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

MANIPULATING THE SAW VOLTAGE AND CURRENT WAVEFORMS TO CONTROL WELDING

PRODUCTIVITY AND QUALITY

J. Pepin1, H. Henein2, D.G. Ivey2, M. Yarmuch3

1PCL Industrial Constructors Inc. 2107 - 4 Street

Nisku, Canada, T9E 7W6 (*Corresponding author: jtpepin@pcl.com, 780-979-8634)

2University of Alberta, Edmonton, Alberta, Canada

7th Floor, ECERF Building, 9107 - 116 Street University of Alberta

Edmonton, Alberta, T6G 2V4

3Weldco Companies 12155 154 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.

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.

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

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.

Figure 5 – The effect of polarity-specific heat input on WFS values, during Constant Current operation of AC-SQ SAW [7]

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.

WFS = 19.93*(EP Heat Input) ‐ 0.04R² = 1

WFS = 27.22*(EN Heat Input) + 0.04

R² = 1

Superimposed WFS = 47.53*(Total 

Heat Input) ‐ 38.22R² = 0.85

Actual WFS = 45.54*(Total Heat Input) ‐ 35.64

R² = 0.70

0

5

10

15

20

25

30

35

40

45

50

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

WFS (mm/s)

Heat Input (kJ/mm)

WFS (EP Heat Input) WFS (EN Heat Input)

WFS (EN + EP Heat Input) WFS (Actual)

Superimposed WFS = 47.529*(Total Heat Input) ‐ 38.216

R² = 0.8468

Actual WFS = 45.539*(Total Heat Input) ‐ 35.638

R² = 0.7027

25

30

35

40

45

1.4 1.5 1.6 1.7

WFS (mm/s)

Heat Input (kJ/mm)

WFS (EN + EP Heat Input) WFS (Actual)

- 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 mm2) (Figure 7).

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 mm3/s). As a result, we are left with

Equations 6 and 7:

331.36 ∗ 0.95 (Eq.6)

229.81 ∗ 1.06 (Eq.7)

Experimental Procedure

AreaPEN = 31.26*(EP Heat Input) + 0.09R² = 0.97

AreaPEN = 21.68*(EN Heat Input) ‐ 0.10

R² = 0.980

20

40

60

80

100

0.0 1.0 2.0 3.0

Norm

alized Area P

EN(m

m2)

Normalized and Adjusted Heat Input (kJ/mm)

Normalized and Adjusted EP Heat Input

Normalized and Adjusted EN Heat Input

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 1.12.0.729) 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.

Table 1- Input and Output Variables of the Phase 1 AC-SQ Welds, Including Traditional Heat Input (Equation 1)

PolarityCurrent(amps)

Voltage(volts)

Balance(%)

Offset(%)

Freq.(Hz)

Current(amps

Voltage(volts)

Power(kW)

TraditionalHeat Input(kJ/mm)

