GMAW Gun Troubleshooting with BWM Services

GMAW Troubleshooting

Reduced to its essential functions, a gas metal arc welding (GMAW) system has two jobs: transfer electricity to the filler wire and cover the weld pool with shielding gas. Gas metal arc guns play a crucial role in both functions, and changing guns changes a number of variables. For instance, different guns transfer electricity with different degrees of electrical efficiency.

Further, consider the effect of thermal efficiency. While components that stay cooler transfer electricity more efficiency, those that get hotter may require a constant fine tune of welding parameters.

Every year Tweco engineers interact with hundreds of end-users. The consistently find that many new product issues are actually caused by continuing the practices developed for their old components. When it comes to troubleshooting GMAW guns, a good amount of the “problems” aren’t really problems, but symptoms of the need to update best practices.

Keeping that in mind, here are 12 real-world examples of GMA “problems,” their root causes and associated solutions.

BWM Services Discusses the Symptoms, Root Causes and Answers to Overcome GMAW Complexities

1. Symptom: erratic arc and/or porosity. Root cause: incorrect contact tip/nozzle relationship.

Nozzles usually fall into four classes: recessed, flush, protruding or adjustable slip. Operators should select the nozzle style that allows them to use the correct contact tip-to-work distance (wire extension is the common term).

More often than not, experienced operators judge the correct distance by the sound of the arc. This is because they know that an erratic arc just does not sound right.

Flux cored wires usually require more preheating of the wire before it touches the workpiece, so an extension is ½ to ¾ in. (depending on the wire type) is common. Because the arc is too far from the contact tip, a recessed or double recessed nozzle should be chosen for welding with wires to avoid running out of gas coverage. Solid wires require less preheating, so operators usually hold an extension of ¼ to 3/8 in. A flush-fit nozzle typically works best for solid wires.

2. Symptom: Unintentionally Long Extension/Erratic Arc. Root Cause: Misperception of Contact Tip-to-Work Distance

While some operators consistently watch the electrode feed into the molten pool, others may focus on the part, joint and GMAW gun geometry, and not look at the arc directly all the time.

Therefore, this second group of operators may focus more on “looking around” the nozzle and into the weld pool. When they do this, they typically “pull away,” which increases extension.

Now they are welding with a greater extension than they are accustomed to for that particular application, and this can cause problems. Fixing the issue typically just requires a greater awareness of what’s going on.

3. Symptom: Microarcing Inside the Conductor Tube. Root Cause: Improper Liner Length

If the liner is too short, the wire will feed without any support. It then begins “snaking” (feeding erratically), and that can create microarcing inside the conductor tube. Different brands of consumables require different liner lengths, so make sure to follow the manufacturer’s recommendations exactly, and measure and cut carefully.

Keep in mind that different styles of tips can require different trim lengths. For instance, a standard tip might require trimming at 10 mm, where an extended tip might require a 30-mm trim.

4. Symptom: Wire Drag and Shavings. Root Cause: Poor Liner Trim Condition

When trimming new liner, a sharp, premium-quality side cutting tools should be used. Orient the cutting tools so that they cut against the  curl of the liner to produce a clean cut. Cutting with the curl tends to produce a burr on the inside edge of the liner that could drag against the wire.

Dull cutting tools or whelpers should never be used to trim a liner. Rather than cleanly cut through the tough piano wire used for liners, they will most likely deform the liner and/or spread out the coals. A cutting disc should never be used, as it can leave a sharp edge that drags against the electrode and creates shavings. Should the cut liner have a burr, utilize a hand file to remove it.

5. Symptom: Erratic Arc, Part 1. Root Cause: Excessive Drag Caused by a Clogged Liner

Wire shavings build up and create excessive friction. This causes the wire to stick or slip. When wire feed speed slows, yet voltage remains constant, the electrode will melt back from the weld pool more quickly and produce and erratic arc.

By far, the leading cause of wire shavings is the wire feeder drive rolls that have been over-tensioned and subsequently crushed the wire.

