitable, as during each polarity-shift cycle, peak current is only temporarily achieved (resulting in reduced deposition rates and base metal penetration) and a significant portion of each cycle is at a prohibitively low voltage (resulting in poorer arc stability). The use of newer inverter power sources allows for welding with a square-wave current and voltage waveforms (referred to henceforth as AC-SQ polarity). AC-SQ ideally undergoes near-instantaneous polarity shifts, which minimizes the arc time at low voltage and maximizes the arc time at peak current [1]. As a result, a default AC-SQ waveform should yield a greater deposition rate than direct current electrode positive (DCEP), but a deeper bead profile penetration and a more stable arc than direct current electrode negative (DCEN). By using the constant current mode of operation, the welding operator can control voltage and current values; any waveform variable manipulation then affects only the wire feed speed (WFS). The common waveform variables include balance, offset, and frequency: Balance is the percentage of each AC cycle spent at EP polarity (Figure 1). As a result, welding using DCEP is the equivalent of welding using AC-SQ with 100% balance; DCEN is likewise the equivalent of 0% balance. The available literature indicates that increasing the balance should increase the amount of base metal fusion (and the depth of penetration), but comes at the cost of achieving a lower WFS [2, 3]. Offset is the percentage increase or decrease to the peak current during the EP and EN phases of the AC cycle (Figure 2). Setting an offset of +x% should increase the peak current during EP polarity by x% and reduce the peak current during EN polarity by x%. Alternatively, setting an offset to a negative value will reduce the EP peak current and increase the EN peak current. The available literature indicates that increasing the offset should reduce the WFS while increasing the amount of base metal fusion [2, 3]. * Electrode positive (EP) polarity occurs when the electrons flow from the base metal towards the consumable electrode. Electrode negative (EN) polarity occurs when the electrons flow from the consumable electrode towards the base metal.

4 Frequency is a measure of the number of complete AC cycles per second (Figure 3). The available literature provides conflicting accounts of the effects of frequency on WFS and penetration, though there is general agreement that the effects should be quite minor [4, 5, 6]. As a result, the effects of frequency are not investigated in this work. Figure 1 Schematic of Current Waveforms with Low and High Balance Settings [7] Figure 2 Schematic of Current Waveforms with Low and High Offset Settings [7] Figure 3 Schematic of Current Waveforms with Low and High Frequency Settings [7] Gupta et al have indicated that polarity effects can become more apparent as the current density is increased (i.e., by using a greater current for a given electrode size, by reducing the electrode size for a given current, or by switching from a solid electrode to a cored electrode for a given electrode size / current amplitude combination) [2]. By extension, the effects of balance, offset and frequency should become more pronounced at greater current density values. Traditional Heat Input, Instantaneous Heat Input, and Polarity-Specific Heat Input Production welding conditions are often controlled by limiting the nominal heat input (Equation 1) [8]. When using direct current polarity (either DCEP or DCEN), welding conditions are controlled by manipulating the primary heat input variables: voltage, current, and travel speed. If the travel speed is measured in mm s -1, heat input is calculated as kj of energy per linear mm of weld. (Eq. 1) However, waveform manipulation can cause rapid changes to the current and voltage during welding, which can render the traditional heat input formula inaccurate [9]. Furthermore, the literature [4, 5, 10] shows that polarity has a significant effect on welding deposition rates (i.e., WFS), the weld bead profile, and the heat

5 affected zone. Combined with the fact that Equation 1 does not incorporate the effects of polarity, it becomes clear that the traditional heat input equation is insufficient for AC-SQ SAW. To account for the shortcomings of Equation 1, previous analysis was performed to determine the current waveform areas under the curve that could be attributed to the EP and EN phases of the AC cycle (Equations 4 and 5, and Figure 4) [7]. (Eq.2) (Eq.3) Figure 4 Schematic of a manipulated AC waveform, illustrating the current to calculate polarity-specific heat input [7] By extension, polarity-specific heat input formulae were produced for the EP and EN phases of the AC cycle (Equations 4 and 5 respectively) [7]. (Eq.4) (Eq.5) Equations 4 and 5 were produced using the following assumptions: Current and voltage waveforms are perfectly square (i.e., each polarity shift is instantaneous). Because the waveforms are perfectly square, frequency should have no impact on WFS or bead profile. Peak current and peak voltage (if offset is set to zero) will be the same regardless of the polarity. The above formulae were used to analyze a series of variable AC waveform welds, and were then compared against recorded WFS values [7] and used to calculate the WFS contribution from the EP and EN polarities (Figure 5). Figure 6 was then produced to more closely demonstrate the good match between the actual and projected WFS values of Figure 5.