ActualWFS

(mm/s)73 AC-SQ 550 28 50% 0% 60 549 32.2 17.7 1.67 37.8574 AC-SQ 550 28 50% 15% 60 551 30.2 16.6 1.57 35.9875 AC-SQ 550 28 50% -15% 60 550 29.9 16.5 1.55 36.5176 AC-SQ 550 28 75% 0% 60 549 29.6 16.3 1.53 32.8777 AC-SQ 550 28 75% 15% 60 551 28.9 15.9 1.50 31.9678 AC-SQ 550 28 75% -15% 60 548 31.0 17.0 1.60 34.7479 AC-SQ 550 28 25% 0% 60 541 31.9 17.2 1.63 44.4380 AC-SQ 550 28 25% 15% 60 546 31.9 17.4 1.64 40.3881 AC-SQ 550 28 25% -15% 60 550 32.6 17.9 1.69 44.2082 AC-SQ 550 28 50% 0% 30 549 32.2 17.7 1.67 37.7083 AC-SQ 550 28 50% 15% 30 550 31.0 17.1 1.61 37.3284 AC-SQ 550 28 50% -15% 30 548 32.2 17.6 1.66 39.1285 AC-SQ 550 28 75% 0% 30 536 29.8 16.0 1.51 35.1186 AC-SQ 550 28 75% 15% 30 552 30.8 17.0 1.61 33.4387 AC-SQ 550 28 75% -15% 30 545 28.9 15.7 1.48 30.0288 AC-SQ 550 28 25% 0% 30 550 32.0 17.6 1.66 42.8389 AC-SQ 550 28 25% 15% 30 549 31.8 17.4 1.65 39.5690 AC-SQ 550 28 25% -15% 30 545 33.4 18.2 1.72 42.6591 AC-SQ 550 28 50% 0% 90 549 31.2 17.1 1.62 38.0792 AC-SQ 550 28 50% 15% 90 550 31.3 17.2 1.62 35.8393 AC-SQ 550 28 50% -15% 90 547 31.9 17.4 1.64 38.3594 AC-SQ 550 28 75% 0% 90 548 27.3 15.0 1.41 29.4395 AC-SQ 550 28 75% 15% 90 549 27.7 15.2 1.43 31.6896 AC-SQ 550 28 75% -15% 90 544 30.8 16.7 1.58 31.8197 AC-SQ 550 28 25% 0% 90 546 31.8 17.4 1.64 40.8798 AC-SQ 550 28 25% 15% 90 546 31.8 17.3 1.64 39.1299 AC-SQ 550 28 25% -15% 90 551 32.2 17.7 1.67 42.27

Input Variables Output VariablesWeld

ID

Table 2 – Calculated Polarity Specific Heat Input Values and Projected WFS Values of the Phase 1 AC-SQ Welds (Equations 4 and 5)

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.

DCEP Heat Input(kJ/mm)

DCEP WFS

(mm/s)

DCENHeat Input(kJ/mm)

DCENWFS

(mm/s)

Theoretical WFS

(mm/s)

% Difference(Theoretical vs. Actual) WFS

73 0.84 16.6 0.84 22.8 39.4 4.1%74 0.90 18.0 0.67 18.2 36.2 0.5%75 0.66 13.1 0.89 24.3 37.4 2.5%76 1.15 22.9 0.38 10.5 33.4 1.5%77 1.20 24.0 0.30 8.1 32.1 0.4%78 1.10 22.0 0.50 13.6 35.6 2.4%79 0.41 8.1 1.22 33.2 41.3 -7.0%80 0.51 10.1 1.13 30.9 41.0 1.6%81 0.33 6.6 1.36 37.0 43.6 -1.3%82 0.83 16.6 0.83 22.8 39.3 4.4%83 0.93 18.4 0.68 18.7 37.1 -0.6%84 0.71 14.1 0.96 26.1 40.1 2.6%85 1.13 22.5 0.38 10.3 32.7 -6.7%86 1.29 25.6 0.32 8.7 34.3 2.6%87 1.02 20.3 0.46 12.6 32.9 9.8%88 0.41 8.2 1.24 33.9 42.1 -1.6%89 0.51 10.2 1.13 30.9 41.1 3.8%90 0.34 6.7 1.38 37.5 44.2 3.7%91 0.81 16.1 0.81 22.0 38.1 0.1%92 0.93 18.5 0.69 18.8 37.4 4.2%93 0.70 13.9 0.95 25.8 39.7 3.4%94 1.06 21.1 0.35 9.6 30.7 4.3%95 1.15 22.9 0.28 7.8 30.6 -3.3%96 1.09 21.7 0.49 13.4 35.1 10.2%97 0.41 8.1 1.23 33.5 41.7 2.0%98 0.51 10.1 1.13 30.7 40.8 4.4%99 0.33 6.6 1.34 36.6 43.2 2.1%

WeldID

Polarity-Specific Heat Input Calculations & Projected WFS

Table 3 – Calculated DCEP Heat Input Values (kJ/mm) For Various Balance and Offset Values

Table 4 – Calculated DCEN Heat Input Values (kJ/mm) For Various Balance and Offset Values

Table 5 – Predicted Total Wire Feed Speed (mm/s) Using The Equations From Figure 5

Table 6 – Predicted Melted Base Metal Per Second (mm3/s)

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.

