PREVENTING ARC BLOW IN ROBOTIC TANDEM MIG WELDING APPLICATIONS

Arc blow can cause a number of welding problems including excessive spatter, undercut, irregular bead width, incomplete fusion, and porosity. What is arc blow and how can it be prevented?

Arc blow, or arc wander, occurs in DC arc welding when the arc stream does not follow the shortest path between the electrode and the workpiece and deflects forward or backward from the direction of travel or, less frequently, to one side.

MAGNETIC ARC BLOW
Magnetic arc blow results from an unbalanced condition in the magnetic field surrounding the arc. This unbalanced condition usually occurs because the arc is located farther from one end of the weld joint than the other end and at varying distances from the workpiece connection.

Visualizing a Magnetic Field
To understand arc blow, it is helpful to visualize a magnetic field. Figure 1 shows Direct Current passing through a conductor (either the welding electrode or the plasma stream between an electrode and a weld joint). A magnetic field surrounds the conductor; its lines of magnetic force, or flux, are represented by concentric circles at right angles to the direction of the current. These circular lines of force diminish in intensity the farther they are from the electrical conductor.

Figure 2 illustrates flux squeezing and distortion at the start and finish of a weld joint. At the start, flux lines concentrate behind the electrode. The arc tries to compensate for this imbalance by moving forward, creating forward arc blow. As the electrode approaches the end of the weld joint, the lines squeeze ahead of the arc. Again, the arc moves in a direction to relieve squeezing, in this case backward and is observed as back blow. At the middle of the joint, welding plates of the same width, the magnetic field is symmetrical, so no arc blow occurs. However, if one plate is wider than the other, side blow could occur at the midpoint of the weld due to flux squeezing.

ARC BLOW IN ROBOTIC TANDEM MIG WELDING
Many welding process advances involve the use of multiple welding arcs for high speeds or high deposition rates. See Figure 3 for a visual depiction of the Tandem Mig Welding process. This type of welding can encounter arc blow problems due to the higher currents ? there is less arc blow with low current than with high. Why? Because the intensity of the magnetic field a given distance from the conductor of electric current is proportional to the square of the welding current.

Specifically, when two arcs are close to each other, their magnetic fields react to cause arc blow on both arcs.

If the arcs are the same polarity, as in Figure 4, the magnetic fields between the arcs oppose each other. This results in a weaker field between the arcs, causing the arcs to blow toward each other.

HOW TO REDUCE ARC BLOW
When arc blow is causing or contributing to such defects as undercut, inconsistent penetration, crooked beads, beads of irregular width, porosity, wavy beads, and excessive spatter, it must be controlled. Possible corrective measures include the following:
? Hold as short an arc as possible to help the arc force counteract the arc blow.

? Angle the electrode with the work opposite the direction of arc blow.
? Make a heavy tack weld on both ends of the seam; apply frequent tack welds along the seam, especially if the fitup is not tight.

? Weld toward a heavy tack or toward a weld already made.
? Weld away from the workpiece connection to reduce back blow; weld toward the workpiece connection to reduce forward blow.

? Wrap the work cable around the workpiece so that the current returning to the power supply passes through it in such a direction that the magnetic field set up will tend to neutralize the magnetic field causing the arc blow.

THE EFFECTS OF FIXTURING ON ARC BLOW
Be aware of the relationship of arc blow to weldment fixturing. Steel fixtures may affect the magnetic field around the arc, and may become magnetized over time. Fixtures used with higher currents and mechanized welding should be designed to minimize arc blow-promoting situations. Fixture-design tips include the following:
? Fixtures for welding the longitudinal seam of cylinders (see Figure 5) should be designed for a minimum of 1 in clearance between the supporting beam and the work. The clamping fingers or bars that hold the work should be nonmagnetic. Do not attach the workpiece cable to the copper backup bar; make the work connection directly to the workpiece if possible.

? Fabricate the fixture from low-carbon steel. This is to prevent the buildup of permanent magnetism in the fixture.

? Do not use a copper strip inserted in a steel bar for a backing, as in (see Figure 6). The steel part of the backup bar will increase arc blow.

? Provide for continuous or close clamping of parts to be seam-welded. Wide, intermittent clamping may cause seams to gap between clamping points, resulting in arc blow over the gaps.

? Do not build into the fixture large masses of steel on one side of the seam only. Counter-balance with a similar mass on the other side.

SUMMARY
By understanding the mechanics of arc blow and how to correctly diagnose it in the weld, operators should be able to eliminate it from their applications and be able to create welds without the problems normally associated with arc blow.

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Geoff Lipnevicius is the operations manager for the automation division at The Lincoln Electric Company, 22800 Saint Clair Avenue, Cleveland, OH 44117-8542, 216-383-8027, Fax: 216-383-8823, www.lincolnelectric.com, [email protected]. He is also a columnist for Melting Point, a welding industry e-newsletter published each quarter by Fabricating & Metalworking.