6 WFS (mm/s) WFS = 27.22* (EN Heat Input) R² = 1 Actual WFS = 45.54* (Total Heat Input) R² = 0.70 Superimposed WFS = 47.53*(Total Heat Input) R² = WFS = 19.93*(EP Heat Input) 0.04 R² = Heat Input (kj/mm) WFS (EP Heat Input) WFS (EN + EP Heat Input) WFS (EN Heat Input) WFS (Actual) Figure 5 The effect of polarity-specific heat input on WFS values, during Constant Current operation of AC-SQ SAW [7] WFS (mm/s) Actual WFS = * (Total Heat Input) R² = Superimposed WFS = * (Total Heat Input) R² = Heat Input (kj/mm) WFS (EN + EP Heat Input) WFS (Actual) Figure 6 The effect of combined heat input on WFS values, during Constant Current operation of AC-SQ SAW [7] Figure 5 shows several sets of data. - Blue diamonds illustrate the isolated effect of EP heat input on WFS (for a range of heat input values). - Red squares illustrate the isolated effect of EN heat input on WFS (for a range of heat input values). - Purple circles illustrate the actual recorded traditional heat input values and corresponding WFS values.

7 - Green triangles were produced using the theoretical EP and EN heat input values of the actual test welds (represented by the purple circles) in combination with the curve-fit equations from Figure 5. Therefore, the green triangles illustrate the predicted WFS values for the actual AC-SQ test welds, given their theoretical EP and EN heat input values. The actual AC-SQ welds (i.e., the purple circles) demonstrated a roughly linear relationship between traditional heat input and actual WFS. However, in contrast, when the heat input values were split into their EP and EN constituents (i.e., the blue diamonds and red squares respectively), stronger linear relationships were revealed. When the polarity-specific heat input values of the actual AC-SQ welds were used to predict the WFS values (i.e., the green triangles), they closely matched the actual WFS values (always falling within 10% of the actual measured WFS values). This work clearly showed that strong linear relationships between polarity-specific heat input and corresponding WFS could be used to accurately predict the actual WFS value of AC-SQ welds. The same methodology was used to produce a relationship between polarity specific heat input and weld bead penetration area (i.e., the expected cross-sectional area of melted base metal, measured in mm 2 ) (Figure 7). Normalized Area PEN (mm 2 ) Area PEN = 31.26*(EP Heat Input) R² = Area PEN = 21.68* 20 0 (EN Heat Input) 0.10 R² = Normalized and Adjusted Heat Input (kj/mm) Normalized and Adjusted EP Heat Input Normalized and Adjusted EN Heat Input Figure 7 The effect of polarity-specific heat input on penetration area (from data using a travel speed of 10.6 mm/s) [7] Note that by multiplying the curve-fit equations from Figure 6 by the travel speed (10.6 mm/s), it is possible to approximate the volume of base metal that is melted each second (in mm 3 /s). As a result, we are left with Equations 6 and 7: (Eq.6) (Eq.7) Experimental Procedure

