Don’t underestimate MIG welding skills

Anybody can weld, right? That’s what many people think, especially when they are talking about gas metal arc welding (GMAW). You turn on the power source, pull the trigger, and lay a bead on a thin piece of sheet metal. Voila!

Those same observers look at gas tungsten arc welding (GTAW) differently. The welder has to hold the torch in one hand and the filler wire in the other, all while working the foot pedal that controls the current to the welding operation. That’s a lot more complicated, right?

That’s not necessarily the case. Surely, GTAW takes a great amount of skill and a deft touch, but GMAW arguably requires much more knowledge and a skilled hand as well. Over the past couple of decades as GMAW has been incorporated into more manufacturing operations because of its speed and efficiency, many have lost appreciation for the skills required to be a topnotch welder of continuous-flow wire.

That shouldn’t be the case.

Selecting the Right Gas

If you want to make a case that GMAW is a more challenging welding process than GTAW, consider shielding gas selection. With GTAW, shielding with the inert argon gas gets the job done most of the time. Sometimes another inert gas, helium, or even a reactive gas such as hydrogen is beneficial for some specialized applications, but for the most part, GTAW is shielded with argon.

Shielding gas selection for GMAW gets a little more complicated. For welding aluminum, magnesium, copper alloys, and other nonferrous metals, the use of argon and helium still prevails. But MIG welders also must consider the reactive gases, such as CO₂ and oxygen, which are necessary for welding carbon and alloy steels. These reactive gases have an effect on transfer mode, penetration profile, mechanical properties, and weld metal chemistry.

Let’s look at all the variables involved with welding a thin sheet of plain carbon or some low-alloy steel. In this case, a welder might have to make the switch from the popular 75 percent argon/25 percent CO₂ mix, which is most often used for welding thin-gauge material in the short-circuit transfer mode, to a blend with higher argon content (greater than 80 percent) if the welder is to achieve the high-energy spray transfer mode commonly used on thicker material. Higher argon content also is required for the advanced pulse spray transfer modes.

The ratio of the gas blend also has an effect on the penetration and the mechanical properties of the weld.

When welding stainless steel or nickel alloys, a welder needs to change to a shielding gas that either eliminates or limits the amount of CO₂ in the gas. Such a gas helps to minimize the presence of carbon that could change the chemistry of the weld metal.

In short, a welder who works on a variety of metals and thicknesses will have to choose among several possible shielding gases.

Keeping Tabs on the Wire Feed System

In manual GTAW the welder skillfully adds the filler wire to the puddle.

In GMAW the wire is fed automatically with the simple pull of a trigger. That may sound easy, but it is not always that simple, even with the modern GMAW wire delivery systems. Simply stated, a lot of things can go wrong.

The welder must understand several adjustments and select the proper hardware to make this system work right. It is important to look at the equipment setup. These wire feed systems should have smooth V-groove drive rolls for solid steel wire, smooth U-groove for aluminum, and knurled for composite (metal-cored) wire. The selection of a spool gun or push/pull system might be necessary to feed soft aluminum. Drive rolls, guides, gun liners, nozzles, and contact tips must be matched for the size and type of wire. The operator also has to make adjustments to the drive roll tension and the reel brake for the proper delivery of wire.

Fine-tuning the Settings

The setup and troubleshooting for semiautomatic GMAW is quite a bit more complex than the manual GTAW process.

Manual GTAW uses a constant-current power source typically set to DC negative polarity for most metals and AC for aluminum and magnesium. The welder adjusts the current level while welding to achieve optimum conditions. He also is responsible for maintaining the correct arc length and travel speed. The welder also has to adjust the postflow timer, balance control, and sometimes pulsing parameters.

Obviously, plenty of skill and process knowledge are required to make high-quality welds with this manual process. But the fact that the welder can make adjustments to current, deposition rate, and travel speed as he welds gives GTAW an advantage over other arc welding processes for intricate work that involves changes in direction and material thickness.

Now let’s look at the semiautomatic GMAW process. The welder first needs to select the consumable electrode and then make appropriate adjustments to the shielding gas, drive rolls, guides, gun, contact tips, nozzle, and liner for the material that is to be welded. Next, parameters need to be adjusted to achieve the proper transfer mode and energy level required to achieve a sound weld. Welders may select conventional short-circuit, globular, spray, or advanced modes of transfer using pulse spray waveforms or modified short-circuit transfer. Adjustments also may be needed to dial in run-in speed, starting parameters, inductance, pulse frequency, and crater-fill features.

In many cases, such as code welding and repeat production work, proven pre-established procedures can be programmed into the machine as a starting point to establish optimum settings. However, more often than not, the welder is required to start the job using rough settings based on prior experience welding similar parts or from using welding equipment manufacturers’ guidelines and then to make adjustments if they are needed. These basic guidelines can be found on websites, in product literature, and on the inside covers of some welding power sources. The basic instructions call for determining material thickness, setting wire feed speed to achieve adequate amperage for fusion, and then adjusting voltage for optimum arc length. Some applications such as aluminum welding also may require adjusting start and crater-fill features.

A welder has a lot to consider when setting up a GMAW job. It is a good idea to test procedures on scrap parts of similar thickness and mass and then dial in all of the necessary parameters.

Developing the Skills

Once all the settings are made, the GMAW process often is perceived as one of the easier processes to use: Point the gun at the correct angle to the joint, keep the correct contact tip-to-work distance, stay on the front of the puddle, and maintain a consistent travel speed. It sounds easy to do, and it is if you are working on a simple, straightforward welding joint.

It gets more complicated with complex joints. Let’s try applying the same basic GMAW approach to a joint that requires changes in direction, forcing the welder to make smooth changes in gun angle while also trying to maintain a steady travel speed and consistent contact tip-to-work distance. The argument could be made that it takes a higher skill level to make quality welds with the GMAW process than the slower, more controlled GTAW process.

Respect for the Wire Welder

GMAW is an important welding process for the fabricating industry, yet many companies underestimate the skill and knowledge required to become a highly productive wire welder. Experienced gas metal arc welders can maximize parts per shift and minimize the amount of rework coming from the welding department.

Experienced and talented welders should be viewed as a competitive advantage for any metal fabricating company.