25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75%-15% 0.29 0.35 0.41 0.48 0.55 0.62 0.69 0.76 0.84 0.92 1.00-10% 0.31 0.38 0.44 0.51 0.58 0.65 0.73 0.80 0.88 0.95 1.03-5% 0.34 0.41 0.48 0.55 0.62 0.69 0.76 0.84 0.91 0.99 1.060% 0.36 0.44 0.51 0.58 0.65 0.73 0.80 0.87 0.94 1.02 1.095% 0.39 0.47 0.54 0.62 0.69 0.76 0.83 0.91 0.98 1.05 1.12

10% 0.42 0.50 0.58 0.65 0.73 0.80 0.87 0.94 1.01 1.08 1.1415% 0.45 0.53 0.61 0.69 0.76 0.84 0.91 0.97 1.04 1.10 1.17

BalanceOffset

25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75%-15% 1.17 1.10 1.04 0.97 0.91 0.84 0.76 0.69 0.61 0.53 0.45-10% 1.14 1.08 1.01 0.94 0.87 0.80 0.73 0.65 0.58 0.50 0.42-5% 1.12 1.05 0.98 0.91 0.83 0.76 0.69 0.62 0.54 0.47 0.390% 1.09 1.02 0.94 0.87 0.80 0.73 0.65 0.58 0.51 0.44 0.365% 1.06 0.99 0.91 0.84 0.76 0.69 0.62 0.55 0.48 0.41 0.34

10% 1.03 0.95 0.88 0.80 0.73 0.65 0.58 0.51 0.44 0.38 0.3115% 1.00 0.92 0.84 0.76 0.69 0.62 0.55 0.48 0.41 0.35 0.29

OffsetBalance

25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75%-15% 37.5 37.0 36.5 36.0 35.6 35.0 34.5 34.0 33.4 32.8 32.2-10% 37.3 36.8 36.3 35.8 35.3 34.8 34.3 33.7 33.2 32.6 32.0-5% 37.1 36.6 36.1 35.6 35.0 34.5 34.0 33.4 32.9 32.4 31.80% 36.9 36.4 35.8 35.3 34.8 34.3 33.7 33.2 32.7 32.1 31.65% 36.7 36.1 35.6 35.1 34.5 34.0 33.5 32.9 32.4 31.9 31.4

10% 36.5 35.9 35.3 34.8 34.3 33.7 33.2 32.7 32.2 31.7 31.215% 36.3 35.7 35.1 34.5 34.0 33.5 32.9 32.5 32.0 31.5 31.0

OffsetBalance

25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75%-15% 362.9 369.3 375.8 382.5 389.4 396.5 403.8 411.3 419.1 427.2 435.4-10% 365.4 372.1 378.9 385.8 392.9 400.2 407.5 415.1 422.7 430.6 438.6-5% 367.9 375.0 382.1 389.3 396.5 403.8 411.2 418.7 426.3 433.9 441.60% 370.6 378.0 385.4 392.8 400.2 407.5 414.9 422.3 429.7 437.0 444.45% 373.5 381.2 388.8 396.4 403.8 411.2 418.5 425.8 433.0 440.1 447.1

10% 376.5 384.5 392.3 400.0 407.5 414.9 422.1 429.2 436.2 443.0 449.715% 379.6 387.9 395.9 403.7 411.3 418.6 425.7 432.6 439.3 445.8 452.1

OffsetBalance

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

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.