8 A series of bead-on-plate welds were performed, using the AC-SQ SAW process. Constant current mode was used for all of the welds, so that WFS would be manipulated by the waveform variables while the current and voltage values could be held constant (ideally). All welds were performed using standard copper contact tips, with a contact-tip-to-work distance (CTWD) of roughly 1 in. (25.4 mm). The heat input variables (voltage, current, and travel speed) and waveform variables (polarity, balance, offset, and frequency) were varied. A Lincoln Electric AC/DC 1000 power source was used with a 3.2 mm (1/8 in.) EN S3 NiCrMo2.5 solid wire with a basic flux. Welds were performed on 11.1 mm (0.438 in.) thick API 5L X70 steel plate. Welding was performed using CC mode, with input settings of 28.0 volts, 550 amps, and a 10.6 mm/s (25 ipm) travel speed (i.e., with a nominal power of 15.4 kw and a nominal traditional heat input value of 1.5 kj/mm). Waveform variables were manipulated to investigate all combinations of key waveform variables: balance (25%, 50%, 75%), offset (-15%, 0%, +15%) and frequency (30 Hz, 60 Hz, 90 Hz). Lincoln Electric Command Centre (version ) was used to record voltage, current and WFS values for each weld, with the current and voltage values subsequently used to calculate traditional and polarity-specific heat input values. Tables 1 and 2 summarize the weld input, traditional and polarity-specific heat input, and WFS values. Weld ID Table 1- Input and Output Variables of the Phase 1 AC-SQ Welds, Including Traditional Heat Input (Equation 1) Polarity Current (amps) Input Variables Voltage (volts) Balance (%) Offset (%) Freq. (Hz) Current (amps Output Variables Voltage (volts) Power (kw) Traditional Heat Input (kj/mm) Actual WFS (mm/s) 73 AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15% AC-SQ % 0% AC-SQ % 15% AC-SQ % -15%

9 Table 2 Calculated Polarity Specific Heat Input Values and Projected WFS Values of the Phase 1 AC-SQ Welds (Equations 4 and 5) Weld ID Polarity-Specific Heat Input Calculations & Projected WFS DCEP Heat Input (kj/mm) DCEP WFS (mm/s) DCEN Heat Input (kj/mm) DCEN WFS (mm/s) Theoretical WFS (mm/s) % Difference (Theoretical vs. Actual) WFS % % % % % % % % % % % % % % % % % % % % % % % % % % % Results and Discussion The combined effects of balance and offset manipulation on the WFS or weld bead profile are not well understood. To better allow a welding operator to select appropriate waveform variables to achieve a suitable WFS and weld bead profile, reference charts were developed. Tables 3 to 6 were developed using the nominal weld input values (i.e., 550 amps, 28 volts, and 10.6 mm/s travel speed) with Equations 4 and 5, as well as the equations listed in Figure 5.

10 Table 3 Calculated DCEP Heat Input Values (kj/mm) For Various Balance and Offset Values Offset Balance 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% -15% % % % % % % Table 4 Calculated DCEN Heat Input Values (kj/mm) For Various Balance and Offset Values Offset Balance 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% -15% % % % % % % Table 5 Predicted Total Wire Feed Speed (mm/s) Using The Equations From Figure 5 Offset Balance 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% -15% % % % % % % Table 6 Predicted Melted Base Metal Per Second (mm 3 /s) Offset Balance 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% -15% % % % % % % By using the data from Table 5, it was possible to produce the following surface plots (Figures 8 and 9) to graphically illustrate the combined predicted effects of balance and offset (for a given nominal welding power) on WFS.

11 Wire Feed Speed (mm/s) % % 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% WFS Figure 8 3D Surface Plot The Combined Predicted Effects of Balance & Offset on WFS (mm/s) for a Nominal Welding Power of 15.4 kw Offset (%) 15% 10% 5% 10% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 5% 0% WFS Figure 9-2D Surface Plot The Combined Predicted Effects of Balance & Offset on WFS (mm/s) for a Nominal Welding Power of 15.4 kw The predicted 3D and 2D surface plots in Figure 8 and Figure 9 show that a combined reduction in both balance and offset is necessary to maximize the WFS; the plot also clearly shows that balance has a greater effect on WFS than offset. Similar surface plots (for other nominal Power values) could be developed to help welding operators better manipulate waveform variables to achieve appropriate specific WFS values.

