MRO Today - Aw, G-MAW!

Aw, G-MAW!

An all-in-one guide to troubleshooting common GMAW
(Gas Metal Arc Welding) problems

by Bruce Morrett, product manager, Hobart Brothers
by and Bill Giese, product manager, Bernard

Although many people tout Gas Metal Arc Welding (GMAW) as an “all-in-one” solution, that doesn’t mean it’s a magic remedy for eliminating your welding problems. This article examines common pitfalls encountered during the normal course of GMA welding, along with trusted ways to correct them.

Porosity
Porosity is a small pocket of gas caught in the weld metal that can appear at any specific point on the weld or along its full length. This discontinuity—whether internal or on the surface of the weld bead—significantly weakens the structural integrity of any weld.

Inadequate shielding gas coverage is a common cause of porosity and is easily corrected by taking some or all of the following measures. First, check the regulator/flow meter for adequate gas flow capacity (increase if necessary) and test gas hoses and the gun for leaks. Eliminate drafts near the welding arc and, if welding outside, shield the arc from the wind with a welding screen.

Also, be certain the nozzle is large enough for your process (too small of a nozzle could cause inadequate shielding gas flow) and remove any spatter from the gun nozzle.

When welding, keep the nozzle one-fourth to one-half inch away from the workpiece and check that you are using the correct contact tip recess for the application. Slow your travel speed and hold the MIG gun near the bead at the end of the weld until the molten metal solidifies (pulling the gun away too soon pulls away the gas coverage before the weld sets).

A dirty workpiece also causes porosity. Clean the surface of the base metal to remove rust, grease, paint, coatings, oil, moisture and dirt prior to welding. You can also use filler wire with added deoxidizers to “clean” up the weld.

Additional causes of porosity include: using the wrong gas (always use a welding-grade shielding gas appropriate to the base metal and filler metal), using too much or the wrong type of anti-spatter (use the correct amount and type for your application) and welding wire that extends too far out of the nozzle (extend no more than one-half inch beyond the nozzle).

Impurities in the base metal, such as sulfur and phosphorous in steel, can be a further cause, remedied by changing the base metal to a different composition (where specifications allow) and wet or contaminated shielding cylinders should be replaced immediately to help prevent porosity.

Undercutting and incomplete fusion
Undercutting occurs when a groove melts in the base metal next to the toe of the weld and is not adequately filled by the weld metal. This discontinuity creates a weaker area at the toe of the weld and could cause cracking.

To correct this problem, reduce the welding current, decrease the welding arc voltage and adjust your electrode angle as needed. Reduce travel speed so that the weld metal completely fills the melted-out areas of the base metal and/or pause at each side of the weld bead when using a weaving technique.

Incomplete fusion (or lack of fusion) is the failure of the weld metal to fuse completely with the base metal or the preceding weld bead in multi-pass applications. Incorrect electrode/work angles that cause the weld metal to get ahead of the arc can be the culprit and should be adjusted accordingly.

For proper welding angles, reference Figure 1 and follow these steps:
• Place the stringer bead in its proper location at the joint, adjusting the work angle or widening the groove to access the bottom during welding.
• Keep the arc on the leading edge of the welding puddle and remember to use a correct gun angle of 0 to 15 degrees.
• If using a weaving technique, momentarily hold the arc on the groove sidewalls when welding.

As seen in Figure 1, proper work angles are important for avoiding GMA welding pitfalls like incomplete fusion.

If correcting the electrode/work angle does not remedy the problem, check to see if the welding puddle is getting ahead of the electrode. Simple adjustments, such as increasing travel speed or using a higher welding current, will correct the problem.

A dirty workpiece could also be the cause of the problem. Always clean the surface of the base metal prior to welding to remove contaminants. If you suspect insufficient heat input could be contributing to incomplete fusion, select a higher voltage range and/or adjusting the wire feed speed as necessary.

Excessive splatter
Spatter occurs when the weld puddle expels molten metal and scatters it along the bead where it then cools and forms a solid mass on the workpiece. Excessive spatter creates a poor weld appearance, lowers the efficiency of the process and leads to incomplete fusion (see following section) in multiple pass welds.

Excessively high wire feed speed and/or voltage settings, along with too long of an electrode extension, or stick-out, are often to blame for spatter; correct the problem by lowering the given settings and using a shorter electrode extension.

Using the wrong contact tip and/or a worn contact tip or having wrong tip to nozzle recess can also lead to an erratic arc and cause excessive spatter. Be certain you have the right contact tips, nozzles and recess parameters for your particular job.

Like porosity, spatter can also be caused by insufficient shielding gas at the welding arc and/or dirty base materials. Ensure proper gas coverage by increasing shielding gas flow at the regulator and minimizing drafts near the welding arc. Be sure to use clean and dry welding wire, and remove all grease, dirt and other contaminants from the base metal.

Excessive (or lack of) penetration
Excessive penetration occurs when the weld metal melts through the base metal and hangs underneath the weld; it is often caused by excessive heat input (See Figure 2). To correct the problem, select a lower voltage range, reduce wire feed speed and increase travel speed.

Lack of penetration and excessive penetration can be remedied by adjusting factors such as voltage, wire feed speed and travel speeds.

Lack of penetration is the shallow fusion between the weld metal and the base metal. An obvious cause (and exact opposite of excessive penetration) is insufficient heat input. Selecting a higher wire feed speed, a higher voltage range and/or reducing travel speed are viable remedies.

Lack of penetration can also be caused by improper joint preparation and/or from the material being too thick. Joint preparation and design must permit access to the bottom of the groove, while also allowing you to maintain proper welding wire extension and arc characteristics.

Wire feed problems: Birdnesting and burnback
Wire feed stoppages and malfunctions of the wire feed system prematurely extinguish the welding arc and create irregularities that may weaken the weld bead. A common form of this stoppage is birdnesting: a tangle of wire that halts the wire from being fed. Fix a bird’s nest by flipping up the drive roll and pulling the wire back out of the gun. Trim off the affected wire and re-thread it through the feeder and back to the gun.

Blockages in the liner, improperly trimmed liners (too short, burred or /pinched) or the wrong liner (too small or large for the electrode diameter) can cause wire feed problems. Replace the liner if you find a blockage, always trim the liner according to the manufacturers direction and be certain you are using the correct size liner for your electrode.

Also be certain you are using the proper feeder drive rolls and tension settings for your electrode and application. If your specifications allow, decreasing drive roll tension, using a larger diameter wire and/or reducing the distance the wire feeds (use shorter cables) can also minimize birdnesting. In certain applications, a push-pull wire feeder may also be a viable option for preventing birdsnesting.

Burnback is the formation of a weld in the contact tip that occurs when the wire feed speed is too slow or if the gun is held too close to the workpiece.

Correcting this problem is easy: increase wire feed speed and the distance of the gun from the workpiece (the nozzle should be no further than one-half inch from the metal). Also remember to replace the contact tip if burnback occurs. Remove the nozzle and the contact tip (which may be melted to the wire), snip the wire, install the new contact tip and replace the nozzle with one that has the appropriate tip recess for the application.

Take home
Quality GMA welds are the result of good welding technique, the proper choice of parameters and the welder’s ability to identify a problem quickly and rectify it.

Armed with some basic information, you can aggressively tackle the most common problems associated with GMA welding without sacrificing time or quality.

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