https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1000&context=ece_facpub

Double-Electrode Arc Welding Process: Principle, Variants, Control and Developments

Alternating current-gas metal arc welding for application to thick plates

This application paper concerns the modeling and control of an innovative welding process, namely, the Consumable Double-Electrode Gas Metal Arc Welding. This innovative process can dramatically increase welding productivity and reduce weld distortion. It has demonstrated the feasibility to double the travel speed for automatic welding but requires controls to realize its unique advantages. To reach this goal, the bypass voltage and base metal current were selected as process outputs to be controlled. The bypass current and main wire feed speed were selected as the inputs and the control system was reduced to two single-input-single-output (SISO) subsystems for convenient implementation and design. Physical analysis and derivation show that these subsystems can be approximated as first-order model systems but their parameters depend on manufacturing conditions. Hence, they were described using first-order interval models whose parameters are unknown but bounded by known intervals. Step response experiments were conducted with selected range of manufacturing conditions to identify a few models for each of the subsystems. These models were then used to derive two interval models. To increase the stability margin, the intervals identified were artificially enlarged. Finally, a prediction-based interval model control algorithm was used to control the resultant interval models and closed-loop control experiments verified the effectiveness of the developed control system.

Interval Model Control of Consumable Double-Electrode Gas Metal Arc Welding Process

This paper evaluates the arc behavior and melting rate in the variable polarity (VP) gas metal arc welding (GMAW) process. Droplet melting analysis was performed using high-speed video that was synchronized with high-speed data acquisition. Melting rate measurements were found to be very dependent on current waveform, polarity, droplet size, and metal transfer, if it occurred, for each waveform period. The transient conditions of current waveform and metal transfer produced rapid changes in arc behavior that influenced the melting at the electrode tip and growing droplet. The concentrated melting theory was developed to explain the significant increase in electrode extension burnoff and droplet growth rate that occurred at short electrode negative (EN) time as a function of current, and during electrode positive (EP) peak pulse when the prepulse droplet volume was small. The highest electrode extension burnoff and droplet growth rate occurred when the arc was permitted to climb over the solid electrode tip producing rapid concentrated melting. Likewise, large molten droplets were found to promote a negative electrode extension burnoff and a decreased droplet growth rate. The arc rooted on large droplets providing additional heating but limited electrode melting. The droplet burnoff rate (DBR) method was developed and found to yield good experimental measurements for the arc melting and resistive heating coefficients used in a second-order melting rate equation developed for a complex waveform process, like VP-GMAW. For the EN period, the EN time affected the melting rate as a function of EN current. The greater melting rate that occurred at low EN time was measured by the changes in the resistive heating coefficient. Concentrated arc melting of the electrode extension at low EN time caused the slope of the burnoff diagram to increase, which represented the resistive heating coefficient. The melting rate of the EP pulse was related to the prepulse droplet volume. Large prepulse droplets decreased the arc melting coefficient, which could he negative, which meant the electrode extension was increasing and the arc length was decreasing in that waveform period.

Arc behavior and melting rate in the VP-GMAW process

Double-electrode gas metal arc welding (DE-GMAW) is a novel process that decouples the melting current into base metal current and bypass current by adding a bypass torch to a conventional GMAW system to establish a bypass arc. This makes it possible to increase the melting current while the base metal current can be controlled at a desired level. Experiments have been done to find the conditions that can assure a stable bypass arc is established/maintained between the welding wire and the bypass torch. To control the base metal current at the desired level, a group of power resistors is added in the bypass loop. The resistance of the power resistor group is adjusted real-time by changing the combination of the resistors, and the change in the resistance results in a change in the bypass current and thus a change in the base metal current. A model has been developed to correlate the change of the resistance needed to achieve the desired base metal current to the deviation of the base metal current from its desired level. Experiments demonstrated that the developed control system can adjust the bypass current in a great range to maintain the base metal current at the desired levels.

Double-electrode GMAW process and control : A novel welding process adds a GTAW torch to a conventional GMAW system to create a bypass arc for increasing melting current while controlling base current

John Eric Talkington

Papers

Variable polarity gas metal arc welding