12 WFS (mm/s) Wire Feed Speed (mm/s) % 50% 75% 0 15% Figure 10-3D Surface Plot The Actual Effects of Balance and Offset on WFS (mm/s) 15% 25% 50% 75% 0 Offset (%) WFS (mm/s) Figure 11-2D Surface Plot The Actual Effects of Balance and Offset on WFS (mm/s) Figure 10 and Figure 11 show 3D and 2D surface plots using the actual weld data. While welds were produced using frequencies of 30, 60, and 90 Hz, the data was averaged for each combination of balance and offset. The predicted plots (Figures 8 and 9) did not reach the same peak WFS as the plots based on the actual data, but this is consistent with the scatter plot in Figure 6. However, overall, the actual weld data did demonstrate a similar general shape, which indicates that the balance setting has a more significant effect than the offset on

13 WFS. However, unlike the predicted model, the actual weld data appears to show that the effect of offset on WFS is very minor when it is set below 0%. By using the data from Table 6, it was possible to produce the following surface plots (Figures 12 and 13) to graphically illustrate the combined predicted effects of balance and offset (for a given nominal welding power) on the amount of base metal that is melted each second (mm 3 /s) during welding. Penetration Volume (mm 3 /s) % 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 15% 0% Volume PEN Figure 12-3D Surface Plot The Combined Predicted Effects of Balance and Offset on Melted Base Metal (mm 3 /s) 15% 10% Volume PEN % % 5% Offset (%) % 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 10% Figure 13-2D Surface Plot The Combined Predicted Effects of Balance and Offset on Melted Base Metal (mm 3 /s) The appears of a kink shape in Figure 11 is the result of a limited number of data points being used to produce the surface map; additional data points would allow the plot to appear more smooth.

14 The predicted 3D and 2D surface plots in Figure 12 and Figure 13 show that an increase in both balance and offset is necessary to maximize the amount of molten base metal. The plots also clearly show that balance has a greater effect on WFS than offset. PEN Volume Penetration Volume (mm 3 /s) % 50% 75% 0 15% Figure 14-3D Surface Plot The Actual Effects of Balance and Offset on Melted Base Metal (mm 3 /s) 15% PEN Volume 25% 50% 75% 0 Offset (%) Figure 15-3D Surface Plot The Actual Effects of Balance and Offset on Melted Base Metal (mm 3 /s) Figure 14 and Figure 15 show 3D and 2D surface plots using the actual weld data (once again, using data averaged from the welds produced using 30, 60, and 90 Hz frequencies). As predicted, increasing balance resulted in a greater amount of molten base metal. However, unlike the predicted plots (Figures 12 and 13), the actual data shows that the amount of molten base metal (i.e., not the amount of deposited melted electrode) can be maximized by increasing balance and decreasing offset. One potential explanation for this unexpected effect could be that the longer exposure to electrode positive (i.e., the greater balance setting) pre-

15 heats a greater amount of base metal. However, by increasing the peak current during the shortened electrode negative portion of the AC cycle, the increased arc plasma pressure may be able to displace molten metal from the weld pool to expose base metal directly to the arc, thereby further increasing the penetration. A similar reasoning was suggested in the literature by Gupta et al. [2], but without additional weld data, it will not be possible to verify this potential explanation. The effects of weld metal transfer mode on bead penetration have been well documented for other processes such as the DC gas metal arc welding (GMAW) process (e.g., GMAW spray transfer causing finger penetration). Such phenomena have become understood by analyzing and correlating different sources of data including waveform measurement, high speed video photography, and destructive weld bead crosssection analysis. While high speed video photography is not possible for the SAW process, it can be performed using AC-GMAW, which is becoming increasingly available. Such research could improve the current understanding of metal transfer modes during AC-SAW, and may determine if the spike in plasma forces during EN are responsible for the increased molten metal produced with lower offset values. Because of the unexpected relationship between waveform variables and bead penetration, a modeled relationship between the waveform variables and bead penetration depth was not prepared. However, for reference, the actual weld data was used to plot a relationship between balance, offset and bead penetration depth. Depth (mm) Penetration Depth (mm) % 50% 75% 0 15% Figure 16-3D Surface Plot The Actual Effects of Balance and Offset on Weld Bead Penetration Depth (mm)