When the wire is crushed, surface shavings and microfines break off at a much higher rate. This clogs the liner faster. This issue was uncovered by Tweco engineers during field trials. Users reported that their test gun worked fine to start, but then the arc became erratic. The real culprit was not the gun, but dirty liners.

The lesson here is that before testing new consumables, filler metals or other components, start with a clean gun liner.

6. Symptom: Erratic Arc, Part 2. Root Cause: Excessive Drag Caused by Worn or Kinked Liner

Liners only last a certain amount of time, and utilizing poor quality wire or wire that’s corroded can speed up the replacement schedule. That said, even when utilizing premium brand wires, operators might need to replace the liner more frequently than they think.

For instance, the Tweco test lab runs a high volume of wire. When running a 1/16-in. solid wire, usually the liner is replaced after two 60-lb spools.

7. Symptom: Erratic Arc, Part 3. Root Cause: Worn Work Clamp or Poor Work Clamp Connection

While not directly related to guns, there are three things worth noting about work clamps/cables.

So much attention gets paid to the gun when troubleshooting that operators may neglect examining the work clamp and cable.

A loose, worn or frayed cable is a recipe for poor GMAW performance, not to mention an increased electrical hazard.

Right size them for the application. An undersized cable generates excessive heat, degrading its ability to conduct electricity. This is why operators may need to turn up the voltage when the cable heats up.

Clamp the cable close to the weld area, and clamp to a clean, bare metal surface.

8. Symptom: Weld Bead Porosity, Part 1. Root Cause: Poor Gas Coverage from a Loose or Deformed Nozzle

All nozzles eventually become loose as they get “beat up” from being dropped, not to mention operators who utilize guns as a chipping hammer.

When nozzles get extremely hot and then banged, they are prone to slipping. If the nozzles slips, there is a chance it could then pull atmosphere into the shielding gas stream and contaminate it. In addition, as the once-round nozzle orifice goes out of shape, it won’t properly direct gas flow.

For the nozzle to provide smooth, laminar gas flow, keep it in good condition and free from spatter buildup.

9. Symptom: Weld Bead Porosity, Part 2. Root Cause: Wrong Type of Gas Nozzle

While some operators have a misconception that smaller nozzles provide better control of the gas flow, that’s not true. For instance, they will use a cone nozzle instead of a standard nozzle. They only reason for a cone nozzle is that it provides access to deep V-grooves and tight corners. A cone nozzle constricts the shielding gas, so its flow will be inherently more turbulent.

The truth is that a large nozzle provides smoother gas flow and better coverage of the molten weld pool. Those familiar with gas tungsten arc welding know that large ups with a gas lens provide the best coverage, and the same is true with GMA nozzles.

10. Symptom: Weld Bead Porosity, Part 3. Root Cause, Turbulent Gas Flow Caused by Undersized Consumables

If the consumables are too small for the application, they get hot quickly. As the gas contacts hot surfaces, it wants to expand but remains constricted by the nozzle. Then, as the hot gas exits the nozzle, it expands rapidly and creates a fast, turbulent gas flow.

If the front-end consumables get too hot, they are most likely undersized for the application. The solution is a large gun with more heat dissipation capabilities or change from an air- to water-cooled gun.

11. Symptom: Weld Bead Porosity, Part 4. Root Cause: Turbulent Gas Flow Caused by Excessive Gas Flow

This is one of the more common errors. Operators believe that if 30 f 3/h provides smooth, laminar flow and good coverage of the weld pool, the faster gas flow may create turbulence and can pull in the surrounding atmosphere.

12. Symptom: Inaccurate Gas Flow Reading. Root Cause: Measuring Flow at the Cylinder, Not at the Gun

With bulk gas systems, the flow rate at each station might vary widely, such as if the first shift has 25 welders drawing on the system and the second shift only has 10 welders. The first shift over-cranks the gas flow settings, and then they may starve the next welding cell down the line.

Test new guns and other components on a dedicated cylinder to eliminate gas flow rate variations.

Furthermore, don’t measure gas flow at the regulator, but at the front of the GMAW gun. For those who must test a gun on a bulk gas system, utilize a two-stage regulator. This will ensure a much more consistent flow rate.

Source: aws.org

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