Understanding GMAW transfer modes

Proper inductance and slope adjustments in short-circuit transfer mode help to ensure a flatter bead appearance with less spatter.

The gas metal arc welding (GMAW) process uses four basic modes to transfer metal from the electrode to the workpiece. Each mode of transfer depends on the welding process, the welding power supply, and the consumable, and each has its own distinct characteristics and applications.

Several variables dictate the type of transfer you use, including the amount and type of welding current, the electrode chemistry, electrode surface, electrode diameter, shielding gas, and the contact tip-to-work distance. Transfer mode also affects your choice of filler metal used.

Which mode is right for you? Choosing wisely can greatly affect your efficiencies and productivity.

Short-circuit Transfer Welding

In short-circuit transfer, the electrode touches the work and short circuits, causing the metal to transfer as a result of the short. This happens at a rate of 20 to more than 200 times per second.

The advantage of the short-circuit transfer is its low energy. This method is normally used on thin material ¼ inch or less, and for root passes on pipe with no backing. It can be used to weld in all positions.

This mode of transfer generally calls for smaller-diameter electrodes, such as 0.023, 0.030, 0.035, 0.040, and 0.045 in. The welding current must be sufficient to melt the electrode, but if it is excessive, it can cause a violent separation of the shorted electrode, leading to excessive spatter. Using adjustable slope and inductance controls can enhance the transfer to minimize spatter and promote a flatter weld profile. Slope adjustment limits the short-circuit amperage, while inductance adjustments control the time it takes to reach maximum amperage. Proper adjustment of these two factors can produce excellent bead appearance and is essential for short-circuit transfer with stainless steel electrodes.

The most predominant solid stainless steel electrodes are ER308L, ER309L, and ER316L. These electrodes are also available in the Si type, such as 308LSi. The LSi types contain more silicon, which increases puddle fluidity and helps the weld puddle to wet out better than the standard alloys. While minor power source adjustments may be needed, both types can be used successfully as long as the specification for the welding consumables permits.

For carbon steel electrodes, the electrode classification dictates the silicon level. ER70S-3 and ER70S-6 are the most widely used. For pipe applications, ER70S-2, ER70S-4, and ER70S-7 are sometimes used for open-root work because they offer lower silicon levels. The lower silicon produces a stiffer puddle and gives you more control of the back bead profile. In an open-root weld, you may use an S-6 type electrode with less inductance than an S-2 type electrode because the S-6 type has a higher level of silicon and the puddle is more fluid.

Maintaining a constant contact tip-to-work distance in short-circuit transfer is important to maintain a smooth transfer.

The most common shielding gas for the short-circuit transfer mode for carbon steel electrodes is 75 percent argon/25 percent CO₂. Numerous three-part shielding gas mixes are also available for carbon steel and stainless steel for this mode of transfer.

Globular Transfer Welding

Globular transfer means the weld metal transfers across the arc in large droplets, usually larger than the diameter of the electrode being used. This mode of transfer ­generally is used on carbon steel only and uses 100 percent CO₂ shielding gas. The method typically is used to weld in the flat and horizontal positions because the droplet size is large and would be more difficult to control if used in the vertical and overhead positions compared to the short-circuit arc transfer. This mode generates the most spatter; however, when higher currents are used with CO₂ shielding and a buried arc, spatter can be greatly reduced. You must use caution with a buried arc because this can result in excessive reinforcement if travel speed isn't controlled.

Stainless steel GMAW electrodes normally aren't used in this mode of transfer because their nickel and chrome content (9 to 14 percent nickel and 19 to 23 percent chromium) creates a higher electrical resistance than carbon steel electrodes. In addition to the electrical resistance differences, the use of 100 percent CO₂ as a shielding gas could be detrimental to the corrosion resistance of the stainless steel electrodes. Carbon steel ER70S-3 and ER70S-6 generally are the electrodes of choice.

Spray Transfer Welding

Spray transfer is named for the spray of tiny molten droplets across the arc, similar to spray coming out of a garden hose when the opening is restricted. Spray transfer usually is smaller than the diameter of the wire and uses relatively high voltage and wire feed speeds or amperage. Unlike short-circuit transfer, once the arc is established, it is on at all times. This method produces very little spatter and is most often used on thick metals in the flat and horizontal positions.

Spray transfer is achieved with high percentages of argon in the shielding gas, generally a minimum of 80 percent. Also called axial spray, this mode uses a current level above what is described as the transition current. The transition current will vary depending on the electrode diameter, shielding gas mixture percentages, and contact tip-to-work distance. When the current level is higher than the transition current, the electrode transfers to the work in very small droplets that can form and detach at the rate of several hundreds per second. Sufficient arc voltage is required to ensure that these small droplets never touch the work, achieving a spatter-free weld. Spray transfer also produces a fingerlike penetration profile.

This transfer mode is used mostly in the flat and horizontal positions because it produces a large weld puddle. High deposition rates can be achieved compared to the other transfer modes. Because of the arc length used, it is also more easily influenced by magnetic fields. If this is not controlled, penetration profile, bead appearance, and spatter levels can be negatively affected.

The major factor in choosing a carbon steel electrode is sometimes the amount of silicate islands that remain on the weld bead surface. This is especially the case if you need to minimize postweld cleaning time or if the finished product will be painted. For this reason, you might choose an ER70S-3, ER70S-4, or ER70S-7 electrode. With stainless steel electrodes, there is little difference in the bead appearance in the Si types because of the higher energy used in this mode of transfer. The wetting action advantage of the Si types is not necessary, and if they are used it usually is a matter of preference. The effect of the chemistry on the transition current is minimal, but a higher voltage may be required with one alloy compared to another to achieve a true spray.

Pulse-Spray Transfer Welding

In the pulse-spray transfer mode, the power supply cycles between a high spray transfer current and a low background current. This allows for supercooling of the weld pool during the background cycle, making it slightly different than a true spray transfer. Ideally, in each cycle one droplet transfers from the electrode to the weld pool. Because of the low background current, this mode of transfer can be used to weld out of position on thick sections with higher energy than the short-circuit transfer, thus producing a higher average current and improved side-wall fusion. Additionally, it can be used to lower heat input and reduce distortion when high travel speeds are not needed or cannot be achieved because of equipment or throughput limitations.

Generally, the same shielding gases used for spray transfer are also used for pulsed-spray mode.

The electrodes you can use include all the standard carbon steel and stainless steel types, along with some of the specialty alloys such as INCONEL® (625), duplex (2209), and superduplex (2509). With a programmable pulse power supply, most solid-wire alloys can be used with a customized pulse waveform.

With all modes of transfer, the wire type will have some effect on the machine settings. In addition, the wire surface will affect the transfer. Manufacturers use different types of arc stabilizers on the wire surface to enhance a smooth transfer. This is why small adjustments must be made when welding with the same type of electrode from different manufacturers.