A Look at GMAW Metal Transfer

Understanding how liquid metal drops are formed and transferred to the weld pool will make you a better welder

By AUGUST F. MANZ, as AWS Fellow

Reprinted with permission: The AWS Welding Journal

This article describes how the liquid metal drop on the arc end of the electrode extension is created and then ejected or transferred into the molten metal weld pool. The piece of wire electrode that forms the drop passes from a wire reel at room temperature and is fed through the contact tip. It then travels from the contact tip into and through the electrode extension zone on its way to the arc zone. During its passage through the electrode extension and toward the arc, it is Joule heated by the current that passes from the contact tip through the electrode extension resistance and to the arc, where it is melted by the heat of the arc.

With GMAW spray transfer, the liquid metal drops are transferred toward the weld pool by combined forces due to surface tension, gravity, and electromagnetic pinch. To be precise, other forces can play a part in the transfer process, but these are the major ones.

When a drop is on the end of the electrode, surface tension acts to keep it from transferring. Both gravity and electromagnetic pinch act to transfer the drop. Then, when the drop touches the weld pool, surface tension acts to complete the transfer of the drop into the pool.

How Much Electrode Makes a Typical Drop?

When a drop is the same diameter as the electrode, a simple calculation will show that it needs a piece of electrode (cylinder) that is 2/3 as long as its diameter.

The Calculation

Drop volume = Vd = (4/3)πrd3

Cylinder volume = Vc = (h)πrc2

Where

Drop radius = rd

Cylinder radius = rc

Cylinder length = h

When

rd = rc and 2r = diameter (D)

Let Vd = Vc and solve for (h)

Then (h) = (2/3)D

For example, when welding with a 0.045-in.-diameter electrode, a drop with the same diameter as the electrode needs a piece of electrode 0.03 in. <(2/3)D> long. If the electrode extension is about 0.3 in. (somewhere typically between 0.25 and 0.5 in.), there is enough material in the electrode extension to make 10 drops

(10 × 0.03 = 0.3).

Each of the 2/3D pieces starts out from a reel of electrode wire at room temperature as it is fed into and through the contact tip on its way to the arc zone. During its travel to the arc zone, the 2/3D piece is heated internally by the current passing through its electrical resistance, according to the Joule heat equation:

I2Rt = Joules (watt seconds)

Where

I = current in amperes

R = resistance in ohms

t = heating time in seconds

Each of the 2/3D pieces can be thought of as sharing its heat with its neighbor. Therefore, the electrode extension’s temperature will average out and rise steadily as each 2/3D piece travels through the electrode extension zone on its way to the arc zone. In this example, by the time a 2/3D piece reaches the arc zone, it can be thought of as having been heated 10 times.

When the 2/3D piece finally reaches the arc zone, it will be partway to melting. Typically for steel, it will have reached a few hundred degrees Celsius. The internal I2Rt heating is almost 100% efficient because the electrode feed rate is greater than the thermal diffusivity (the speed of heat travel in the electrode extension metal) and thermal conductivity. Only a small amount of heat is lost due to radiation and convection.

What Happens to Drop Size When the 2⁄3D Length is Changed?

A calculation will show that when twice as much electrode is used for each drop (2 × 2/3D), the drop diameter will be 1.26 times the electrode diameter. When half as much electrode is used for each drop (1/2 × 2/3D), the drop diameter will be 0.794 times the electrode diameter. The range of 1.26 to 0.794 illustrates that a typical drop diameter does not vary a great deal. Therefore, the remainder of this discussion will be based on the 2/3D length.

What Happens to the 2⁄3D Piece When It Reaches the Arc?

When a 2/3D piece finally reaches the arc on the end of the electrode extension, it begins to be heated by the watts of the arc. The arc watts are created by the current passing through the arc’s anode voltage or cathode voltage, as the case may be. The arc watts cause a rapid rise in the temperature of the 2/3D piece. As the heat of the arc watts is transferred into the 2/3D piece, more of it melts and it becomes a drop.

With GMAW spray transfer, as shown in Fig. 1, the drop is transferred from the end of the electrode and through the arc by the combined forces of gravity, electromagnetic pinch, and surface tension. When the transferred drop merges with the weld pool, surface tension completes the transfer.

With GMAW short circuit transfer (GMAW-S), as shown in Fig. 3 (Ref. 2), the continuing electrode feed rate causes the drop to contact the surface of the weld pool, where it is transferred by the forces mainly due to surface tension and electromagnetic pinch. The effect of gravity is minimal.

With GMAW-S, although the electromagnetic pinch force can help transfer the liquid drop, it is the surface tension force that dominates the transfer. Some power supplies even decrease or eliminate the current at the instant of transfer, thus minimizing or eliminating the electromagnetic pinch force. This allows only the surface tension force to complete the drop transfer into the pool.

As a consequence of this dominance of the surface tension force and the uniformity of the drop size, the drop transfer sequence does not vary greatly. Transfer time is relatively constant for a specific material, welding condition, etc. For example, with steel electrodes, the measured transfer time ranges between 0.002 and 0.003 s per transfer.

What Happens to Drop Size as the Current Magnitude Changes?

With lower current, the electromagnetic pinch force is less. Therefore, the drop will increase in size because gravity is the only force left to overcome the retaining force of surface tension.

With higher current, the electromagnetic pinch force is greater. Therefore, the drop will decrease in size because the pinch force will assist gravity in overcoming the surface tension retaining force.

What Can You Do to Promote Better Metal Transfer?

The best metal transfer occurs when drop size and drop frequency are stable, in other words, when drops have the same size, shape, and frequency of transfer. Therefore, it is essential that surface tension is uniform. Uniform surface tension depends on the surface chemistry (composition) of the drop. This requires excellent gas shielding (type and amount). Do not be careless in setting the gas flow rate.

Uniform electrode wire surface condition is very important for maintaining a stable surface tension and feed rate. Keep the wire shielded from dust and contamination. A clean electrode wire will have a more-stable feed rate. A stable feed rate helps promote a stable drop size.

Most important is your welding technique. Be careful to maintain a consistent arc length and manipulation when you weave or move the arc. Stability promotes uniform drop size. Finally, do not fuss with the power source settings. Use the settings recommended for the welding condition. Remember, everything you do can affect drop size and frequency. Stable transfer makes the best welds.  WJ

References

1. Welding Handbook, Tenth Edition, Volume 1, Welding and Cutting Science and Technology, Part I. Miami, Fla.: American Welding Society.

2. Manz, A. F. 1963. Advances in power supplies for industry. Welding Journal 42(9): 719–724. Work Consulted

Koellhoffer, L., Manz, A. F., and Hornberger, E. G. 1988. Welding Processes and Practices. Hoboken, N.J.: John Wiley and Sons, pp. 343, 346.

Fig. 1 — Schematic representation of GMAW. (Source: Adapted from Linnert, G. E., 1994, Welding Metallurgy, 4th ed., Miami, Fla.: American Welding Society, Fig. 6.12.)