| Abstract Scope |
Fall voltages are important characteristics that researchers and engineers seek to understand for applications including power source design, and welding output prediction (e.g. heat input and bead width). Currently, there is limited understanding of how the voltage set at the machine is isolated to distinct fall voltages that make up the total voltage loss (VTot) in the GMAW system. This has led to a costly trial and error approach to achieve desired settings for a successful weld. In a typical welding system, the anode and cathode fall voltages (Van and Vcath) are the expected main contributors for heat input. However, all fall voltages including the contact tip (VCT), electrode extension (VEE), arc column (VAC), and welding leads (VWL) must be accounted for to understand heat input and the required voltage set at the machine. Previous work has posed important questions around the definition of arc length (LAC), which is required to determine VAC. Understanding the relationship between voltage and LAC is paramount to controlling the GMAW process. The self-regulating arc in GMAW allows for the quantification of effects to LAC from changing variables such as VTot, welding current (Iw), and contact tip to work piece distance (LCTWD). A Phantom v210 camera was used to capture high speed videography and synchronized with an external data acquisition (DAQ) system (National Instruments USB-6351) to collect video, current, and voltage data at 8.1 kHz over 0.5 s. The camera was placed perpendicular to the direction of travel and centered to capture the space between the gas cup and the bottom of the weld pool. An 800 nm narrow band pass filter with an aperture set at f/22, and an exposure time of 120 μs was used to view the metal vapour and ionized gas dominant regions, which facilitated identifying the point of arc attachment to the consumable. Voltage sensor probes had electrical contact at the contact tip and ground terminal (placed within 150 mm of the weld zone) to exclude welding lead and ground cable voltage losses from the measurements (VM). All welding was performed using a Lincoln Power Wave S500 power source and a Lincoln 84 Dual Power Feed unit set to program 5 for Gas Shield Standard Constant Voltage. A Lincoln Magnum PRO Al G450A K3355-2 gun was used with a 8 m #2 (35 mm) lead cable and a 3 m 4/0 grounding cable.
Four sets of experiments with aluminum (AlS1, AlS2, AlS3, and AlS4) were performed (each set with up to 6 tests, denoted Tn, n=1,2,3,4,5,and 6). All welds were performed with direct current electrode positive (DCEP) in the 1G position with a 10° push angle. Sets AlS1 and AlS3 varied LCTWD, and sets AlS2 and AlS4 varied voltage setting for each weld trial. Time averaged LAC measurements ranged from 1.6 to 12.2 mm and had a consistent standard deviation (σ) range of 0.2 to 0.5 mm between all weld sets. The consistent range of σ values shows that time averaged LAC measurements are more precise than single point measurements. Arc length measurements allowed for a determination of V’AC by plotting VM vs. LAC for AlS2 and AlS4, which consisted of varying voltage settings with a constant LCTWD and wire feed speed (Uc) for the respective sets. Considering V’AC as the slope of the linear fit (least sum of squares), then V’AC= 0.62 V mm-1. From the limited data analyzed, a consistent correlation of -0.07 mm A-1 was observed between LAC and Iw. Varying LCTWD was found to have a negligible effect on LAC considering the narrow operable range of LCTWD governed by adequate gas shielding. Per the linear correlations obtained, there is a 0.01 to 0.1 mm change in LAC for every 1 mm change in LCTWD. The average value of the sum of the anode and cathode fall voltages (VF) was found to be 18.3 ± 0.4 V and approximately independent of Iw. The percent contribution for VF ranged from ≈ 70 to 85%, and attributed to the range of voltages used in the experiments. It was confirmed that VAC is a major contributor to VM with ≈ 14 to 29% contribution that varied between sets depending on the voltage setting. Minor contributors to VM were VEE and VCT, each contributing <1%. Analytical equations were used to predict respective voltage contributions for aluminum procedures from the Lincoln Procedure Hand Book (LPHB) using the experimental values obtained for VF, V’AC, and an empirical relationship to identify an expected LAC. The main contributors to VTot were VF and VAC with an overall average of 64 ± 3%, and 34 ± 3%. All other constituents were be minor contributors to overall voltage loss, with averages of 0.8 ± 0.1% (VCT), 0.02 ± 0.01% (VEE), and 1.1 ± 0.1% (VWL). In conclusion: arc column electric field (V’AC) was 0.62 V mm-1 from an empirical relationship (VM=0.62LAC+18.5) considering arc length (LAC) and a measured voltage (VM); future work will include experiments with different material grades and consumable diameters at varying Iw ranges to verify the impact of Iw on V’AC; time weighted LAC measurements ranged from 1.6 to 12.2 mm with a standard deviation between 0.2 to 0.5 mm; time weighted measurements are critical for welds with smaller LAC as the arc becomes more erratic approaching short circuit transfer mode; a consistent decrease in LAC was observed with increasing Iw (-0.07 mm A-1) and is subject for further study due to limited data supporting this specific observation; LAC was approximately independent of change in contact tip to work piece distance (LCTWD) with an increase between 0.01 and 0.1 mm for every 1 mm increase in LCTWD and considered negligible for the operable range of LCTWD (4 mm range in this study); the sum of the anode and cathode (VF) had an average of 18.3 ± 0.4 V and was approximately independent of current; lastly, expected LAC and percent contributions for each fall voltage constituent were determined for a set of aluminum procedures from the LPHB. |