‐15%

0%

15%

3031323334353637383940

25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75%

Wire Feed Speed (mm/s)

Balance (%)

39‐40

38‐39

37‐38

36‐37

35‐36

34‐35

33‐34

32‐33

31‐32

30‐31

WFS

‐15%

‐10%

‐5%

0%

5%

10%

15%

25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75%

Balance (%)

39‐40

38‐39

37‐38

36‐37

35‐36

34‐35

33‐34

32‐33

31‐32

30‐31

WFS

Offset (%

)

Figure 10 - 3D Surface Plot – The Actual Effects of Balance and Offset on 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

‐15%

0

15%

3031323334353637383940

25%

50%

75%

Wire Feed Speed (mm/s)

Balance (%)

39‐40

38‐39

37‐38

36‐37

35‐36

34‐35

33‐34

32‐33

31‐32

30‐31

WFS (mm/s)

‐15%

0

15%

25% 50% 75%

Offset (%

)

Balance (%)

39‐40

38‐39

37‐38

36‐37

35‐36

34‐35

33‐34

32‐33

31‐32

30‐31

WFS (mm/s)

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 (mm3/s) during welding.

Figure 12 - 3D Surface Plot – The Combined Predicted Effects of Balance and Offset on Melted Base Metal (mm3/s)

Figure 13 - 2D Surface Plot – The Combined Predicted Effects of Balance and Offset on Melted Base Metal (mm3/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.

‐15%

0%

15%

360370380390400410420430440450460

25%

30%

35%

40%

45%

50%

55%

60%

65%

70%

75%

Penetration Volume (mm

3/s)

Balance (%)

450‐460

440‐450

430‐440

420‐430

410‐420

400‐410

390‐400

380‐390

370‐380

360‐370

VolumePEN

‐15%

‐10%

‐5%

0%

5%

10%

15%

25%

30%

35%

40%

45%

50%

55%

60%

65%

70%

75%

Balance (%)

450‐460

440‐450

430‐440

420‐430

410‐420

400‐410

390‐400

380‐390

370‐380

360‐370

Offset (%

)

VolumePEN

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.

Figure 14 - 3D Surface Plot – The Actual Effects of Balance and Offset on Melted Base Metal (mm3/s)

Figure 15 - 3D Surface Plot – The Actual Effects of Balance and Offset on Melted Base Metal (mm3/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%

0

15%

350360370380390400410420430440450460470480

25%

50%

75%

Penetration Volume (mm

3/s)

Balance (%)

470‐480

460‐470

450‐460

440‐450

430‐440

420‐430

410‐420

400‐410

390‐400

380‐390

370‐380

360‐370

350‐360

PENVolume

‐15%

0

15%

25% 50% 75%

Balance (%)

470‐480

460‐470

450‐460

440‐450

430‐440

420‐430

410‐420

400‐410

390‐400

380‐390

370‐380

360‐370

350‐360

Offset (%

)

PENVolume

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 cross-

section 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.

Figure 16 - 3D Surface Plot – The Actual Effects of Balance and Offset on Weld Bead Penetration Depth (mm)

‐15%

0

15%

4.004.254.504.755.005.25

5.50

5.75

6.00

25%

50%

75%

Penetration Depth (mm)

Balance (%)

5.75‐6.00

5.50‐5.75

5.25‐5.50

5.00‐5.25

4.75‐5.00

4.50‐4.75

4.25‐4.50

4.00‐4.25

Depth (mm)

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.

‐15%

0

15%

25% 50% 75%

Penetration Depth (mm)

Balance (%)

5.75‐6.00

5.50‐5.75

5.25‐5.50

5.00‐5.25

4.75‐5.00

4.50‐4.75

4.25‐4.50

4.00‐4.25

Offset (%

)

Depth (mm)

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.

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Fabrication, pp. 28 – 31, March 2002.

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Journal. Volume 19 (3): pp. 228 - 233. 1987.

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