MANIPULATING THE SAW VOLTAGE AND CURRENT …

1 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, Ivey2, M. Yarmuch3 1 PCL Industrial Constructors Inc. 2107 - 4 Street Nisku, Canada, T9E 7W6 (*Corresponding author: 780-979-8634) 2 University of Alberta, Edmonton, Alberta, Canada 7th Floor, ECERF Building, 9107 - 116 Street University of Alberta Edmonton, Alberta, T6G 2V4 3 Weldco 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.

2 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.

3 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.

4 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.

5 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).

6 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).

7 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).

8 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.

9 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 ( , 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].

10 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.)