Gas Metal Arc Transfer Modes: Part 2 – Spray and Pulsed Transfer

In this second and final article on transfer characteristics, we will discuss the Spray and Pulsed GMAW Transfer modes.

Spray Transfer

Spray transfer takes place with higher currents and voltages then Globular Transfer. Spray transfer is also dependent on the size of the electrode and argon rich shielding gas mixtures with a minimum of 85% Argon. As you increase the wire feed the current increases achieving a transition change from globular to spray transfer (known as the transition current) there is an increased flow of droplets across the arc and the diameter of the droplets become smaller and are typically less than the diameter of the wire. The transfer therefore takes place in the form of a fine spray also known as axial spray, coupled with deep penetration and a relatively large weld pool. 

This can lead to difficulty using spray transfer on thinner material as it can cause burn-through. This large weld pool is also very difficult to control and maintain during all positional welding, so it is mainly used with thicker materials in the flat or horizontal positions, principally on steel and stainless steels. Aluminum and other nonferrous metals can be welded in all positions in the spray mode because the weld pool solidifies quickly, maintaining a smaller and, thus, more manageable pool.

Spray transfer achieves greater productivity over globular and short-circuiting transfers that use lower currents and wire feed. Increasing the wire feed and voltage to achieve spray transfer generates higher deposition rates. Spray transfer has a smooth and quiet arc with minimal spatter and has nice bead appearance with finger type penetration profiles.

Most structural welding codes and standards accept GMAW Spray Transfer mode for critical structures and offer many prequalified joints.

Figure 1 shows the Spray Transfer of smaller droplets projected into the weld pool.

Figure 1, Image of Spray Transfer

Typical shielding gas mixtures for spray transfer use a high argon with CO2 or Oxygen (O2) mixture. The most common mixtures for steel are 85-92% Argon + 15-8% CO2 and for stainless steel 98% argon + 2% O2, or a mixture of Helium, CO2 and O2.

For aluminum the mixture is 100% Argon or a mixture of Argon and Helium depending on the thickness of the materials. Figure 2 shows the different penetration profiles for spray transfer showing the finger like penetration over globular transfer.

Figure 2, Effect of argon rich shielding gas mixture on penetration and bead shape 

Pulsed Transfer (GMAW-P)

Pulsed transfer is achieved using specially designed power sources that allow the change from high peak current level (at a level of spray transfer current range) to a lower background current (at a level of globular transfer current range) in the form of pulses at a high controlled rate. Pulsed transfer is a form of spray transfer.

The peak current increases above the transition to spray transfer and then drops to a background level below the transition from spray transfer thus in each pulse a nominal droplet of metal is detached and transferred into the weld pool. 

The average current in pulsed transfer is much lower than that of spray transfer and mirrors the globular transfer range however the bead appearance is that of spray transfer, with minimal spatter. The lower average current also means a smaller weld pool and lower heat input which allows for out of position welding and welding of thinner materials with larger wire diameters. Due to the higher peak currents, the depth of penetration is the same and, in some cases, better than that of conventional spray transfer. 

Pulsed transfer of aluminum offers some benefits over conventional spray transfer. Due to the lower heat input generated when using pulsed transfer, then larger diameter wires can be used to weld aluminum which reduces the feeding problems associated with smaller diameter wires. Also, there is improved control of the arc in out of position welds and the bead appearance is better than that of convectional spray transfer.

Figure 3 shows the pulsed transfer peak and background current settings related to the droplet detachment.

Figure 3, Pulsed Spray Transfer

Benefits of using pulsed transfer over spray transfer are:

• Out of position welding on ferrous and non-ferrous metals, 

• Minimal to no spatter, 

• Quality welds, including on thinner metals, 

• due to the lower heat input, larger diameter wires can be used to weld thinner materials.

Modern power sources today allow a synergic pulsed programing mode to control the peak current, background current, pulse width and the frequency of pulsing. These parameters are pre-programmed for a given application and use of shielding gas, material and wire diameter. This means by selecting the wire feed, wire size and shielding gas on just one knob all the parameters mentioned above will change at the same time to match the selection. 

There are many different types of pulsed welding equipment available from different manufacturers for controlling pulse programing options and therefore, care should be taken when selecting a pulse GMAW power source. Make sure you know what you want and then talk to your welding supplier who can help you select the correct power source for your needs.