Methods of Improving Productivity pulsed GMAW

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Methods of Improving the Productivity of Consumable-electrode Arc Welding Processes By Marc Purslow | Applications Engineer, Arc Welding     IntroductionConsumable-electrode arc welding processes such as gas metal arc welding (GMAW), flux cored arc welding (FCAW), and submerged arc welding (SAW) are widely used in manufacturing. This article will briefly describe how using pulsed GMAW, improving electrode burn-off efficiency, and taking advantage of the effect of resistive heating can increase deposition rates and boost productivity.          Pulse vs. Constant VoltagePulsed GMAW waveforms are used to achieve spray transfer at a lower average current than welding in constant voltage (CV) mode by employing a peak current and a background current. The peak current promotes spray transfer while the background current maintains the arc between pulses. Figure 1 illustrates the difference in current/wire-feed-speed (WFS) ratios for 0.045-inch diameter wire for the GMAW process in pulsed and CV welding modes. At an output current of 300 amps, the WFS when operating in CV mode was 422 inches per minute (ipm), while the WFS when operating in pulse mode was 535 ipm. This represents a 26.8% increase in deposition rate at the same operating current and heat input.       Resistive Heating of the ElectrodeFor a given WFS, the current output of the power supply is significantly affected by the contact tip to work distance (CTWD). Due to the effect of resistive (I2R) heating, increasing the CTWD will decrease the current needed to melt the electrode at a constant WFS. This effect can also be used to increase the deposition rate while maintaining the same current and heat input. The degree of resistive heating is greater for smaller-diameter wires since they have a higher electrical resistance.    Figure 2 illustrates the effect of resistive heating on 0.063-inch diameter solid wire using the GMAW process. At a CTWD of 0.75 inches, the current output of the power supply was 300 amps. Increasing the CTWD to 1.125 inches reduced the current output by 23% to 230 amps. In this example, maintaining the 1.125 inch CTWD and restoring the current to the original 300 amp level would result in a sizable increase in WFS. The figure also illustrates that welding in CV mode with the extended CTWD still required 300 amps, which supports previous conclusions of the reduction in average current when using pulsed GMAW.     Electrode Burn-off EfficiencyThe current level used for a given diameter of wire can significantly affect the burn-off efficiency, with higher currents typically resulting in greater efficiency.  While increasing the current alone may not be an option due to heat-input limitations, material thickness, or an open-root joint preparation, increasing the travel speed to maintain a consistent current/travel-speed ratio may resolve this issue. Figure 3 illustrates the increase in pounds of filler metal consumed per kWh of welding power at three different current settings for submerged arc welding in constant current mode with a 3/32 inch diameter electrode. Increasing the welding current from 250 amps to 425 amps resulted in a 21.4% increase in the amount of steel filler metal consumed per kWh of welding power.