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
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.