16 15% Depth (mm) 0 Offset (%) Penetration Depth (mm) % 50% 75% Figure 17-2D Surface Plot The Actual Effects of Balance and Offset on Weld Bead Penetration Depth (mm) The 3D and 2D surface plots in Figure 16 and Figure 17 show that increasing balance clearly increases bead penetration, but the effect of offset is not completely clear. The peak penetration depth is achieved by using a maximum balance setting combined with a minimum offset setting. Future Work While the predicted and actual effects of balance and offset on WFS were aligned, the predicted and actual effects of balance and offset on base metal penetration were not. It is suspected that a negative offset can increase base metal penetration by increasing the peak EN current enough to push the molten metal aside, thereby exposing unmelted base metal directly to the welding arc. While it is not possible to record weld metal transfer modes of AC-SQ submerged arc welding, recording similar videos using AC-GMAW could potentially help create a better understanding of the effects of waveform variables on metal transfer mode, and by extension, their effects on weld bead geometry (e.g., base metal penetration volume and depth). Conclusions 1. Polarity-specific heat input equations were used to illustrate that maximum WFS was achieved during constant current operation of AC-SAW by reducing both offset and balance. Balance had a more significant effect on WFS than offset. The predicted relationship between balance, offset, and WFS closely matched the actual data.

17 2. Polarity-specific heat input equations were used to illustrate that maximum base metal fusion (i.e., volume melted per unit time) was achieved during constant current operation of AC-SAW by increasing both offset and balance. However, the actual data illustrated that maximum base metal fusion was achieved by increasing balance while minimizing offset. Additional work will be necessary to determine how the negative offset is contributing to an increased base metal fusion. 3. Actual data was used to illustrate that maximum weld bead penetration depth was achieved by increasing balance and minimizing offset. Acknowledgments The authors would like to acknowledge EVRAZ Inc. NA for providing the materials, facilities, and some of the funding to execute the welds investigated in this report. The authors would also like to acknowledge Alberta Innovates - Technology Futures (AITF) for providing the materials, facilities and equipment necessary to further investigate and analyze the completed welds. Finally, the authors would like to acknowledge PCL Industrial Constructors Inc. for supporting the analysis, preparation, and presentation of this information at the 2013 CWA Canweld Conference.

18 Bibliography 1 K. Fisher, Subarc Technology Breakthrough: Technology Increases SAW Deposition Rates, Welding Design and Fabrication, pp , March Gupta, P.C., Ghose, P.K., Sharma, S.K., Effect of Polarity on Melting Rate in Submerged Arc Welding. Indian Welding Journal. Volume 19 (3): pp P. Jiluan, Arc Welding Control, Woodhead Publishing Limited, Cambridge, England, M.A.R. Yarmuch, B.M. Patchett, Variable AC Polarity GTAW Fusion Behavior in 5083 Aluminum, Welding Journal, v86 (7): pp , J. Pepin, Effects of Submerged Arc Weld (SAW) Parameters on Bead Geometry and Notch-Toughness for X70 and X80 Linepipe Steel. M.Sc. thesis. University of Alberta Lincoln Electric. Something New is Happening in the Submerged Arc Welding Process. < (visited June 1, 2012). 7 J. Pepin, M. Yarmuch, G. Casey, H. Henein, D.G. Ivey, C. Penniston, Using Polarity-Specific Heat Input to Evaluate the Effects of AC Submerged Arc Welding (SAW) Waveform Variables on Deposition Rates and Weld Bead Profiles. ASM Trends in Welding Conference ASME Committee on Welding and Brazing. ASME Boiler and Pressure Vessel Code Section IX: Qualification Standard for Welding and Brazing Procedures, Welders, Brazers, and Welding and Brazing Operators. ASME Lincoln Electric. True Energy TM : Instantaneous Energy Values for the Weld. Document NX < (visited September 20, 2013) J. Pepin, C. Penniston, H. Henein, D.G. Ivey, L. Collins, D. Boyd, Using Semipenetration Ratio To Characterise Effects Of Waveform Variables On Bead Profile And Heat Affected Zone With Single Electrode Submerged Arc Welding. Canadian Metallurgical Quarterly