globular transfer mode is sometimes referred to as “the accident that occurs when trying to change between spray transfer to short circuit.” insinuating that there isn’t an intentional reason for a welder to choose this mode when using GMAW. While its uses are limited, there are very real reasons a welder or procedure may call for the use of globular transfer mode.

Globular transfer mode is named such because of how the wire melts off the end of the consumable wire. As the welding wire melts, it forms a large glob like droplet that are 2-3xs the size of the wire diameter. The globs fall off very erratically and aggressively, often resulting in an inconsistent bead appearance and heavy spatter. The behavior in which these droplets are cast off is very unpredictable but can be managed with a variety of techniques or gas choices. For example, a welder may choose to lower their arc voltage, effectively producing a shorter arc. Additionally, this process can be used in congruence with a shielding gas containing a higher percentage of an inert or non-reactive gas, such as Argon, to create more arc stability and a greater chance of the globs reaching the weld pool.

When used with 100% CO2, globular transfer produces excellent penetration, but the chance of an unpredictable arc and spatter also increases. To offset this, lowering the weld current rather than the voltage is recommended. The effects on the weld in this scenario would be a weld puddle that is more fluid, deeper, and essentially lower than the base metal. This can create undercut (a weld discontinuity) but also contains more of the spatter within the pool. This technique is called a buried arc.

As spatter increases, so does post-weld clean up, which costs time and money. The more spatter produced also decreases electrode efficiency. In fact, globular transfer has the lowest electrode efficiency of all the transfer modes, at only 85-89%.

Despite the challenges and potential weld discontinuities of using globular transfer, this mode produces a deeper penetration than GMAW-S and allows for higher wire feed speeds (WFS).

If the cost of using an inert gas is an issue, but post-weld clean-up time is not a factor or consideration, globular transfer may be a solution for thicker joints requiring a deeper penetration than is offered by GMAW-S.

Globular transfer is not recommended for vertical and overhead positions.

Most GMAW procedures will call for spray transfer or short circuit transfer versus globular.

GMAW welding fundamentals for ferrous metals

Ferrous metals, or metals that contain the element iron, such as carbon steel, cast iron, stainless steel, wrought iron, and mild steel, can all be welded using GMAW or GMAW-P.

Each of these metals requires its own care and consideration on gas, heat input, post-heating, and cooling. For example, mild steel is very common and can be used with a shielding gas of 75% argon and 25% CO2. However, while also common, stainless steel would work best with either a richer argon mix or a helium mix gas. Both use v-grooved drive rollers, steel coiled gun liners, and they can be purchased in the same diameter.

GMAW welding produces no slag crust on the top of the weld and, therefore, can be pushed or pulled. A pulling or ‘drag’ motion of the weld can produce deeper penetration but leave the weld face leaner and more protruded. Whereas pushing the weld puddle may leave the weld with a smoother weld bead contour but have slightly shallower weld penetration. The choice to push vs pull often depends on the material and how thick it is.

With stainless steel MIG welding, it is recommended to weld in a slight up-and-down zig-zag motion when the material is thicker than ¼” in the flat and vertical positions. When the material is less than, then angle the zig-zag motion at an almost diagonal motion.

Mild steel joints have an array of oscillations and weave patterns a welder may choose depending on the position and thickness of the metal.

A T-joint in the overhead position may call for the welder to create a small J shape like pattern. Favoring the top plate but then sweeping through the bottom quickly to help offset the pull of gravity on the weld. Alternatively, a series of up-and-down zig-zags or even a steady hand with a pull position may prove useful.

The wire feed speed to voltage ratio depends on the diameter of the wire, type of transfer mode, type of metal, weld position, and gas mixture.

GMAW welding fundamentals for non-ferrous metals

Contrary to some belief, non-ferrous GMAW welding typically uses DCEP, and often utilizes the pulse setting we read about earlier, for greater heat control. Because some weld wire, such as aluminum, is a rather soft material the risk of jamming the machine increases. To avoid this, it’s recommended to keep the GMAW gun whip less than 6 feet in length, or use a push/pull gun or spool gun (see Figure 9.9). Several welding jobs that require a lot of aluminum welding such as boat building, use a spool gun (Figure 9.8 ) for their portability and that it cuts down on jamming.

Aluminum welding will also require that the steel coil liner for ferrous metals, that runs along the inside of the MIG whip, be replaced with a plastic liner. Additionally, U-groove drive rollers need to be placed into the machine instead of the V-groove rollers, and the tension between the rollers needs to be adjusted appropriately. Consult the manufacturer of your machine or the manual provided by them for their recommendations on replacing the liner, and tension for the wire type and thickness.

Drive roller tension should be 1 or less for a push-pull gun system on aluminum.

Recall that when welding aluminum you should also change out the shielding gas. Using 100% argon is very common when welding non-ferrous metals, but a mix may also be used.

Attributions

1. Figure 10.19: GMAW application by Mgschuler is released under CC BY 3.0